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National Advisory Committee for Aeronautics, Report - The Air Forces on a Model of the Sperry Messenger Airplane without Propeller

This is a report on a scale—effect research which was made in the variable density wind
tunnel of the National Advisory Committee for Aeronautics at the request of theArmyAir Serv-
ice. A 1/10 scale model of the Sperry Mwsenger airplane with USA—5 wings was tested without
a propeller at various Reynolds Numbers up to the full scale value. Two series of tests were
made: The first on the original model which was of the usual simplified construction, and the
second on a modified model embodying a great amount of detail.

While the present report is of a preliminary nature, the work has progressed far enough to
show that the scale effect is almost entirely confined to the drag. In the tests so far conducted,
the drag at any given angle of attack within the normal flying range is found to vary as (1771)“.

The exponent n is constant for any one angle of attack, and ranges from —0.045 at large angles
of attack to -—0.17 at small angles.

It was also found that the model should be geometrically similar to the full-scale airplane
if the test data are to be directly applicable to full scale. If the condition of geometric simi-
larity be fulfilled, the data obtained at a full-scale value of Reynolds Number agree very closely
with free-flight data. The variable density wind tunnel therefore appears to be a very promis-
ing instrument for procuring test data free from scale efi’ect. It is also admirably suited for
studying the scale effect and obtaining information which is necessary in an interpretation of
the results obtained in atmospheric wind tunnels at low values of the Reynolds Number.