turboprop airplane but additional factors are 
available to influence the specific endurance at 
low altitude. In other words, low altitude 
endurance can be improved by shutting down 
some powerplants and operating the remaining 
powerplants at higher, more efbcient power 
setting. Many operational factors could decide 
whether such procedure would be a suitable 
technique. 
TURBOJET POWERED AIRPLANE. In- 
creasing altitude has a powerful effect on both 
the range and endurance of the turbojet air- 
plane. As a result of this powerful effect, the 
typical turbojet airplane will achieve maxi- 
mum specific endurance at or near the tropo- 
pause. Also, the maximum specific range will 
be obtained at even higher altitudes since the 
peak specific range generally occurs at the 
highest altitude at which the normal rating of 
the engine can sustain the optimum aero- 
dynamic conditions. At low altitude cruise 
conditions, the engine speed necessary to sus- 
tain optimum aerodynamic conditions is very 
low and the specific fuel consumption is rela- 
tively poor. Thus, at low altitude, the air- 
plane prefers the low speeds to obtain 
(&/CD)- but the powerplant prefers the 
higher speeds common to higher engine effi- 
ciency. The compromise results in maximum 
specific range at flight speeds well above the 
optimum aerodynamic conditions. In a sense, 
low altitude cruise conditions are engine 
dictated. 
Altitude is the one most important factor 
affecting the specific range of the turbojet 
airplane. Any operation below the optimum 
altitude will have a noticeable effect on the 
range capability and proper consideration 
must be given to the loss of range. In addi- 
tion, turbojet airplanes designed specifically for 
long range will have a large percent of the 
gross weight as fuel. The large changes in 
gross weight during cruise will require partic- 
ular methods of cruise control to extract the 
maximum flight range. A variation from the 
optimum flight path of cruise (constant Mach 
NAVWEPS OO-EOT-80 
AIRPLANE PERFORMANCE 
number, cruise-climb, or whatever the appro- 
priate technique) will result in a loss of range 
capability. 
The failure of an engine during the optimum 
cruise of a multiengine turbojet airplane will 
cause a noticeable loss of range. Since the 
optimum cruise of the turbojet is generally a 
thrust-limited cruise, the loss of part of the 
total thrust means that the airplane must 
descend to a lower altitude. For example, if a 
twin-engine jet begins an optimum cruise at 
35,000 ft. (e=O.31) and one powerplant fails, 
the airplane must descend to a lower altitude 
so that the operative engine can provide the 
cruise thrust. The resulting altitude would be 
approximately 16,030 ft. (~=0.61). Thus, the 
airplane will experience a loss of the range 
remaining at the point of engine failure and 
loss could be accounted for by the reduced 
velocity (TM) and the increase in specific fuel 
consumption (c~) from the higher ambient air 
temperature. In the case of the example air- 
plane, engine failure would cause a 30 to 40 
percent loss of range from the point of engine 
failure. Of course, the jettisoning of expend- 
able weight items would allow higher altitude 
and would increase the specific range. 
Maximum endurance in the turbojet air- 
plane varies with altitude but the variation is 
due to the changes in ‘fuel flow necessary to 
provide the thrust required at (I./D),... The 
low inlet air temperature of the tropopause 
and the greater engine speed reduce the specific 
fuel consumption to a minimum. If the single- 
engine turbojet airplane is at low altitude 
and must hold or endure for a period of time, 
a climb should begin to take advantage of the 
higher specific endurance at higher altitude. 
The altitude to which to climb will be deter- 
mined by the quantity of fuel remaining. In 
the case of the multiengine turbojet at low 
altitude, some slightly different procedure