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National Advisory Committee for Aeronautics, Technical Notes - Drag of Exposed Fittings and Surface Irregularities on Airplane Fuselages

Measurements of drag were made on fittings taken from a
typical fuselage to determine whether the difference between
the observed full size fuselage drag and model fuselage drag
could be attributed to the effects of fittings and surface ir-
regularities found on the full size fuselage and not on the
model.

There are wide variations in the drag coefficients for the
different fittings. In general those which protrude little
from the surface or are well streamlined show very low and al—
most negligible drag. The measurements show, howeyer, that a
large part of the difference between model and full scale test
results may be attributed to these fittings.

The drag of airplane fuselages is usually found from tests
on carefully polished models without surface irregularities or,
at most, with idealized outlines of the engine, cockpit, and
pilot's head. It is evident to the most casual observer that ac—
tual airplane fuselages are dotted with cowl fittings, step
plates, wire and strut fittings, screw heads and bolts which, in
the aggregate, cover a not inconsiderable portion of the surface.
Inasmuch as the drag of a streamline body is largely due to the
friction of the air on the surface, one may surmise that a consid~
erable discrepancy may exist between smooth model drag and actual
rough airplane fuselage drag. This discrepancy is often over—
looked because the increase in drag due to the surface irregulari~
ties present on the airplane but usually omitted from models, is
about equal to the decrease in drag due to the increased scale of
the airplane over that of the model. With a view to determining
whether this difference of drag could be attributed to the protrude
ing fittings and surface irregularities of the full size fuselage,
and also to throw light on the magnitude and distribution of these
drag effects, tests were made on small parts. Some were taken
from a "Messenger" fuselage, and others were duplicates of parts
found there. These tests were made in the sixrinch, open—throat
wind tunnel of the National Advisory Committee for Aeronautics,
at Langley Field, Virginia.