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Sunday, November 8, 2020

Formation of Wave drag




Wave Drag is a force, or drag, that retards the forward movement of an airplane, in both supersonic and Transonic Flight, as a consequence of the formation of shock waves.

Wave drag is caused by the formation of shock waves around the aircraft in supersonic flight or around some surfaces of the aircraft whilst in transonic flight. Whilst in cruise, most civil jet aircraft fly in the mach .75 to .85 speed range. Although shock waves are typically associated with supersonic aircraft, they also form on an aircraft traveling at less than the speed of sound on areas of the aircraft, such as the aerofoils, where local airflow is accelerated to sonic speed and then decelerated, once again, back to subsonic speed. The shockwave (and associated wave drag) forms at the point the airflow becomes subsonic.

As the aircraft continues to accelerate, the area of the wing experiencing supersonic flow increases, the shockwave moves further back on the wing and becomes larger. Boundary layer separation also increases with the increase in speed and if the speed is allowed to increase beyond the limiting mach number (MMO), severe buffeting, Mach Tuck, or "upset" (loss of control) may occur.

Most modern jet powered aircraft are engineered to operate at transonic air speeds. Transonic airspeeds see a rapid increase of drag from about Mach 0.8, and it is the fuel costs of that drag that typically limits the airspeed. Attempts to reduce wave drag can be seen on all high-speed aircraft; most notable is the use of swept wings, but another common form is a wasp-waist fuselage as a side effect of the Whitcomb area rule.

*Note

Shock wave formation causes an increase in drag. One of the principal effects of a shock wave is the formation of a dense high pressure region immediately behind the wave. The instability of the high pressure region, and the fact that part of the velocity energy of the airstream is converted to heat as it flows through the wave, is a contributing factor in the drag increase, but the drag resulting from airflow separation is much greater. If the shock wave is strong, the boundary layer may not have sufficient kinetic energy to withstand airflow separation.

The drag incurred in the transonic region due to shock wave formation and airflow separation is known as “wave drag.” When speed exceeds the critical Mach number by about 10 percent, wave drag increases sharply. A considerable increase in thrust (power) is required to increase flight speed beyond this point into the supersonic range where, depending on the airfoil shape and the AOA, the boundary layer may reattach.

Sources:

https://www.flightliteracy.com/high-speed-flight-part-two-…/

https://www.flight-mechanic.com/high-speed-aerodynamics-sh…/

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