percent and, when the wing is at a height  equal to one-tenth the span (h/b=O.l),  the  reduction in induced drag is 47.6 percent.  Thus, a large reduction in induced drag will  take place only when the wing is very close  to the ground.  Because of this variation,  ground effect is most usually recognized  during  the liftoff of takeoff or prior to touchdown on  landing.  The reduction of the tip or trailing vortices  due to ground effect alters the spanwise lift  distribution and reduces the induced angle of  attack. In this case, the wing will require  a lower angle of attack in ground effect to  produce the same lift coefficient.  This effect  is illustrated by the lift curves of figure 6.9  which show that the airplane in ground effect  will develop a greater slope of the lift curve.  For the wing in ground effect, a lower angle  of  attack is necessary  to produce the same lift  coefficient or, if a constant angle of attack is  maintained, an increase in lift coefficient will  result.  Figure 6.9 illustrates the manner in which  ground effect will alter the curve of thrust re-  quired versus velocity.  Since induced drag  predominates at low speeds, the reduction of  induced drag due to ground effect will cause  the most significant reduction of thrust re-  quired (parasite plus induced drag) only at  low speeds. At high speeds where parasite  drag predominates, the induced drag is but  a small part of the total drag and ground  effect causes  no significant change in thrust re-  quired.  Because ground effect involves the  induced effects  of airplane when in close prox-  imity to the ground, its effects are of greatest  concern during the takeoff and landing.  Ordi-  narily, these are the only phases of flight in  which the airplane would be in close proximity  to the ground.  GROUND EFFECT ON SPECIFIC FLIGHT  CONDITIONS.  The overall influence of  ground effect is best realized by assuming that  the airplane descends  into ground effect while  maintaining a constant lift coefficient and,  3&l  NAVWEPS CKLBOT-BO  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  thus, a constant dynamic pressure  and equiva-  lent airspeed. As the airplane descends  into  ground effect, the following. effects will take  place:  (1) Because  of the reduced induced angle  of attack and change in lift distribution, a  smaller wing angle of attack will be required  to produce the same lift coefficient.  If a  constant pitch attitude is maintained as  ground effect is encountered, an increase in  lift coefficient will be incurred.  (2) The reduction in induced flow due to  ground effect causes a significant reduction  in induced drag but causes  no direct effect on  parasite drag.  As a result of the reduction  in induced drag, the thrust required at low  speeds  will be reduced.  (3) The reduction in downwash due to  ground effect will produce a change in longi-  tudinal stability and trim.  Generally, the  reduction in downwash at the horizontal  tail increases  the contribution to static longi-  tudinal stability.  In addition, the reduction  of downwash at the tail usually requires  a greater up elevator to trim the airplane at  a specific lift coefficient. For the conven-  tional airplane configuration, encountering  ground effect will  produce a nose-down  change in pitching moment. Of course, the  increase in stability and trim change associ-  ated with ground effect provide a critical re-  quirement of adequate longitudinal control  power for landing and takeoff.  (4) Due to the change in upwash, down-  wash, and tip vortices, there will be a change  in position error of the airspeed system, as-  sociated with ground effect.  In the majority  of cases,  ground effect will cause an increase  in the local pressure  at the static source and  produce a lower indication of airspeed and  altitude.  During the landing pha~c  of flight, the effect  of proximity to the ground plane must be  understood and appreciated. If the airplane  is brought into ground effect with a constant  angle of attack, the airplane will experience 