NAVWEPS OO-EOT-SO  BASIC AERODYNAMICS  45  of such a device. To illustrate a few principles  of flap management, figure 1.19 presents the  lift and drag curves of a typical airplane in the  clean and flap down configurations.  When the flaps are lowered for landing essen-  tially the same items must be considered. Ex-  tending the flaps will cause these. changes to  take place:  (1) Lowering the flaps requires retrim-  ming to balance the nose down moment  change.  (2) The increase in drag requires a higher  power setting to maintain airspeed and  altitude.  (3) The angle of attack required to pro-  duce the same lift coefficient is less, e.g.,  flap extension tends to cause the airplane to  “balloon.”  In order to appreciate some of the factors  involved in flap management, assume that the  airpIane has just taken off and the flaps are  extended. The pilot should not completely  retract the flaps until the airplane has sufficient  speed. If the flaps are retracted prematurely  at insufhcient airspeed, maximum lift coefi-  cient of the clean configuration may not be  able to support the airplane and the airplane  will sink or stall.  Of course, this same factor  must be considered for intermediate flap posi-  tions between fully retracted and fully ex-  tended. Assume that the airplane is allowed  to gain speed and reduce the flight lift coefii-  cient to the point of flap retraction indicated  on figure 1.19. As the configuration is altered  from the “cluttered”  to the clean configura-  tion, three important changes take place:  An additional factor which must be consid-  ered when rapidly accelerating after takeoff,  or when lowering the flaps for landing, is the  limit  airspeed for flap extension. Excessive  airspeeds in the flap down configuration may  cause structural damage.  (1) The reduction in camber by flap re-  traction changes the wing pitching moment  and-for the majority of airplanes-requires  retrimming to balance the nose up moment  change. Some airplanes feature an automat-  ic retrimming which is programmed with  flap deflection.  (2) The retraction of flaps shown on  figure 1.19 causes  a reduction of drag coeffi-  cient at that lift  coefficient. This drag  reduction improves the acceleration of the  airplane.  In many aircraft the effect of intermediate  flap deflection is of primary importance in  certain critical operating conditions.  Small  initial deflections of the flap cause noticeable  changes in C ’ s,, without large changes in drag  coefficient. This feature is especially true of  the airplane equipped with slotted or Fowler  flaps (refer to fig. 1.17).  Large flap deflections  past 30’ to 33’ do not create the same rate of  change of Cs- but do cause greater changes in  CD. A fact true of most airplanes is that the  first 50 percent of flap deflection causes mwc  than half of the total change in Cr.- and the  last 50 percent of flap deflection causes mo~c  than half of the total change in Cs.  (3) The retraction of flaps requires an  increase in angle of attack to maintain the  same lift coefficient. Thus, if airplane accel-  eration is low through the flap retraction  speed range, angle of attack must be in-  creased  to prevent the airplane from sinking.  This situation is typical after takeoff when  gross weight, density altitude, and tempera-  ture are high.  However, some aircraft have  such high acceleration through the flap re-  traction speed that the rapid gain in air-  speed requtres much less noticeable attitude  change.  The effect of power on the stall speed of an  airplane is determined by many factors.  The  most important factors affecting this relation-  ship are powerplant type (prop or jet), thrust-  to-weight ratio, and inclination of the thrust  vector at maximum lift.  The effect of the  propeller is illustrated in figure 1.20. The  slisstream velocity behind the propeller is  different from the free stream velocity depend-  ing on the thrust developed. Thus, when the  propeller driven airplane is at low air+ceds 