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National Advisory Committee for Aeronautics, Technical Notes - Experiments with an Airfoil Model on Which the Boundary Layer is Controlled Without the Use of Supplementary Equipment

This report describes tests made in the Variable Density
Wind Tunnel of the National Advisory Committee for Aeronautics
to determine the possibility of controlling the boundary layer
on the upper surface of an airfoil by use of the low pressure
existing near the leading edge. The low pressure was used to
induce flow through slots in the upper surface of the wing.
The tests showed that the angle of attack for maximum lift was'
increased at the expense-of a reduction in the maximum lift
coefficient and an increase in the drag coefficient.

It is well known that the boundary layer on the upper sur-
face of an airfoil grows thicker as the angle of attack is ins
creased. The thickening and loss of energy continue until the
boundary layer has insufficient energy to overcome the adverse
pressure gradient, and reversal of flow and separation may oc-
cur, accompanied by an increase in drag, and finally by a re-
duction in lift.

Many attempts have been' made to delay the separation either
by removing the boundary layer by suct ion (References l to 5) or
by adding energy to the layer by blowing air from the interior
of the wing out through backward—opening slots in the upper sur—
face (References 5 to 7). Some of these attempts have been suc—
cessful in that the maximum lift coefficient or the angle of at—
tack for maximum inner both, has tea" increased. Most of the
methods employed; haétteé neces31tate the use of mechanical de—
vices, such as blowers, independent of the wing itself, and con—
sequently they have had little application.

This report describes tests to determine the possibility of
delaying the separation by use of the low pressure existing near
the leading edge. Preliminary tests, Which were made in 1926,
indicated that air could be sucked through slots in the upper
surface into the interior of the wing by the action of a dis-
charge slct located in the lowhpressure region on the upper sur—
face near the leading edge. An airfoil of this type will be
"called an internal—circulation airfoil in this report. A model
was constructed in 1927 but the tests fiéaé unavoidably delayed
until 1930 when force tests were made of the model in the Vari-
able Density' Wind Tunnel of the Nationa1 Advisory Committee for
Aeronautics.