NAVWEPS OO-SOT-80 
AIRPLANE PERFORMANCE 
The forces acting on the airplane during a 
climb are shown by the illustration of figure 
2.21. When the airplane is in steady flight 
with moderate angle of climb, the vertical 
component of lift is very nearly the same as the 
actual lift. Such climbing flight would exist 
with the lift very nearly equal to the weight. 
The net thrust of the powerplant may be in- 
clined relative to the flight path but this effect 
will be neglectec! for the sake of simplicity. 
Note that the weight of the aircraft is vertical 
but a component of weight will act aft along 
the flight path. 
If it is assumed that the aircraft is in a steady 
climb with essentially small inclination of the 
flight path, the summation of forces along the 
flight path resolves to the following: 
Forces forward= Forces aft 
where 
T= thrust available, lbs. 
D= drag, lbs. 
W= weight, lbs. 
v=flight path inclination or angle ,of 
climb, degrees (“gamma”) 
This basic relationship neglects some of the 
factors which may be of importance for air- 
planes of very high climb performance. For 
example, a more detailed consideration would 
account for the inclination of thrust from the 
flight path, lift not equal to weight, subse- 
quent change of induced drag, etc. However, 
this basic relationship will define the principal 
factors affecting climb performance. With 
this relationship established by the condition 
of equilibrium, the following relationship 
exists to express the trigonometric sine of the 
climb angle, y: 
T-D sin y=- W 
This relationship simply states that, for a 
given weight airplane, the angle of climb (7) 
depends on the difference between thrust and 
drag (T-D), or excess thrust. Of course, 
when the excess thrust is zero (T-D=0 or 
T=D), the inclination of, the flight path is 
zero-and the airplane is in steady, level flight. 
When the thrust is greater than the drag, the 
excess thrust will allow a climb angle depend- 
ing on the value of excess thrust. Also, when 
the thrust is less than the drag. the deficiency 
of thrust will allow an angle ~of descent. 
The most immediate interest in the climb 
angle performance involves obstacle clearance. 
The maximum angle of climb would occur 
where there exists the greatest difference be- 
tweenthrust available and thrust required, i.e., 
maximum (T-D). Figure 2.21 illustrates the 
climb angle performance with the curves of 
thrust available and thrust required versus 
velocity. The thrust required, or drag, curve 
is nss,~pued to be ppw=n*~r;.rP nc CnmP +-+a! y.“- ..I‘..&. c “I ““IILL ‘, y 
airplane configuration which could be powered 
by either a turbojet or propeller type power- 
plant. The thrust available curves included 
are for a characteristic propeller powerplant 
and jet powerplant operating at maximum 
output. 
The thrust curves for the representative pro- 
peller aircraft show the typical propeller thrust 
which is high at low velocities and decreases 
with an. increase in velocity. For the pro- 
peiler powered airplane, the maximum excess 
thrust and angle of climb will occur at some 
speed just above the stall speed. Thus, if it 
is necessary to clear an obstacle after takeoff, 
the propeller powered airplane will attain 
maximum angle of climb at an airspeed con- 
veniently close to-if not at-the takeoff 
speed. 
The thrust curves for the representative jet 
aircraft show the typ~ical turbojet thrust which 
is very nearly constant ~with speed. If the 
thrust available is essentially constant with 
speed, the maximum excess thrust and angle 
of climb will occur where the thrust required 
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