NAVWE,PS OD-80T-80  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  the forces acting on the airplane, i.e., lift,  thrust, weight, and drag.  ANGLE OF ATTACK VERSUS AIRSPEED.  In order to achieve equilibrium in the vertical  direction, the net lift must equal the airplane  weight.  This is a contingency of steady, level  flight or steady climbing and descending flight  when the flight path inclination is slight.  A  refinement of the basic lift equation defines the  relationship of speed, weight, lift coefficient,  etc., for the condition of lift equal to weight.  V=17.2  y  J  TP  or  where  V=velocity, knots (TAX)  VE=equivalent airspeed, knots (EAS)  W=gross weight, lbs.  S= wing surface  area, sq. ft.  W/S= wing loading, psf  g=altitudc density ratio  C,= lift coefficient  From this relationship it is appreciated that a  given configuration of airplane with a specific  wing loading, W/S, will achieve lift equal to  weight at particular combinations of velocity,  V, and lift coefficient,  C,. In steady flight, each  equivalent airspeed demands a particular vaIue  of C,, and each value of C, demands a particular  equivalent airspeed to provide lift equal to  weight.  Figure 6.1 illustrates a typical lift  curve for an airplane and shows the relation-  ship between C, and OL,  angle of attack. For  this relationship, some specific value of a will  create a certain value of C, for any given aero-  dynamic configuration.  For the conditions of steady flight with  a given airplane, each angle of attack corre-  sponds to a specific airspeed. Each angle of  attack produces a specific  value of CL and each  value of C, requires a specific value of equiva-  350  lent airspeed to provide lift equal to weight.  Hence, angle of attack is the primary .control of  airspeed  in mad3 fright. If an airplane is es-  tablished in steady, level flight at a particular  airspeed, any increase in angle of attack will  result in some reduced airspeed common to the  increased C,.  A decrease in angle of attack  will  result in some increased airspeed com-  mon to the decreased  CL. As a result of the  change in airspeed, the airplane may climb or  descend  if there is no change in powet setting  but the change in airspeed was provided by  the change in angle of attack. The state of  the airplane during the change in speed will  be some transient condition between the  original and final steady state conditions.  Primary control of airspeed in steady flight  by angle of attack is an important principle.  With some configurations of airplanes, low  speed flight will bring about a low level of  longitudinal stick force stability and possi-  bility  of low  airplane static  longitudi-  nal stability.  In such a case, the “feel”  for  airspeed will  be light and may not furnish  a ready reference  for easy control of the air-  plane.  In addition, the high angles of attack  common to low speed flight are likely to pro-  vide large position errors to the airspeed indi-  cating system. Thus, proper control of air-  speed will be enhanced by good “attitude”  flying or-when  the visual t;eference  field is  poor-an angle of attack indicator.  RATE OF CLIMB  AND .DESCENT.  In  order for an airplane to achieve  ‘ equilibrium at  constant altitude, lift must be equal to weight  and thrust must be equal to drag.  Steady,  level flight requires equilibrium in both the  vertical and horizontal directions.  For the  case of climbing or descending flight condi-  tions, a component of weight is inclined along  the flight path direction and equilibrium is  achieved when thrust is not equal to the drag.  When the airplane is in a steady climb or  descent, the rate of climb is related by the  following expression: 