given to the airplane size. For geometrically 
similar airplanes, a certain deflection of the 
I!!. ailerons will produce a fixed value of zlr mde- 
pendent of the airplane size. However, the 
roll rate of the geometrically similar airplanes 
at a given speed will vary inversely with the 
span, b. 
If 
Pb - 
~-constant 
p=(constant) 7 
( ) 
Thus, the smaller airplane will have an ad- 
vantage in roll rate or in time to accelerate 
through a prescribed angle of roll. For ex- 
ample, a one-half scale airplane will develop 
twice the rate of roll of the full scale airplane. 
This relationship points to the favor of the 
small, short span airplane for achieving high 
roll performance. 
An important variable affecting the rate of 
roll is the true airspeed or flight velocity, V. 
If a certain deflection of the ailerons creates a 
Pb specific value of -7 the rate of roll varies 2V 
directly with the true airspeed. Thus, if the 
roll helix angle is held constant, the rate of 
roll at a particular true airspeed will not be 
affected by altitude. The linear variation of 
roll rate with airspeed points out the fact that 
high roll rates will require high airspeeds. 
The low roll rates at low airspeeds are simply 
a consequence of the low flight speed and this 
condition may provide a critical lateral con- 
trol requirement for satisfactory handling 
qualities. 
Figure 4.30 illustrates the typical rolling 
paformance of a low speed airplane. When 
the ailerons are at full deflection, the maximum 
roll helix angle is obtained. The rate of roll 
increases linearly with speed until the control 
forces increase to limit of pilot effort and full 
control deflection cannot be maintained. Past 
NAVWEPS OO-BOT-BO 
STABILIJY AND CONTROL 
the critical speed, with some limited amount 
of force applied by the pilot (usually the limit 
of lateral force is assumed to be 30 lbs.), the 
Pb ailerons cannot be held at full deflection, ~~ 
drops, and rate of roll decreases. In this exam- 
ple, the rolling performance at high speeds is 
limited by the ability of the pilot to maintain 
full deflection of the controls. In an effort to 
reduce the aileron hinge moments and control 
forces, extensive application is made of aerody- 
namic balance and various tab devices. How- 
ever, 100 percent aerodynamic balance is not 
always feasible or practical but a sufficient 
Pb value of - must be maintained at high speeds. ZV 
Rather than developing an extensive weight 
lifting program mandatory for all Naval 
Aviators, mechanical assistance in lateral con- 
trol can be provided. If a power boost is 
provided for the lateral control system, the 
rolling performance of the airplane may be 
extended to higher speeds since pilot effort 
will not be a limiting factor. The effect of a 
power boost is denoted by the dashed line 
extensions of figure 4.30. A full powered, 
irreversible lateral control system is common 
for high speed airplanes. In the power oper- 
ated system there is no immediate limit to the 
deflection of the control surfaces and none of 
the aberrations in hinge moments due to com- 
pressibility are fed back to the pilot. Control 
forces are provided by the stick centering 
lateral bungee or spring. 
A problem particular to the high speed is 
due to the interaction of aerodynamic forces 
and the elastic deflections of the wing in 
torsion. The deflection of ailerons creates 
twisting moments on the wing which can cause 
significant torsional deflections of the wing. 
At the low dynamic pressures of low flight 
speeds, the twisting moments and twisting 
deflections are too small to be of importance. 
However, at high dynamic pressures, the 
deflection of an aileron creates significant 
303