The range sf the reciprocating powered air- 
plane can be augmented by the use of ground 
effect. When the airplane is close to the 
ground or water surface the reduction of in- 
duced drag increases the maximum lift-drag 
ratio and causes a corresponding increase in 
range. Of course, the airplane must be quite 
close to the surface to obtain a noticeable in- 
crease in (L/D),., and range. The difficulty in 
holding the airplane at the precise altitude 
without contacting the ground or water will 
preclude the use of ground effect during ordi- 
nary flying operations. The use of ground 
effect to extend range should be reserved as 
a final measure in case of emergency. Because 
of the very detrimental effect of low altitude 
on the range of the turbojet, ground effect will 
not be of a particular advantage in an attempt 
to augment range. 
The most outstanding examples of the use 
of ground effect are shown in the cases of multi- 
engine airplanes with some engines inoperative. 
When the power loss is quite severe, the air- 
plane may not be capable of sustaining altitude 
and will descend. As ground effect is en- 
countered, the reduced power required may 
allow the airplane to sustain flight at extremely 
low altitude with the remaining powerplants 
functioning. In ground effect, the recipro- 
cating powered airplane will encounter a 
greater (L/D),, which occurs at a lower air- 
speed and power required and the increase in 
range may be quite important during emer- 
gency conditions. 
INTERFERENCE BETWEEN AIRPLANES IN 
FLIGHT 
During formation flying and inflight refuel- 
ing, airplanes in proximity to one another will 
produce a mutual interference of the flow pat- 
terns and alter the aerodynamic characteristics 
of each airplane. The principal effects of this 
interference must be appreciated since certain 
factors due to the mutual interference may 
enhance the possibility of a collision. 
NAVWEPS D&ROT-R0 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC ‘PROBLEMS OF FLYING 
One example of interference between air- 
planes in flight is shown first in figure 6.10 with 
the effect of lateral separation of two airplanes 
flying in line abreast. A plane of symmetry 
would exist halfway between two identical air- 
planes and would furnish a boundary of flow 
across which there would be no lateral com- 
ponents of flow. As the two airplane wing 
tips are in proximity, the effect is to reduce the 
strength of the tip or trailing vortices and re- 
duce the induced velocities in the vicinity of 
wing tip. Thus, each airplane will experience 
a local increase in the lift distribution as the 
tip vortices are reduced and a rolling moment is 
developed which tends to roll each airplane 
away from the other. This disturbance may 
provide the possibility of collision if other air- 
planes are in the vicinity and there is delay in 
control correction or overcontrol. If the wing 
tips are displaced in a fore-and-aft direction, 
the same effect exists but generally it is of a 
lower magnitude. 
The magnitude of the interference effect due 
to lateral separation of the wing tips depends 
on the proximity of the wi.ig tips and the ex- 
tent of induced Pov;. This implies that the 
interference v-r 1 e grealest when the tips 
are very close AL-L the airplanes are operating 
at high lift coefficients. An interesting ramifi- 
cation of this effect is that several airplanes in 
line abreast with the wing tips quite close will 
experience a reduction in induced drag. 
An indirect form of interference can be en- 
countered from the vortex system created by a 
preceding airplane along the intended flight 
path. The vortex sheet rolls up a considerable 
distance behind an airplane and creates consid- 
erable turbulence for any closely following air- 
plane. This wake can prove troublesome if air- 
planes taking off and landing are not provided 
adequate separation. The rolled-up vortex 
sheet will be strongest when the preceding air-