Use for questions related to the transition between subsonic and supersonic flows, like flight at speeds close to Mach 1 or supercritical wing design.
Transonic speeds are those speeds at which the airflow around an aircraft (or other object) contains both subsonic and supersonic areas; this is typically the case between approximately mach 0.8 and mach 1.2, although the precise boundaries vary depending on the shape of the aircraft.
At speeds below approximately mach 0.8, the airflow around the aircraft is entirely subsonic, and no untoward effects are seen on aircraft performance or handling.
As the speed of the aircraft increases through the lower critical mach number (often referred to simply as the critical mach number, as most aircraft cannot fly fast enough to encounter the upper critical mach number) - which is typically somewhere near mach 0.8, although, depending on the aircraft's design, it can be below mach 0.7 or above mach 0.9 - areas of supersonic airflow appear, accompanied by shockwaves wherever the supersonic airflow changes direction sharply or decelerates back to subsonic speed (if the ambient air is sufficiently humid, these shockwaves produce visible areas of condensation behind them). These shockwaves greatly increase the drag on the aircraft (this drag is known as wave drag), which makes flight at transonic speeds relatively inefficient; the speed at which this effect first becomes noticeable - usually just above the (lower) critical mach number - is known as the drag divergence mach number, or simply the divergence mach number. The shockwaves also change the pressure distribution over the aircraft's wings and tail, which can lead to various unpleasant effects (such as mach tuck, an extremely dangerous phenomenon whereby exceeding the lower critical mach number causes the aircraft to lose pitch control and nose down into a steep dive, which accelerates the aircraft further and steepens the dive).
Accelerating towards mach 1, the areas of supersonic flow continue to grow (as do their associated shockwaves, with a concomitant increase in wave drag), at the expense of the subsonic areas surrounding them. When the aircraft breaks the sound barrier, the airflow far form the wing switches from subsonic to supersonic, but the airflow over the wing itself changes fairly little, with large areas of both subsonic and supersonic airflow. As the aircraft's speed continues to increase, the remaining areas of subsonic flow - and the shockwaves bounding them - shrink, until the upper critical mach number (typically somewhere around mach 1.2) is reached, when the last areas of subsonic flow disappear and the airflow around the aircraft becomes entirely supersonic. In the upper portion of the transonic speed range, the decrease in wave drag as the areas of subsonic flow, and their associated shockwaves, shrink and finally disappear overcomes the normal increase in drag at higher speeds, and, in this speed range, the total amount of drag experienced by the aircraft actually decreases as its speed increases.
At yet higher speeds, the airflow remains entirely supersonic, and drag once again increases with speed.
Alternatively, here is a diagram of how the airflow around a wing changes with increasing speed (the shapes and locations of the transitions between subsonic and supersonic flight are intended merely to show the general pattern of what happens at different speeds, not the precise details of the airflow over a particular airfoil):
As flying faster has numerous advantages (for civilian aircraft, it lets you get passengers and/or cargo from point A to point B more quickly, which not only makes your customers happier, but also allows you to make more flights - and, thus, more profit - in the same amount of time; for military aircraft, it lets you catch a slower-moving enemy plane that you're chasing, or outrun them if they're chasing you), a great deal of aircraft-design focuses on delaying the onset of the transonic regime, and on minimizing its negative effects.
- One way to do this is by using supercritical airfoils (so named for their more desirable aerodynamics at speeds exceeding the lower critical mach number), which have a higher lower critical mach number than older airfoil designs, and generate much weaker shockwaves during transonic flight; as a result, the onset of wave drag is delayed, and it increases much more slowly, compared to aircraft using non-supercritical airfoils. As an added bonus, supercritical airfoils also have superior low-speed performance, and, as such, are often used even with aircraft that will never experience transonic flight.
- Another option is to use wing-sweep (usually backwards, but occasionally forwards; either works for this purpose), which makes the wing appear, from the airflow's perspective, to have a thinner, less shockwave-promoting profile.
- The wing can simply made very thin instead of sweeping it, but this creates problems with finding space to put things such as fuel, landing-gear, or the actuators for the aircraft's control-surfaces.
- Finally, we have the transonic area rule (also known as the Whitcomb area rule, after one of its discoverers, or simply the area rule), which involves carefully shaping the aircraft so that it avoids abrupt changes in its cross-sectional area between different locations along the aircraft's longitudinal axis. For this reason, aircraft flying at transonic speeds frequently have fuselages that are narrower in the wing region; in jetliners, where creating such a "bottlenecked" or "wasp-waisted" fuselage is only possible to a limited extent due to the need to avoid intruding into the passenger cabin, the wings have large, prominent anti-shock bodies (nowadays usually integrated into other aircraft components, such as the aerodynamic fairings covering the tracks for the aircraft's flaps) extending back from their trailing edges in order to smooth out the aircraft's area distribution and keep its total cross-sectional area from decreasing sharply just behind the wings, and other aircraft components (such as engines, tail surfaces, etc.) are carefully placed in order to create the smoothest possible cross-sectional area distribution.
Thanks to all these developments, jets - both military (fighters, bombers, and large transports) and civilian (all post-first-generation jetliners) - now routinely cruise at low transonic speeds without large drag or handling penalties.
For more information about the transonic speed regime and its associated effects, see Wikipedia's articles on: