Speed effects

EVERYTHING HERE IS PROVISIONAL*

A word of caution

Designing the kind of aircraft that we want to fly in KSP with a realistic aerodynamic model is not at all trivial. One of the reasons is the quite extreme speed range that we desire to fly aircraft in.

The ratio between the absolute maximum and minimum speeds of a modern light aircraft is about 5; airliners, while obviously much faster, also achieve the same ratio. A KSP player designing an SSTO, on the other hand, will probably try for a speed ratio of 20. This is a large number in real life, even though high performance fighter aircraft like the F-15 can achieve it (and for example, the max/min ratio for ther SR-71 was probably 13).

Our aircraft are most commonly supersonic: and flying around or above the sound barrier can alter the behavior of an airplane in very significant ways.

Shuttles and SSTOs reentering from orbit will briefly experience flight at hypersonic speeds, which has yet again different characteristics. Though the typical reentry speed in KSP is low enough that this won't be nearly as important as it was for the real-world Space Shuttle.

Effects of high speeds

Flight characteristics in the supersonic regime can differ markedly from subsonic: it's not uncommon for an aircraft to develop instabilities only when supersonic. The most problematic axis is often yaw, since the effectiveness of tailfins decreases with higher Mach.

In the pitch axis, most supersonic aircraft experience a phenomenon called "Mach tuck", i.e. a tendency to nosedive. This is due to the center of lift moving backwards from its subsonic position when the airflow around the wings starts to become supersonic. This will happen before the aircraft as a whole reaches supersonic speeds: it is the so called transonic speed range.

Mach tuck is not a dynamical instability: actually, it makes the aircraft more stable in that sense. But if the control authority in pitch is insufficient, the tendency to pitch down might overpower the controls and send the aircraft into an unrecoverable dive.

Another common reason for losing control of very high speed aircraft at altitude is the reduction in engine thrust, if the engines are mounted below the wings (like they often are in real-world fighters). The pitch-up moment from the engines might mask a pronounced tendency to nose down during the acceleration phase, but be insufficient to keep the aircraft level when the ceiling approaches.

Drag will also increase and peak in the transonic zone, before decreasing in the fully supersonic regime (but almost never reaching subsonic values again): real aircraft often have to use afterburners to temporarily boost their engines to get over the drag peak around Mach 1. No stock engines and few modded ones have afterburners in KSP, but the thrust/weight ratio of aircraft will often be very high in real-world terms and might make this effect unnoticeable.

Drag and lift curves in function of Mach number will be calculated by FAR in the "Static stability" pane.