NAVWEPS 06801-80 
AIRPLANE PERFORMANCE 
near the speed for (L/D&-. There is no direct 
relationship which establishes this situation 
since the variation of propeller efficiency is the 
principal factor accounting for the variation 
of power available with velocity. In an ideal 
sense, if the propeller efficiency were constant, 
maximum rate of climb would occur at the 
speed for minimum power required. How- 
ever, in the actual case, the propeller efficiency 
of the ordinary airplane will produce lower 
power available at low velocity and cause the 
maximum rate of climb to occur at a speed 
greater than that for minimum power required. 
The power curves for the representative. jet 
aircraft show the near linear variation of power 
available with velocity. The maximum rate 
of climb for the typical jet airplane will occur 
at some speed much higher than that for max- 
imum rate of climb of the equivalent propeller 
powered airplane. In part, this is accounted 
for by the continued increase in power avail- 
able with speed. Note that a 50 percent in- 
increase in thrust by use of an afterburner may 
cause an increase in rate of climb of approxi- 
mately 100 percent. 
The climb performance of an airplane is 
affected by many various factors. The con- 
ditions of maximum climb angle or climb rate 
occur at specific speeds and variations in speed 
will produce variations in climb performance. 
Generally, there is sufficient latitude that small 
variations in speed from the optimum do not 
produce large changes in climb performance 
and certain operational items may require 
speeds slightly different from the optimum. 
Of course, climb performance would be most 
critical at high weight, high altitude, or dur- 
ing malfunction of a powerplant. Then, opti- 
mum climb speeds are necessary. A change 
in airplane weight produces a twofold effect 
on climb performance. First, the weight, W, 
appears directly in denominator of the equa- 
tions for ,both climb angle and climb rate. 
In addition, a change in weight will alter the 
drag and power required. Generally, an in- 
crease in weight will reduce the maximum rate 
156 
of climb but the airplane must be operated at 
some increase of speed to achieve the ,smaller 
peak climb rate (unless the airplane is compres- 
sibility limited). 
The effect of altitude on climb performance 
is illustrated by the composite graphs of figure 
2.22. Generally, an increase in altitude will 
increase the power required and decrease the 
power available. Hence, the climb perform- 
ance of an airplane is expected to be greatly 
affected by altitude. The composite chart of 
climb performance depicts the variation with 
altitude of the speeds for maximum rate of 
climb, maximum angle of climb, and maximum 
and minimum level flight airspeeds. As alti- 
tude is increased, these various speeds finally 
converge at the absolute ceiling of the airplane. 
At the absolute ceiling, there is no excess of 
power or thrust and only one speed will allow 
steady level flight. The variation of rate of 
climb and maximum level flight’ speed’ with 
altitude for the typical propeller powered air- 
plane give evidence of the effect of supercharg- 
ing. Distinct aberrations in these curves take 
place at the supercharger critical altitudes and 
~blower shift points. The curve of time to 
climb is the result of summing.up the incre- 
ments of time spent climbing through incre- 
ments of altitude. Note that approach to the 
absolute ceiling produces tremendous increase 
in the time curve. 
Specific reference points are established by 
these composite curves of climb performance. 
Of course, the absolute ceiling of the airplane 
produces zero rate of climb. The service ceiling 
is specified as the altitude which produces a 
rate of climb of 100 fpm. The altitude which 
produces a rate of climb of 500 fpm is termed 
the combat ceiling. Usually, these specific refer- 
ence points are provided for the airplane at