GE F-16 In-Flight Analyses

                                       Basic Aerobatics

                                       
Ralf Vandebergh


                                                                  Introduction

The main page Realistic Landings with Google contains an
alyses of the GE F-16 aerodynamic accuracy in the landings compared to real-life F-16 HUD video footage. On this sub-page you will find analyses of the GE F-16 in-flight accuracy using comparisons of some easy aerobatic maneuvers. First there is a simulation of an aileron roll which is a close copy of an aileron roll from a real-life F-16 HUD video. Then the same procedure will be repeated for a looping with the GE F-16 in comparison with a looping of a real-life F-16. The results of these analyses tell more about the in-flight aerodynamic accuracy of the Google Earth F-16 flight simulator.

The material of these analyses can also be useful to learn more about the principles of some basic aerobatic maneuvers.

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                                   Basic Aerobatics with the GE F-16


                                                          Aeronautic Roll Maneuver

T
here are many variants of the aeronautic roll maneuver of which most of them are basically used in aerobatic demonstrations. The roll consists of a rotation about the longitudinal axis of the aircraft. This maneuver can be either a single rotation or multiple rotations while the rotation speed can be very different. The easiest and most simplistic variant of the roll is the aileron roll which doesn't require any attitude corrections but it is a straight executed maneuver, often used in test flights, and performed just by applying aileron input.

The amount of aileron input (how far you hold the stick to the left or the right) determines the roll rate. Because the aileron roll is an uncorrected roll, the nose of the aircraft drops during the maneuver, an effect of the lost of lift. For that reason, the aileron roll usually starts at a relative high pitch attitude. As the nose is raised shortly before execution of the roll, the aircraft starts to gain altitude which compensates for the lost of lift. At the end of the roll, the altitude therefore should not vary much from the start of the maneuver. The visible effect of the loss of lift during the aileron roll is the pitch decrease during the roll. The amount of decrease depends on the roll rate and therefore directly on the amount of aileron input.


                                                    Reproducing an F-16 Aileron Roll


The reproduction of a real-life aileron roll performed at a particular altitude and airspeed is not easy when precise values are persued. Only a little differentiation in airspeed or altitude at the start of the roll, or a little difference in the roll-rate and other influences on the exact configuration will cause different values in the analyses, the comparisons between the maneuver in real-life and in the simulation. For this reason, approximate values (a margin of 10 knots for airspeed and 100 feet for altitude) are sufficiënt to give an indication on the overall accuracy of the in-flight aerodynamics in aerobatic maneuvers.

As in the real-life F-16 HUD video can be seen, to start the roll, the nose (the cross on the real-life HUD display) is lifted to a pitch angle between 10 and 15 degrees. At that moment the airspeed is around 394 knots and the altitude around 21.500 feet. At the 180 degrees point (flying upside down) the airspeed has dropped a little while the altitude has increased a bit. This is a result of the high pitch maneuver at the start of the roll. The pitch attitude at the 180 degrees point is around 3 degrees as a result of the loss of lift during the roll. At the 360 degrees point when the roll is completed, the airspeed has increased again while the pitch angle is just over zero degrees. The altitude has increased again to 22000 feet because the aircraft is still climbing as a result of the high pitch maneuver at the start of the roll. Thus, thanks to this maneuver, we end the aileron roll at an altitude close to the altitude at the start of the roll.



                                                           Aileron Roll Simulation

I made several attempts to reproduce the real-life F-16 aileron roll as precisely as possible. The following result was the closest match.

Simulated flight by Ralf Vandebergh



                                                      F-16 Aileron Roll  Analysis


The screenshot set below shows 3 phases in the aileron roll: The start, the 180 degrees point and the end (360 degrees point). The real-life HUD screenshots are on top and the screenshots from the simulation are displayed at the bottom.
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0 degree point (roll start)
At the start the pitch angle is between 10 and 15 degrees while the altitude is around 21.500 feet (+/_ 100 feet). The airspeed is almost exactly 394 knots. To reproduce the real-life aileron roll parameters in the simulation it takes some practice to meet these values as precisely as possible; you have to take into account that the airspeed and altitude changes quickly with every change in pitch.

180 degrees point (inverted)
At 180 degrees roll angle, the airspeed has decreased to around 385 knots and we see a very precise match with a difference in the order of 1 knot between real-life and simulation. The altitude at this point has increased close to 22000 feet in both the real-life video and the simulation with a difference within 100 feet margin).

360 degrees point (roll completion)
To start with, a great and obvious match in accuracy at the completion of the roll is the attitude of the aircraft nose and the position of the flight path vector (marked in red). In the upper real-life screenshots we recognize them as the cross (nose attitude or pitch) and the small circle (flight path vector). The pitch attitude is just below 5 degrees in the real-life display and and has the same position in the simulation. The position of the flight path vector symbol is just above the zero-pitch line (the horizontal white line in the real-life display) in both the real-life display and the simulation.

Further we see a small but interesting difference in airspeed. Although close to 10 knots margin, the airspeed has dropped with 5 knots from the 180 degrees point to the 360 degrees point in the simulation were it increased with 6 knots in the real-life roll. This difference may be caused by one or more unknown aerodynamic factors that influences the configuration and was not factored in the simulation. The altitude however, at the completion of the roll and still close to 22000 feet matches greatly with the real-life roll and within 100 feet margin.


In conclusion, and taking into account the difficulty of precisely reproducing the maneuver we can say that the GE F-16 flight simulator has very good accuracy in this real-life aileron roll comparison, Even if we ignore the airspeed and altitude data in this analysis, the good aerodynamic accuracy can already be noticed from the fact that the pitch attitude and the flight path vector end at almost exactly the same position as in the real-life aileron roll at the completion of the roll.



  
                                                      A more Advanced Aileron Roll  

As the roll rate is maximum in an aileron roll, the loss of lift due to the inverted wing is minimal. As can be seen in the upper screenshots, there is a lowering in pitch angle of around 10 degrees between the start and the completion of the roll. The amount of decrease in pitch angle depends on the roll rate and if we make a rotation at very low speed the nose drops to very low pitch angles with the result that the aircraft is diving towards the ground. To produce an aileron roll at a low rotation speed while still maintaining level flight, the loss of lift has to be corrected using rudder and elevator input by correcting the pitch angle as the aircraft makes a rotation around its longitudinal axis. This maneuver is called a slow roll. The following simulation is made with the GE F-16 flight simulator and demonstrates roughly the principle.

Simulated flight by Ralf Vandebergh




                                                      Aeronautic Looping Maneuver


                                                                         F-16 Loop in Theory

The aerobatic loop is a full elevator maneuver were the aircraft continues to increase the pitch angle until the start attitude is reached again. This results in a 360 degrees circular flight path. First the aircraft is accelerated to build op sufficiënt velocity, in case of the F/16 usually around Mach 1 (speed of sound) which is realized by a nose down maneuver preceding the loop. Then the nose is raised to increase the pitch angle. Instead of stopping the pitch-up maneuver as is the case in a normal climb, the aircraft continues to fly nose up and will turn to an inverted attitude when exceed the 90 degrees angle point were the nose of the aircraft is pointed straight upward. After this point, the pitch indicator on the HUD display will turn 180 degrees and count backwards. At this 90 degrees point the aircraft must have left sufficient velocity to make the turn to the inverted angle. If not, the aircraft will pitch back to lower angles to return to the start position. If beyond this difficult 90 degrees point and the inverted 180 degrees point, the gravity will pull back acceleration of the aircraft as soon as the nose starts to point nose down. From that point the airspeed will increase until the point that the nose is pointing towards the horizon when the start attitude is reached again.


                                                                                             Pitch Characteristics

Because a loop, in fact, is a 360 degrees pitch-up maneuver, characteristics are also similar. This means that the aircraft reaches a stall point if the airspeed is not sufficiënt during the maneuver. Decisive is the airspeed around the 90 degrees point; When the start speed is too low or when the elevator deflection is too small, the aircraft will reach this decisive point at a too low speed. If the airspeed is too low around the 90 degrees point, the aircraft will not be able to make the turn to an inverted attitude thus not giving gravity the chance to pull the nose down in order to complete the loop. The nose will sink more or less back to the the start position.


                                                                                         Looping Test Flights

In order to produce a more detailed discription of the loop characteristics, I made several loops with the slow and the fast GE aircraft. When in an SR-22 loop at 160 knots start speed full elevator deflection is applied there is a point around 55 degrees and 120 knots that there is a stall were the loop is shortly interruped. But soon the aircraft is able to complete the loop. When starting the loop at 130 knots with no flaps the continuation of the loop is only partially, the aircraft turns to the inverted position before reaching the 90 degrees point. The character of the loop changes when flaps are applied. Flaps increase the pitch angle where the stall point occurs as they bring the stall point closer to the 90 degrees point and the inverted position. When starting the loop at 130 knots with full flaps, the stall point is raised high enough to the inverted position and the aircraft is able to complete the loop. The following effects that I encountered during the testflights are similar for both types of aircraft: When too small elevator deflection is applied when starting the loop, thus increasing the size of the loop, the airspeed decreases too fast in order to complete the loop. If more elevator deflection is applied at the point where the pitch angle stops to increase and starts to decrease, it will not help to complete the loop as the increased angle of attack causes the airspeed to drop even more. If the loop was started with excess power available, adding the extra power will also barely help to complete the loop. The extra speed added in the climb will not overcome the drag. However, applying full elevator deflection at this point has some effect but it is no guarantee that it will help to complete the loop.



                                                             F-16 Looping Analysis


Simulated flight by Ralf Vandebergh                  The GE F-16 simulated looping as used for the analyses below                                          

In the following comparison we see 3 stages of a real-life F-16 looping compared to 3 stages of a simulated F-16 looping made with the GE flight simulator as seen in the upper video. As mentioned before, duplicating a real-life aerobatic maneuver is not easy to accomplish as not every single (aerodynamic) factor in the real flight is known.

0 degree point  (loop start)
The upper set of pictures shows the 3 stages of the real-life F-16 looping and the lower set of pictures the same 3 stages of the simulated looping. For this comparison I have tried to copy the known circumstances as accurately as possible. In the first picture we see the start of the looping at the 0 degrees point at around 8900 feet altitude. The airspeed at this point is around 560 knots. Both values are closely duplicated in the simulation in the lower picture.

270 degrees point  (nose-down point)
The next point that is chosen for the analyses is the nose-down attitude at 270 degrees into the looping. At this point, the aircraft already has overcome several aerodynamic phases when reaching this attitude thus it will be interesting to see how the airspeed and altitude data compare at this point in the maneuver. We see that both in the real flight and the simulation the airspeed is not far from roughly 225 knots, a reduction in airspeed of more than half the airspeed at the start of the loop. The altitude at this 270 degrees point is a very close match of 19100 feet in both the real flight and the simulation. This means that the altitude at this point has increased with roughly 10000 feet from the start of the loop.

360 degrees point  (loop completion)
The next step in the analyses is the completion of the loop at the 360 degrees point. The airspeed in both the real flight and the simulation is not far from 300 knots, higher then at the 270 degrees point but lower then at the 0 degrees point. The altitude at the 360 degrees point in both the real flight and the simulation is close to 14000 feet which is (as expected) lower then at the 270 degrees point but higher then at the 0 degrees start point.

Considering the close match in airspeed and altitude values it can be concluded that the conducted simulated F-16 looping must be a nearly perfect duplication of the real-life F-16 looping. More importantly it can be concluded that the aerodynamic accuracy of the GE F-16 flight simulator in the loop maneuver is very good considering the close match in data in all 3 stages into the looping.
  


                                                                             
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                                        Visualizing Angle of Attack in an F-16 Landing


Visualizing the angle of attack in an F-16 approach and landing was one of my smaller projects with the GE flight simulator. The simulated F-16 final approach in the video below is a combination of an unstabilized approach (decelerating approach) in the longest part of the trajectory and a stabilized approach in the last part. The unstabilized part is flown in the way we see in the real-life F-16 HUD footage as shown earlier, a slowly but nearly constant decrease in airspeed. Because this is a relatively steep descent, coming from over the mountains, the descent has to be made at a high angle of attack at the lowest possible speed. This is the technique as demonstrated on the main page chapter Steep Landings, section Steep Low Speed Approach. In this case the airspeed is stabilized at 135 knots, only 10 knots above stall speed.

Continuation of the unstabilized approach is not possible in this case because we need the close-to-stall speed much earlier in the approach then usually to make the steep descent possible. As we can not decrease the speed further, we have to use a stabilized approach at this close-to stall speed for a successful landing. The nose is raised at 170 knots in this steep approach (in a nominal approach this is around 190 knots). While the pitch angle increases, the airspeed drops. When 135 knots comes into view, some power is added to stabilize the speed. The flare angle is 15 degrees while at the same time the power is set to zero.

This simulation visualizes the changing angle of attack (AoA) of this F-16 final approach.
The uninterrupted yellow line marks the flight path vector while the dotted yellow line marks the aircraft nose attitude or pitch angle and the difference between these 2 lines marks the AoA. When the 2 yellow lines would actually align, a red line appears indicating that the AoA is zero. At that moment the pitch attitude and the flight path vector are pointed in the same direction. We see that in the first part of the simulation the AoA is negative as the dotted line is below the uninterrupted line, indicating that the nose is pointed below the flight path vector. The AoA becomes larger as we approach the runway and is maximum at the moment of the flare. This maximum angle can not be marked further in the video as the flight path vector symbol is too low and out of view.

Simulated flight by Ralf Vandebergh

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