NAVWEPS OO-EOT-RO  AtRPtANE PERFORMANCE  value, or, if reverse  thrust is available, a maxi-  mum negative value for minimum landing dis-  tance. Lift and drag are produced as long as  the airplane has speed and the values of lift  and drag depend on dynamic pressure and  angle of attack. Braking friction results when  there is a normal force on the braking wheel  surfaces  and the friction force is the product of  the normal force and the coe&cient of braking  friction.  The normal force on the braking  surfaces  is some part of the net of weight and  lift, i.e., some other part of this net may be  distributed to wheels which have no brakes.  The maximum coefficient of braking friction is  primarily a function of the runway surface  con-  dition (dry, wet, icy, etc.) and rather inde-  pendent of the type of tire for ordinary condi-  tions (dry, hard surface runway).  However,  the operating coefficient of braking friction is  controlled by the pilot by the use of brakes.  Figure 2.34 illustrates the typical variation  of the various forces acting on the aircraft  throughout the landing roll.  If it is assumed  that the aircraft is at essentially constant angle  of attack from the point of touchdown, CL and  CD are constant and the forces of lift and drag  vary as the square of the velocity.  Thus, lift  and drag will decrease  linearly with 4 or V’  from the point of touchdown.  If the braking  coefficient is maintained at the maximum  value, this maximum value of coefficient of  friction is essentially constant with speed and  the braking friction force will  vary as the  normal force on the braking surfaces.  As the  airplane nears a complete stop, the velocity  and lift approach zero and the normal force on  the wheels approaches the weight of the air-  plane.  At this point, the braking friction  force is at a maximum.  Immediately after  touchdown, the lift: is quite large and the  normal force on the wheels is small. As a re-  sult, the braking friction force is small.  A  common error at this point is to apply exces-  sive brake pressure without sufficient normal  force on the wheels.  This may develop a skid  with a locked wheel and cause the tire to blow  out so suddenly that judicious use of the brakes  is necessary.  The acceleration of the airplane during the  landing roll is negative (deceleration) and will  be considered to be in that sense. At any in-  stant during the landing roll the acceleration  is a function of the net retarding force and the  airplane mass. From Newton ’ s second law of  motion:  The coefficient of braking friction can reach  peak values of 0.8 but ordinarily values near  0.5 are typical for the dry hard surface  runway.  Of course, a slick, icy runway can reduce the  maximum braking friction coefficient to values  as low as 0.2 or 0.1:  If the entire weight of  the airplane were the normal force on the brak-  ing surfaces,  a coefficient of braking friction of  0.5 would produce a deceleration of %g, 16.1 ft.  per sec.a Most airplanes in ground effect  rarely produce lift-drag ratios lower than 3 or  4.  If the lift of the airplane were equal to the  weight, an L/D = 4 would produce a decelera-  tion of xg, 8 ft. per sec.*  By this comparison  it should be apparent that friction braking  offers the possibility of greater deceleration  than airplane aerodynamic braking.  To this  end, the majority of airplanes operating from  The net retarding force on the airplane, Fr, is  the net of drag, D, braking friction, F, and  thrust, T.  Thus, the acceleration (negative)  at any instant during the landing roll is :  where  or  B = Fr/M  a=g 0+/W)  a= acceleration, ft. per seca (negative)  Fr=net retarding force, lbs.  g= gravitational acceleration, ft. per sec.’  W=weight, lbs.  M= mass, slugs  = Wig  d=$ (Df F--T) 