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National Advisory Committee for Aeronautics, Technical Notes - Preliminary Investigations on Boundary Layer Control by Means of Suction and Pressure with the USA 27 Airfoil

The tests described in this report constitute a prelimi—
nary investigation of airfoil boundary layer control, as carried
out in the atmospheric wind tunnel of the Langley Memorial Aero—
nautical Laboratory, from February to August, 1927.

Tests were made on a U.S.A. 27 airfoil section with var—
ious slot shapes and combinations, and at various amounts of
pressure or suction on the slots.

The lift of airfoils can be increased by removing or by
accelerating the boundary layer.

Removing the boundary layer by suction is more economical
than to accelerate it by jet action. Gauze—covered suction
slots apparently give the best results.

When not in operation, all suction slots tested had a det—
rimental effect upon the aerodynamic characteristics of the airm
foil which was not apparent with the backward—opening pressure
slots.

Thick, blunt—nose airfoils would seem to give best results
with boundary layer control.

It is known that the performance characteristics of air—
planes would be materially improved if the departure of the air
flow from the upper surface of airfoils could be made as small
at large angles of attack as it is at small angles. The cause
of the departure of the actual flow of air around airfoils from
that which Would exist in an inviscid fluid is found in the ac—
tion of viscosity and has its root in the layer of retarded air
(boundary layer) close to the airfoil. At small angles of attack
the flow retarded by the aetion of viscosity produces a thin
boundary layer over the surface while at large angles the entire
flow over the upper surface becomes discontinuous and turbulent.

The lift may be expected to increase with the angle of at—
tack so long as continuous flow over the upper surface can be
maintained.‘ The object of controlling the flow of air in the
boundary layer is to prevent, in so far as is possible, the ap—
pearance of disbontinuous flow over the upper surface of the
airfoil which gives rise to an increase of profile drag and,
finally, a decrease of lift (burble) as the angle of attack is
increased.