Shock Stall
Summary
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.
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.
The pitching moment is generally subject of a transonic spike of the type shown in
Fig. 2, and often changes sign
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.
Effects on Loads and Moments
Figure 1: Lift/Drag characteristics at transonic speeds
Longitudinal Stability.
Figure 2: Longitudinal characteristics at transonic speeds
Selected References
