NAVWEPS 00-ROT-80 
OPERATING STRENGTH LIMITitTIONS 
twisting deformation will be great enough to 
nullify the effect on aileron deflection and the 
aileron effectiveness will-be zero. Since speeds 
above this point create rolling moments op- 
posite to the direction controlled, this point 
is termed the “aileron reversal speed.” Oper- 
ation beyond the reversal speed would create 
an obvious control difficulty. Also, the ex- 
tremely large twisting moments which produce 
loss of aileron effectiveness create large twist- 
ing moments capable of structural damage. 
In order to prevent loss of aileron effective- 
ness at high airspeeds, the wing must have 
high torsional stiffness. This may be a feature 
difficult to accomplish in a wing of very thin 
section and may favor the use of inboard ailer- 
ons to reduce the twisted span length and 
effectively increase torsional stiffness. The use 
of spoilers for lateral control minimizes the 
twisting moments and alleviates the reversal 
problem. 
Divergcm is another phenomenon common 
to flight at high dynamic pressures. Like 
aileron reversal, it is an effect due to the inter- 
action of aerodynamic forces and elastic deflec- 
tions of the structure. However, it differs 
from aileron reversal in that it is a violent 
instability which produces immediate failure. 
Figure 5.5 illustrates the process of instability. 
If the surface is above the divergence speed, 
any disturbance precipitates this sequence. 
Any change in lift takes place at the aerody- 
namic center of the section. The change in 
lift ahead of the elastic axis produces a twist- 
ing moment and a consequent twisting deflec- 
tion. The change in angle of attack creates 
greater lift at the ac., greater twisting deflec- 
tion, more lift, etc., until failure occurs. 
At low flight speeds where the dynamic 
pressure is low, the relationship between aero- 
dynamic force buildup and torsional deflection 
is ‘stable. However, the change in lift per 
angle of attack is proportional to ‘vz but the 
structural torsional stiffness of the wing re- 
mains constant. This relationship implies 
that at some high speed, the aerodynamic force 
buildup may overpower the resisting torsional 
stiffness and “divergence” will occur. The 
divergence speed of the surfaces must be suf- 
ficiently high that the airplane does not en- 
counter this phenomenon within the normal 
operating envelope. Sweepback, short span, 
and high taper help raise the divergence speed. 
F/titter involves aerodynamic forces, inertia 
forces and the elastic properties of a surface. 
The distribution of mass and stiffness in a 
structure determine certain natural frequencies 
and modes of vibration. If the structure is sub- 
ject to a forcing frequency near these natural 
frequencies, a resonant condition can result 
with an unstable oscillation. The aircraft is 
subject to many aerodynamic excitations while 
in operation and the aerodynamic forces at 
various speeds have characteristic properties 
for rate of change of force and moment. The 
aerodynamic forces may interact with the 
structure in a fashion which may excite or 
negatively damp the natural modes of the 
structure and allow flutter. Flutter must not 
occur within the normal flight operating en- 
velope and the natural modes must be damped 
if possible or designed to occur beyond the 
limit speed. A’typical flutter mode is illus- 
‘trated in figure 5.5. 
Since the problem is one of high speed flight, 
it is generally desirable to have ‘very high 
natural frequencies and flutter speeds well 
above the normal operating speeds. Any 
change of stiffness or mass distribution will 
alter the modes and frequencies and thus allow 
a change in the flutter speeds. If the aircraft 
is not properly maintained and excessive play 
and flexibility exist, flutter could occur at 
flight speeds below the limit airspeed. 
Compres&ility pmblems may define the limit 
airspeed for an airplane in terms of Mach num-