NAVWEPS D&ROT-R0  APPLICATION OF AERODYNAMICS  TO SPECIFIC  ‘ PROBLEMS OF FLYING  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.  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 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.  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-  planes is large, high gross weight, and operat-  ing at high lift coefhcients.  At times this tur-  bulence may be falsely attributed to propwash  or jetwash.  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.  INTERFERENCE BETWEEN AIRPLANES IN  FLIGHT  383 