twisting deflections which reduce the effec- 
tiveness of the aileron, e.g., downward deflec- 
tion of an aileron creates a nose down twist of 
the wing which reduces the rolling moment 
due to aileron deflection. At very high speeds, 
the torsional deflection of the wing may be 
so great than a rolling moment is created 
opposite to the direction controlled and “aile- 
ron reversal” occurs. Prior to the speed for 
aileron reversal, a serious loss of roll helix 
angle may be encountered. The effect of this 
aeroelastic phenomenon on rolling perform- 
ance is illustrated in figure 4.30. 
To counter the undesirable inceractiuo be- 
tween aerodynamic forces and wing torsional 
deflections, the trailing edge ailerons may be 
moved inboard to reduce the portion of the 
span subjected to twisting moments. Of 
course, the short span, highly tapered wing 
planform is favorable for providing relatively 
high stiffness. In addition, various configura- 
tions of spoilers may be capabIe of producing 
the required rolling performance without the 
development of large twisting moments. 
CRITICAL REQUIREMENTS, The critical 
conditions for requiring adequate lateral con- 
trol power may occur at either high speed or 
low speed depending on the airplane configura- 
tion and intended use. In transonic and super- 
sonic flight, compressibility effects tend to 
reduce the effectiveness of lateral control de- 
vices to produce required roll helix angles. 
These effects are most significant when com- 
bined with a loss of control effectiveness due to 
aeroelastic effects. Airplanes designed for 
high speed flight must maintain suflicient 
lateral control effectiveness at the design dive 
speed and this is usually the predominating 
requirement. 
During landing and takeoff, the airplane 
must have adequate lateral control power to 
contend with the ordinary conditions of flight. 
The lateral controls must be capable of achiev- 
ing required roll helix angles and acceleration 
through prescribed roll dispIacements. Also, 
the airplane must be capable of being con- 
NAVWEPS OO-UOT-80 
STABILITY AND CONTROL 
trolled in a sideslip to accomplish crosswind 
takeoff and landing. The lateral control dur- 
ing crosswind takeoff and landing is a par- 
ticular problem when the dihedral effect is 
high. Since the sweepback contributes a large 
dihedral effect at high lift coefficients, the 
problem is most important for the airplane 
with considerable sweepback. The limiting 
crosswind components must be given due re- 
spect especially when the airplane is at low 
gross weight. At low gross weight the speci- 
fied takeoff and landing speeds will be low and 
the controlled angle of sideslip will be largest 
for a given crosswind velocity. 
MISCELLANEOUS STABILITY PROBLEMS 
There are several general problems of flying 
which involve certain principles of stability as 
well as specific areas of longitudinal, direc- 
tional and lateral stability. Various condi- 
tions of flight will exist in which certain 
problems of stability (or instability) are un- 
avoidable for some reason or another. any 
of the following items deserve consideration 
because of the possible unsafe condition of flight 
and the contribution to an aircraft accident. 
LANDING GEAR CONFIGURATIONS 
There are three general configurations for the 
aircraft landing gear: the tricycle, bicycle, and 
“conventional” tail wheel arrangement. At 
low rolling speeds where the airplane aerody- 
namic forces are negligible, the “control-fixed” 
static stability of each of these configurations 
is determined by the side force characteristics 
of the tires and is not a significant problem. 
The instability which allows ground loops 
in an aircraft with a conventional tail wheel 
landing gear is quite basic and can be appre- 
ciated from the illustration of figure 4.31. Cen- 
trifugal force produced by a turn must be 
balanced and the aircraft placed in equilibrium.