NAWEPS OD-ROT-RO 
HIGH SPEEO AERODYNAMICS 
general effect and contributes less damping to 
longitudinal pitching oscillations. These ef- 
fects can become so significant at high Mach 
numbers that the aircraft might require com- 
plete synthetic stabilization. 
PLANFORM EFFECTS. The development 
of surfaces for high speed involves considera- 
tion of many items in addition to the airfoil 
sections. Taper, aspect ratio, and sweepback 
can produce major effects on the aerodynamic 
characteristics of a surface in high speed flight. 
Sweepback produces an unusual effect on the 
high speed characteristics of a surface and has 
basis in a very fundamental concept of aero- 
dynamics. A grossly simplified method of 
visualizing the effect of sweepback is shown in 
figure 3.14. The swept wing shown has the 
streamwise velocity broken down to a com- 
ponent of velocity perpendicular to the leading 
edge and a component parallel to the leading 
edge. The component of speed perpendicular 
to the leading edge is less than the free.stream 
speed (by the cosine of the sweep angle) and 
it is this velocity component which determines 
the magnitude of the pressure distribution. 
The component of speed parallel to the lead- 
ing edge could be visualized as moving across 
constant sections and; in doing so, does not 
contribute to the pressure distribution on the 
swept wing. Hence, sweep of a surface pro- 
duces a beneficial e&ct ‘in high speed flight 
since higher flight speeds may be obtained be- 
fore components of speed perpendicular to the 
leading edge produce critical conditions on the 
wing. This is one of the most important ad- 
vantage of sweep since there is an increase in 
critical Mach number, force divergence Mach 
number, and the Mach number at which the 
drag rise will peak. In other words, sweep will 
delay the onset of compressibility effects. 
Generally, the effect of wing sweep will 
apply to either sweep back or sweep forward. 
While the swept forward wing has been used 
1 in rare instances, the aeroelastic instability of 
such a wing creates such a problem that sweep 
back is more practical for ordinary applica- 
tions. 
In addition to the delay of the onset of com- 
pressibility effects, sweepback will reduce the 
magnitude of the changes in force coefficients 
due to compressibility. Since’ the component 
of velocity perpendicular to the leading edge is 
less than the free stream velocity, the magni- 
tude of all pressure forces on the wing will be 
reduced (approximately by the square of the 
cosine of the sweep angle). Since compressi- 
bility force divergence occurs due to changes in 
pressure distribution, the use of sweepback will 
“soften” the force divergence. This effect is 
illustrated by the graph of figure 3.14 which 
shows the typical variation of drag coeiIicient 
with Mach number for various sweepback 
angles. The straight wing shown begins drag 
rise at M=O.lO, reaches a peak near M=l.O, 
and begins a continual drop past M= 1.0. Note 
that the use of sweepback then deh+y~ the drag 
rise to some~ higher Mach number and wdms 
the magnitude of the drag rise. 
In view of the preceding discussion, sweep- 
back will have the following principal ad- 
vantages : 
(1) Sweepback will delay the onset of all 
compressibility effects. Critical Mach num- 
ber and force divergence Mach number will 
increase since the velocity component affect- 
ing the pressure distribution is less than the 
free stream velocity. Also, the peak of drag 
rise is delayed to some higher supersonic 
speed-approximately the speed which pro- 
duces sonic flow perpendicular to the leading 
edge. Various sweeps applied to wings of 
.moderate aspect ratio will produce these 
approximate effects in transonic flight: 
Sweep angle(k) 
Revised Jaanuar~ 1965 
226