NAVWEPS O&ROT-R0  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  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.  where  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  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.  The relationship of friction force, normal  force, braking torque, and rolling  torque is  illustrated in figure 6.11.  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.  387  ,L= F/N  p =coefFicient of friction (mu)  F =friction force, Ibs.  N = normal force, lbs. 