High-speed Flight

Compressibility

High-speed flight is flight near, but below the speed of sound.  Below high-speed—what is regarded as slow-speed flight—the movement of air around an aircraft during flight does not involve compression of the airflow—what is referred to as compressibility.  Instead, the behaviour of slow-speed airflow entails the rules of aerodynamics discussed thus far—the flow of air is like the flow of water around rocks in a stream, where the flow accelerates or slows, based on size and surface features of obstructions to the flow of water.  In contrast, high-speed flight is different.  Because of the speed of the airframe and wings, etc., the air cannot move out of the way; instead of flowing around the wing, for example, the air cannot get out of the way fast enough, resulting in a compression of the air particles.  Compressibility introduces radical changes in aerodynamic principles of high-speed flight.

As the aircraft moves through the air, a pressure wave is propagated ahead of the aircraft,6 which, as Linda D. Pendleton describes, effectively warns the air molecules that lie in the path of the aircraft that the wings, fuselage, etc., are coming, enabling them to organize and smoothly streamline around the structure of the aircraft.  As the aircraft speed increases and approaches the speed of the propagated pressure wave, less and less warning is provided and smooth orderly flow is lost.  Linda D. Pendleton describes the situation as follows:

As speed increases, the aircraft comes closer and closer to the air particles before they are “warned” by the advancing pressure wave and can change direction.  The greater the aircraft speed, the fewer air particles will be able to move out of its path.  As a consequence, the air particles begin to pile up in front of the aircraft, and the air density increases.7

The pressure wave, then, conditions the air particles, allowing them to get out of the way of the airframe; as the aircraft approaches high-speed flight, the pressure wave can no longer radiate in front of the aircraft structure, and the air particles begin to compress—i.e, the phenomenon of compressibility develops.  The speed of the pressure wave is, of course, the speed of sound, and compressibility occurs when the aircraft itself approaches the speed of sound.

The concept of compressibility goes a long way to explain the thin design of leading edges of high-speed wings and fuselage designs, and aeronautical engineers attempt to reduce as much as possible the effects of compressibility.  Essentially, compressibility inhibits laminar flow, and instead of the air particles accelerating smoothly, the speed of airflow decreases dramatically.

Transonic Flight

The speed of air flow over the upper camber of a wing varies, and it follows, therefore, that portions of the air flowing over the wing will attain the speed of sound—Mach 1.0 (M 1.0)—first, while other portions of airflow remain subsonic.  As indicated in the depiction to the right, supersonic speed first develops at the area of maximum airfoil thickness.  While this portion has attained M 1.0, remaining portions of the wing, as well as the airframe, remain subsonic.  When this happens, an aircraft is said to have reached its critical Mach number (MCRIT), and this marks the beginning of the transonic speed range.  Pendleton writes as follows:

Critical Mach number . . is an aircraft limitation to be heeded.  If the aircraft is flown at speeds in excess of its critical Mach number, numerous unsettling and potentially disastrous events occur. . .  Boundary layer separation on the control surfaces might cause the surfaces to rapidly oscillate, which is called buzz.  This can cause metal fatigue problems in both the control surfaces and the hinge fittings joining the control surface to the wing . .  Shock-induced separation of the airflow over the control surfaces causes them to be in an area of turbulent, nonstreamlined air that causes a loss of effectiveness.  When there is a shock wave in front of the control surface, deflection of the surface cannot influence the airflow in front of the shock wave . . The wings might begin to twist due to compressibility effects.  The airfoil shape over the length of the wing is seldom constant, and the differing onset of formation of shock waves and movements of centers of pressure cause this effect.  This effect is unnerving at the least. . .  Severe buffeting will almost surely be the next compressibility effect that is sure to show up.  The extremely turbulent airflow separated from the wings begins to bang against the tail surfaces in a manner that is violent and irregular and disturbs the flow patterns around these surfaces causing them to buffet . . This buffeting, if allowed to continue, has been known to cause separation of the tail from the aircraft . . .  If all this has not caused the pilot to slow down and escape compressibility effect by now, a more violent effect is about to occur.  Loss of longitudinal stability will cause the aircraft to “tuck under” . .  The important point is that the first signs of compressibility effects call for immediate pilot action.  The airspeed must be reduced, and the nose must be eased up.  The power reduction has to be fast, for when the tuck starts and the aircraft starts into a dive, the situation is going to get rapidly worse.  The increased speed will cause the separation of the airflow to become more pronounced, and the severity of the buffet will become greater.  The greater the turbulence over the tail, the greater will be the elevator angle and the stick force required to pull out of the dive.  Some have not been successful in this manoeuvre, and some have lost the tail of the aircraft before they had time to begin recovery.8

Generally speaking, corporate jets and airline jets are designed to cruise at speeds between M .7 and M .9.  Exceeding these speed will produce Mach tuck and the other rather unpleasant flight characteristic described by Pendleton.  So the question arises as to how the pilot is advised as to the proximity of the aircraft’s speed relative to MCRIT.  Effective communication is complicated by variations in Mach 1.0, based on altitude and temperature.  While slow-speed aircraft are speed-limited by Vne, high-speed aircraft are speed limited by the expression MMO—the maximum operating speed relative to the speed of sound.  MMO is displayed automatically on the airspeed indicators of high speed by what is referred to as the barber pole—a self-adjusting needle that predicts the MMO based on ambient temperature and pressure altitude.  An aural over-speed warning device is also wired to the system—referred to as a clacker.