NAVWEPS 00-8OT-80 
AIRPLANE PERFORMANCE 
with altitude above the tropopause. This is 
due in great part to the more rapid decay of 
engine thrust in the stratosphere. 
During a power off descent the deficiency of 
thrust and power define the angle of descent 
and rate of descent. TWO particular points 
are of interest during a power off descent: 
minimum angle of descent and minimum rate 
of descent. The minimum angle of descent 
would provide maximum glide distance through 
the air. Since no thrust is available from the 
power plant, minimum angle of descent would 
be obtained at (L/D)-. At (L/D),, the 
deficiency of thrust is a minimum and, as 
shown by figure 2.22, the greatest proportion 
between velocity and power required is ob- 
tained. The minimum rate of descent in 
power off flight is obtained at the angle of 
attack and airspeed which produce minimum 
power required. For airplanes of moderate 
aspect ratio, the speed for minimum rate of 
descent is approximately 75 percent of the 
speed for minimum angle of descent 
RANGE PERFORMANCE 
The ability of an airplane to convert fuel 
energy into flying distance is one of the most 
important items of airplane performance. The 
problem of eficient range operation of an air- 
plane appears of two general forms in flying 
operations: (1) to extract the maximum flying 
distance from a given fuel load or (2) to fly a 
specified distance with minimum expenditure 
of fuel. An obvious common denominator for 
each of these operating problems is the “spe- 
cific range, ” nautical miles of flying distance 
per lb. of fuel. Cruise flight for maximum 
range cond.itions should be conducted so that 
the airplane obtains maximum specific range 
throughout the flight. 
GENERAL RANGE PERFORMANCE. 
The principal items of range performance can 
be visualized by use of the illustrations of figure 
2.23. From the characteristics of the aero- 
dynamic configuration and the powerplant, the 
conditions of steady level flight will define 
various rates of fuel flow throughout the range 
of flight speed. The first graph of figure 2.23 
illustrates a typical variation of fuel flow versus 
velocity. The specific range can be defined by 
the following relationship: 
nautical miles 
specific raw= lbs, of fuel 
nautical miles/hr. 
‘pecific range= lbs. of fuel/hr. 
thus, 
specific range = velocity, knots 
fuel flow, lbs. per hr. 
If maximum specific range is desired, the flight 
condition must provide a maxinium of velocity 
fuel flow. This particular point would be 
located by drawing .a straight line from the 
origin tangent to the curve of fuel flow versus 
velocity. 
The general item of range must be clearly 
distinguished from the item of endurance. The 
item of range involves consideration of flying 
distance while endurance involves consideration 
of flying time. Thus, it is appropriate to define 
a separate term, “specific endurance.” 
specific endurance= flight hours 
lb. of fuel 
specific endurance = flight hours/hr. 
lbs. of fuel/hr. 
then, 
specific endurance= 1 
fuel flow, lbs. per hr. 
By this definition, the specific endurance is 
s&ply the reciprocal of the fuel ~flow. Thus, 
.ifl.maximum endurance is desired, the flight 
condition ‘must provide a minimum of fuel 
flow. This point is readily appreciated as the 
lowest point of the curve of fuel flow versus 
velocity. Generally, in subsonic performance, 
the speed at which maximum endurance is 
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