Important Issues in Flight Simulation

Important Issues in Flight Simulation

Level of Modeling

The most common error I have encountered in flight simulation involves the issue of the level of modeling chosen for the simulation. Consider the issue of stability. When you pull back on the stick from level flight, and hold the stick slightly aft of center, the nose begins to rise, and the plane begins immediately to climb. As the nose rises the airspeed begins to decay very slowly and almost imperceptibly, and as this occurs, the rate of climb also falls off. Due to the inherent longitudinal stability, this reduced airspeed has the effect of changing the trim to more nose-down, so that with the stick held steady, the rate at which the nose rises begins to decay. If held long enough, the pitch attitude will eventually equilibrate at some nose-up angle, at a reduced airspeed, and at a rate of climb much smaller than that experienced when the climb was initiated, as suggested in Figure 1.

Figure 1

The pilot's psychological conception of the action of the elevator control is rather different however, since he generally thinks of the stick as controlling the pitch attitude, as suggested in Figure 2, i.e. he pulls back until the nose reaches the desired climb attitude, and then returns the stick to the neutral position for the steady climb.

Figure 2

The business of compensating for the change in trim is, for the experienced pilot, an unconscious and automatic act. There are even more complex and subtle effects such as the rotational inertia as the aircraft begins to rotate in pitch, so that to initiate a pitching-up at a constant rate, one would actually have to give an initial pulse of stick-aft, followed by neutral stick as the nose rotates upwards until the desired pitch angle is attained, at which point a pulse of stick-forward would be required in order to stop the upward rotation and maintain the pitch at the new climb angle, as suggested in Figure 3.

Figure 3

This rotational inertia effect is superimposed on the other effects, and again is totally unconscious, so that in order to perform the constant-rate pitch-up depicted in Figure 2, the actual control input would be something like Figure 4.

Figure 4

Of course only a pilot performing precision aerobatics cares to control his rate of pitch so crisply, and most pilots would perform a much smoother and simpler version of this control pattern. The question is, at what level should the simulation model these complex and subtle effects? The common assumption is that the more physically accurate the simulation is, the better the simulation. The fallacy in this thinking is that the simulation is not analog and continuous, but is discrete and step-wise, so that these subtle effects, if fully simulated, result in a jerky and unpredictable response. Furthermore, the real pilot has a visual input and mechanical output which are orders of magnitude more fine grained than that available to the simulator pilot, and in fact real the pilot often makes pitch changes which are smaller than the pitch represented by one pixel on the screen, using stick forces which are less than one notch of the computer joystick. Also, the human visual system has specialized motion detectors which perceive motion at much higher resolution than the perception of absolute edges, but this motion information is lost in the simulation where a slow movement of the nose is converted into a series of jumps. The result of all of this is that the simulation cannot model physical reality to the resolution available in real flight, and when it attempts to do so, it introduces dynamics at a level which is invisible to the pilot, and that appear therefore to the pilot as random and chaotic movements of the plane.

It is more appropriate for a simulator to reproduce the subjective feel of real flight than to accurately reproduce the physical forces of flight. The Microsoft Flight Simulator is an example of a simulator that is excessively "realistic" physically, resulting in an unrealistic subjective feel. Things which are easy and natural in real flight, such as maintaining a steady climb or turn, require inordinate effort in this simulator, which leaves no spare attention to devote to the real operations of flying, such as deciding what altitude to climb to, or heading to turn to. I once flew a flight simulator on the Amiga (sorry, I forget what it was called- it had an aircraft carrier) which had the very simplest control dynamics, but which produced an excellent simulation. If you pulled the stick back a bit, the nose would rise smoothly and steadily, until you released the stick, at which point the nose locked on to its current pitch angle and stayed there, just as in Figure 2. This is nothing like the physical reality of a plane in flight, but it very much reproduces the subjective perception of flying, since the pilot can devote his full attention to where he wants to fly, rather than how to do it. The simulation is, in effect, not only reproducing the forces of flight, but is also taking over some of the automatic low level functions of the pilot. The exact level at which this trade-off is optimal depends on the speed and resolution of the computer, as well as the design objectives of the simulation. Given a fast computer with high resolution, the simulation can afford to be more physically realistic without loss of subjective feel. A simulator designed for a fast machine will however be highly unsatisfactory on a slower machine.

Field of View

The restricted field of view available on the computer monitor is perhaps the greatest single obstacle to generating a realistic flight experience. The benefit of the all-around view in the real plane can be appreciated if one attempts to fly a plane wearing blinkers (like the "hood" worn for training in instrument flight). The view of the wingtips seen in peripheral vision gives a fine indication of the rate of roll, not available in the straight-ahead view where one is looking along the axis of the roll. An effortless look out the side is also essential when making the turn from downwind to final on the landing approach, and the left wing view available in most all simulators is really no substitute for an easy glance to the left because it lacks [1] the sense that one is looking sideways (since your head is still pointed straight ahead) and [2] the view of the nose of the plane in peripheral vision while looking left in a real plane. Flying a plane while looking at the left wing is easy and natural even to student pilots, but is extremely difficult in a flight simulator.

The all-around view provided by peripheral vision is most important when flying in combat in a dogfight simulation. I have two simulators on my Mac at home- Hellcats, and Chuck Yeager's Air Combat. Hellcats is the superior of the two in almost all respects, except for one feature available in Air Combat, which is the plane-to-plane view, whereby the "camera" is positioned along the extended line of sight joining you and your enemy, so that both of you are in view on the screen, although you see your own plane from the outside. It takes a little practice to fly the plane using this outside view, especially when your plane is headed towards the "camera", in which case a roll to the left appears as a rotation to the right, a problem familiar to radio control model flyers. The advantage offered by this external view is so great however, that this single feature makes Air Combat by far the superior all-around simulator.

Figure 5 illustrates a typical scenario where this view is indispensable. You have just made a pass at the enemy after a dive from above, and perhaps had a shot at him. You are now very close to him but overshooting, and you must do some kind of loop to get back around for another pass. At the point marked A however, in a simulation, you lose sight of the enemy at the most critical moment due to the "tunnel vision" offered by the computer screen. Since things are happening so fast at this critical juncture, there is no point trying to track the enemy by switching to different views. In Hellcats your only hope is to just pull back hard and fly round and round in circles watching your radar and hoping to get another glimpse at the enemy! In Air Combat however you can enjoy what is the most interesting spatial challenge of dogfighting.

Figure 5

In a real dogfight the pilot is rarely looking out the front windshield, his head is constantly gyrating this way and that in order to avoid losing sight of the enemy even for an instant. When pulling back hard (which he is doing almost constantly) the pilot usually looks in the direction that the aircraft is turning, i.e. "straight upwards" relative to the plane, and the strategy is to attempt to point that upward acceleration vector at a point in space trailing slightly behind the enemy aircraft, marked C in the figure, i.e. to maneuver into position B, from whence it is fairly easy to get back onto his tail. Notice that if at point A, your plane were to roll slightly left or right, it would have a big effect on your final position, placing you either too close above, or too far behind the enemy respectively. The bank angle must therefore be controlled throughout the half-loop to steer the "straight up" acceleration vector as close as possible towards point C, compensating also for the evasive gyrations of your opponent (who at this point would do best to roll right in order to point his acceleration vector towards your tail!).

The plane-to-plane view is an example of the way a well designed simulator can compensate for the problems inherent in the computer representation. In a real dogfight the pilot's head tracks the enemy effortlessly, and he maintains a good spatial sense of the essential distances and angles. As a consequence his full attention can be on maneuvering his plane. In a good simulation therefore this function should also be effortless and automatic. Ironically, the plane- to-plane view is available in Hellcats too, but only in "instant replay" mode, so the programmers went to all the trouble of creating this view, but did not bother to make it available while flying real-time! Likewise, in Air Combat, the plane-to-plane view is only available with targets which have been "locked on", but to do so one must first maneuver so that they appear in the front windshield view and then hit RETURN, which establishes the lock. You cannot toggle back and forth between plane-to-plane views of different targets, so you are really handicapped when flying against more than one opponent. After shooting down one opponent you must "lock on" to the next before you can see where he is, except that you must see him in the front windshield view in order to "lock on" to him! You wind up gyrating wildly looking this way and that just to find the next opponent! Since it is easy for the real pilot to search the sky quickly (at least for a nearby plane) the plane-to-plane view should be available in the simulation to locate planes without first having to "lock on". The plane-to-plane view, or plane-to-runway view should also be available during the landing approach, where a good spatial sense is essential to a successful landing approach. It is irritating that these features could have been added with virtually no extra effort if only the designers had realized the importance of these functions!

Simulating the Rudder

The rudder is one of the most obscure controls of the airplane, and few people really understand what purposes it serves. It does not serve to "turn" the airplane, since that is achieved by tilting the lift vector of the wings in the desired direction, and "pulling" through the turn with the elevator. I have heard it said that the purpose of the rudder is to correct residual errors left by the designer, and it is true that most of the use of the rudder serves to prevent the plane from yawing, rather than to initiate an intended yaw. Most of these unintended yaw forces result from the turning of the propeller, and require a small amount of right or left rudder to be held constantly depending on airspeed and power setting. Another yawing force called "adverse yaw" results from application of the aileron, which makes one wing generate more lift than the other for rolling, which in turn generates asymmetrical drag resulting in a yaw in a direction opposite to the roll. All of these rudder effects are compensated for automatically and effortlessly by the pilot (if he is any good!) and therefore it is appropriate for them to be absent in a computer simulation. Many simulators do simulate adverse yaw rather uselessly, especially if they do not provide for an analog rudder control to synchronize with the analog aileron deflection!

There are two uses for the rudder however which generate an intended yaw, and these useful functions are rarely implemented properly in computer simulations! They are the slip and the skid. The slip is an essential maneuver for fine control during landing, especially in crosswinds. Consider the situation depicted in Figure 6 A.

Figure 6

The plane is very close to the runway on the landing approach, and is parallel to the centerline, but offset to the right. Without the rudder this would require a banked turn to the left followed by a banked turn to the right. In fact the pilot is constantly making last minute adjustments of this sort just before touching down, which would result in a constant waggling of the wings, typical in flight simulator landings, during which one wheel is sure to touch down prematurely and out of alignment. The proper correction is shown in Figure 6 B where the pilot banks the plane to the left, but applies just enough right rudder to prevent the plane from turning to the left. The nose therefore remains parallel to the centerline, but the plane slips sideways until it is back in line with the centerline. When landing in a crosswind this side slip is essential if you want to touch down with the wheels aligned parallel to the centerline and without any sideways drift due to the wind. In a proper crosswind landing therefore the upwind wheel hits the ground first, i.e. the plane lands with the wings banked in the slip, as shown in Figure 6 C. Many simulators allow you to establish a slip, as in Figure 6 B, but they do not provide for the sideways shift which should result from the slip, which makes it impossible to use the side slip for landing corrections!

The skid, although aerodynamically identical to the slip, is used in a different manner when, for example flying in close formation with another plane, as shown in Figure 7 A. If the following plane wants to "tuck in" closer to the leader, for example, the proper procedure is to leave the wings level and kick the right rudder. This exposes the left side of the fuselage to the slipstream which pushes the plane laterally to the right. Opposite aileron must be applied to prevent the plane from rolling to the right due to the right rudder. A proper banked turn could also be used in this case, but again, formation flying requires constant corrections which would result in a constant waggling of the wings, which is both awkward and not very pretty! Instead, in formation flying, the pilot holds his wings parallel to those of the leader, and operates the rudder like the rudder of a boat, steering the plane laterally without banking the wings. Again, most simulators do not allow this fine control, which makes formation flying virtually impossible. Notice that in formation flying, right rudder is used to slide to the right, while on the landing approach right rudder (or more correctly, left bank) is used to slide to the left.

Figure 7

The skid is also used in air combat to make fine adjustments to the lateral aim of the guns, as shown in Figure 7 B. In this case the intention is not to move the plane laterally, but just to point the guns, although it does have the secondary effect of sliding you "in trail" behind the target- which is not always a good thing if you fly into his prop wash, which will throw off your aim, or will cover your windshield with oil if he is streaming a trail of it. Many flight simulators do not allow for this kind of steering of the nose. In the situation shown in Figure 7 B, for example, immediate action is required, which means that without the ability to skid, the plane would have to be banked 90 degrees to the right and the elevator pulled back briskly but briefly to line up for a shot. If in the meantime the enemy has pulled up just a bit, this now requires a left bank back to level flight, followed by another brisk tug on the elevator. In other words you can only correct one dimension at a time with 90 degree rolls in between, which in air combat would be intolerable! In real life you just kick the rudder to line up the guns.

Finally, there is one more obscure use of the rudder as a defensive maneuver in air combat. Figure 7 C illustrates a "deflection shot" where the target is moving rapidly at right angles to your line of fire, which requires you to "lead" your aim, shooting into the point in space which your opponent is going to occupy in the time it takes for your bullets to arrive there, indicated by the solid arrow in the figure. To do this you must have an accurate estimate of the enemy's speed and direction. The enemy can throw off your calculations by applying hard right rudder in a skid, so that his actual path through the air will be as indicated by the gray arrow in the figure. In the absence of nearby fixed objects for reference, your estimate of his direction is governed by the direction his plane is pointing, which because of the slip is now no longer the same as the direction he is actually moving, with the result that your bullets will pass harmlessly over his head.


Many of the principles discussed here are not necessarily difficult to implement, and indeed my belief in maintaining a simple level of simulation should simplify the programming and increase the performance of the system. In my view it is essential for a flight simulation to consider more than just the physics of flight, and to include considerations of human perception and the limitations of the computer as a vehicle for simulation in order to strike the right balance in the many trade-offs that must be made. I would be happy do demonstrate the subtle and complex effects discussed here in an actual airplane if you are interested.

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