NAVWEPS OO-EOT-RO  BASIC AERODYNAMICS  Thus, wirh rhe higher lift coefficienr  available,  less dynamic pressure is required to provide  the necessary  lift.  Because  of the stated variation of stall speed  with C-,  large changes in CL- are necessary  to produce significant changes in stall speed.  This effect is illustrated by the graph in figure  1.16 and certain typical values are shown  below:  Percent  increase  in CL.  .~.  2 10 so loo 300  Percent reduction  in stall speed 1  5 18  29 50  The contribution of the high lift devices must  be considerable to cause large reduction in  stall speed. The most elaborate combination  of flaps, slots, slats, and boundary layer con-  trol throughout the span of the wing would  be required to increase C,-  by 300 percent.  A common case is that of a typical propeller  driven transport which experiences  a 70 per-  cent increase in CzIM1  by full flap deflection.  A typical single engine jet fighter with a thin  swept wing obtains a 20 percent increase in  CL- by full flap deflection. Thin airfoil sec-  tions with sweepback impose distinct limita-  tions on the effectiveness  of flaps and the 20  percent increase in CL- by flaps is a typical-  if not high-value  for such a configuration.  One factor common to maximum lift condi-  tion is the angle of attack and pressure  distri-  bution.  The maximum lift coefficient of a  particular wing configuration is obtained at  one angle of attack and one pressure  distribu-  tion.  Weight, bank angle, load factor, density  altitude, and airspeed have no direct effect on  the stall angle of attack.  This fact is sufficient  justification for the use of angle of attack indi-  cators and stall warning devices which sense  pressure distribution on the wing.  During  flight maneuvers, landing approach, takeoff,  turns, etc. the airplani will stall if the critical  angle of attack is cxcccdcd. The airspeed ar  which stall occurs will  be determined by  weight, load factor, and altitude but the stall  39  HIGH LIFT DEVICES  There are many different types of high lift  devices used to increase the maximum lift co-  efficient for low speed flight.  The high lift  devices applied to the trailing edge of a section  consist of a flap which is usually 15 to 25 per-  cent of the chord.  The deflection of a flap  produces the effect of a large amount of camber  added well aft on the chord. The principal  types of flaps are shown applied to a basic sec-  tion of airfoil.  The effect of a 30’ deflection of  a 25 percent chord flap is shown on the lift  and drag curves of figure 1.17.  An airplane may be designed to be “stall-  proof” simply by reducing the effectiveness  of  the elevators. If the elevators are not power-  ful enough to hold the airplane to high angles  of attack, the airplane cannot be stalled in any  condition of flight.  Such a requirement for a  tactical military airplane would seriously re-  duce performance. High lift coefficients near  the maximum are required for high maneuver-  ability and low landing and takeoff speeds.  Hence, the Naval Aviator must appreciate the  effect of the many variables affecting the stall  speed and regard “attitude  flying,”  angle of  attack indicators, and stall warning devices  as techniques which allow more precise control  of the airplane at high lift conditions.  angle of attack is unaffected. At any parricu-  lar altitude, the indicated stall speed is a func-  tion of weight and load factor. An increase  in altitude will produce a decrease  in density  and increase the true airspeed at stall.  Also,  an increase  in altitude will alter compressibility  and viscosity effects and, generally speaking,  cause the in,&ztcd stall speed to increase.  This parti&lar  consideration is usually sig-  nificant only above altitudes of 20,000 ft.  Recovery from stall involves a very simple  concept. Since stall is precipitated by an  excessive angle of attack, the angle of attack  must be dccmmd. This is a fundamental princi-  ple which is common to any airplane. 