percent of the stall speed for propeller air-  planes or 90 percent of the stall speed for jet  airplanes. This feat must be accomplished on  a smooth runway at all normal service takeoff  loading conditions.  Figure 4.19 illustrates the principal forces  acting on an airplane during takeoff toll.  When the airplane is in the three point attitude  at some speed less than the stall speed, the  wing lift will be less than the weight of the  airplane. As the elevators must be capable  of rotating to the takeoff attitude, the critical  condition will be with zero load on the nose  wheel and the net of lift and weight supported  on the main gear. Rolling friction resulting  from the normal force on the main gear creates  an adverse nose down moment.  Also, the  center of gravity ahead of the main gear  contributes a nose down moment and this  consideration could decide the most aft loca-  tion of the main landing gear during design.  The wing may contribute a large nose down  moment when flaps are deflected but this  effect may be countered by a slight increase  in downwash at the tail.  To balance these  nose down moments, the horizontal  tail  should be capable of producing sufficient nose  up moment to attam the takeoff attitude. at  the specified speeds.  The propeller airplane at takeoff power may  induce considerable slipstream velocity at the  horizontal tail which can provide an increase  in the e&iency  of the surface. The jet  airplane does not experience a similar magni-  tude of this effect since the induced velocities  from the jet are relatively small compared  to the slipstream velocities from a propeller.  At landing, the airplane must have suthcient  control power to ensure adequate control at  specified landing speeds. Adequate landing  control is usually assured if the elevators are  capable of holding the airplane just off the  runway at 105 percent of the stall speed. Of  course, the most critical requirement will exist  when the c.g. is in the most forward position,  flaps are fully extended, and power is set at  idle.  This configuration will  provide the  most stable condition which is most demand-  ing of controllability.  The full deflection of  flaps usually provides the greatest wing diving  moment and idle power will produce the most  critical (least) dynamic pressure at the hoti-  zontal tail.  The landing control requirement has one  particular difference from the maneuvering  control requirement of free flight.  As the  airplane approaches the ground surface, there  will be a change in the three-dimensional flow  of the airplane due to ground effect. A wing in  proximity to the ground plane will experience  a decrease  in tip vortices and downwash at  a given lift coefficient. The decrease  in down-  wash at the tail tends to increase the static  stability and produce a nosedown moment from  the reduction in download on the tail.  Thus,  the airplane just off the runway surface will  requite additional control deflection to trim  at a given lift coefficient and the landing con-  trol requirement may be critical in the design  of longitudinal control power.  As an example of ground effect, a typical  propeller powered airplane may requite as  much as 15” more up elevator to trim at CL-  in ground effect than in free flight away from  the ground plane. Because  of this effect, many  aitplaneshavesufIicientcontrolpowertoachieve  full stall out of ground effect but do not have  the ability to achieve full stall when in close  proximity to the ground.  In some cases  the effectiveness  of the control  surface is adversely affected by the use of trim  tabs. If trim tabs are used to excess  in ttim-  ming stick forces, the effectiveness of the  elevator.may be reduced to hinder landing or  takeoff control.  Each of the three principal conditions re-  quiting adequate longitudinal control are ctit-  ical for high static stability.  If the forward  c.g. limit is exceeded, the airplane may en-  counter a deficiency of controllability in any  of these conditions.  Thus, the forward c.g.  177  LANDING  CONTROL REQUIREMENT  NAVWEPS 00-BOT-80  STABILITY AND CONTROL 