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National Advisory Committee for Aeronautics, Report - Characteristics of Clark Y Airfoils of Small Aspect Ratios

This report presents the results of a series of wind-
tunnel tests showing the force, moment, and autorotational
characteristics of Clark Y airfoils having aspect ratios
varying from 0.5 to 3.
An airfoil of rectangular plan form was tested with
rectangular tips, faired tips, and semicircular tips.
Tests were also made on one airfoil of circular plan
form and two airfoils of elliptical plan form.
The tests revealed a marked delay of the stall and a
decided increase in values of maximum lift coefiicient
and maximum resultant force coefi‘lcient for aspect ratios
of the order of 1 as compared with the values for aspect
ratios of 2 and 3. The largest value of 03,“, was 2.17
with a wing of circular plan form and an aspect ratio
of 1.27. The same wing gave a 01,“, of 1.85 and an
L/D ratio of 1.63 at 45° angle ofattaclc.
Wings having aspect ratios of about 1 were found to
have moment characteristics more favorable to stability
than those having larger aspect ratios. Decreasing the
aspect ratio greatly reduced ranges and rates of autoro-
tation based on a given span and air speed. Results,
when reduced to infinite aspect ratio by conventional
formulas, indicate that such formulas are not applicable
for aspect ratios less than 1.5. It is apparent that the
plan form and tip shape of the wing are of major im-
portance among the factors affecting airfoil character-
istics at aspect ratios of 1.5 and smaller.
In recent years there has been an increasing demand
for an airplane suited to the needs of the private
owner. Without going into a discussion of the problem
it may be said that such an airplane should be capable
of descending along a steep path at such a low rate
of speed that it will be unnecessary for the pilot to
alter the direction of the flight path or the speed
when near the ground in order to make a satisfactory
landing.