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• April 23, 2016 Create Date
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National Advisory Committee for Aeronautics, Report - Airfoil Section Characteristics as Affected by Protuberances

The drag and interference caused by protuberance
from the surface of an airfoil have been determined in the
N. A. C. A. variable-density wind tunnel at a Reynolds
Number of approximately 3,100,000. The efiects of vari-
ations of the fore-andraft position, height, and shape of
the protuberance were measured by determining how the
airfoil section characteristics were afiected by the addition
of the various protuberances extending along the entire
span of the airfoil. The results provide fundamental
data on which to base the prediction of the effects of actual
short-span protuberances. The data may also be applied
to the design of air brakes and spoilers.
The ideal airplane, aerodynamically, may be con-
sidered as one having only the drag due to skin friction
and the minimum induced drag associated with its lift.
Prof. B. M. Jones in England has shown that actual
airplanes fall far short of such an ideal. Interference
effects, it seems, must be blamed for a considerable part
of the energy wasted in producing the turbulence
associated with the comparatively large drag of actual
airplanes.
The National Advisory Committee for Aeronautics
has planned a series of investigations dealing with the
subject of aerodynamic interference. The investiga-
tions will, it is hoped, lead to the discovery of the cause
of the serious adverse effects and will provide data that
may be applied to the solution of practical problems of
design. An examination of present-day airplanes, both
military and commercial, has led to the belief that a
considerable part of the adverse interference arises
from small projecting objects, such as fittings, tubes,
wires, rivet heads, lap joints, butt straps, filler caps,
inspection plates, and many other projections from the
main surfaces that may be considered together as pro-
tuberances.