the ship. When the airplane slows to sub- 
sonic, the shock wave travels out ahead of the 
airplane in a form which is somewhat spheri- 
cal. Because there are density variations 
through the shock wave, the shock wave 
moving ahead of the airplane can cause aber- 
rations in light waves and it may appear.to 
the pilot as if a large sheet of clear cellulose 
or plastic were in front of the airplane. In 
addition, the density variation and initial 
shape of the wave leaving the airplane may 
cause reflection of sunlight which would 
appear as a sudden, brilliant “flash” to the 
pilot. 
Of course, the wave released by decelerating 
to subsonic speed can travel out ahead of the 
airplane and traverse a populated area to cause 
a sonic boom. The initial direction of the 
released wave will be the flight path of the 
airplane at the instant it decelerates to sub- 
sonic speed. To be sure, the released wave 
should not be aimed in the direction of a popu- 
lated area, even if a considerable distance 
away. There are instances where a released 
wave has been of an audible magnitude as far 
as 30 to 40 miles ahead of the point of release. 
The released pressure wave will be of greatest 
intensity when created by a large, high drag 
configuration at low altitude. Since the wave 
intensity decreases rapidly with distance away 
from the source, the boom will be of strongest 
audibility near the point of release. 
It should become apparent that sonic booms 
are a byproduct of supersonic aviation and, 
with supersonic flight becoming more common- 
place, the problem is more perplexing. The 
potential of sonic booms is mostly of the 
audible nature and nuisance of the disturbance. 
The damage potential of the ordinary sonic 
boom is quite small and the principal effects 
are confined to structures which are extremely 
brittle, low strength, and have characteristic 
high residual stresses. In other words, only 
the extremes of pressure waves generated by 
airplanes in flight could possibly cause cracked 
plaster and window glass. Such materials 
NAVWEPS OD-ROT-80 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
are quite prone to sharp dynamic stresses and, 
when .superimposed on the high residual 
stresses common to the products and building 
construction, slight but insignificant damage 
may result. Actually, the most objectionable 
feature of the sonic boom is the audibility and 
the anxiety or apprehension caused by the 
sharp, loud noise which resembles a blast. 
The pressure jump through the shock waves 
in the immediate vicinity of the airplane is 
much greater than those common to the 
audible “booms” at ground level. Thus, 
airplanes in close formation at supersonic 
speeds may encounter considerable interference 
between airplanes. In addition, to eliminate 
even the most remote possibility of structural 
damage, a high speed airplane should not 
make a supersonic pass close to a large air- 
plane which may have low limit load factor 
and be prone to be easily disturbed or damaged 
by a strong pressure wave. 
HELICOPTER PROBLEMS 
The main difference between helicopter and 
an airplane is the main source of lift. The 
airplane derives its lift from a fixed airfoil 
surface while the helicopter derives lift from 
a rotating airfoil called the rotor. Hence, the 
aircraft will be classified as either “fixed- 
wing” or “rotating wing.” The word “heli- 
copter” is derived from the Greek words 
meaning “helical wing” or “rotating wing. ” 
Lift generation, by a “rotating wing” enables 
the helicopter to accomplish its unique mission 
of hovering motionless in the air, taking off 
and landing in a confined or restricted area, 
and autorotating to a safe landing following 
a power failure. Lift generation by “rotating 
wing’ ’ is also responsible for some of the 
unusual problems the helicopter can encounter. 
Since the helicopter problems are due to par- 
ticular nature of .the rotor aerodynamics, the