The scenario for the six views is as follows: Figure 6 shows six views of a wing, with each view showing the Mach number getting higher. If an airplane designed to fly subsonic, perhaps at a Mach number of 0.80, flies too fast and enters transonic flight, some noticeable changes will take place with respect to the airflow over the wing. In supersonic flight, the aerodynamic center moves back to 50 percent of the wing’s chord, causing some significant changes in the airplane’s control and stability. For a subsonic airfoil, the aerodynamic center, or the point of support, is approximately 25 percent of the way back from the wing leading edge. High-Speed AirfoilsTransonic flight is the most difficult flight regime for an airplane, because part of the wing is experiencing subsonic airflow and part is experiencing supersonic airflow. Figure 5 shows an expansion wave on a supersonic airfoil. Behind the expansion wave the velocity increases, and the static pressure and density decrease. An expansion wave will occur at the point where the direction of flow changes. When supersonic air is flowing over the top of a wing, and the wing surface turns away from the direction of flow, the air will expand and follow the new direction. For this reason, supersonic air, when given the opportunity, wants to expand outward.
View B is the wing of an airplane in supersonic flight, with the sound pressure waves piling up toward the wing leading edge.Įxpansion WaveEarlier in the discussion of high-speed aerodynamics, it was stated that air at supersonic speed acts like a compressible fluid. Figure 3A shows a wing in slow speed flight, with many disturbances on the wing generating sound pressure waves that are radiating outward. If the shock waves reach the ground, and cross the path of a person, they will be heard as a sonic boom. This piling up of sound energy is called a shock wave. At this point the sound energy starts to pile up, initially on the top of the wing, and eventually attaching itself to the wing leading and trailing edges. When the speed of the airplane reaches the speed of sound, however, the pressure waves, or sound energy, cannot get away from the airplane. For a slow-moving airplane, the pressure waves travel out ahead of the airplane, traveling at the speed of sound. Shock WavesSound coming from an airplane is the result of the air being disturbed as the airplane moves through it, and the resulting pressure waves that radiate out from the source of the disturbance. If an airplane flies faster than Mach 5, it is said to be in hypersonic flight. Supersonic speed is from Mach 1.20 to 5.0. At this speed, the shock wave which formed on top of the wing during transonic flight has moved all the way aft and has attached itself to the wing trailing edge. When an airplane is flying at supersonic speed, the entire airplane is experiencing supersonic airflow. Transonic speed is typically between Mach 0.80 and 1.20. The speed at which the shock wave forms is known as the critical Mach number. The shock wave also causes the center of lift to shift aft, causing the nose to pitch down. Stability problems can be encountered during transonic flight, because the shock wave can cause the airflow to separate from the wing. The shock wave forms 90 degrees to the airflow and is known as a normal shock wave. Over the top of the wing, probably about halfway back, the velocity of the air will reach Mach 1 and a shock wave will form. When an airplane is flying at transonic speed, part of the airplane is experiencing subsonic airflow and part is experiencing supersonic airflow. How fast an airplane can fly and still be considered in subsonic flight varies with the design of the wing, but as a Mach number, it will typically be just over Mach 0.8. Keep in mind that the air accelerates when it flows over certain parts of the airplane, like the top of the wing, so an airplane flying at 500 mph could have air over the top of the wing reach a speed of 600 mph. Subsonic, Transonic, and Supersonic FlightWhen an airplane is flying at subsonic speed, all of the air flowing around the airplane is at a velocity of less than the speed of sound, which is known as Mach 1.
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