INTERFERENCE BETWEEN AIRPLANES IN FLIGHT

During formation flying and inflight refuel­ing, airplanes in proximity to one another will produce a mutual interference of the flow pat­terns 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.

One example of interference between air­planes 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 air­planes and would furnish a boundary of flow across which there would be no lateral com­ponents 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 re­duce 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 air­planes 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 ex­tent of induced flow. This implies that the interference vr ’ ‘ e greatest when the tips are very close aixa the airplanes are operating at high lift coefficients. An interesting ramifi­cation 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 en­countered 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 consid­erable turbulence for any closely following air­plane. This wake can prove troublesome if air­planes taking off and landing are not provided adequate separation. The rolled-up vortex sheet will be strongest when the preceding air­planes is large, high gross weight, and operat­ing at high lift coefficients. At times this tur­bulence may be falsely attributed to propwash or jet wash.

Another important form of direct inter­ference is common when the two airplanes are in a trail position and stepped down. As shown in figure 6.10, the single airplane in flight de­velops upwash ahead of the wing and down – wash behind and any restriction accorded the flow can alter the distribution and magnitude of the upwash and downwash. When the trailing airplane is in dose proximity aft and below the leading airplane a mutual interference takes place between the two airplanes. The leading airplane above will experience an effect which would be somewhat similar to encountering ground effect, i. e., a reduction in induced drag, a reduction in downwash at the tail, and a change in pitching moment nose down. The trailing airplane below will experience an effect which is generally the opposite of the airplane above. In other words* the airplane below will experience an increase in induced drag, an increase in downwash at the tail, and a change in pitching moment nose up. Thus, when the airplanes are in close proximity, a definite collision possibility exists because of the trim change experienced by each airplane. The magnitude of the trim change is greatest when the airplanes are operating at high lift coefficients, e. g., low speed flight, and when the airplanes are in close proximity.

In formation flying, this sort of interference must be appreciated and anticipated. In cross­ing under another airplane, care must be taken to anticipate the trim change and adequate clearance must be maintained, other­wise a collision may result. The pilot of the leading aircraft will know of the presence of the trailing airplane by the trim change experienced. Obviously, some anticipation is necessary and adequate separation is necessary to prevent a disturbing magnitude of the trim change. In a close diamond formation the leader will be able to ‘‘feel” the presence of the slot man even though the airplane is not within view. Obviously, the slot man will have a difficult job during formation maneuvers because of the unstable trim changes and greater power changes required to hold position.

A common collision problem is the case of an airplane with a malfunctioning landing gear. If another airplane is called to inspect the malfunctioning landing gear, great care must be taken to maintain adequate separation and preserve orientation. Many instances such as this have resulted in a collision when the pilot of the trailing airplane became dis­oriented and did not maintain adequate sepa­ration.

During inflight refueling, essentially the same problems of interference exist. As the receiver approaches the tanker from behind and below, the receiver will encounter the downwash from the tanker and require a slight, gradual increase in power and pitch attitude to continue approach to the receiving position. While the receiver may not be visible to the pilot of the tanker, he will anticipate the receiver coming into position by the slight reduction in power required and nose down change in pitching moment. Ade­quate clearance and1 proper position must be maintained by the pilot of the receiver for a collision possibility is enhanced by the rela­tive positions of the airplanes. A hazardous condition exists if the pilot of the receiver has excessive speed and runs under the tanker in close proximity. ‘ The trim change expe­rienced by both airplanes may be large and unexpected and it may be difficult to avoid a collision. ‘

In addition to the forms of interference previously mentioned, there exists the possi­bility of strong interference between airplanes in supersonic flight. In this case, the shock waves from one airplane may strongly affect the pressure distribution and rolling, yawing, and pitching moments of an adjacent air­plane. It is difficult to express general rela­tionships of the effect except that magnitude of the effects will be greatest when in close proximity at low altitude and high q. General­ly, the trailing airplane will be most affected.