NAVWEPJ OO-ROT-80 
AIRPLANE PERFORMANCE 
to airplane factors. The turboprop power- 
plant prefers operation at low inlet air tem- 
peratures and relatively high power setting to 
produce low specific fuel consumption. While 
an increase in altitude will increase the mini- 
mum power required for the airplane, the 
powerplant achieves more efficient operation. 
As a result of these differences, maximum en- 
durance of the multiengine turboprop airplane 
at low altitudes may require shutting down 
some of the powerplants in order to operate 
the remaining powerplants at a higher, more 
efficient power setting. 
EFFECT OF ALTITUDE ON ENDUR- 
ANCE, TURBOJET AIRPLANES. Since the 
fuel flow of the turbojet powered airplane is 
proportional to thrust required, the turbojet 
airplane will achieve maximum specific endur- 
ance when operated at minimum thrust re- 
quired or (L/D),. In subsonic flight, 
(L/D)m~ occurs at a specific value of lift 
coefBcient for a given airplane and is essentially 
independent of weight or altitude. If a given 
weight an~d configuration of airplane is oper- 
ated at various altitudes, the value of the 
minimum thrust required is unaffected by the 
curves of thrust required versus velocity shown 
in figure 2.27. Hence, it is apparent that the 
aerodynamic configuration has no prefeience 
for altitude (within compressibility limits) 
and specific endurance is a function only of 
engine performance. 
The specific fuel consumption of the turbojet 
engine is strongly affected by operating RPM 
and altitude. Generally, the turbojet engine 
prefers the operating range near normal rated 
engine speed and the low temperatures of the 
stratosphere to produce low specific fuel con- 
sumption. Thus, increased altitude provides 
the favorable lower inlet air temperature and 
requires a greater engine speed to provide the 
thrust required at (L/D)-. The typical 
turbojet airplane experiences an increase in 
specific endurance with altitude with the peak 
values occurring at or near the tropopausc. 
For example, a typical single-engine turbojet 
airplane will have a maximum specific endur- 
ance at 35,ooO ft. which is at least 40 percent 
greater than the maximum value at sea level. 
If the turbojet airplane is at low altitude and 
it is necessary to hold for a considerable time, 
maximum time in the air will be obtained by 
beginning a climb to some optimum altitude 
dependent upon the fuel quantity available. 
Even though fuel is expended during the climb, 
the higher altitude will provide greater total 
endurance. Of course, the use of afterburner 
for the climb would produce a prohibitive re- 
duction in endurance. 
OFl4X’TIMUM RANGE AND ENDUR- 
ANCE 
There are many conditions of flying oper- 
ations in which optimum range or endurance 
conditions are not possible or practical. In 
many instances, the off-optimum conditions 
result from certain operational requirements 
or simplification of operating procedure. In 
addition, off-optimum performance may be the 
result of a powerplant malfunction or failure. 
The most important conditions are discussed 
for various airplanes by powerplant type. 
RECIPROCATING POWERED AIR- 
PLANE. In the majority of cases, the recipro- 
cating powered airplane is operated at’an engine 
dictated cruise. Service use will most probably 
define some continuous power setting which 
will give good service life and trouble-free 
operation of the powerplant. When range or 
endurance is of no special interest, the simple 
expedient is to operate the powerplant at the 
recommended power setting and accept what- 
ever speed, range, or endurance that results. 
While such a procedure greatly simplifies the 
matter of cruise control, the practice does not 
provide the necessary knowledge required for 
operating a high performance, long range 
airplane. 
The failure of an engine on the multiengine 
reciprocating powered airplane has interesting 
ramifications. The first problem appearing is