= Charles D. Harris
{c}
<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>.<Wind-tunnel Investigation of Effects of Rear Upper Surface Modification on an NASA Supercritical Airfoil by Charles D. Harris and James A. Blackwell, Jr.>[Byline, Summary and Introduction, printed pp. 1–2]{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.<Wind-tunnel Investigation of Effects of Rear Upper Surface Modification on an NASA Supercritical Airfoil by Charles D. Harris and James A. Blackwell, Jr.>[Printed pp. 4–6, model geometry and measurement apparatus]{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,
$$
\Delta D=qS\Delta C_D
$$
gives \b[$-2.5\,\mathrm N$ 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.
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