NAVWEPS DD-BOl-80 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
When the runway surface is dry, brush- 
finished concrete, the maximum value for the 
coefficient of friction for most aircraft tires is 
on the order of 0.6 to 0.8. Many factors can 
determine small differences in this peak value 
of friction coefhcient for dry surface conditions. 
For example, a soft gum rubber composition 
can develop a very high value of coefficient of 
friction but only for low values of surface 
shearing stress. At high values of surface 
shearing stress, the soft gum rubber will shear 
or scrub off before high values of friction co- 
efficient are developed. The higher strength 
compounds used in the production of aircraft 
tires produce greater resistance to surface shear 
and scrubbing but the harder rubber has lower 
intrinsic friction coeflicient. Since the high 
performance airplane cannot afford the luxury 
of excessive tire weight or size, the majority 
of airplane tires will be of relatively hard 
rubber and will operate at or near the rated 
load capacities. As a result, there will be 
little difference between the peak values of 
friction coefficient for the dry, hard surface 
runway for the majority of aircraft tires. 
If high traction on dry surfaces were the 
only consideration in the design of tires, the 
result would be a soft rubber tire of extreme 
width to create a large footprint and reduce 
surface shearing stresses, e.g., driving tires on 
a drag racer. However, such a tire has many 
other characteristics which are undesirable 
such as high rolling friction, large size, poor 
side force characteristics, etc. 
When the runway has water or ice on the 
surface, the maximum value for the coefficient 
of friction is reduced greatly below the value 
obtained for the dry runway condition. When 
water is on the surface, the tread design be- 
comes of greater importance to maintain con- 
tact between the rubber and the runway and 
prevent a film of water from lubricating the 
surfaces. When the rainfall is light, the peak 
value for friction coefficient is on the order of 
0.5. With heavy rainfall it is more likely 
that sufficient water will stand to form a liquid 
film between the tire and the runway. In this 
case, the peak coefficient of friction rarely 
exceeds 0.3. In some extreme conditions, the 
tire may simply plane along the water without 
contact of the runway and the coefficient of 
friction is much lower than 0.3. Smooth, 
clear ice on the runway will cause extremely 
low values for the coefficient of friction. In 
such a condition, the peak value for the co- 
efhcient of friction may be on the order of 0.2 
or 0.15. 
Note that immediately past the incipient 
skidding condition the coefficient of friction 
decreases with increased slip speed, especially 
for the wet or icy runway conditions. Thus, 
once skid begins, a reduction in friction force 
and rolling torque must be met with a reduc- 
tion in braking torque, otherwise the wheel 
will decelerate and lock. This is an important 
factor to consider in braking technique because 
the skidding tire surface on the locked wheel 
produces considerably less retarding force than 
when at the incipient skid condition which 
causes the peak coeflicient of friction. If the 
wheel locks from excessive braking, the sliding 
tire surface produces less than the maximum 
retarding force and the tires become relatively 
incapable of developing any significant side 
force. Stop distance will increase and it may 
be difficult-if not impossible-to control the 
airplane when full slip is developed. In addi- 
tion, at high rolling vel,ocities on the dry sur- 
face runway, the immediate problem of a skid- 
ding tire is not necessarily the loss of retard- 
ing force but the imminence of tire failure. The 
pilot must insure that the application of brakes 
does not produce some excessive braking torque 
which is greater than the maximum rolling