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National Advisory Committee for Aeronautics, Technical Notes - Notes on the Construction and Testing of Model Airplanes

In most wind tunnels the construction and test of a model
aerofoil or fuselage is standardized, but in the endeavor to
secure a non—turbulent air flow and great refinement in force
measurements: the general requirements for construction and
tests of-model airplanes or seaplanes have been neglected.

The average‘airplane model is constructed to scale from general
assembly plans with detail depending on the skill of the model
maker. As one result the cost of a model is very high, As
another result the test data is frequently unreliable.

The following notes are intended to supplement Mr. F. H,
Norton’s paper on "The Construction of Models for Tests in Wind
Tunnels" (National Advisory Gommittee for Aeronautics‘ Report
No. 74}. It will be shown that the construction of sn'airplane
model can and should be simplified in order to obtain the most
reliable test data.

The general purpose of the usual wind tunnel tests on a
model airplane is to obtain the aerodynamic characteristics,
static balance, and the efficiency of the controls for the
particular combination of wings, tail surfaces, fuselage and
landing gear employed in the design. These parts must be exact
scale reproductions. Any appreciable variation from scale rem
production must be in the remaining parts of the model, i.e.,
struts, wires, fittings, control horns, radiators, engines
and the various attachments_found exposed to the wind in special
airplanes.

Obviously the wings, tail surfaces, fuselage and landing
gear must be rigidly held in the proper relation, one to another,
preferably with means for minor adjustments. These reQuirements
are easily met in models of internallyebraced monoplanes, but
they usual y cause trouble in models of multiplanes, particular—
ly then an attempt is made to provide an exact scale reproduc—
tion of the interplane bracing in accordance with the popular
conception of model requirements. Although model struts and
fittings are very minute and difficult to make, their use would
be justified if a model so constructed was rigid and capable of
minor adjustments and if it in test fairly represented the full
scale airplane.

igure 1 represents an average attempt to secure an exact
scale repro‘uct ion in a wind tunnel model. The result is a
flimsy affair which is not only difficult to line up, but also
iiffi wlt to keep adj Ms ed. The values of dr ag obta:.ned from
tests are from 30% to 50% higher than the values obtained from
calculations, which allow for a normal scale effect. This great
difference in drag is due to the abnormal scale effect for strwlts
1:1 wires at 10? values of VL. Fi~ure 3 is included to illus»
trate the magnitude of this scale effect.