BRAKING PERFORMANCE 
For the majority of airplane configurations 
and runways conditions, the airplane brakes 
furnish the most powerful means of deceler- 
ation. While specific techniques of braking 
are required for specific situations, there 
are various fundamentals which are common 
to all conditions. 
Solid ftictim is the resistance to relative 
motion of two surfaces in contact. When 
relative motion exists between the surfaces, 
the resistance to relative motion is termed 
“kinetic” or “sliding” friction; when no 
relative motion exists between the surfaces, 
the resistance to the impending relative mo- 
tion is termed “static” friction. The minute 
discontinuities of the surfaces in contact are 
able to mate quite closely when relative 
motion impends rather than exists, so static 
friction will generally exceed kinetic friction. 
The magnitude of the friction force between 
two surfaces will depend in great part on the 
types of surfaces in contact and the magnitude 
of force pressing the surfaces together. A 
convenient method of relating the friction 
charactersitics of surfaces in contact is a 
proportion of the friction force to the normal 
(or perpendicular) force pressing the surfaces 
together. This proportion defines the coeffi- 
cient of friction, ~1. 
,L= F/N 
where 
p =coefFicient of friction (mu) 
F =friction force, Ibs. 
N = normal force, lbs. 
The coefficient of friction of tires on a runway 
surface is a function of many factors. Runway 
surface condition, rubber composition, tread, 
inflation pressure, surface friction shearing 
stress, relative slip speed, etc., all are factors 
which affect the coefficient of friction. When 
the tire is rolling along the runway without 
the use of brakes, the friction force resulting is 
simple rolling resistance. The coefficient of 
NAVWEPS O&ROT-R0 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
rolling friction is of an approximate magnitude 
of 0.015 to 0.030 for dry, hard runway surface. 
The application of brakes supplies a torque 
to the wheel which tends co retard wheel rota- 
tion. However, the initial application of 
brakes creates a braking torque but the initial 
retarding torque is balanced by the increase in 
friction force which produces a driving or 
rolling torque. Of course, when the braking 
torque is equal to the rolling torque, the wheel 
experiences no acceleration in rotation and the 
equilibrium of a constant rotational speed is 
maintained. Thus, the application of brake 
develops a retarding torque and causes an 
increase in friction force between the tire and 
runway surface. A common problem of brak- 
ing technique is application of excessive brake 
pressure which creates a braking torque greater 
than the maximum possible rolling torque. 
In this case, the wheel loses rotational speed 
and decelerates until the wheel is stationary 
and the result is a locked wheel with the tire 
surface subject to a full slip condition. 
The relationship of friction force, normal 
force, braking torque, and rolling torque is 
illustrated in figure 6.11. 
The effect of slip velocity on the coefficient 
of friction is illustrated by the graph of figure 
6.11. The conditions of zero slip corresponds 
to the rolling wheel without brake application 
while the condition of full, 100 percent slip 
corresponds to the locked wheel where the 
relative velocity between the tire surface and 
the runway equals the actual velocity. With 
the application of brakes, the coefficient of 
friction increases but incurs a small but meas- 
urable apparent slip. Continued increase in 
friction coefficient is obtained until some max- 
imum is achieved then decreases as the slip 
increases and approaching the 100 percent slip 
condition. Actually, the peak value of co- 
efficient of friction occurs at an incipient skid 
condition and the relative slip apparent at this 
point consists primarily of elastic shearing 
deflection of the tire structure. 
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