NAVWEPS OO-ROT-80 
AIRPLANE PERFORMANCE 
normal force and increased braking friction 
force to cope with the increased mass. Also, 
the higher landing speed at the same CL and 
CD produce an average drag which increased in 
the same proportion as the increased weight. 
Thus, increased gross weight causes like in- 
creases in the sum of drag plus braking friction 
and the acceleration is essentially unaffected. 
To evaluate the effect of gross weight on 
landing distance, the following relationships 
are used: 
the effect of weight on landing velocity is 
if the net retarding force increases in the 
same proportion as the .weight, the accel- 
eration is unaffected. 
the effect of these items on landing dis- 
tance is, 
or 
$2 w* 
s,=w, 
In effect, the minimum landing distance will 
vary directly as the gross weight. For ex- 
ample, a 10 percent increase in gross weight 
at landing would cause: 
a 5 percent increase in landing velocity 
a 10 percent increase in landing distance 
A contingency of the previous analysis is the 
relationship between weight and braking ftic- 
tion force. The maximum coefficient of brak- 
ing friction is relatively independent of the 
usual range of normal forces and rolling speeds, 
e.g., a 10 percent increase in normal force would 
create a like 10 percent increase in braking 
friction force. Consider the case of two air- 
planes of the same type and c.g. position but 
of ~diffetent gross weights. If these two air- 
planes are rolling along the runway at some 
speed at which aerodynamic forces are negli- 
gible, the use of the maximum coefficient of 
braking friction will bring both airplanes to 
a stop in the same distance. The heavier ait- 
plane will have the gteater mass to decelerate 
but the greater normal force will provide a 
greater retarding friction force. As a result, 
both airplanes would have identical accelera- 
tion and identical stop distances from a given 
velocity. However, the heavier airplane 
would have a greater kinetic energy to be dis- 
sipated by the brakes and the principal differ- 
ence between the two airplanes as they reach 
a stop would be that the heavier airplane 
would have the hotter brakes. Therefore, 
one of the factors of braking performance is the 
ability of the brakes to dissipate energy with- 
out developing excessive temperatures and 
losing effectiveness. 
To appreciate the effectiveness of modern 
brakes, a 30,000-lb. aircraft landing at 175 
knots has a kinetic energy of 41 million ft.-lbs. 
at the instant of touchdown. In a minimum 
distance landing, the brakes must dissipate 
most of this kinetic energy and sach brake must 
absotb an input power of approximately 1,200 
h.p. for 25 seconds. Such requirements for 
brakes are extreme but the example serves to 
illustrate the ptoblems of brakes for high 
performance airplanes. 
While a 10 percent increase in landing 
weight causes : 
a 5 percent higher landing speed 
a 10 percent greater landing distance, 
it also produces a 21 percent increase in the 
kinetic energy of the airplane to be dissipated 
during the landing roll. Hence, high landing 
weights may approach the energy dissipating 
capability of the brakes. 
The s&t of wind on landing distance is large 
and deserves proper consideration when pre- 
dicting landing distance. Since the airplane 
will land at a particular airspeed independent 
of the wind, the principal effect of wind on 
landing distance is due to the change in the 
ground velocity at which the airplane touches 
down. The effect of wind on acceleration 
duting the landing distance is identical to the 
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