BRAKING PERFORMANCE

For the majority of airplane configurations and runways conditions, the airplane brakes furnish the most powerful means of deceler­ation. While specific techniques of braking are required for specific situations, there are various fundamentals which are common to all conditions.

Solid friction 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 mo­tion 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 coeffi­cient of friction, Ц.

n = F/N

where

n = coefficient of friction (mu)

F = friction force, lbs.

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 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 to retard wheel rota­tion. 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 brak­ing 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 meas­urable apparent slip. Continued increase in friction coefficient is obtained until some max­imum is achieved then decreases as the slip increases and approaching the 100 percent slip condition. Actually, the peak value of co­efficient 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.

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 coefficient 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 co­efficient 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 coefficient. 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 be­comes of greater importance to maintain con­tact 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 co­efficient 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 reduc­tion 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 coefficient 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 addi­tion, at high rolling velocities on the dry sur­face runway, the immediate problem of a skid­ding tire is not necessarily the loss of retard­ing 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 torque and particular care must be taken when the runway conditions produce low values of friction coefficient and when the normal force on the braking surfaces is small. When it is difficult to perceive or distinguish a skidding condition, the value of an antiskid or auto­matic braking system will be appreciated.

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 condi­tions. 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 per­cent of speed reduction from the point of touch­down, 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 land­ing distance, aerodynamic drag will be a prin­cipal source of deceleration only for the initial portion of landing roll for very high drag con­figurations 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 pro­duce the maximum normal force on the braking surfaces. (See figure 6.11.) The pilot will be able to influence the normal force on the brak­ing surfaces during the initial part of the land­ing roll when dynamic pressure is large and aerodynamic forces and moments are of conse­quence. 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 air­plane with the tricycle landing gear. Since the airplane lift may be considerable immedi­ately after landing, retraction of flaps or ex­tension of spoilers immediately after touch­down 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 usu­ally has no brakes and any normal force dis­tributed to this wheel is useful only for pro­ducing 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 with­out 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 aft stick is a significant increase in braking friction force. Of course, the flaps should not be retracted while still airborne and aft stick should be used just enough without lifting the nosewheel off the runway. These tech­niques are to no avail if proper use of the

brakes does not produce the maximum coeffi­cient of friction. The incipient skid condi­tion will produce the maximum coefficient of friction but this peak is difficult to recognize and maintain without an antiskid system. Judicious use of the brakes is necessary to obtain the peak coefficient of friction but not develop a skid or locked wheel which could cause tire failure, loss of control, or consider­able reduction in the friction coefficient.

The capacity of the brakes must be sufficient to create adequate braking torque and produce the high coefficient of friction. In addition, the brakes must be capable of withstanding the heat generated without fading or losing effectiveness. The most critical requirements of the brakes occur during landing at the maximum allowable landing weight.

TYPICAL ERRORS OF BRAKING TECH­NIQUE. Errors in braking technique are usu­ally coincident with errors of other sorts. For example, if the pilot lands an airplane with excessive airspeed, poor braking technique could accompany the original error to produce an unsafe situation. One common error of of braking technique is the application of braking torque in excess of the maximum possible rolling torque. The result will be that the wheel decelerates and locks and the skid reduces the coefficient of friction, lowers the capability for side force, and enhances the possibility of tire failure. If maximum brak­ing is necessary, caution must be used to modulate the braking torque to prevent lock­ing the wheel and causing a skid. On the other hand, maximum coefficient of friction is obtained at the incipient skidding condition so sufficient brake torque must be applied to produce maximum friction force. Intermittent braking serves no useful purpose when the objective is maximum deceleration because the periods between brake application produce only slight or negligible cooling. Brake should be applied smoothly and braking torque modulated at or near the peak value to insure that skid does not develop.

One of the important factors affecting the landing roll distance is landing touchdown speed. Any excess velocity at landing causes a large increase in the minimum stop distance and it is necessary that the pilot control the landing precisely so to land at the appropriate speed. When landing on the dry, hard surface runway of adequate length, a tendency is to take advantage of any excess runway and allow the airplane to touchdown with excess speed. Of course, such errors in technique cannot be tolerated and the pilot must strive for precision in all landings. Immediately after touchdown, the airplane lift may be considerable and the normal force on the braking surfaces quite low. Thus, if excessive braking torque is applied, the wheel may lock easily at high speeds and tire failure may take place suddenly.

Landing on a wet or icy runway requires judicious use of the brakes because of the re­duction in the maximum coefficient of friction. Because of reduction in the maximum attain­able value of the coefficient of friction, the pilot must anticipate an increase in the mini­mum landing distance above that applicable for the dry runway conditions. When there is considerable water or ice on the runway, an increase in landing distance on the order of 40 to 100 percent must be expected for similar conditions of gross weight, density altitude, wind, etc. Unfortunately, the conditions likely to produce poor braking action also will cause high idle thrust of the turbojet engine and the extreme case (smooth, glazed ice or heavy rain) may dictate shutting down the engine to effect a reasonable stopping distance.