and fatigue damage. By minimizing the 
amount of total time spent at high power 
setting, greater overhaul life of the powerplant 
can be achieved. This should not imply that 
the-takeoff rating of the engine should not be 
used. Actually, the use of the full maximum 
power at takeoff will accumulate less total 
engine wear than a reduced power setting at 
the same RPM because of less time required to 
climb to a given altitude or to accelerate to a 
given speed. 
The most severe rate of wear and fatigue 
damage occurs at high RPM and low MAP. 
High RPM produces high centrifugal loads 
and reciprocating iuertia loads. When the 
large reciprocating inertia loads are not cush- 
ioned by high compression pressures, critical 
resultant loads can be produced. Thus, op- 
erating time at maximum RPM and MAP must 
be held to a minimum and operation at mari- 
mum RPM and low MAP must be avoided. 
AIRCRAFT PROPELLERS 
.The aircraft propeller functions to convert 
the powerplant shaft horsepower into propul- 
sive horsepower. The basic principles of pro- 
pulsion apply to the propeller in that thrust is 
produced by providing the airstream a mo- 
mentum change. The propeller achieves high 
propulsive ef?iciency by processing a relatively 
large mass flow of air and imparting a rela- 
tively small velocity change. The momentum 
change created by propeller is shown by the 
illustration of figure 2.18. 
The action of the propeller can be idealized 
by the assumption that the rotating propeller 
is simply an actuating disc. As shown in fig- 
ure 2.18, the inflow approaching the propeller 
disc indicates converging streamlines with an 
increase in velocity and drop in pressure. The 
converging streamlines leaving the propeller 
disc indicate a drop in pressure and increase in 
velocity behind the propeller. The pressure 
change through the disc results from the distri- 
bution of thrust over the area of the propeller 
NAVWEPS OO-EOT-80 
AIRPLANE PERFORMANCE 
disc. In this idealized propeller disc, the pres- 
sure difference is uniformly distributed over the 
disc area but the actual case is rather different 
from this. 
The final velocity of the propeller slipstream, 
V,, is achieved some distance behind the pro- 
peller. Because of the nature of the flow pat- 
tern produced by the propeller, one half of the 
total velocity change is produced as the flow 
reaches the propeller disc. If the complete 
velocity increase amounts to Za, the flow veloc- 
ity has increased by the amount II at the pro- 
peller disc. The propulsive e$icien~, vp, of the 
ideal propeller could be expressed by the fol- 
lowing relationship: 
output power ?%I= . mput power 
TV 
‘I’= T(V+a) 
where 
v,=propulsive efficiency 
T=thrust, lbs. 
V=fligkt velocity, knots 
IJ = velocity increment at the 
propeller disc, knots 
Since the final velocity, Vs, is the sum of total 
velocity change 2a and the initial velocity, 
V,, the propulsive efliciency rearranges to a 
form identical to that for the turbojet. 
2 VP’ 
1+ k 0 
So, the same relationship exists as with the 
turbojet engine in that high efficiency is de- 
veloped by producing thrust with the highest 
possible mass flow and smallest necessary 
velocity change. 
The actual propeller must be evaluated in a 
more exact sense to appreciate the effect of 
nonuniform disc loading, propeller blade drag 
forces, interference flow between blades, etc. 
With these differences from the ideal Propeller, 
145