(4) Sweepback contributes to lateral sta- 
bility in rhe same sense as dihedral. When 
the swept wing aircraft is placed in a side- 
slip, the wing into the wind experiences an 
increase in lift since the sweep is less and 
the wing away from the wind produces less 
lift since rhe sweep is greater. As shown in 
figure 3.15, the swept wing aircraft in a 
sideslip experiences lift changes and a sub- 
sequent rolling moment which tends to 
right the aircraft. This lateral stability 
conrribution depends on the sweepback and 
the lift coefficient of the wing. A highly 
swept wing operating at high lift coeflicient 
usually experiences such an excess of this 
lateral stability contribution that adequate 
controllability may be a significant problem. 
As shown, the swept wing has certain im- 
portant advantages. However, the use of 
sweepback produces certain inevitable disad- 
vantages which are important from the stand- 
point of both airplane design and flight oper- 
ations. The most important of these disad- 
vantages are as follows: 
(1) When sweepback is combined with 
taper there is an extremely powerful tendency 
for the wing to stall tip first. This pattern 
of stall is very undesirable since there would 
be little stall warning, a serious reduction 
in lateral control effectiveness, and the for- 
ward shift of the center of pressure would 
contribute to a nose up moment (“pitch up” 
or “stick force lightening”). Taper has its 
own effect of producing higher local lift 
coefhcients toward the tip and one of the 
effects of sweepback is very similar. All 
outboard wing sections are affected by the 
upwash of the preceding inboard sections 
and the lift distribution resulting from sweep- 
back alone is similar to that of high taper. 
An additional effect is the tendency to 
develop a strong spanwise flow of the bound- 
ary layer toward the tip when the wing is at 
high lift coefficients. This spanwise flow 
produces a relatively low energy boundary 
layer near the tip which can be easily sep- 
NAVWEPS 00-801-80 
HIGH SPEED AERODYNAMICS 
arated. The combined effect of taper and 
sweep present a considerable problem of tip 
stall and this is illustrated by the flow pat- 
terns of figure 3.16. Design for high speed 
performance may dictate high sweepback, 
while structural efficiency may demand a 
highly tapered planform. When such is the 
case, the wing may require extensive aero- 
dynamic tailoring to provide a suitable stall 
pattern and a lift distribution at cruise condi- 
tion which reduces drag due to lift. Wash- 
out of the tip, variation of section camber 
throughout span, flow fences, slats, leading 
edge extension, etc., are typical devices used 
to modify the stall pattern and minimize 
drag due to lift at cruise condition. 
(2) As shown by the lift curve of figure 
3.15 the use of sweepback will reduce the lift 
curve slope and the subsonic maximum lift 
coefficient. It is important to note this 
case is definitely subsonic since sweepback 
may be used to improve the transonic ma- 
neuvering capability. Various sweep angles 
applied to wings of moderate aspect ratio 
produce these approximate effects on the 
subsonic lift characteristics: 
sweep Angle (A): 
O”................................. 0 
w................................ 4 
300. 14 
450.......... 30 
M)Q................................ yl 
The reduction of the low speed maximum 
lift coefficient (which is in addition to that 
lost due to tip stall) has very important 
implications in design. If wing loading is 
not reduced, stall speeds increase and sub- 
sonic maneuverability decreases. On the 
other hand, if wing loading is reduced, the 
increase in wing surface area may reduce 
the anticipated benefit of sweepback in the 
transonic flight regime. Since the require- 
ments of performance predominate, certain 
increases of stall speeds, takeoff speeds, 
251