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National Advisory Committee for Aeronautics, Technical Notes - Experiments with a Counter Propeller

The following tests were conducted:

A - Preliminary test to determine the rotation in
the slipstream of a model propeller. the observed data
to serve as a basis for a counter-propeller design.

3 — Test of the model propeller alone in the usu—
al manner.

C — Test of the model propeller in combination
with the counter—propeller.

The propeller selected for the investigation
was the U.S. Navy type model F, three—foot diameter and
three—foot geometrical pitch. It is completely described
in N.A.C.A. Technical Report Ho. 237 entitled "Tests on
Thirteen Navy Type Model Propellers.

This pitch ratio was chosen because preliminary trials
showed that for the same thrust and velocity of advance,
the helix angles of the slipstream elements, as measured
from the axial direction, vary directly with the pitch ra—
tio. Since any gain with a counter—propeller must result
from recovering some part of the rotational energy of the
slipstream, it appeared that a high-pitch propeller would
offer the greater opportunity for improvement.
2 N.A.C.A.'Techflical Note No. 453

The direction of the slipstream elements was measured
with a cylindrical yaw head. Observations were made along
two radial lines, one about one—half inch from the trail—
ing edge of the propeller and the other three inches far—
ther to the rear. The general wind velocity was about 60
feet per second. Propeller rotative speeds were adjusted
to give 8 pounds, 16 pounds and 27 pounds thrust, thus pro—
viding three points in the range of v/nD through which
the propeller would normally operate. Slipstream direc—
tions were observed for each of the three thrusts. The
angularity of the elements, relative to_the_anialfidiregtiqn
is shown in Figure‘lJ Close tb the trailing edge it is
somewhat greater than at points three inches to the rear.
It varies inversely as the radius and directly as the‘ _ ‘
thrust." It should be noted that the angles measurEd'take
no account of any radial yelocity, But only of the tangen-
tial and axial components.