NAVWEPS DD-BOT-BO  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  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.  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 combined effect of flap retraction and  aft stickis 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 tech-  niques are to no avail if proper use of the  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.  390 