During a subsonic flight, the air molecules around the aircraft move in a smooth, gradual pattern. The pressure wave ahead of the aircraft nose signals the air molecules of an approaching body. The air molecules make sufficient space for the subsonic aircraft to pass. In a supersonic flight, however, the aircraft flies faster than the pressure wave, so no prior indication is made.
The body of the aircraft collides with the air molecules resulting in violent movements and the creation of shock areas. The air around the flying aircraft is compressed. The disturbed air molecules in the aircraft’s wake (after it has passed) create a pressure wave.
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Photo: Boom Supersonic
Supersonic shock waves condition the air during a supersonic flight. Whether the air conditioning around the body of the aircraft or the engines, shock waves increase the static pressure, decreasing the airspeed.
Normal shocks
The normal shock wave creates a 90-degree angle to the free-stream airflow. The shockwave moves relative to the fluid (air) and creates a supersonic zone around the object (aircraft).
While it propagates through a medium, the abrupt and discontinuous change in the fluid characteristics is observed. The flow properties downstream of the normal shockwave are in the subsonic region.
Oblique shocks
Unlike normal shocks, oblique shock waves create an inclination with respect to the incident upstream airflow. When supersonic air molecules encounter a wedge, the flow tends to compress due to being wrapped in itself. Behind the oblique shockwave, the air density, temperature, and pressure increase, resulting in a decrease in air velocity. With the shockwave at an incidence, the airflow is directed away from the object.
It is noteworthy that the airfoil shape can also condition the air in the supersonic flow. When the flow passes through the convex area of the airfoil, expansion fans are created within the supersonic region. Across such expansion, the air density, temperature, and pressure decrease.
Supersonic wave drag
In addition to friction and induced drag, supersonic flights also incur wave drag. Supersonic wave drag can be due to the overall volume of the aircraft (mainly the fuselage) and lift. During the supersonic cruise, the air compresses as the aircraft comes in contact with it and expands when it passes through it.
The compression of air exerts a force in the opposite direction of travel. Past the midpoint, a suction pressure force is exerted in the same direction due to the expansion of air. The two phenomena in the supersonic region dictate the magnitude of volume wave drag on aircraft. Hence, a thin and long fuselage would produce much lower drag at supersonic speeds.
Photo: Boom Supersonic
Wave drag due to lift generated by wings is also a combination of compression and expansion of air in the supersonic region. During the cruise, the wing’s top leading edge is nearly parallel to the free-stream flow. This is where the compression is minimal. However, as the air passes further downstream of the leading edge, much lower static pressure is observed.
Behind the shockwave, the pressure returns to the ambient level. On the bottom leading edge of the wing, an oblique shock is observed, whereas a gradual lower pressure further downstream on the wing is observed. A long slender wing would produce much lower drag due to less-violent air movements around it.
What are your thoughts on the conditioning of air through shock waves at supersonic speeds? Tell us in the comments section.
