Stabilized Approach to the End
              
                    Professionally Flying with Google

                              A Virtual Aerodynamic Experiment

                                       Ralf Vandebergh

                                                    
                                                                     Index


                                                   I   Introduction                                                                  
                                       
                                                  1   Changing Stability by Decalage

                                                  2   A Stable Aircraft Landing

                                                  3   Self-Stabilization of the GE SR-22

                                                  4   Stable Landings with the GE F-16

                                                  5   GE F-16 Stabilized Approach in Practice

                                                  6   Unstable Aircraft and Power for Altitude

                                                  7   GE F-16 Precision Landing

                        The Experiment  8   Handsfree Landing of the GE F-16




                          
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                                                                    Site Purposes


--  Demonstrating why glidepath correction methods depend on aircraft type 

--  Demonstrating behave of power-for-altitude glidepath corrections on pitch unstable aircraft

--  Demonstrating how a perfectly for a stabilized approach trimmed F-16 can land itself fully handsfree

--  Including Google Earth SR-22 & F-16 flight simulator accuracy tests on pitch & speed stability at low & high angle of attack

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Introduction


Preface


This website contains an aerodynamic experiment with the F-16 flight simulator in the Google Earth program. The F-16 aircraft, known for its neutral or relaxed pitch stability (close to unstable in pitch) will be landed several times handsfree on a runway, thus without any touch of the controls. The Google Earth flight simulator is particularly suited for this demonstration as its integrated 2 types of aircraft are  perfectly usable to show how pitch stability can be manipulated to approach each others pitch stability characteristics. In real life, the F-16 stability is regulated computer controlled but this is not the case if running a flight simulator as the one integrated in the google Earth software. To still be able to land the F-16 simulator fully handsfree, it must be flown in a highly stable configuration. In the virtual experiment at the end of this page, the Google Earth (GE) F-16 flight simulator will be landed 6x on a runway without touching any controls, requiring it to be trimmed for a perfectly stabilized approach. Therefore, glidepath or altitude adjustments during the approach must be as subtle as possible and established with caution. I found that to accomplish this, a particular method which is usually only applied on pitch/speed stable aircraft, worked best: Correcting the altitude with the power. Therefore, demonstrated on this website are the advantages but also the disadvantages of power-for-altitude corrections on aircraft with weak pitch - and speed stability.


Strong pitch stability and corresponding speed stability of many light aircraft make it possible to adjust the altitude with the throttle (engine power) on a final approach while the trim speed remains relatively stable. On large jet aircraft this method is usually not applied because the
much higher approach speed in combination with the slower engine response is rather ineffective in glidepath corrections compared to the immediate pitch adjustments using the elevator. When changing the engine power of an aircraft with strong stability in the pitch axis, energy is converted more into a pitch change (for example a climb) then into airspeed. When the engine power of an aircraft with weak stability in the pitch axis is changed, energy is converted more into airspeed then into a pitch change. Adjusting the altitude on a final approach with this type of aircraft works therefore more effectively by using the elevators instead of by using the engine power. This also implements that changing the airspeed with the engine power works far more effectively on aircraft with weak pitch stability then on aircraft with strong pitch stability. To change the airspeed on a pitch stable light aircraft, changing the pitch attitude is the better and especially safer method as any change in engine power would immediately result more into a climb or descent then into a changed airspeed. This difference can be demonstrated perfectly with the 2 aircraft in the Google Earth flight simulator: the F-16 (has low pitch stability) and the SR-22 (has high pitch stability).

On this page is demonstrated that this difference is no absolute constant but that the behavior by aircraft type is different with the angle of attack due to changing decalage (difference between the angle of the wing and the tailplane). The more an aircraft with high pitch stability is flown at a lower angle of attack, the more energy is converted into airspeed and the less energy into a pitch change. This way it is approaching the stability characteristics of an aircraft with low pitch stability. Conversely
, the more an aircraft with low pitch stability is flown at a higher angle of attack, the more energy is converted into a pitch change and the less energy into airspeed. In this case, the aircraft approaches the stability characteristics of an aircraft with high pitch stability. The principle of manipulating the longitudinal stability of an aircraft using changes in the angle of attack will be more precisely explained in the section 'Changing stability by decalage'.


                                                                              Ralf Vandebergh,  astronautics journalist

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                                                        Preview of handsfree landing experiment 5, the most difficult of the 6 handsfree landings when the GE F-16
                                                        was landed handsfree in the flare attitude at lowest possible speed.
This requires to configure the aircraft for
                                                        a long trajectory near the edge of stall speed. The video shows the phases from trimming until touchdown.
                                                        The last subtle heading corrections before the handsfree trajectory are made using the rudder, as ailerons are
                                                        already trimmed in this phase.
Note the poor sight on the runway due to the high pitch attitude, a difficulty.




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                                                Google Earth F-16, Aerodynamic Facts  


If you are used to ride a bike with a carbon frame but then perhaps you switch to an aluminium frame, you will experience less stability in the corners. As the brain tends to correct more and more for the loss of feeling stability, after sufficient time you will be able to take corners with an increased feeling of stability again. In fact we could say that this is a kind of artificial stability. In this matter, there is a relation with the F-16 aircraft. The F-16 is designed for low stability thus for high maneuverability. In real-life, artificial stability of the aircraft is created using computer control (fly-by-wire). The Google Earth F-16 flight simulator, an integrated function in the software, offers a great opportunity to experience the true aerodynamics of the F-16, as it would perform purely without computer control. A new user of this GE F-16 flight simulator will note that it is very sensitive in flight, a direct demonstration of the true and unmanipulated flight properties of the F-16. A virtual aerodynamic experiment on this page will demonstrate that the GE F-16
can even land itself on a runway when perfectly trimmed and stabilized on a final approach. This is called the handsfree landing experiment. As the title of this page suggests, in this experiment will be demonstrated what happens if a stabilized approach is flown literally to the end without the execution of a flare maneuver as usually is the case in nominal aircraft landings. Instead, the aircraft will be trimmed to an attitude that is similar to the angle of attack used for the flare and it will land itself in that attitude automatically.


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         To learn more about the Google Earth F-16 Aerodynamics in Flight visit:


                                                                              GE F-16 In-Flight Analyses


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                                                     1.  Introductory Remarks


                                                                                 Changing Stability by Decalage


                                                                                                      
                                                The definition of Decalage and its role within Aircraft Stability
 

Although the word 'decalage' is used regularly within aircraft aerodynamics, the exact role of the mechanism seems almost unexplained, and even on the world wide web it's hard to find any satisfying definition. Generally known is that decalage is about the angle of incidence difference between the main wing and the tail wing. Some sources mention that decalage is related to pitch stability and here is where things get confusing: The general function of pitch stability or longitudinal stability is attributed to the position of the center of lift relative to the center of gravity (static margin). As an aeronautics journalist I decided to find out an appropriate discription about the true role of decalage within aircraft stability in order to make an attempt to fill the gap of the missing information. This is the result:
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Decalage comes into play in every flying system which has more then 1 angle of attack involved. A flying system with only 1 angle of attack is a flat plat and only this system can function properly without the mechanism of decalage. This is because a flat plate with the center of lift behind the center of gravity automatically corrects itself in the pitch attitude. As soon as there are more angles of attack into play, it is a requirement to ensure that these angles are orientated in the right way relative to each other. This can be illustrated with the following example of an aircraft with a main wing and a tail: Even when the center of lift is behind the center of gravity, the aircraft can not correct its pitch attitude after a disturbance when the tail wing has a higher angle of attack then the main wing. In this situation, the lift in the center of lift of the aircraft works in the wrong direction. This proves that pitch stability depends not only on static margin but also on decalage.

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How Decalage works in Practice

Any technical book about aircraft will mention that to create stability in the pitch axis, the tailplane (horizontal stabilizer) must have a lower angle of attack then the main wing. This is partially correct, it is more correct to state that the wing surface in the back must have a lower angle of attack to create a properly working stability mechanism. This is important to note because some aircraft (canards) have the horizontal stabilizer in the front. Even more interesting, there are aircraft configurations with 2 wings of the same size (tandem wing) in which both wings can either work as a main wing or as a horizontal stabilizer.

The key to create longitudinal stability in a wing configuration with more then 1 angle of attack is that the wing surface in the back must generate the biggest moment in order to counteract a disturbance which is coming from the front. Note that the definition of a 'configuration' in this relation is very broad; A configuration could also be a system with no separated aerodynamic surfaces such as a flying wing were the reflex trailing edge in fact is the same surface as the main wing. This works similarly as a normal aircraft configuration with separated wings and tail. It can be mathematically relatively easily explained why a lower angle of attack creates the biggest pitching moment. The following examples explain how decalage works within its role in aircraft stability.


Positive Decalage
Imagine the main wing has an angle of attack of 2 degrees and the horizontal stabilizer an angle of attack of 1 degrees. When there is a pitch down disturbance of -1 degrees, the nose pitches down -1 degrees due to this disturbance. At the moment of the disturbance, the new angle of attack of the main wing becomes 2 degrees -1 degrees = 1 degree. This represents a 50% lowering in angle of attack. For the horizontal stabilizer the new angle of attack becomes 1 degree - 1 degree = 0 degrees. This represents a 100% lowering in angle of attack. As the horizontal stabilizer experiences more lowering in angle of attack then the main wing, it also produces more negative lift. This downward lift creates a pitching moment, thus the tail goes down and the nose moves back up: the aircraft returns to its original trim attitude so we have longitudinal stability. The greater the aircraft angle of attack / decalage is, the more energy will convert into a pitch change instead into airspeed after a change in the engine power. This is because the greater negative lift on the horizontal stabilizer - as a result of the lower AoA - creates a pitching moment when the thrust is increased. At a low positive angle of attack / decalage, there is a pitching moment as well as soon as the airspeed increases. This pitching moment is significantly slower and this is because energy converts first into airspeed after the engine change. Note that the wing and tail angles in this example are not representative for real life aircraft but just to demonstrate the principle in an easy way. In reality the difference in angle of attack usually is bigger (about 3 degrees).

Zero Decalage
In the next example the aircraft is trimmed for zero decalage. This means we increase the angle of the horizontal stabilizer relative to the angle of the main wing until there is no (zero) difference in angle. If the stabilizer and the main wing have the same angle, we have zero decalage. In the example of the pitch down disturbance, there is no difference in lowering of angle of attack thus no pitching moment. The aircraft will not return to the original trim attitude but experiences neutral longitudinal stability: it will not pitch up nor pitch down. Therefore at zero decalage, energy will convert maximum into airspeed after a change in the engine power. This example also explaines the link between the neutral pitch stability of the F-16 and the minimal decalage in cruise conditions. The situation changes considerably in high angle of attack conditions when positive decalage is created by the up-stabilator deflection which causes downward lift. This pressure on the stabilator creates some positive longitudinal stability.

Negative Decalage
In the last example the aircraft is trimmed for a negative decalage. Now we increase the angle of the horizontal stabilizer relative to the angle of the main wing. In this condition we have the inverted situation as in the first example. Now the horizontal stabilizer has a higher angle of attack then the main wing. Or we can say that the main wing has a lower angle of attack then the horizontal stabilizer. The effect in a pitch down disturbance is that now the main wing experiences more lowering in angle of attack then the horizontal stabilizer thus produces more negative lift thus more downward force. The result is that the nose pitches down even more. The aircraft now experiences negative longitudinal stability. At a negative decalage, energy will convert again into a pitch change but the opposite effects of positive decalage occur: When the engine power is increased, theoretically a pitch down maneuver will occur
. In practice however we may notice a deviation from this effect (see section aerodynamic decalage).

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Effects of Negative Decalage on Stable and Unstable Aircraft

Much is written about positive decalage in aerodynamic resources but the effects of negative decalage are barely discribed due to their unimportance in normal stable flight. Pitch stability or instability is characterized by static margin (SM) which is the distance between the center of gravity (CG) and center of lift (CL) of an aircraft. This center of lift is characterized as the neutral point (NP), the neutral point between all lift acting on the aircraft and therefore mostly dominated by the main wings. Note that the NP is just the center of lift of the entire aircraft and determines nothing about positive or negative sum of lift as an outcome. Static margin is positive when the CG is ahead of the CL and negative if it is behind. SM is expressed as percent and determined by taking the NP percentage of the main aerodynamic chord (MAC) minus the CG percentage of the MAC. (measured from the chord leading edge).The outcome is a positive value if the CG is ahead of the NP and a negative value if the CG is behind the NP. In the case of the F-16 the static margin is around -0.04 of MAC (-4 percent). To visualize this: The center of gravity is in this case just slightly behind the neutral point, namely -4 percent of the MAC. The aerodynamic effect is that the aircraft is only very slightly unstable in pitch, it is almost neutrally stable (which would be at 0.00 of MAC).

Negative Decalage and Positive SM
An aircraft behaves differently when negative decalage is combined with positive or negative static margin. Negative decalage and positive SM causes an unstabilizable pitch-down maneuver in normal undistored flight. This is the effect of negative decalage on a pitch stable aircraft and it is actually caused by the fact that the positive lift on the horizontal stabilizer now works in the same direction as the pitching moment around the center of gravity instead of counteracting it. A cardboard model airplane with positive pitch stability resembles negative decalage when it is thrown in an inverted attitude. Once in the air, the airplane will correct itself back into positive decalage attitude as soon as possible.

Negative Decalage and Negative SM
Negative SM and negative decalage causes only an unstabilizable pitch-down maneuver if there is a pitch-down disturbance. In this case the clockwise rotation of the CL around the CG in undistored flight is counteracted by the positive lift of the horizontal stabilizer. This means the aircraft can fly level as long as there is no pitch disturbance. This is the effect of negative decalage on a pitch unstable aircraft. An aircraft with negative longitudinal stability with the center of lift ahead of the center of gravity
is not able to stabilize itself in the case of a pitch disturbance, even though the horizontal stabilizer is producing positive lift. As we have in this case negative decalage, the surface with the lower angle of attack (wing) is in the front. Imagine an angle of incidence of the wing of -1 degree and an angle of incidence of the horizontal stabilizer of +1 degree. When there is a -2 degree pitch-down disturbance, the wing new angle of attack becomes -1 -2 degree = -3 degrees. This is a 300% lowering in angle of attack. The new angle of attack of the horizontal stabilizer becomes +1 -2 = -1 degrees. This is a 200% lowering in angle of attack. Thus the wing experiences more lowering in angle of attack and the difference creates a pitch-down moment. As the horizontal stabilizer is producing positive lift, it is not able to counteract this nose-down pitching moment, instead it helps the pitch-down moment.

(See also chapter 'Relation between aircraft forces and decalage'.
)

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                                                                     Importance of Aerodynamic Decalage


When the theory as described in the previous section is tried out with the GE flight simulator, we will find that all works perfectly except some effects of negative decalage. When trimming the GE SR-22 or F-16 for a slightly negative angle of attack we have negative decalage (higher angle of attack of tail, lower angle of attack of wing). According to theory, when increasing the engine power, the nose should pitch down because of the lower angle of attack of the wing which would create a pitching moment in the downward direction. In practice, this effect actually not occurs but the opposite is happening: we experience a slow pitch up maneuver.

The effect we experience with the GE flight simulator can be expained by the asymmetric wing camber (more upper camber, less lower camber) which causes that the wing is still capable of producing lift at zero or slightly negative angles of attack. This is especially due to the high airspeed that is corresponding to a negative angle of attack / negative decalage. This high airspeed is caused by the fact that lift and weight work in the same direction at a negative angle of attack. At a positive angle of attack both forces are counteracting and are equal when the aircraft is flying level (it is not ascending nor descending). Only a symmetric wing would produce zero lift at zero angle of attack. An example of a symmetric wing is a flat plate which will start to produce positive lift as soon as the AoA is positive and negative lift as soon as the AoA is negative. Thus in the case of a symmetric airfoil, the aircraft indeed would pitch down when the engine power is increased at negative decalage. 

Due to the asymmetric wing camber and its shifted zero lift line, it is only practically useful to measure decalage relative to the zero lift lines of the wing and the horizontal stabilizer (aerodynamic decalage).




Aerodynamic Effects of Flaps

The flaps increase the angle of attack and the camber of the wing and shift the zero lift line upward. Thus when flaps are increased, the geometric decalage becomes larger but the aerodynamic decalage becomes even larger. At high angle of attack, deployed flaps can be used to create greater positive decalage at lower airspeeds. When using deployed flaps to create a greater negative angle of attack under cruise conditions and negative geometric decalage, we should note that the aerodynamic decalage could be zero or even positive in the same condition.

When deploying flaps in
the GE SR-22 and GE F-16 flight simulator we experience a pitch-up maneuver. A user will notice that this pitch-up movement becomes greater the more percent flaps are added (in fact the more the flaps are lowered). This effect is present in real life as well with different types of aircraft. Although the deployment of flaps in fact creates a nose-down pitching moment caused by the increased lift of the wing and the movement of the center of pressure to the rear, the torque between the CG and the CL is mostly not large enough to let this nose-down moment happen. The reason is that the increased angle of attack of the wing caused by the flaps causes a greater difference in angle between wing and horizontal stabilizer. A higher angle of attack of the wing is similar to a lower angle of attack of the horizontal stabilizer, providing more negative lift and creating more downforce thus pitches the nose up. This is in fact the same as a greater upward deflection of the elevator for example on take-off. The much longer arm between the wing and the horizontal stabilizer creates a greater moment then the shorter arm between the CG and the CL.

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Practical Usage of Pitch Stability                   


A perfect aircraft landing starts with a well executed final approach. To stay on the proper glidepath to the runway, generally small altitude corrections are required and these can be done in 2 different ways. For aircraft with strong pitch (longitudinal) stability these corrections can be made by changes in the engine power (throttle). An increase in power will cause the aircraft to raise the nose (increased pitch) while the airspeed remains stable. The advantage of this method is that both the altitude and the airspeed are controlled by just one control input, namely the throttle. The airspeed is controlled by this method using the proper nose attitude (pitch). When for example the approach speed should be 100 knots, but we have 90 knots, then we should lower the nose to pick up airspeed until we have the required 100 knots. Because this way the glidepath angle has become steeper, we lose altitude more quickly so we have to increase the power to gain altitude. In real-life this generally is the standard method teached to students on light aircraft. The greatest advantage of this method basically is to rule out stall occurance caused by a pitch-up maneuver without correcting the airspeed. Demonstrated
 on this page is how pitch stability and speed stability is related and how it can be manipulated. Further on this site is demonstrated what effects occur if the power-for-altitude method is used on low pitch stable aircraft.

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Difference between real-life flying and simulator flying

An important note should be made about a delay in the airspeed as visible on the instrument panel in a real aircraft. This delay is not present in the GE flight simulations. In real live (light) aircraft, the pitch attitude (nose attitude) is an important indicator for the airspeed. This is because a certain pitch angle corresponds to a certain airspeed and when holding that attitude, it can directly be used to estimate the right airspeed without delay as on the instrument panel. Instructors teach their students therefore to focus on the right pitch attitude trim before making glidepath corrections. Note that this technique only applies to aircraft that on approach are flown 'pitch for airspeed, power for altitude' such as the Google Earth SR-22.
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The Google Earth flight simulator may look simplistic but it is  an amazing tool of technology. The picture below shows how it was done in the 1960's by NASA. For their flight simulators, a camera was moved over an artificial landscape on a wall. Screenshot from the documentary: NASA's Fastest Experimental Supersonic Airliner (Documentarytube).


        
                        


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                                                                 2.  A Stable Aircraft Landing

 

                                                Technique of the Stabilized Approach

The simplicity and the requirement to control all settings fully manually in the Google Earth flight simulator are pre-eminently suited for trying out principles of aircraft landing basics such as the stabilized approach. In air traffic, the stabilized approach is prefered for flight safety and for lowering the workload of pilots and controllers. Not all pilots agree that the stabilized approach is the best way to land and much depends on the type of aircraft. Originally this type of approach was attuned to heavy passenger jets which were involved with high drag and slower power response. High drag causes a high sink rate and compensating with power can take too long due to the slow power response as it takes several seconds for the engines to spool up. The solution was a stabilized airspeed and a stable sink rate (vertical speed) in the final approach to the runway.

There is a difference in the definition of a stabilized approach in general aviation and in aerotechnical terms. For professional airliner pilots it means a list of proceedings before the landing including the technical part (stable speed, stable descent rate) but also completing checklists and setting the aircraft in the landing configuration. Here we just look at the aerotechnical meaning of the stabilized approach. The goal is to fly the final approach to the runway at a constant speed and glide angle. In a well stabilized approach, a minimal number of control adjustments need to be done. Most aircraft are stabilized in the last 1000 feet above runway elevation. In a perfect stabilized approach, the aircraft would theoretically fly itself to the runway and would land without any touch of the controls. Just because this is only pure theoretically, the idea developed that this would be an interesting experiment to try out using the GE flight simulator.

This experiment with the Google Earth F-16 will be executed in the section 'Handsfree Landing, the experiment'. First some preparing test flights.


                                            Screenshot of one of the GE F-16 stabilized approaches from the 'Handsfree Landing Experiment' on this site
                                                           The final approach was flown at a fixed airspeed of 140 knots and a vertical speed of -800 feet per minute


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                                                 Introduction into Aircraft Stability

                                               

                                                          Relation between Wing/Tail Forces & Angles


The position of the center of grafity (CG) and center of lift (CL) of an aircraft are directly related to the decalage thus the difference in angle between the wing and the horizontal stabilizer. If the CG is at a short distance in front of the CL (low positive static margin), the decalage should be relatively small (minimal difference in angle between wing and tailplane). If the CG is moved more to the front to create a larger moment arm between the CG and the CL (high positive static margin), decalage must be increased as well. With the CG more in the front, the longer moment arm must be compensated. This is realized by a lower angle of attack of the horizontal stabilizer. The greater negative lift ensures a pitch down moment of the tail to compensate for the nose down pitching moment. An aircraft is more stable the longer the moment arm between the CG and CL and thus the larger the decalage. But it is also less maneuverable in this condition while the large decalage causes excessive downward trim drag. Therefore even some passenger aircraft are designed with the CG very close to the CL (very low static margin) and very small decalage causing less trim drag while stability is artificially created using computer control. If the center of gravity is located behind the center of lift, the lift force creates a clockwise rotation around the center of gravity. The horizontal stabilizer must in this case provide positive lift to counteract this moment so it must be set in a higher angle of attack configuration then the wing. This automatically means that the decalage must be negative in order to realize level flight with a negative static margin. A positive decalage in this case would increase the clockwise rotation. On the other hand, when a negative decalage is combined with a positive static margin, thus with the CG in front of the CL, the positive lift force on the horizontal stabilizer is helping the natural counterclockwise rotation of the CL around the CG as would occur on a tail-less aircraft. The large moment arm of the tail increases this nose-down pitching moment.


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                                                                   Principle of Trim as explained by NASA

On most aircraft, the center of gravity of the airplane is located near the center of pressure of the wing. If the center of pressure of the wing is aft of the center of gravity, its lift produces a counter-clockwise rotation about the cg. A positive lift force from the tail produces a counter-clockwise rotation about the cg. To trim the aircraft it is necessary to balance the torques produced by the wing and the tail. But since both rotations are counter-clockwise, it is impossible to balance the two rotations to produce no rotation. However, if the tail lift is negative it then produces a clockwise rotation about the cg which can balance the wing rotation.

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                                                                       Neutral & Negative Pitch Stability

An aircraft designed to be highly maneuverable (F-16) has the center (CL) of lift located very close to the center of gravity (CG) or even the CL in front of the CG. Pitch stability is 100% neutral if the negative gravity force is at the same spot as the positive lift force. Such an aircraft is much more maneuverable then a stable aircraft that has the negative gravity force at a larger distance in front of the positive lift force. This is thanks to the lack of any momentum arm (torgue) between the CG and CL. In this design, decalage should be zero under cruise conditions and the tailplane produces in these conditions zero up - or downforce. In the case of true negative pitch stability were the CL is at larger distance in front of the CG and the horizontal stabilizer provides positive lift, the positive lift force of the wings and the positive lift force of the horizontal stabilizer act in the same direction providing even more maneuverability. But this design is even harder to fly without the help of computers.

 
                                           Principle of Neutral & Negative Pitch Stability

Because of having the CL very close to the CG, the horizontal stabilizer attitude of the F-16 is close to neutral under cruise conditions and produces not much up - or downforce. Due to the neutral position, the difference between the angle of attack of the wing and the angle of attack of the tail (decalage) is minimal. After a disturbance in pitch, the stabilizing moment is practically zero. When flying at a negative angle of attack, the horizontal stabilizer has a leading edge up attitude (comparable to descending attitude), thus a higher angle of attack then the main wing (negative decalage). After a pitch down disturbance, the tail will raise even more because there is no stabilizing moment of the tailplane (remember, the tailplane should have a lower angle of attack for pitch stability), bringing the nose of the aircraft down even more so there is more pitch instability. In the case of deployed flaps however, there is some stability gain. This is because the flaps increase the angle of attack of the wing thus decrease the negative decalage resulting in more longitudinal stability. To get a neutrally stable aircraft as the F-16 without fly-by-wire control flying level requires a perfect undistored condition. This can be easily tried out using the GE F-16 simulator. One will experience that getting the elevator deflection in the right position to reach a constant level flight condition is not easy to accomplish. This is caused by the very high maneuverability of the aircraft were every touch of the elevator causes a force in the upward or downward direction. Eventually, after some time the pitch oscillations will practically entirely dempen out and yet a condition of nearly perfect level flight will be reached as long as the controls remain untouched.



                                                 Increased Pitch Stability at High AoA


When flying the GE F-16 flight simulator, one of the things I experienced from the beginning was a significantly increased pitch stability at high angles of attack. This increased stability is very helpful in the landings that occur always at a high angle of attack. Several attempts to explain this condition aerodynamically as a result of CL-position changes due to high AoA failed until I took a closer look at the actual positions of the horizontal stabilizer during different phases into the flight. Then it became clear that the effect is caused by the increased decalage at higher angles of attack in combination with a positive static margin:

When the F-16 is trimmed for a high angle of attack, the horizontal stabilizer has its leading edge down thus a negative angle of attack. This means it produces lift in the downward direction. In the first place, due to this action, the center of lift moves to the rear. As we have seen in the introduction (section decalage) we also now have positive decalage as the angle of attack of the horizontal stabilizer is lower then that of the main wing. If there is a pitch down disturbance, a stronger downward force on the horizontal stabilizer will occur with the result that the tail moves down more. This causes the nose to raise again thus the aircraft is able to return to its original trim attitude. This moment can not be created because of the rearward movement of the CL relative to the CG which causes that the F-16 is no longer neutrally stable.

At low angles of attack as is the case in high speed flight, the horizontal stabilizer is in more neutral position thus produces not enough downward force to be able to bring the nose up. When the leading edge of the horizontal stabilizer is up, it produces positive lift while the wing is now the surface with the lower angle of attack. This situation in flight can occur when the aircraft is trimmed for a negative angle of attack. A pitch down disturbance results now in more lift on the horizontal stabilizer in the upward direction, causing the nose to pitch down even more. This explaines the high degree of longitudinal instability at low angles of attack.

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Principle of increased pitch stability in high AoA flight

Increased pitch stability can be reached by setting the wing in the back (in most aircraft configurations the horizontal stabilizer) to a lower incidence angle, in practice often leading edge down. In high speed, low angle of attack flight, like cruise, this is resulting with a neutral stable aircraft like the F-16 in an ascent. However, In low speed, high angle of attack flight, like in the pre-landing phase, the negative lift on the horizontal stabilizer is not sufficient for an ascent and it is possible to fly level with an increased longitudinal stability.

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When executing the simulated pitch down disturbances for these tests, I even clearly felt the increased stability of the GE F-16 in high AoA configuration because the force that was required to create a disturbance was significantly bigger then in the low AoA configuration.


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                                   GE Flight Simulator Pitch
Stability Accuracy


                                                                        Test  
                                                                                      
An aircraft trimmed for a certain pitch angle (or airspeed) can respond in different ways to disturbances in the balance. Longitudinal or pitch stability determines the effect on the pitch angle after a disturbance. The aircraft can start to oscillate (climb and descent) around the originally trimmed attitude and eventually return to the original attitude. In this case the aircraft has positive static pitch stability and is static stable. Note that it is not necessary for a positive stable aircraft to return to exactly the original attitude and speed. It is sufficient that it returns to the approximate original situation. This is what we also see in the simulations. (For instance, an original pitch attitude of 10 degrees can eventually become 7 degrees). If the pitch attitude remains at a fixed angle after a pitch disturbance the aircraft has neutral static pitch stability. If an aircraft pitches away from the originally trimmed setting without returning to the approximate originally trimmed attitude and the shift in pitch angle tends to increase, it has negative static pitch stability (no stability). The 2 aircraft available in the Google Earth flight simulator have both different stability. Fighter jets like the F-16 are intentionally designed to be unstable because of increased maneuverability. Dynamic pitch stability determines the chararcteristics of the oscillations that occur after a pitch disturbance. When an aircraft has positive pitch stability it returns to the original pitch attitude. Dynamic stability determines the characteristics of this attitude return. When the amplitude of the climbs and descents around the original attitude becomes smaller over time the aircraft has positive dynamic stability. If the amplitude not increases or decreases, the aircraft has neutral dynamic stability. 

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                                                 Static Pitch Stability Test 1  

                                                          
Low Angle of Attack


In the following simulations with the GE SR-22 and F-16, aerodynamic accuracy is tested according to static pitch stability and compared to the expectations known from the real aircraft. The video on the left shows the F-16 in level flight and the video on the right shows the SR-22 in level flight. To analyse how both aircraft respond in order to check static pitch stability, a similar powerful disturbance in pitch is executed
. As can be seen, the F-16 doesn't recover the original pitch attitude but tends to keep pitching down with an increasing negative angle. This demonstrates the poor static pitch stability of the aircraft. What we in fact experience is that the F-16 pitches up and down but with very long and slow phugoid oscillations. With other words: the F-16 has very low static pitch stability or is close to be unstable. As this corresponds to the behave of the real-life F-16, it can be concluded that the Google Earth F-16 is accurate in this aerodynamic static stability test. The Cirrus SR-22 is one of the aircraft that has positive stability which makes these types of aircraft easier to fly. After the pitch disturbance of the SR-22, it clearly starts to recover the original attitude and the negative pitch angle not further increases like the F-16. Instead, it pitches up and down with decreasing amplitude until it has restored the original pitch trim attitude.




             F-16  Pitch-Down Disturbance                                SR-22 Pitch-Down Disturbance


                                                                                                                             Simulated flights by Ralf Vandebergh



                                        Static Pitch Stability Test 2  

                                                             High Angle of Attack



More interesting in the context of this site is the static pitch stability test at high angle of attack as the stability of the F-16 trimmed for high AoA plays a great role in the demonstrated stabilized approaches with the F-16 and the handsfree landing experiment as displayed later on this page. As explained in the introduction, the stability of an unstable aircraft can be improved by increasing the angle of attack.

In the next simulations both aircraft are trimmed for a certain pitch attitude and airspeed to test static pitch stability on the aircraft in a high angle of attack configuration. The F-16 on the left is trimmed for 10 degrees and 150 knots. The SR-22 on the right is trimmed for a similar pitch angle of 10 degrees but at 75 knots as it is a much slower aircraft. Interestingly, the F-16 now responds different on a pitch disturbance. Now the pitch angle not further increases negatively causing a long nose-down dive. Instead the aircraft nose now starts to raise after reaching the zero pitch point. From this point the F-16 starts to oscillate around the original pitch attitude of 10 degrees but with an amplitude that becomes smaller over time: the phugoid oscillations dempen out. This means that in a high AoA attitude, the GE F-16 has positive stability and is statically pitch stable
.
However, the SR-22 in high AoA shows better dynamic stability as the oscillations dempen out quicker after a similar pitch disturbance as can be seen in the video on the right.

As mentioned earlier, it is not necessary for an aircraft to return to exactly the original trim in order to be marked as static stable but it is sufficient that it makes at least a small change towards the original attitude and airspeed. This is what we see in the SR-22 simulation on the right.




             F-16 Pitch-Down Disturbance                               SR-22 Pitch-Down Disturbance


                                                                                                                            Simulated flights by Ralf Vandebergh
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                                    3. 
Self-Stabilization of the GE SR-22


                                         Experiencing an 'Automatic' Stabilized Approach

A particularly easy-to-realize stabilized approach - this could be called an 'auto-stabilized approach' - can be achieved using the Cirrus SR-22 in the Google Earth flight simulator as a glider (power off) when descending from a relatively high angle to the runway. When on final approach, set power to zero and pitch up waiting for airspeed to decrease to around 70 knots. Then keep the nose at the zero-pitch line and just wait. There will be a point that the aircraft stabilizes itself to a zero pitch angle (nose pointed to horizon) while the speed stabilizes at around 66 knots or close to that value. Most of the times I tried out this zero-pitch approach, the aircraft stabilized itself similar or close to these values while it was descending to a point somewhere near the mid of the runway. The aircraft 'feels' like it is trimming itself automatically and very little pitch and power control imputs are required to achieve a stabilized configuration. The self-stabilizing effect demonstrated here can be explained by the tendency of a longitudinal stable aircraft to restore the original pitch attitude after a disturbance, in this case 0 degrees.



                                                               Stabilized Approach Simulations
                                                         

These 2 simulations demonstrate the perfect stabilized approach after 'self-stabilization' of the SR-22 and that it is well repeatable. Note the fixed position of the flight path marker on the runway during the entire trajectory of the final approach. The almost unchanging vertical speed is another indication for the straight glide path. There is a small difference in vertical speed of around 90 feet per minute between the 2 flights, 760 feet per minute average left and 850 feet per minute right. This is caused mainly by small differences in the glide path angle. In the left video the aiming point is around the mid of the dark area in the center of the runway, exactly between the 4 short white marks, while in the right video the aiming point is at the beginning of this dark area. As visible in the video's, small aileron corrections which are necessary to align with the centerline (heading corrrections) have no noticeable influence on the stabilization. 

 

                                                                                             Simulated flights by Ralf Vandebergh

                                                                                      

                                      Screenshot of one of my SR-22 'self-stabilization' flights. Marked are values for speed, vertical speed and pitch attitude
                                                                        


                                                          Variables of the Approach

Between the 2 video's above there is a certain difference in glidepath angle causing a difference in vertical speed while the value for the airspeed of 66 knots between the 2 flights is similar. In the 2 screenshots of SR-22 approaches shown below there is a huge difference in glidepath angle while the aiming point on the runway is similar. If we still would like to fly the final approach in both cases at a pitch angle of 0 degrees, we have to change the airspeed as can be seen in the picture on the left (75 knots) compared to the picture on the right (65 knots).The aiming point in both pictures is at the start of the dark area in the mid of the runway. In the left picture the approach is less steep and to descent at a zero-pitch angle, an airspeed of 75 knots is required to set the right glidepath angle. In the right picture the approach is steeper and now an airspeed of 65 knots is required if we would like to descent at a zero-pitch attitude. In practice however, things are different:
  

                                                                                                
In real-life flying practice, the approach speed is not adjusted to set a certain pitch angle like the 0 degrees pitch as described above but the pitch angle is adjusted to set the desired approach speed. If the desired approach airspeed is 65 knots we would need to lower the approach speed in the left simulation below with 10 knots, from 75 to 65 knots. To accomplish this, the pitch angle should be increased while the power (altitude) is adjusted at the same time. Now the aircraft is trimmed for a certain airspeed.This illustrates that the aircraft must be trimmed different for every different glide path angle, airspeed and power setting. It further illustrates that every change in one of these variables requires an adjustment in the other variables
.







                                                SR-22 Stabilized Approach in Practice
                                                                   

In the next simulated flight of the GE SR-22 we will approach from a less steep glidepath angle at the same airspeed as in the auto-stabilized approach shown in the previous section. Now we are close to the actual practical purpose of the stabilized approach: descending at a stable airspeed to a certain aiming point on the runway to make a precision landing without the need of major corrections. In the next simulation we try to land in the short dark area at the front of the runway very close to the adjacent river, thus requiring quite some precision.
A helpful tool for precision landings and part of the display is the flight path vector marker (FPV) that marks the exact flight path of the aircraft. For a precise landing, the FPV symbol is placed on an aiming point on the runway. The pitch angle marker displays the attitude of the aircraft nose. When descending towards the runway at a decreasing airspeed, the angle of attack must be increased to compensate for the lost of lift. The increasing angle of attack (AoA) is visible as a growing distance between the FPV marker and the pitch angle marker as nicely demonstrated in this GE F-16 video.


                                    
Screenshot from the stabilized approach video below, now flying to an aiming point on the runway using the FPV-marker




                                                                 Precision Landing using Power for Altitude                                               
                                          

The next simulation is a demonstration of the approach technique as generally used for light aircraft in real-life. This means that the aircraft is trimmed for a certain approach airspeed and the entire approach to landing is flown without actually changing the elevator deflection. Corrections in the glidepath are made with very small changes in the engine power. This is the so called 'pitch for airspeed, power for altitude method'.

In the simulated flight below, the Google Earth Cirrus SR-22 is landed on a small dark area at the front of the runway close to a river. This point is chosen as it is perfect to demonstrate a precision landing. In real-life, aircraft never land intentionally this close to the runway end. We approach from a relatively normal glide path angle at an airspeed of 65 knots. The video shows how the process of fully manually trimming and stabilization is done. The nose is raised to decrease the airspeed to the desired approach speed of 65 knots which in this case is corresponding to a pitch angle of 2,5 to 3 degrees. In real-life there is a trimming wheel that has to be adjusted after a change in elevator deflection. In the GE simulations we don't experience the pressure on the controls as in a real-life aircraft with the result that the trimming of an aircraft feels less or more like the use of a trim wheel.


Shortly before touchdown, some small power adjustments are made which are necessary to make little corrections in the glide patch angle. With this method, we don't touch the yoke to change the glidepath as this would change the approach speed which we already have trimmed. As can be seen, the small power corrections for the altitude don't have much influence on the airspeed, the result of speed stability. Note that the descent is stabilized until the flare is executed. In this demo the flare angle is almost as high as the F-16's normal approach pitch angle of 11 degrees. In daily practice, the flare angle of the SR-22 is much lower because the landing airspeed is taken higher then demonstrated in this simulation which shows the landing at an airspeed very close to stall speed. With a perfect flare angle and timing, the landing can still be made very softly.

Simulated flight by Ralf Vandebergh

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                              4.  Stable Landings with the GE F-16

                                                         Stabilized or Unstabilized?


In real-life we see that landings with the F-16 are mostly unstabilized. Analyses of real-life video's of F-16 approaches and landings quickly show that mostly no constant airspeed is held in the final approach but the airspeed is decreasing at a relatively steady rate to the desired touchdown speed (decelerating approach). However there will be some phases in the approach were the airspeed is held constant for short periods of time. In this real-life F-16 HUD video we see that the speed is kept around 212- 214 knots for a few seconds in the last turns before the final approach and then the speed is further dropped to the desired landing speed. The absence of real-life stabilized F-16 landing footage sparked my interest to try it in simulations to study how a stabilized approach as it is used generally in aeronautics would look like when flown with an F-16. At the end there is the handsfree landing experiment that will show what happens if a stabilized approach is flown literally to the end. In the coming GE simulations I have successfully tried out principles of the stabilized approach used in general aviation, especially when it comes to glide path correction methods.



                                                             Glidepath Corrections  

                                                       Pitch
or Power for Altitude?


An approach is considered stabilized if the path to the aiming point is straight, only small corrections of that glide path are acceptable, otherwise it's an unstabilized approach. A discussion that goes on for years in the world of aeronautics is the thread around whether to use engine power or pitch attitude to control the airspeed or whether to use engine power or pitch attitude to control the altitude. In practice this depends mainly on the aircraft type. In a pitch stable aircraft it is likely to correct the glidepath with the power while the approach airspeed is adusted using the pitch attitude. With this technique, the aircraft is trimmed for a steady approach speed which is managed by adjusting the pitch attitude for the appropriate airspeed and keep that pitch attitude / airspeed. In real-life this is done by changing the elevator deflection and release control pressure on the yoke by adjusting the trim wheel. With the GE flight simulator we do not feel the pressure on the yoke thus if we trim the aircraft, it is like using the trim wheel on a real-life aircraft. When there is a deviation from the proper glidepath to the runway, the elevator is not touched to correct the pitch (remember the aircraft is in a fixed pitch attitude to maintain a fixed airspeed) but the power is used to correct the glidepath. A change in power will cause a change in lift on the horizontal stabilizer thus a pitch change of the aircraft. For example: If we are under the proper glidepath and need to go up, we add power. The higher airspeed will cause the horizontal stabilizer to produce more lift in the downward direction thus the tail moves down and the nose moves up. Another logical effect of the pitch-up maneuver after the power increase is that the airspeed not further increases but remains at a relatively steady value: we experience
speed stability.



                                                      Speed Stability & Aircraft Type

The F-16 has weak speed stability which means that the airspeed remains not unchanged when the power is increased in a trimmed configuration. An Aircraft with strong speed stability (like the SR-22), and trimmed for a certain airspeed and pitch has a fixed airspeed when the power is increased to raise or lower the altitude. Thus for a speed stable aircraft the power for altitude method works best. The idea behind this method is that an aircraft that is trimmed for a certain airspeed holds that airspeed when the glidepath is corrected by power corrections. This way, the final approach can be flown at a stable airspeed while glidepath corrections have little influence on the airspeed.

As the F-16's speed stability is weak, the airspeed remains not stable when the power is changed to correct the glidepath
. With the changed airspeed the aircraft would need a change in pitch angle to correct the airspeed. In the landings, this change in pitch causes a deviation from the proper glidepath angle which can not be corrected using the power as any power increase will again change the airspeed (due to the speed instability). Therefore, on speed unstable aircraft the altitude is mostly corrected by changing the pitch angle and the airspeed is corrected using the engine power. On fast aircraft there is an important additional negative effect when correcting the airspeed using the pitch attitude on the final approach: The glidepath angle changes very rapidly with only a minimal effect of airspeed change. This way, an effective airspeed control is difficult.



                                                                                   Speed Stability Explained

With the GE flight simulator it is easy to try the mechanism of speed stability to better understand the principle. When the power is increased on a pitch stable aircraft which is trimmed for an airspeed, the airspeed starts to increase as a logical reaction. As a result of the higher airspeed, the horizontal stabilizer produces more lift in the downward direction causing to raise the aircraft nose. Due to this pitch-up maneuver, the airspeed again decreases and the nose is dropping. This way, the aircraft starts to oscillate. When the pitch response on the power change is fast enough, there is little time between the airspeed increase (due to power) and the pitch increase with the result that the airspeed stabilizes. The same happens with an airspeed decrease and pitch decrease. However, as you increase or decrease the power excessively on the pitch stable GE SR-22, the pitch response has a little delay causing a deterioration in speed stability. But as for glidepath corrections during the landing phase only small power changes are necessary, the speed stability in the final approach to landing regime is optimal.



                                                         Speed In-stability Analyzed

The poor speed stability of the F-16 is an effect of the weak pitch stability of the aircraft. In a condition were the aircraft is used were it is designed for, namely flying at high velocity, the angle of attack is very low or even negative. In this condition, the horizontal stabilizer (stabilators in case of the F-16) are in close to neutral position thus providing almost no downward force. In the case of flying at a slightly negative angle of attack, the stabilators even provide some upward force. As a result, an increase in power barely causes the horizontal stabilizer to produce lift in downward direction to bring the nose up. The result is that the airspeed increases much more because it is not immediately counteracted by the nose-up maneuver. Eventually, when the airspeed builds up high enough, yet there will be some extra lift and a very slow pitch-up maneuver will occur. However, because the pitch response on the power change is not fast enough, there is too much time between the airspeed increase and the pitch increase with the result that the airspeed not stabilizes.

The slow pitch response of the GE F-16 in my tests was particularly obvious in the landings when correcting the glidepath using the power. With every small change of power, the airspeed changed considerably which was resulting in an unstable airspeed. When correcting the glidepath with the pitch angle, the airspeed can be adjusted using the power.
This is a reason to prefer the pitch-for-altitude method on pitch unstable aircraft.
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                                               GE Aircraft Speed Stability Tests

                                    


Speed Stable Aircraft  (GE SR-22)
Speed stability is an important factor in aircraft control, especially during the final approach phase. Positive speed stability is the tendency of an aircraft to keep or return to a certain airspeed in a trimmed configuration. In practice this means that when the aircraft is trimmed for a certain airspeed, the configuration will automatically keep the airspeed at that fixed value when the power is increased or decreased to ascent or descent. As a result the airspeed remains practically unchanged in the final approach to landing. In the case of a pitch disturbance it is the tendency to return to the original trimmed pitch attitude and airspeed. The SR-22 in the GE flight simulator is a
good example of a speed stable aircraft.

Speed Unstable Aircraft  (GE F-16)
A speed unstable aircraft (poor speed stability) will not keep the trimmed airspeed at a fixed value when the engine power is changed. Thus when using the power for altitude method on a speed unstable aircraft, the airspeed will not remain as it was when making a glidepath correction. With the changed airspeed the aircraft would need a change in pitch to correct for the airspeed. In the landings, this change in pitch would cause a deviation from the proper glide path angle which can not be corrected with the power as any power increase will again change the airspeed (due to the speed instability). Therefore, on speed unstable aircraft the altitude (glidepath) is mostly corrected by changing the pitch angle and the airspeed is corrected using the engine power. (see previous section) A good example of a speed unstable aircraft is the F-16 in the GE flight simulator.

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                                                                                  To the Test

                                                In the video's below, speed stability accuracy of the Google Earth flight simulator is tested.

In the simulation on the left we see the SR-22 trimmed for an airspeed of just above 100 knots while in level flight. As can be seen the altitude is just above 4500 feet. Then the power is increased at the marked moment. We see that the aircraft starts to climb at a vertical speed of around 440 feet per minute. When looking at the airspeed we see that it is still around 100 knots and practically unchanged. Even if we increase the power more and the vertical speed increases up to 1000 feet per minute, the airspeed still remains at 100 knots.

In the simulation on the right we see the F-16 trimmed for an airspeed of 140 knots while flying level. To maintain level flight, the nose of the F-16 has to be raised to around 12 degrees pitch angle. That is the reason why we actually don't see the horizon in this simulation. When increasing the power, an immediate increase in airspeed is the result. Although for this climbing condition a higher airspeed certainly is no problem, it demonstrates that the airspeed remains not unchanged when increasing the power of the trimmed configuration. This indicates that the aircraft has poor speed stability or is said to be speed unstable.



                             Speed Stable  (SR-22)                                                          Speed Unstable (F-16)
      
 

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                                                             Speed Stability & Angle of Attack

In the speed stability tests, the GE F-16 was trimmed for a high angle of attack to fly level at an airspeed of 140 knots. Although the speed stability as expected is weaker then that of the GE SR-22, it is in fact still much better then when flying at lower angles of attack. The following test results explain: When flying at 30.000 feet at 250 knots and 0 (zero) degrees angle of attack, the airspeed increases to 260-270 knots when a certain amount of power is added. When flying at a similar altitude of 30.000 feet but at 150 knots and 10 degrees angle of attack, with that same amount of power increase, the airspeed remains at 150 knots or increases slightly to 155 knots. Thus, in this condition the speed stability is much better. In both tests, the altitude is similar to rule out differences due to air density.

The improved speed stability experienced at high angles of attack is caused by the increased pitch stability at high angles of attack as we have seen previously as both are effects of increased decalage (longitudinal dihedral).


A greater decalage (a lower angle of attack for the horizontal stabilizer then for the main wing) is resulting in a better pitch response on a power change causing better speed stability. Conversely, the speed stability is weaker when flying at lower angles of attack due to the smaller decalage that is resulting in a lower tail lift production causing a slower pitch response when changing the power, which is causing the airspeed to outperform the lift. This fact also means that a speed stable aircraft can experience a deterioration in speed stability at lower angles of attack. Otherwise, in higher angle of attack conditions, due to excessive tailplane lift the situation can occur that the lift again is outperforming the airspeed with the result that the speed stability is deteriorating as well. This i
llustrates that speed stability is not a fixed value and that it is only optimal within a certain airspeed/angle of attack regime. However, the speed stability of a pitch stable aircraft will be optimal when close to the landing-angle of attack configuration as can be perfectly demonstrated during the landings with the GE SR-22 flight simulator. It also explaines why we can not use the power-for-altitude method optimally in every flight phase.

With the pitch stable GE SR-22, a clear deterioration in speed stability can be experienced if flying at negative angles of attack (airspeed increase) or at very high angles of attack (airspeed decrease) This concludes that the optimum speed stability of the GE SR-22 is situated somewhere in between.



                                                                                  Speed Stability & Altitude

Another important factor in speed stability is the altitude. When flying with the GE F-16 at 7500 feet at 10 degrees angle of attack and 150 knots, the airspeed increased to 170 knots when the power was increased with a certain amount. For comparison: At an altitude of 30.000 feet and a similar angle of attack, the airspeed remained close to 150 knots. This indicates that the speed stability improves at higher altitudes and deteriorates at lower altitudes. To investigate this observation further, I did tests at 45000 feet with an airspeed of 150 knots and 12,5 degrees AoA. With the same amount of power change, the speed stability appeared at its best, at least for a certain power range: the airspeed remained at 150 knots.

 
                                                                              Altitude Test on Speed Stability

To investigate what happens exactly with the speed stability of the GE F-16 when flying at low or high altitudes I did the following tests. As we have seen, the speed stability depends on the ratio between the produced airspeed and the produced lift. At a certain amount of power there is a certain amount of airspeed and a certain amount of lift. In the following test, the time is counted in which it takes to change the airspeed with an amount of 50 knots with a certain amount of power increase. Starting at 250 knots and ending at 300 knots. The longer it takes to change the airspeed with these 50 knots, the better the speed stability is. This is done at different altitudes. In all tests the flaps setting is 80%.


                               At    2300 feet it takes  7 seconds to go from 250 knots to 300 knots with a fixed amount of power increase

                               At 12.000 feet it takes 11 seconds to go from 250 knots to 300 knots with a fixed amount of power increase

                               At 30.000 feet it takes 27 seconds to go from 250 knots to 300 knots with a fixed amount of power increase


The test clearly illustrates that the time which is necessary to increase the airspeed with 50 knots is the longest at the highest altitude. This concludes that in these simulations, the speed stability is indeed better at higher altitudes. This condition can be explained by the deteriorating airspeed capability at higher alititudes compared to the deteriorating lift capability at higher altitudes. Apparently in this condition the deterioration in airspeed is greater then the deterioration in lift. As a result, when increasing the power the airspeed is not longer outperforming the lift as is the case at lower altitudes were we are used to experience a great increase in airspeed and a low increase in lift of the F-16.


                            Demonstrating improved speed stability of the GE F-16 at an altitude of 41000 feet. Left: flying at an airspeed of 160 knots
                                                      with only 50% power applied. Right: When applying 100% power, the airspeed still remains close to 160 knots.
                                               Note also the changed pitch attitude from 6 to 10 degrees as a result of the power increase which is resulting in a climb.

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                                   5.   GE F-16 Stabilized Approach in Practice


                                                                      Principle

The following simulation demonstrates the principle of the F-16 stabilized approach as I developed it for the GE flight simulations. The technique is similar to the approach
as demonstrated in the Cirrus SR-22 simulations. The difference is the much higher approach speed of the F-16. In the simulation below, the F-16 is stabilized at 140 knots and a pitch angle of 10 degrees. This airspeed is lower then the usual approach speed of the F-16 but having control of this configuration 140 / 10 will be very helpful in the execution of the final handsfree landing experiment. The power is zero from the beginning of the final approach, the landing gear is down and full flaps are set. When reaching the approximate airspeed  of 190 knots, which is for me an indication that I'm on the proper glide path (see landing analyses mainpage) the pitch is increased to decrease the airspeed to the desired approach speed. When reaching the chosen approach speed of 140 knots, a little power is added. And soon the aircraft is stabilized at 140 knots and 10 degrees pitch.

Some words about the flare in this video. The simulation demonstrates a nice slow and long flare in which the airspeed makes a transition from 143 knots to 125 knots, the stall speed in the landing configuration. During the same trajectory, the pitch angle makes a transition from 10 degrees to 15 degrees. The stall point is reached at the optimal height above the runway and the aircraft settles on the runway with a nice soft touch down. The maximum flare angle of 15 degrees enables the landingspeed to reach the maximum low value of 125 knots while still touching down softly.

Simulated flight by Ralf Vandebergh

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                                  6Unstable Aircraft and Power for Altitude


                             
                                   Advantages & Disadvantages in Stabilized Approach

As mentioned earlier, a perfectly stabilized approach shouldn't require any corrections, however very small corrections are acceptable and sometimes necessary to stay on the proper glidepath. Once stabilized at the desired approach speed and corresponding pitch angle, any adjustments can easily de-stabilize the configuration and the aircraft must be retrimmed. To realize a virtual handsfree F-16 landing, the main experiment on this website, it is important to find a correction method that has the lowest influence on the trim. To find out the method for glide path corrections that seems to work best, I tried both the methods of altitude adjustment by power and altitude adjustment by pitch. Here is a summery of tested landing methods.


Method 1:  Nominal decelerating approach
Used mostly in real-life F-16 approaches and in fact the easiest way to land. The airspeed is decelerating at a constant rate while the touchdown point is reached at exactly the right airspeed. Although this type of approach is an unstablized approach the power is used for the airspeed and the pitch is used for the altitude. This GE F-16 flight simulation is a good example of a decelerating (unstabilized) approach: California landing.

Method 2:  Stabilized approach - Pitch-for-Altitude
The difference with the first method is that the power is increased at the moment that the right approach speed is reached. This is resulting in a stabilized airspeed. To correct small deviations of the glidepath (altitude) with the pitch, the airspeed is corrected with the power. Example: When the intitial approach speed is 140 knots and the glidepath is corrected by lowering the nose, the airspeed can change to 150 knots and has be corrected by decreasing the power (power for airspeed).

Method 3:  Stabilized approach - Power-for-Altitude
Similar to the previous method but the power is used to control the glidepath. Example: When the initial approach speed is 140 knots and the glidepath is corrected by decreasing the power, the airspeed can change to 130 knots and has to be corrected by decreasing the pitch (pitch for airspeed.) The result is a new deviation from the glidepath. Therefore this method is only preferable when glidepath corrections are only very small thus when approaching from a perfect glidepath angle. Note that there is no airspeed change when using this method with a speed stable aircraft. Thus there is no need for a pitch change. This is the key why this method in real-life is used mainly on speed stable aircraft.



                                                       Optimal Method for Trim

I found that when the F-16 was trimmed for a stabilized approach, the trimmed configuration was less disturbed when correcting the glidepath with the power (method 3). This can be explained by the slow pitch response on the power changes which has a positive effect on the finetuning, rather then pitch corrections which are usually of more radical character. The disadvantage is the delay between the power change and the pitch response.  In practice it means that if the FPV-symbol appears too high (above the aiming point on runway), power must be reduced and if the FPV-symbol appears too low (under the aiming point on runway), power must be increased. If done carefully, little power corrections have little influence on the trimmed pitch attitude. With the pitch angle well stabilized, we only need to take care of the power and watch whether the FPV-marker is too far below or above the aiming point on the runway. This way it is well possible to make precision landings as demonstrated in the next section.




                                                         Effect of Pitch Oscillations on Glidepath Corrections

Glidepath corrections with the power need to be done in small amounts. It is easy to overcorrect with the result that the aircraft climbs or descents more then required. It is important to realize that every change in power initiates a change in pitch which has the same effect as a pitch disturbance as demonstrated in the static pitch stability tests. This means that after a power change which is resulting in a pitch change, the aircraft starts to oscillate in the stabilizing proces to return to the original trim attitude. Therefore we should take into account that during these oscillations, the aircraft nose will also pitch down after we have corrected for pitch up and will also pitch up after we have corrected for pitch down. The initial reaction is to correct for this effect with the result that the phugoid oscillations only increase and the effect is becoming worse. With the GE-SR-22 flight simulator it can be experienced that just waiting until the oscillations have dempen out after a power correction will mostly bring the aircraft close to the desired touchdown point. 
In fact, the strong pitch stability of the SR-22 causes the aircraft less or more to react excessively on the power corrections with the result that the pitch oscillations further increase. It is important to note that the oscillations act faster on an aircraft with strong pitch stability as the SR-22 because of the greater tendency to return to the original trim attitude and that the oscillations act slower on an aircraft with weak pitch stability like the F-16. Note that the static pitch stability of the F-16 significantly improves in the high angle of attack landing configuration as demonstrated in earlier tests.

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                                                                                                  Positive Effect

In my
GE-F-16 landing simulations, the delay in stabilization clearly showed a positive effect on glidepath corrections using the power. Instead of waiting for the phugoid oscillations to dempen out as seems to work best with a stable aircraft like the SR-22, on the F-16 it appears to work better to apply quick small power corrections to stay on the proper glidepath / keeping the flight path marker on track. In fact on the F-16, waiting for the oscillations to dempen out would last too long to be able to make a precision landing and intervention by correcting the glidepath with the elevator pitch would be required. Summarizing: According to these GE simulations it can be concluded that the slower pitch response of the F-16 due to the weak longitudinal stability has a positive effect on precision landings using the power-for-altitude method.

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                                            Unstable Aircraft and t
he Airspeed Problem

An interesting advantage of glidepath corrections using the power-for-altitude method is that there is no direct need to keep an eye on the airspeed. In the GE simulations it is just a matter of controlling the Flight Path Vector Marker (FPV) with the power and keeping it at the aiming point on the runway, which ensures a landing at an acceptable airspeed. If the approach speed becomes too low, the aircraft will sink too much and the FPV- marker will sink too and will need adjustment. However, when using the power-for-altitude method on a speed unstable aircraft like the F-16, there will be differences in the airspeed during glidepath corrections. I
n the next simulations, the power for altitude correction method is used to demonstrate how this method looks in an F-16 final approach. The main difference is that the airspeed not remains at a fixed value when the altitude is corrected using the power, as would be the case with a speed stable aircraft like the SR-22. However, as long as power corrections are small, the changes in airspeed will remain small too.

This concludes that when using the power-for-altitude method on a speed unstable aircraft, the glidepath angle should be taken as precisely as possible as too big corrections will result in an unstable approach speed. The advantage however is that the slower pitch response due to the poor longitudinal stability causes slower oscillations with the result that the aircraft not overreacts when applying power corrections
. Instead of waiting for the oscillations to dempen out, it is now possible to make quick small power correcions for the glidepath with the result that it is easier to make precision landings.




                                                                 
Power for Altitude Simulation 
                                                   Note the changes in airspeed as a result of the power corrections for the glidepath.
                                                                               This is caused by the F-16 speed instability  
                                                        

Simulated flight by Ralf Vandebergh

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                                               7.  GE F-16 Precision Landing



The goal is to touch down in the short dark area at the front of the runway close to the river just like the precision landing made with the SR-22 as demonstrated in an earlier chapter. The difference is that the approach is at a much higher airspeed (144 knots compared to 65 knots for the SR-22) and that the roll-out of the F-16 is much longer. The higher approach speed leaves in principle less time for power-for-altitude adjustments then with a slower aircraft but it  works fine especially thanks to the slower pitch oscillations due to the weak longitudinal stability (as described in the previous chapter). The final approach is flown  with only small power adjustments to maintain the proper glidepath, or to keep the FPV-marker at the front of the runway.
The power is set to zero at the right moment to reach the proper vertical velocity in combination with the flare that is needed to reach the intended touchdown point, the short dark area at the front of the runway.


                                                        
                                                      Measuring Airspeed Reduction by Flare


The simulation is also a good demonstration to show the difference in vertical speed and the difference in the final airspeed of a landing with a flare compared to a landing with no flare. Before the exectution of the flare we approach the aiming point at a vertical speed of close to 1100 feet per minute and an airspeed of 142 knots and we would have touched down at that rate if no flare was executed and with the approach pitch angle of 7 degrees. Now with the flare we have landed at a vertical speed of 630 feet per minute at an airspeed of 137 knots and a pitch angle of 13 degrees. This means that the flare executed over an angle of 6 degrees from 7 up to 13 degrees caused a reduce in airspeed of 142-137= 5 knots.

Simulated flight by Ralf Vandebergh



                                             F-16 Stabilized Approach like an Airliner

The F-16 final approaches in the previous simulations are all flown at an airspeed somewhere between 140 and 145 knots. This speed in fact is relatively low if compared to the standard approach airspeed from real-life F-16 landings. Additionally, as previously mentioned, F-16 final approaches are mostly not flown at a stable airspeed (stabilized approach) but an an unstable airspeed with constant rate of decrease (decelerating approach). The main reason for the minimal approach speed in my simulations is that these are test flights for the handsfree landing experiment where it is necessary to land at a high pitch angle that is set early in the approach. This is because the controls can not be touched anymore later in the approach to execute a flare maneuver.

In aeronautics a standard is used to determine the approach speed (Vref) which can be easily calculated by stall speed in landing configuration x 1,3.  The stall speed of the F-16 can be determined as somewhere between 120 and 125 knots. So for the F16 we calculate: Stall speed 120 knots x 1,3 = Vref=156 knots. Or stall speed 125 knots x 1,3 = Vref=162 knots.

If we want to fly a stabilized approach in accordance with the regulations applied in general aviation, we should approach with no higher airspeed then Vref+20 knots or not less then Vref which in our case is 156 to 162 knots. The sink rate (vertical speed) should not be higher then 1000 feet per minute in the final approach. In the next simulation I tried as best as possible to meet these requirements. We see that after stabilizing, the approach speed is maintained around 155 knots with some small variations caused by small glide path corrections while the sink rate is kept neatly around 500 feet per minute. Note that in real-life flights the practical approach speed is Vref+corrections, for example for wind, wind shear or icing.

Simulated flight by Ralf Vandebergh


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                                                 F-16 Lowest Possible Landing Speed
                                                                                        
                                                                                                Flare to the Max


In most of the simulated flights were I persued a lowest practical landing speed with the F-16, the most minimal speed was 125 knots which is already lower then an average real-life F-16 landing. The following examples show 2 different landing/flare techniques to investigate what creates the difference between a landing at maximum low speed and a landing at the usual touchdown speed of around 135 knots.

The simulation on the left demonstrates that it is possible to touch down at even 120 knots. Coming from a stabilized approach at 140 knots, the nose is raised for a flare to 15 degrees while the power is still on and the airspeed is dropping just under 140 knots. Then the power is set to zero just at the moment of touching the 15 degrees maximum flare angle. Now the speed is dropping rapidly while the landing is relatively soft at 120 knots.

In the simulation on the right, the stabilized approach is flown like airliner regulations, calculated at Vref 156 knots. Due to the higher approach speed, the touchdown point now is further down the runway. To get rid of excessive speed, the power is cut at 150 knots. In contrast with the left simulation, this is done before the flare is executed and not during the flare. Then the nose is raised to a flare angle of 13 degrees. Touchdown is at 135 knots, a normal value for an F-16 landing in real-life. Note also that there is no stall alarm now, indicating that the aircraft actually has landed with excessive speed, although this is strictly aerodynamic terms were the aircraft should optimally land at exactly the stall speed.



                            F-16  Lowest Speed Landing                               F-16  Nominal Speed Landing

 

                                                                                                                            Simulated flights by Ralf Vandebergh

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                       8.  Handsfree Landing of the GE F-16
 
                                             
The Experiment

                              

                                                                   Technique

As mentioned earlier, this experiment is a simulation of a landing of an F-16 without fly-by-wire control at a perfectly stabilized approach, visualizing that the aircraft virtually can land in this configuration without the touch of any controls. This means that the aircraft will be trimmed in a way that in the last part of the final approach no adjustments for landing have to be applied at all. During the landing, no use of the elevator will be made for pitch adjustments to control airspeed or altitude, no use of power adjustments for altitude or airspeed and no use of ailerons and rudder to control heading towards the runway. In practice it turned out that in a long final approach, such as used for the experiments, only the last part of the trajectory is suited to fly entirely handsfree. Though handsfree landing experiment 6 (update) shows that it is certainly not impossible to fly a handsfree trajectory to the runway over a longer distance. The reason for the difficulty is for one reason because some heading corrections are necessary in the early stage of the final approach to prevent a too large shift from the runway centerline at the point of touchdown. The same applies for power corrections which will be used to adjust glidepath deviations. At a certain point, which will be marked in the simulated flight video's every time, the aircraft flies in the trimmed configuration at the last control inputs given before this marked point appears.

This point will be marked with: 'From now on hands are fully off the controls'.



                                                                  Preparation

From the preparing testflights, this is a summery of aspects which are important for the execution of the GE F-16 handsfree landing experiment.


- Static pitch stability:  Although the static pitch stability of the F-16 is close to negative or in more simple terms: the static pitch stability is poor, making it in real life only flyable using Fly-by-wire technology, the pitch stability is increased when flying at a high angle of attack as during the landing phase. In practice this means that during the pre-handsfree trajectory, small glidepath corrections are possible without disturbing the trimmed configuration. During the handsfree trajectory this means that it is possible to fly a very stable approach.

- Glidepath correction method:  Important during the pre-handsfree trajectory is the choise of the right method to adjust small deviations in the glidepath angle. The right method will not disturb the trimmed configuration which is set for the handsfree approach. From the testflights it is clear that the configuration is less disturbed when correcting the glidepath using the power (power for altitude method).

-Speed stability:  As the relevant GE F-16 testflights have demonstrated, the speed stability of the F-16 is poor when flying in cruise conditions when the angle of attack is low. This is a true property of the F-16 as the aircraft is flying at a minimal decalage in these conditions. However, the speed stability is much better at the high angles of attack used for the landing which is a positive factor. As explained in the speed stability tests: When using the power for glidepath adjustment, it is important that the proper glidepath for the desired approach speed is taken as precisely as possible to prevent too big corrections that could disturb the trimmed configuration for the handsfree landing.



                                            Trimming the F-16 for the Experiment

Because the landing will occur with no hands on the controls, every control input must be made early in the approach and must be stable before the last final handsfree trajectory until touchdown starts. After several GE test flights it turned out that there is an optimal setting to trim the F-16 for a handsfree landing. I found that this is the airspeed 140 / pitch 10 degrees configuration as mentioned in the section F-16 stabilized approach in practice. This setting appears to be optimal because the touchdown will be made at a pitch angle and corresponding airspeed that ensures a soft as possible landing without a flare, thus comparable to a flare-less landing of an F-16. An early trimming to a pitch angle of 10 degrees is required because during the handsfree landing there is no possibility at all to change the angle of attack to a higher value to realize a softer landing.
The airspeed of 140 knots is the right corresponding value to this particular pitch angle to fly the stabilized approach. Also it is close to the nominal touchdown speed of an F-16. The final experiment 5 will involve a handsfree touchdown at an angle of attack corresponding to that of an F-16 landing with flare.

In this video the relation between the decreasing airspeed and corresponding pitch angle in the final approach is explained while trimming to the final 140 knots - 10 degrees configuration used for the F-16 stabilized approaches on this site. The right pitch angle for every airspeed is determined by the position of the flight path vector marker. In this simulation, the upward movement is stopped shortly at every marked point to display the corresponding airspeed/pitch values for every marked point on top, left in the video.



                                     Handsfree Landings at different Angles of Attack


For the experiment a nominal trimming of 140 knots / 10 degrees is chosen to ensure a soft as possible landing as described above. Nevertheless it is also possible to trim the GE F-16 for different landing speeds and different corresponding pitch angles. For instance, landing at a higher airspeed of 160 knots requires the aircraft to be trimmed to a pitch angle of 5 degrees. (160 / 5 configuration). Landing at a lower airspeed of 130 knots requires a pitch angle of around 14 degrees, see Handsfree Landing Experiment 5. The realization of this 130 / 14 configuration for a handsfree landing is difficult but rather interesting because of the possibilities for a softer landing thanks to the higher angle of attack of this true flare attitude. The 160 / 5 configuration could be interesting to try out a faster handsfree landing at a lower angle of attack.


                                                           What do we expect?

The main goal of this experiment is an automatic landing with a fully manually trimmed aircraft and particularly the F-16 is interesting because of the poor longitudinal stability. A secondary goal is to practice the theoretically aerodynamic correct statement that an aircraft would reach an aiming point when it is flying the stabilized approach literally until touchdown. This is because the descent graphic of a perfect stabilized approach should involve a straight line towards the aiming point on the runway. In most landings, the aiming point is actually never reached because the aircraft landing requires a certain distance to make the transition from the approach attitude to the final flare. So in practice the aircraft usually lands at a point at a certain distance behind the aiming point. In this virtual experiment however, we would expect that the aircraft should land at nearly exactly the aiming point because this point is set with the last control inputs before completely keeping the hands off the controls.
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                                                    Handsfree Landing Experiment 1  

                                                                     
Execution



In the simulated flight in the video below it can be seen that the proceedings as demonstrated in the previous chapters are used to trim the GE F-16 to the proper 140 knots / 10 degrees configuration. The video starts with the turn to final approach, then the 190 knots point were the nose is raised above the zero-pitch line, indicating that the glide path angle is nominal. At that point the power is already off and flaps are set maximum while the landing gear is lowered after the turn to final. The nose it raised slowly to increase the pitch and further reduce the airspeed. At 140 knots, a little power is added to stabilize the speed. At that moment, the pitch angle should be close to 10 degrees.

As can be seen, the FPV-symbol is not yet in the perfect position at the front of the runway, so some small glidepath corrections are made using small power adjustments. Some small yaw movements for heading correction are also made using the rudder. Ailerons are not further used here because the deviations are only minimal. After the last small adjustments, these are the control inputs that should land the aircraft on the runway by itself. The vertical speed is 590 feet per minute just before touchdown and decreases interestingly slightly to 580 feet per minute at the moment of touchdown without any change of inputs. The aircraft remains perfectly trimmed at 141 knots and 10 degrees until the point of touchdown.

As can be seen, the landing is off the centerline because the small heading corrections that are usually applied to land at the centerline can not be made in this case as we can not touch the controls. The aircaft lands with power on because of the power addition in the final approach to stabilize the airspeed as one of the last control inputs before the handsfree trajectory.

The absence of a landing flare results in a touchdown at a vertical speed of 580 feet per minute which is a rougher landing then with a flare but it is not particularly a hard landing. As we have previously seen, there are flare-less landings in real life for instance on aircraft carriers.

Simulated flight by Ralf Vandebergh



                                               Another Shift from the Aiming Point


In the practice of most aircraft landings, the actual aiming point that was chosen for a stabilized approach is never reached. This is because the required flare maneuver will shift the actual landing point several meters down the runway behind the aiming point. One of the demonstrations of the handsfree landing experiment is to visualize what happens if no flare maneuver is performed, thus the touchdown should theoretically happen at the actual aiming point.

In practice: If we carefully analyze in handsfree landing experiment 1 the position of the aiming point by comparing the FPV-symbol at pre-landing and at where the aircraft actually landed, it appears that the actual touchdown point is in front of the aiming point and that the actual aiming point once again is not reached. The left screenshot shows the point before the landing and we see that the FPV-symbol is visible just above the dark area at the front of the runway. The right screenshot shows the point of touchdown and shows that the real touchdown point is still in the dark area at the front of the runway, so in front of the aiming point. This forward shift however is much smaller then the backward shift that exists with a flare maneuver were the true landing point is behind the aiming point.






                                                  Handsfree Landing Experiment 2  

                                                           
Prove of Repeatability



This time the simulated flight video is without instructions, except the marked point were the handsfree trajectory starts. This flight looks very similar to experiment 1 with only small but interesting differences. This time the vertical speed shortly before touchdown is 580 feet per minute and turning to 590 feet per minute at touchdown, exactly the reversed values as seen in experiment 1. The trimming is almost exactly the same with 141 knots airspeed and 10 degrees pitch in the approach and a slight decrease to 140 knots at touchdown while the pitch angle stays at exactly the same value. The very equal values as seen in both experiment 1 and 2 prove that this experimental handsfree landing is repeatable. Again the landing is slightly off the centerline because any control inputs on the heading adjustments are disabled due to the handsfree landing.

Simulated flight by Ralf Vandebergh



                                                      Handsfree Landing Experiment 3 

                                                               
Precision Landing



The goal of this third handsfree landing is to perform a precision touchdown just as previously practiced. Like the precision landing with the SR-22 and later with the F-16 in the preparing test flights on this page, the aiming point is in the short area at the front end of the runway. But the difference is that the landing now will be made fully with the hands off the controls. This means practically that all proceedings as demonstrated in the previous sections must be executed in a perfect way. Especially important in this case is the perfect glide path angle that must be adjusted and stabilized before the handsfree trajectory starts and remain stabilized until the landing. As can be seen in the video, small power corrections that have been made to adjust the glide path angle have resulted in a slightly lower approach speed of 139 knots compared to experimental flights 1 and 2 while the pitch angle with 10 degrees remains unchanged. As a result experimental flight 3 shows a bit higher vertical speed of 720 feet per minute in the last part of the final approach until touchdown. Because of the required aircraft roll-out phase, certainly only the touchdown can be made in the short dark area at the runway front end and it is not possible to come to a full stop in that area.

Simulated flight by Ralf Vandebergh




                                                  Handsfree Landing Experiment 4 

                                                                
Out of Routine


This time the landing is made at another airport then the usual test location used for most of my simulations. This is basically to get out of the routine of a final approach to a certain airport and
to test if I can make the handsfree landing on another airport without the usual landing routine. Experiment 3 was involved with a slightly lower approach speed while the pitch was unchanged. Experiment 4 has some other changes in settings. This time the approach is made with a slightly higher approach speed of 142 knots and a slightly lower pitch angle of 9 degrees. As a result the touchdown occurs with a vertical speed of 740 feet per minute, just slightly higher then then the vertical speed in handsfree landing experiment 3.

Simulated flight by Ralf Vandebergh




                                                    Handsfree Landing Experiment 5 

                                                        
Landing in Flare Attitude



The goal is to trim the aircraft to a higher angle of attack and corresponding higher pitch attitude then in the previous 4 experimental flights. In fact the aircraft will be trimmed to the actual flare-attitude which should result in a softer handsfree landing then in the previous 4 experiments in the 10 degrees/140 knots configuration. In nominal landings, the flare must be executed shortly before touchdown with the requirement that the pilot touches the controls. The flare of an F-16 usually is a transition from an initial pitch angle of 10 or 11 degrees to 13 degrees or maybe 15 degrees of pitch angle. In this experiment I trimmed the aircraft with success to a pitch angle of 14 degrees and a corresponding airspeed of 130 knots.

It was much more difficult then trimming to the standard configuration of 10 degrees and 140 knots, also due to the poor sight on the runway caused by the high pitch angle. In the video, the entire final approach procedure can be seen. Turning final at around 200 knots with already power off, reaching final at 190 knots were the nose is raised above zero pitch. Then the normal precedure is followed. As airspeed decreases, the nose is raised to keep the FPV-marker at the front of the runway. Then the aircraft is carefully trimmed to 14 degrees pitch angle and 130 knots airspeed. When reaching this attitude, the stall alarm shows up for a long time and this is exactly what should happen: We approach in the actual landing-flare attitude and this should involve an airspeed close to stall speed.

The handsfree phase is marked in 2 stages this time. First there is a mark for the point were 'the hands are fully off the controls except the rudder'. This is because some very small heading corrections are necessary to reach a touchdown point as close as possible to the centerline. Since after the sensitive last pitch trim for the handsfree landing, hands needs to be off the stick, small heading corrections are made using the rudder. (In real life rudder control is done with the pedals but on the virtual F-16, I personally control it with keys, so it is still considered as hand-touched here). Then there is a mark for 'hands are fully off the controls including rudder', meaning that the aircraft controls now are fully untouched and the aircraft flies completely on itself until touchdown.

This handsfree final approach in the maximum angle of attack (flare) configuration results in a nice soft touchdown close to a nominal flare landing. The touchdown takes place at a vertical speed of 370 feet per minute, compared to
580-720 feet per minute in the other 4 handsfree landing experiments.

Simulated flight by Ralf Vandebergh




                                                                Handsfree Landing Experiment 6  (Update) 

                                                     Long Handsfree Final Approach
  

 
Although this handsfree landing technically is very similar to the previous experiments, the handsfree trajectory of the final approach until touchdown is much longer then usual and covers almost the entire video. A long handsfree approach like this requires a very precise adjustment of the controls - especially in the lateral (directional) control - to not miss the runway at all. A handsfree landing very close to the runway centerline is almost not possible this way. This final experiment is unmarked but note the very constant airspeed and pitch angle and the absence of any adjustments in the display. Watch video of final Handsfree Landing Experiment 6, Long Final Approach.
                                                                            
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                                                  Conclusions

                                                                 GE F-16 Handsfree Landing Experiment




- As the previous 5 experimental flights with the GE F-16 flight simulator demonstrate, it is virtually possible to land an  F-16 aircraft in its pure aerodynamic configuration - thus without artificial stability - on a stabilized approach without any touch of the controls. To managed this, the aircraft must be trimmed for the proper pitch angle, airspeed and corresponding vertical speed to make a landing possible. The aircraft must be fully trimmed for this landing configuration before the handsfree trajectory starts as the controls are further untouched until touchdown. 

- The handsfree F-16 landings in the first 4 experimental flights are all made at a pitch angle of 10 degrees or close to 10 degrees, an airspeed of 139 to 141 knots and a vertical speed of 580 to 740 feet per minute. Experimental flight 5, the final and most difficult handsfree landing where the aircraft is landed in the true flare attitude, has different values with a pitch angle of 14 degrees, an airspeed of 130 knots and a vertical speed of 370 feet per minute.

- The high angle of attack necessary for the landing changes the initial close-to-negative longitudinal (pitch) stability of the F-16 into low positive longitudinal stability,  making it possible to fly a virtual handsfree stabilized approach. It also enables a pre-handsfree trajectory with minor corrections in glidepath angle whithout destabilizing the -for a landing - trimmed configuration
.


- The handsfree descent trajectory in these experimental flights is resulting in a landing without a flare maneuver because no input on the controls is possible. Therefore, the landing is generally rougher then a landing with flare but still within practical limits. This is because the aircraft was trimmed to a pitch angle of 10 degrees early in the approach, which is close to the 11 to 13 degrees pitch angle that is common in real-life F-16 landings.

- Experiment 5 shows that it is even possible to land handsfree with the GE F-16 trimmed to a flare-attitude as in a slow  F-16 approach and landing with flare. Trimming to this attitude seems more difficult then a handsfree landing in the standard 10 degrees pitch / 140 knots configuration. The difficulty is caused by the fragility of the trim at the edge of stall speed. Additionally it is more difficult to set the proper glidepath for the handsfree landing trajectory which is caused by the very poor sight on the runway due to high pitch trim for a high angle of attack. In fact, only the runway-end is visible in this attitude, which disables a precision handsfree landing as is demonstrated in experiment 3.

- The aiming point, interestingly, is not exactly reached in the virtual handsfree landings as in the expectations based on the aerodynamic principle were the true landing point would be exactly the same as the aiming point in a perfect stabilized approach. The true landing point is in all handsfree landings located at a short distance in front of the aiming point. However, the true landing point is much closer to the aiming point then in a landing with an executed flare maneuver were the true landing point is located behind the aiming point and at a greater distance from the aiming
point.


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