To understand how and why shockwaves affect flying controls we need to start by looking at what exactly shock waves are. A shock wave is an instantaneous increase in static pressure. When air flows through a shockwave the static pressure, temperature and density, all increase abruptly, while the velocity decreases. If the shockwave is very intense the reduction in velocity is sufficient to cause the boundary layer to separate from aircraft surfaces, thereby causing the airflow to become turbulence.
Shockwaves affect flying controls in the following ways:
1. In subsonic flight the pressure changes caused by the deflection of a control surface are felt not just on the control itself, but ahead of the control hinge line. So, for example deflecting an aileron may affect the pressure distribution all the way to the wing leading edge. But pressure changes cannot move forward through shockwaves. So if for example a shockwave forms on the hinge line of an aileron, the pressure changes caused by aileron deflection will be confined to the surfaces of the aileron. This will obviously reduce the control forces that the aileron is able to generate.
2. If a shockwave located ahead of an aileron causes the boundary layer to separate, this will directly reduce the ability of the aileron to generate control forces.
3. If intense shockwaves form on the upper surfaces of the wings, the resulting boundary separation will reduce the lift being generated and also envelope the rear surface of the aircraft (including the tail plane) in turbulent airflow. This will directly reduce the ability of the elevators to generate control forces.
4. The shockwave-induced loss of lift from the wings will reduce the downwash passing over the tail plane and elevators. This will alter the angle of attack that is experienced by the tail plane and elevators.
5. As an aircraft accelerates through the transonic speed range the formation of shockwaves, and their rearward movement, will cause the C of P of the wings to move aft. This rearward movement of the C of P, coupled with the loss of downwash over the tail plane, will cause the aircraft to pitch nose down (Mach tuck under).
6. As airspeed increases, the increasing dynamic pressure will increase the stick forces that must be applied to move the control surfaces.
The following design features are typically employed to overcome/minimize the above problems:
1. Use powerful hydraulic systems to reduce the stick forces that are required to move the control surfaces when flying at high speeds.
2. Use sweepback and reduce the thickness and camber of aerofoils in order to delay the formation of shockwaves and to reduce their intensity.
3. Use stabilators/all-flying-tail-planes (instead of separate elevators) to increase the pitch control authority. Increase the size of the other control surfaces.
4. Place the tail plane on the top of the fin to position it outside of the downwash and turbulent airflow produced by the wing.
5. Locate vortex generators ahead of ailerons to prevent shockwave induced boundary layer separation.
6. With very long thin wings, aileron deflection can cause wing twisting to the extent that the effects of the controls are reversed. Raising an aileron for example, may twist the wing trailing edge down, thereby increasing the angle of attack of that part of the wing. So instead of producing a wing-down rolling moment, the overall result would be a wing-up rolling moment. This problem is prevented by fitting an addition set of ailerons further inboard where the wing is stiffer. The ailerons at the wingtips are used in low speed flight then locked in neutral as speed increases. In high speed flight only in inboard ailerons are used.
The above lists are by no means exhaustive.