Charles D. Harris

Charles D. Harris is the first named author, with James A. Blackwell, Jr., of the 1972 NASA report testing rear-upper-surface changes on a supercritical airfoil. The experiment examined a local off-design velocity peak while also checking the consequences at the design condition. The published summary reports that the particular attempted change adversely affected design-condition drag.^[ref]

The tests covered Mach numbers from $0.60$ to $0.81$ in a transonic wind tunnel. The models differed in rear-upper-surface shape while retaining their trailing-edge thickness; pressure measurements and a wake rake connected the geometric change to aerodynamic observations.^[ref]

An original comparison shows why an improvement needs its operating condition. Stipulate that a proposed modification changes drag coefficient by $-0.001$ at one condition and $+0.002$ at another. With the same reference dynamic pressure $q=10000\mathrm{Pa}$ and area $S=0.25{\mathrm{m}}^2$ for this arithmetic example,gives $-2.5\mathrm{N}$ and $+5.0\mathrm{N}$, respectively. The sign of the benefit reverses. These invented coefficients are not values reported by Harris; they isolate why suppressing one local feature cannot establish lower total drag throughout an operating range.

A useful experimental comparison therefore records the Mach number, Reynolds number, angle of attack, loading and surface condition with the measured pressure coefficient distribution and drag. Changes of boundary-layer transition or mounting effects can alter the result. The documented negative outcome is scientifically informative because it tests a specific geometric proposal rather than assuming that a locally smoother-looking flow must improve complete performance.