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, 
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