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.
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
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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