NAVWEPS OO-ROT-RO 
AIRPLANE PERFORMANCE 
The governing of the engine-propeller combi- 
nation will allow operation throughout a wide 
range of power and speed while maintaining 
efficient operation. 
If the envelope of maximum propeller dfi- 
ciency is available, the propulsive horsepower 
available will appear as shown in the second 
chart of figure 2.19. The propulsive power 
available, Pa, is the product of the propeller 
efficiency and applied shaft horsepower. 
The propellers used on most large reciprocating 
engines derive peak propeller efficiencies on the 
order of s,=O.85 to 0.88. Of course, the peak 
values are designed to occur at some specific 
design condition. For example, the selection 
of a propel!er for a !ong rasge transport wsuld 
require matching of the engine-propeller com- 
bination for peak efhciency at cruise condjtion. 
On the other hand, selection of a propeller for 
a utility or liaison type airplane would require 
matching of the engine-propeller combination 
to achieve high propulsive power at low speed 
and high power for good takeoff and climb 
performance. 
Several special considerations must be made 
for the application of aircraft propellers. In 
the event of a powerplant malfunction or 
failure, provision must be made to streamline 
the propeller blades and reduce drag so that 
flight may be continued on the remaining op- 
erating engines. This is accomplished by 
feathering the propeller blades which .stops 
rotation and incurs a minimum of drag for the 
inoperative engine. The necessity for feather- 
ing is illustrated in figure 2.19 by the change 
in equivalent parasite area, Af, with propeller 
blade angle, 8, of a typical instaliation. When 
the propeller blade angle is in the feathered 
position, the change in parasite drag is at a 
minimum and, in the case of a typical multi- 
engine aircraft, the added parasite drag from 
a single feathered propeller is a relatively small 
contribution to the airpfane total drag. 
At smaller blade angles near the Rat pitch 
position, the drag added by the propeller is 
very large. AC these small blade angles, the 
propeller windmilling at high RPM can create 
such a tremendous amount of drag that the 
airplane may be uncontrollable. The propel- 
ler windmilling at high speed in the low range 
of blade angles can produce an increase in para- 
site drag which may be as great as the parasite 
drag of the basic airplane. An indication of 
this powerful drag is seen by the hclieopter in 
autorotation. The windmilling rotor is ca- 
pable of producing autorotation rates ofdcscent 
which approach that of a parachute canopy 
with the identical disc area laading. THUS, 
the propeller windmilling at high speed and 
small blade angle can produce an cffccti+e 
drag coefficient of the disc area which compares 
with tha~t of a parachute canopy. The drag 
and yawing moment caused by loss of power 
at high engine-propeller speed is considerable 
and the transient yawing displaccmcnt of the 
aircraft’ may produce critical loads for the 
vertical tail. For this reason, automatic 
feathering may be a necessity rather than a 
luxury. 
The large drag which can be produced by 
the rotating propeller can be utilized to im- 
prove the stopping performance of the air- 
plane. Rotation of the propekr blade to 
small positive values or negative values with 
applied power can produce large drag or re- 
verse thrust. Since the thrust capability of the 
propeller is quite high at low speeds, very 
high deceleration can be provided by reverse 
thrust alone, 
The qs&zg limitatiar of the pmpcllcr are 
closely associated with those of the Rower- 
plant. Overspeed conditions are critical be- 
cause of the large centrifugal loads and blade 
twisting moments produced by an excessive 
rotative speed. In addition, the propeller 
blades will have various vibratory modes and 
certain operating limitations may hc necessary 
to prevent exciting resonant conditions.