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National Advisory Committee for Aeronautics, Technical Notes - Flight Investigation in Climb at High Speed of a Two Blade and a Three Blade Propeller

As part of a. flight program at the NACA to obtain information on
general propeller aerodynamic characteristics, an investigation has been
made of a two-blade and a three—blade propeller on a. slender-nose fighter
airplane in climb and at high speed.

In climbs, the propeller efficiency varied with both change in
operating engine power and change in blade number. For normal rated
engine power (900 hp and 2600 rpm) the propeller efficiency was higher
than for military power (1200 hp and 3000 rpm), being on the order of
1|- percent higher at 12,000 feet with a three-blade propeller. With a
two-blade propeller, the propeller efficiency was approximately the same
for normal rated and military power at altitudes below 12,000 feet. At
altitudes above 12,000 feet, the propeller efficiency for the military-
power condition increased by about 6 percent at 20,000 feet because of the
power drop when the critical altitude was exceeded. A change in blade
number from three to two resulted in a decrease in propeller efficiency
from 8 to 11+ percent for the normal-rated-power condition and about
6 to 7 percent for the military-power condition. EEhis loss in effi-
ciency was due to increasing the power loading'per blade which took
place when the blade number was changed.

In high-speed flight at a Mach number of 0.7, propeller efficiency
increased 17 percent when the power coefficient per blade was increased
from 0.07 to 0.17 at the normal engine rotational speed of 2600 rpm; thus
the propeller efficiency is shown to increase with power coefficient at
higher speeds. Further improvement might have been obtained if the
propeller had been tested at higher loadings, since the values of effi-
ciency continued to increase up to the highest loadings used in the
tests. Compressibility losses occurred at high speed whenever a tip
Mach number of O .9 was reached and increased in severity with further
increases in tip Mach number. The main sources of efficiency loss
were the shank and tip sections of the blade. Tip compressibility
losses could be minimized by reducing rotational speed. When the tip
Mach number was reduced from O 96 to 0 82 at the same blade power
coefficient (0 .13) and advance ratio (2.5) , the propeller efficiency
increased by 1+ percent.