NAVWEPS DD-BOT-BO 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
BRAKING TECHNIQUE. It must be 
clearly distinguished that the techniques for 
minimum stop distance may differ greatly from 
the techniques required to minimize wear and 
tear on the tires and brakes. For the majority 
of airplane configurations, brakes will provide 
the most important source of deceleration for 
all but the most severe of icy runway condi- 
tions. Of course, aerodynamic drag is very 
durable and should be utilized to decelerate the 
airplane if the runway is long enough and the 
drag high enough. Aerodynamic drag will be 
of importance only for the initial 20 to 30 per- 
cent of speed reduction from the point of touch- 
down. At speeds less than 60 to 70 percent of 
the landing speed, aerodynamic drag is of little 
consequence and brakes will be the principal 
source of deceleration regardless of the runway 
surface. For the conditions of minimum land- 
ing distance, aerodynamic drag will be a prin- 
cipal source of deceleration only for the initial 
portion of landing roll for very high drag con- 
figurations on very poor runway conditions. 
These cases are quite limited so considerable 
importance must be assigned to proper use of 
the brakes to produce maximum effectiveness. 
In order to provide the maximum possible 
retarding force, effort must be directed to pro- 
duce the maximum normal force on the braking 
surfaces. (See figure 6.11.) The pilot will be 
able to influence the normal force on the brak- 
ing surfaces during the initial part of the land- 
ing roll when dynamic pressure is large and 
aerodynamic forces and moments are of conse- 
quence. During this portion of the landing 
roll the pilot can control the airplane lift and 
the distribution of normal force to the landing 
gears. 
First to consider is that any positive lift will 
support a part of the airplane weight and reduce 
the normal force on the landing gear. Of 
course, for the purposes of braking friction, it 
would be to advantage to create negative lift 
but this is not the usual capability of the air- 
plane with the tricycle landing gear. Since 
the airplane lift may be considerable immedi- 
ately after landing, retraction of flaps or ex- 
tension of spoilers immediately after touch- 
down will reduce the wing lift and increase 
the normal force on the landing gear. With 
the retraction of flaps, the reduced drag is more 
than compensated for by the increased braking 
friction force afforded by the increased normal 
force on the braking surfaces. 
A second possible factor to control braking 
effectiveness is the distribution of normal force 
to the landing gear surfaces. The nose wheel 
of the tricycle landing gear configuration usu- 
ally has no brakes and any normal force dis- 
tributed to this wheel is useful only for pro- 
ducing side force for control of the airplane. 
Under conditions of deceleration, the nose- 
down pitching moment created by the friction 
force and the inertia force tends to transfer 
a significant amount of normal force to nose 
wheel where it is unavailable to assist in 
creating friction force. For the instant after 
landing touchdown, the pilot may control 
this condition to some extent and regain or 
increase the normal force on the main wheels. 
After touchdown, the nose is lowered until the 
nose wheel contacts the runway then brakes 
are applied while the stick is eased back with- 
out lifting the nose wheel back off the runway. 
The effect is to minimize the normal force on 
the nose wheel and increase the normal force 
on braking surfaces. While the principal 
effect is to transfer normal force to the main 
wheels, there may be a significant increase in 
normal force due to a reduction in net lift, i.e., 
tail download is noticeable. This reduction 
in net lift tends to be particular to tailless or 
short coupled airplane configurations. 
The combined effect of flap retraction and