NAVWEPS 00-801-80 
HIGH SPEED AERODYNAMICS 
(1) The wing lift curve slope is reduced 
for a given aspect ratio. This is illustrated 
by the lift curve comparison of figure 3.15 
for the straight and swept wing. Any 
reduction of lift curve slope implies the 
wing is less sensitive to changes in angle of 
attack. This is a beneficial effect only when 
the effect of gusts and turbulence is con- 
sidered. Since the swept wing has the 
lower lift curve slope it will be less sensitive 
to gusts and experience less “bump” due 
to gust for a given aspect ratio and wing 
loading. This is a consideration particular 
to the aircraft whose structural design shows 
a predominating effect of the gust load 
spectrum, e.g., transport, cargo, and patrol 
types. 
(2) “Divergence” of a surface is an aero- 
elastic problem which can occur at high 
dynamic pressures. Combined bending and 
twisting deflections interact with aerody- 
namic forces to produce sudden failure of 
the surface at high speeds. Sweep forward 
will aggravate this situation by “leading” 
the wing into the windstream and tends to 
lower the divergence speed. On the other 
hand, sweepback tends to stabilize the 
surface by “trailing” and tends to raise the 
divergence speed. By this tendency, sweep- 
back may be beneficial in preventing di- 
vergence within the anticipated speed range. 
(3) Sweepback contributes slightly to the 
static directional-or weathercock-stability 
of an aircraft. This effect may be appre- 
ciated by inspection of hgure 3.13 which 
shows the swept wing in a yaw or sideslip. 
The wing into the wind has less sweep and 
a slight increase in drag; the wing away 
from the wind has more sweep and less 
drag. The net effect of these force changes is 
to produce a yawing moment tending to 
retarn the nose into the relative wind. 
This directional stability contribution is 
usually small and of importance in tailless 
aircraft only. 
(2) Sweepback will reduce the magnitude 
of change in the aerodynamic force coeffi- 
cients due to compressibility. Any change 
in drag, lift, or moment coefbcients will be 
reduced by the use of sweepback. Various 
sweep angles applied to wings of moderate 
aspect ratio will produce these approximate 
effects in transonic flight. 
00 ............................... 0 
150. ............. ................ 5 
M” .............................. 15 
45’.............................. 35 
600 .............................. 60 
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These advantages of drag reduction and preser- 
vation of the transonic maximum lift coefficient 
are illustrated in figure 3.14. 
Thus, the use of sweepback on a transonic 
aircraft will reduce and delay the drag rise and 
preserve the maneuverability of the aircraft 
in transonic flight. It should be noted that a 
small amount of sweepback produces very 
little benefit. If sweepback is to be used at all, 
at least 30’ to 33’ must be used to produce any 
significant benefit. Also note from figure 3.14 
that the amount of sweepback required to 
d&y drag rise in supersonic flight is very large, 
e.g., more than 60° necessary at M=2.0. By 
comparison of the drag curves at high Mach 
numbers it will be appreciated that extremely 
high (and possibly impractical) sweepback is 
necessary to delay drag rise and that the lowest 
drag is abtained with zero sweepback. There- 
fore, the planform of a wing designed to operate 
continuously at high Mach numbers will tend 
to be very thin, low aspect ratio, and unswept. 
An immediate conclusion is that sweepback is 
a device of greatest application in the regime of 
transonic flight. 
A few of the less significant advantages of 
sweepback are as follows: 
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Revised January l%S