NAVWEPS oo-80~~80 
AIRPLANE PERFORMANCE 
obtained is approximately 75 percent of the 
speed for maximum range. 
A more exact analysis of range may be ob- 
tained by a plot of specific range versus velocity 
similar to the second graph of figure 2.23. Of 
course, the source of these values of specific 
range is derived by the proportion of velocity 
and fuel flow from the previous curve of fuel 
flow versus velocity. The maximum specific 
range of the airplane is at the very peak of the 
curve. Maximum endurance point is located 
by a straight line from the origin tangent to 
the curve of specific range versus velocity. 
This tangency point defines a maximum of 
(nmi/lb.) per (nmi/hr.) or simply a maximum 
of (hrs./lb.). 
While the very peak value of specific range 
would provide maximum range operation, long 
range cruise operation is generally recom- 
mended at some slightly higher airspeed. 
Most long range cruise operation is conducted 
at the flight condition which provides 99 per- 
cent of the absolute maximum specific range. 
The advantage of such operation is that 1 
percent of range is traded for 3 to 5 percent 
higher cruise. velocity. Since the higher cruise 
speed has a great number of advantages, the 
small sacrifice of range is a fair bargain. The 
curves of specific range versus velocity are 
affected by three principal variables: airplane 
gross weight, altitude, and the external aero- 
dynamic configuration of the airplane. These 
curves are the source of range and endurance 
operating data and are included in the per- 
formance section of the flight handbook. 
“Cruise control” of an airplane implies that 
the airplane is operated to maintain the recom- 
mended long range cruise condition through- 
out the flight. Since fuel is consumed during 
cruise, the gross weight of the airplane will 
vary and optimum airspeed, altitude, and 
power setting can vary, Generally, “cruise 
control” means the control of optimum air- 
speed, altitude, and power setting to maintain 
the 99 percent maximum specific range condi- 
tion. At the beginning of cruise, the high 
initial weight of the airplane will require spe- 
cific values of airspeed, altitude,’ and power 
setting to produce the recommended cruise 
condition. As fuel is consumed and the air- 
plane gross weight decreases, the optimum ai,r- 
speed and power setting may decrease or the 
optimum altitude may increase. Also, the 
optimum specific range will increase. The 
pilot must provide the proper cruise control 
technique to ensure that the optimum condi- 
tions are maintained. 
The final graph of figure 2.23 shows a typical 
variation of specific range with gross weight 
for some particular cruise operation. At the 
beginning of cruise the gross weight is high 
and the specific range is low. As fuel is con- 
sumed, and the gross weight reduces, the 
specific range increases. .This’ type of curve 
relates the range obtained by the expenditure 
of fuel .by the crosshatched area between the 
gross weights at beginning and end of cruise. 
For example, if the airplane begins cruise at 
18,500 Jbs. and ends cruise at 13,000 lbs., 5,500 
lbs. of fuel is expended. If the average spe- 
cific range were 0.2 nmi/Jb., the total range 
would be: 
range=(0.2)$ (5,500) lb. 
= 1,100 nmi. 
Thus, the total range is dependent on both 
the fuel available and the specific range. When 
range and economy of operation predominate, 
the pilot must ensure that the airplane will be 
operated at the recommended long range cruise 
condition. By this procedure, the airplane 
will be capable of its,maximum design operat- 
ing radius or flight distances less than the 
maximum can be achieved with a maximtim of 
fuel reserve at the destination. 
RANGE, PROPELLER DRIVEN AIR- 
PLANES. The propeller driven airplane com- 
bines the propeller with the reciprocating 
engine or the gas turbine for propulsive power. 
In the case of either the reciprocating engine or