Ch14 Transonic Flow
－drag divergence phenomena, rapid shift of center of
pressure unsteady and somewhat unpredictable effects of
shock waves on the control surfaces
－Typical cruise Mach number of jet transports (0.75-0.83)
( now concept to push
MDD →1 )
c.f.
14.7.4 The Transonic Area Rule and the Supercritical Airfoil
－Product of transonic wind tunnels at Langley by Richard Whitcomb
－to reduce drag in the transonic regime
Area Rule: the cross-sectional area of the body should have a smooth
variation with longitudinal distance along the body; there should be no
rapid or discontinuous changes in the cross-sectional area distribution.
( wasp-like or cokebottle shape for the
body; the body crosssection should be
decreased in the
vicinity of the wing. )
The area rule reduces the peak transonic drag by a considerable
amount.
Supercritical airfoil: shaped somewhat flat on the top surface in order
to reduce the local Mach number inside the supercritical region below
what it would be for a conventional airfoil under the same flight
conditions.

The shock strength is lower; the B.L. separation is less severe.
MDD ↑
i.e. Mcr, supercritical airfoil = Mcr, an equivalent standard airfoil
MDD, supercritical airfoil >> MDD, standard airfoil (slightly >Mcr)

“Super-critical”