Another important form of direct inter- 
ference 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 de- 
velops 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 close proximity aft and below the 
leading airplane a mutual interference takes 
place betweetrthe two airplanes. The leading 
airplane above will experience an effect which 
would be somewhat similar to encountermg 
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 cross- 
ing under another airplane, care must be 
taken to anticipate the trim change and 
adequate clearance must be maintained, other- 
wise 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 
NAVWEPS OO-ROT-80 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
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”airpIane 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 pilo: of the trailing airplane became dis- 
oriented and did not maintain adequate sepa- 
ration. 
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 reduttion in power required and 
nose down changein pitching moment. Ade- 
quate clearance and, proper position must be 
maintained by the pilot of the receiver for a 
collision possibility is enhanced by the rela- 
tive 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 expe- 
rienced by both airphines 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 possi- 
bility 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 air- 
Pl ane. It is difficult to express general rela-