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National Advisory Committee for Aeronautics, Technical Notes - Further Experimental Studies of Area Suction for the Control of the Laminar Boundary Layer on a Porous Bronze NACA 64A010 Airfoil

A loweturbulence wind—tunnel investigation was made of an
NACA 61+A010 airfoil having a porous surface to determine the
reduction in section total-drag coefficient that might be obtained
at large Reynolds numbers by the use of area suction. Initial results
of this investigation have been reported previously. The present paper
presents the results of additional tests of the same airfoil model
equipped with a porous skin of lower porosity.

Full—chord laminar flow was maintained by application of area
suction up to a Reynolds number of approximately 20 x 106. At this
Reynolds number, combined wake and suction drags of the order of
38 percent of the drag for a smooth and fair NACA 6hAOlO airfoil with-
out boundary-layer control were obtained. ’It seems likely from the
results that attainment of full-chord laminar flow by means of
continuous suction through a porous surface will not be precluded by
a further increase in Reynolds number provided that the airfoil surfaces
are maintained sufficiently smooth and fair and provided that outflow
of air through the surface is prevented.

A recent two-dimensional wind-tunnel investigation of area suction
applied to an NACA 61mm airfoil (reference 1) indicated that full-
chord laminar flow could be obtained at Reynolds numbers at least up to
approximately 8 x 106. The resultant total-drag coefficients (wake
drag plus the drag equivalent of the Suction power required) were lower
than the drag for the NACA 6hAOlO airfoil without boundary—layer control.
An increasingly nonuniform chordwise distribution of inflow occurred
with an increase in Reynolds number because of the high porosity of the
porous bronze skin. (See reference 1.) Excessive suction—flow rates
were required, therefore, to prevent boundary-layer transition caused
by outflow of air through the airfoil surface; the excessive suction
flows resulted in no reduction in total drag at Reynolds numbers greater
than approximately 8 x 106.

In an attempt to improve the chordwise inflow distribution and
thus obtain full-chord laminar flow and reductions in total drag at
larger values of the Reynolds number, the same airfoil model used in
the original investigation (reference 1) has been retested with a
porous sintered-bronze surface of much lower porosity. The investi-
gation was made in the Langley two-dimensional lowuturbulence pressure
tunnel at Reynolds numbers up to approximately 20 X 106. wake-drag
coefficients, suction-flow coefficients, and suctionnair pressure-loss
coefficients were measured.