= Wave drag
\b[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>.<Introduction to Aircraft Performance and Static Stability by Earll Murman>[PDF page 7, Transonic Effects on Airfoil Cd, Cl]{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.<A Study of the Zero-Lift Drag-Rise Characteristics of Wing-Body Combinations Near the Speed of Sound by Richard T. Whitcomb>[Report abstract, matched-area comparisons and indented-body tests]{ref} The related NASA account describes testing the proposed <area rule> using a specially prepared wing/body model in the Langley transonic tunnel.<Richard T. Whitcomb by NASA>[Area-rule passage, prioritized model and tunnel comparison]{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 $C_{D,w}=0.012$ gives
$$
D_w=qS C_{D,w}=72\ \mathrm N.
$$
At a speed of 250 m/s, overcoming that component alone requires mechanical <power> $D_wU=18$ 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.
Back to article page
