value, or, if reverse thrust is available, a maxi- 
mum negative value for minimum landing dis- 
tance. Lift and drag are produced as long as 
the airplane has speed and the values of lift 
and drag depend on dynamic pressure and 
angle of attack. Braking friction results when 
there is a normal force on the braking wheel 
surfaces and the friction force is the product of 
the normal force and the coe&cient of braking 
friction. The normal force on the braking 
surfaces is some part of the net of weight and 
lift, i.e., some other part of this net may be 
distributed to wheels which have no brakes. 
The maximum coefficient of braking friction is 
primarily a function of the runway surface con- 
dition (dry, wet, icy, etc.) and rather inde- 
pendent of the type of tire for ordinary condi- 
tions (dry, hard surface runway). However, 
the operating coefficient of braking friction is 
controlled by the pilot by the use of brakes. 
The acceleration of the airplane during the 
landing roll is negative (deceleration) and will 
be considered to be in that sense. At any in- 
stant during the landing roll the acceleration 
is a function of the net retarding force and the 
airplane mass. From Newton’s second law of 
motion: 
B = Fr/M 
or 
where 
a=g 0+/W) 
a= acceleration, ft. per seca (negative) 
Fr=net retarding force, lbs. 
g= gravitational acceleration, ft. per sec.’ 
W=weight, lbs. 
M= mass, slugs 
= Wig 
The net retarding force on the airplane, Fr, is 
the net of drag, D, braking friction, F, and 
thrust, T. Thus, the acceleration (negative) 
at any instant during the landing roll is : 
d=$ (Df F--T) 
NAVWEPS OO-EOT-RO 
AtRPtANE PERFORMANCE 
Figure 2.34 illustrates the typical variation 
of the various forces acting on the aircraft 
throughout the landing roll. If it is assumed 
that the aircraft is at essentially constant angle 
of attack from the point of touchdown, CL and 
CD are constant and the forces of lift and drag 
vary as the square of the velocity. Thus, lift 
and drag will decrease linearly with 4 or V’ 
from the point of touchdown. If the braking 
coefficient is maintained at the maximum 
value, this maximum value of coefficient of 
friction is essentially constant with speed and 
the braking friction force will vary as the 
normal force on the braking surfaces. As the 
airplane nears a complete stop, the velocity 
and lift approach zero and the normal force on 
the wheels approaches the weight of the air- 
plane. At this point, the braking friction 
force is at a maximum. Immediately after 
touchdown, the lift: is quite large and the 
normal force on the wheels is small. As a re- 
sult, the braking friction force is small. A 
common error at this point is to apply exces- 
sive brake pressure without sufficient normal 
force on the wheels. This may develop a skid 
with a locked wheel and cause the tire to blow 
out so suddenly that judicious use of the brakes 
is necessary. 
The coefficient of braking friction can reach 
peak values of 0.8 but ordinarily values near 
0.5 are typical for the dry hard surface runway. 
Of course, a slick, icy runway can reduce the 
maximum braking friction coefficient to values 
as low as 0.2 or 0.1: If the entire weight of 
the airplane were the normal force on the brak- 
ing surfaces, a coefficient of braking friction of 
0.5 would produce a deceleration of %g, 16.1 ft. 
per sec.a Most airplanes in ground effect 
rarely produce lift-drag ratios lower than 3 or 
4. If the lift of the airplane were equal to the 
weight, an L/D = 4 would produce a decelera- 
tion of xg, 8 ft. per sec.* By this comparison 
it should be apparent that friction braking 
offers the possibility of greater deceleration 
than airplane aerodynamic braking. To this 
end, the majority of airplanes operating from