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National Advisory Committee for Aeronautics, Report - Joint Report on Standardization Tests on NPL RAF15 Airfoil Model

This report contains the wind-tunnel test data obtained in the United States on a 36' by 6 inch
R. A. F. 15 airfoil model prepared by the British Aeronautical Research Committee for international
trials. Tests were made in cooperation with the National Advisory Committee for Aeronautics at
the Bureau of Standards, Langley Memorial Aeronautical Laboratory, Massachusetts Institute of
Technology, and MoOoolc Field.

In addition to brief descriptions of the various wind tunnels and methods of testing, the report
contains an analysis of the test data. It is shown that while in general the agreement is quite satis-
factory there are two cases in which it is unsatisfactory. Since the lack of agreement in the latter
is probably wplained by errors known to be inherent in the methods of determining and applying
corrections in these particular tests, it is concluded that the agreement obtained is more a matter of
technique than a wind-tunnel characteristic.

During the early development of experimental aerodynamics it was found that test data on
the same wing section from difierent wind tunnels frequently showed rather large and important
lack of agreement that could not be ignored. This condition led many engineers to distrust all
wind-tunnel test data and for many years prevented the wind tunnel from receiving the atten-
tion and credit it deserves. The situation has been greatly improved in recent years owing to
the general adoption of more careful test methods and the application of corrections now known
to be necessary. Since it is a matter of some interest, a few of the more important advances
will be discussed briefly.

The early attempts which were made to find the cause or causes of lack of agreement in
wind-tunnel tests on airfoil models centered chiefly on interference effects from the method of
attachment to the balance. One of the first papers on this subject is an appendix to a report
by Bairstow, Pannell, Lavender, Fage, and Cowley.1 _ It was pointed out in this paper that the
so-called “crank-spindle” method of attachment was unreliable. Concerning this, the report
says, “We have been unable to find any means, of supporting a model airfoil from its center
which does not involve disturbance of flow of air round the aerofoil to a considerable extent;
with the best of such arrangements we have yet found the residual correction after subtracting
the resistance of the spindle alone is of the order of 20 per cent on the minimum drag.” The
next important paper on the subject is by Pannell and Campbell.2 By this time it was generally
recognized that unless great precautions were taken, good agreement could not be obtained
in tests on the same model with difierent methods of support in the same wind tunnel, while
good agreement between two tunnels usingthe same method of supporting the model was dis-
appointingly rare.