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National Advisory Committee for Aeronautics, Technical Notes - The Reduction in Drag of a Forward Sloping Windshield

This paper gives results of a short investigation of
the drag of a forward—sloping closedrcabin windshield.
The drag of the windshield in both the original and a final
modified form was determined from tests in the variable—
density wind tunnel. The final form of the windshield
was arrived at by modifying the original as the result of
flow obserVatiens in the N A. C. A: Emoke tunnel. The in—
vestigation was made primarily to study the utility of
the N.A.C.A. smoke tunnel as applied to the problem of re—
ducing the drag of objects for which the full dynamic
scale could not be approached in the smoke tunnel, but de-
signers should find the results of the flow observations
and drag measurements of value because they show that most
of the large drag added by the original windshield is_elim~
inated by the modification of the windshield to the final
form.
The windshield of the modern highnspeed transport air—
plane appears in many instances to be conspicuously lack-
ing in aerodynamic refinement. A short investigation of
such a windshield was therefore .COnsidered desirable. The
investigation consisted of drag tests in the variabl6~ '
density tunnel of a given type of windshield and of a med-
ified form of the windshield arrived at as a result of air-
flow observations made with model windshields in_the N.A.C.A.
smore tunnel. Thus. the investigation also permittfid a
study of the utility of smoke—tunnel flow observations
applied to a practical engineering problem.
Various pieces of apparatus previously designed for
air~flow observations have permitted Fests to'be made;—
only at very low values of the Reynolds Number,_ most of 1
the tunnels being of the order of l to 3 inches in diam—
eter. The National Advisory Committee for Aeronautics,
in order to allow the direct observation of air flows at
larger values of the Reynolds Humber and to permit greater
ease of construction of the models, has designed and con—
structed a smoke—flow tunnel having a throat 2 feet in di—
ameter;. -The dynam1o scale attainable with this equipment
is still rather low, the velocity being limited to aLout
10 feet per second. However, by using enlarged replicas,
full-scale Reynolds Numbers may be reached for small ob—
jects such as round and streamline wires. For most of the
flows in which the aeronautical engineer is interested,
however, full-scale values of the Reynolds Number cannot
be approached.