Copyright © A. Filippone (1999-2003). All Rights Reserved.

High Speed Aerodynamics

Shock Stall


The increasing Mach number has some unfortunate effects on a wing: sudden increase of drag, corresponding decrease of lift, and longitudinal instability. The precise effect is dependent on the geometry of the wing (camber, thickness ratio, etc.). The figures below show some characteristic behavior.

When flow separation occurs, triggered by a shock wave, the lift coefficient starts to decrease (Fig. 1, top), while the drag increases sharply (Fig. 1 bottom). This phenomenon is called shock stall. It has some analogies with the airfoil stall, past the angle of CLmax.

Effects on Loads and Moments

Typical lift and drag coefficients are shown in Fig. 1 as a function of the transonic Mach number.

Lift. The loss of lift is due to the boundary layer separation on the upper side. At the Mach number increases, the chock moves downstream, the amount of separation decreases, and the airfoil recovers part of its lift, until the free stream becomes supersonic. Beyond that point, the lift decreases again, though more gradually.

Drag. Fig. 1 (bottom) shows the transonic drag rise at different lift coefficients (e.g. different angles of attack of the same wing). The freestream Mach number at which drag rise takes place decreases with the increasing lift coefficient. Typical values for old low speed airfoils of M are of the order 0.45 to 0.65. The transonic behavior can be greatly improved with the supercritical wing sections.

Stall on CL

Stall on CD
Figure 1: Lift/Drag characteristics at transonic speeds

Longitudinal Stability.

The pitching moment is generally subject of a transonic spike of the type shown in Fig. 2, and often changes sign

Stall on CM
Figure 2: Longitudinal characteristics at transonic speeds

The use of thin supercritical wing sections, low lift at operation point, and wing back sweep are among the most effective methods of postponing the shock stall to higher speeds.

Selected References

  • Clancy JC. Aerodynamics, John Wiley, New York, 1975.

  • Moulden TH. Fundamentals of Transonic Flow, John Wiley, 1984.

  • AGARD AR-82. The Effects of Buffetting and Other Transonic Phenomena on Maneuvering Combat Aircraft, 1975.

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Copyright © A. Filippone (1999-2003). All Rights Reserved.