NAVWEPS 00-ROT-80 
HIGH SPEED AERODYNAMICS 
and landing speeds usually will be accepted. 
While the reduction of lift curve slope may 
be an advantage for gust considerations, 
the reduced sensitivity to changes in angle 
of attack has certain undesirable effects in 
subsonic flight. The reduced wing lift 
curve slope tends to increase maximum lift 
angles of attack and complicate the problem 
of landing gear design and cockpit visi- 
bility. Also, the lower lift curve slope 
would reduce the contribution to stability 
of a given tail surface area. 
(3) The use of sweepback will reduce 
the effectiveness of trailing edge control 
surfaces and high lift devices. A typical 
example of this effect is the application of 
a single slotted flap over the inboard 60 
percent span to both a straight wing and a 
wing with 35” sweepback. The flap applied 
to the straight wing produces an increase 
in maximum lift coefficient of approxi- 
mately 50 percent. The same type flap 
applied to the swept wing produces an 
increase in maximum lift coefficient of 
approximately 20 percent. To produce some 
reasonable maximum lift coefficient one a 
swept wing may require unsweeping the 
flap hinge line, application of leading edge 
high lift devices such as slots or slats, and 
possibly boundary layer control. 
(4) As described previously, sweepback 
contributes to lateral stability by producing 
stable rolling moments with sideslip. The 
lateral stability contribution of sweepback 
varies with the amount of wing sweepback 
and wing lift coefficient-large sweepback 
and high lift coefficients producing large 
contribution to lateral stability. While sta- 
bility is desirable, any excess of stability will 
reduce controllability. For the majority of 
airplane configurations, high lateral sta- 
bility is neither necessary nor desirable, but 
adequate control in roll is absolutely neces- 
sary for good flying qualities. An excess of 
lateral stability from sweepback can aggra- 
vate “Dutch roll” problems and produce 
marginal control during crosswind takeoff 
and landing where the aircraft must move in 
a controlled sideslip. Therefore, it is not 
unusual to find swept wing aircraft with 
negative dihedral and lateral control de- 
vices designed principally to meet cross wind 
takeoff and landing requirements. 
(5) The structural complexity and aero- 
elastic problems created by sweepback are of 
great importance. First, there is the effect 
shown in figure 3.17 that swept wing has a 
greater structural span than a straight wing 
of the same area and aspect ratio. This effect 
increases wing structural weight since 
greater bending and shear material must be 
distributed in the wing to produce the same 
design strength. An additional problem is 
created near the wing root and “carry- 
through” structure due to the large twisting 
loads and the tendency of the bending stress 
distribution to concentrate toward the trail- 
ing edge. Also shown in figure 3.17 is the 
influence of wing deflection on the spanwise 
lift distribution. Wing bending produces 
tip rotation which tends to unload the tip 
and move the center of pressure forward. 
Thus, the same effect which tends to allay 
divergence can make an undesirable contri- 
bution to longitudinal stability. 
EFFECT OF ASPECT RATIO AND TIP 
SHAPE. In addition to wing sweep, plan- 
form properties such as aspect ratio, and tip 
shape, can produce significant effects on the 
aerodynamic characteristics at high speeds. 
There is no particular effect of aspect ratio on 
critical Mach number at high or medium 
aspect ratios. The aspect ratio must be less 
than four or five to produce any apparent 
change in critical Mach number. This effect 
is shown for a typical 9 percent thick sym- 
metrical airfoil in the graph of figure 3.18. 
Note that very low aspect ratios are required 
to cause a significant increase in critical Mach 
number. Very low aspect ratios create the