Wave drag

Wave drag is the component of aerodynamic pressure drag associated with the compressible wave field produced by a moving body, including the effects of shock waves. It becomes important in transonic flow and supersonic flow. A freestream Mach number below one does not exclude locally supersonic regions and shocks around an accelerating flow over an airfoil.^[ref]

Wave drag can occur at zero lift, because a body of finite thickness displaces the flow. Its identification therefore differs from that of lift-induced drag, which depends on the finite wing's loading and wake. A measured high-speed increase in total drag can also include changed flow separation and other losses; attributing the entire increment to one mechanism requires an explicit decomposition.

Richard T. Whitcomb's report compared wing/body combinations with bodies of revolution having the same axial area distribution. Compensating for the wing's area by indenting the body substantially reduced the zero-lift drag-rise increment near sonic speed.^[ref] The related NASA account describes testing the proposed area rule using a specially prepared wing/body model in the Langley transonic tunnel.^[ref] The observations support a design principle for the tested regime rather than proving that a shape's area distribution determines all of its drag.

For a stated decomposition, the wave-drag coefficient is $C_{D,w}=D_w/(qS)$, using common dynamic pressure $q$ and aerodynamic reference area $S$. An original illustrative estimate with $q=20,000$ Pa, $S=0.30$ m${}^2$ and stipulated $C_{D,w}=0.012$ givesAt a speed of 250 m/s, overcoming that component alone requires mechanical power $D_{w}U=18$ kW. These are constructed values, not measurements from Whitcomb's report.

A useful apparatus comparison retains force-balance data, surface pressure measurements and flow visualization while recording Mach number, Reynolds number and lift. Observing a shock locates part of the wave field; it does not, by itself, measure the integrated wave-drag force.