design features which result in a large positive 
dihedral effect are high sweepback, high wing 
position, or large, high vertical tail, When 
the inertia axis is inclined below the aero- 
dynamic axis at low or negative angles of 
attack, the roll induced inertia couple results 
in proverse yaw. 
Depending on the flight condition where the 
roll coupling problem exists, four basic types 
of airplane behavior are possible: 
(1) Coupled motion stable but unacceptabk. 
In this case the motion is stable but proves 
unacceptable because of poor damping of the 
motion. Poor damping would make it 
dificult to track a target or the initial am- 
plitudes of the motion may be great enough 
to cause structural failure of loss of control. 
(2) Coupled motion stable and acceptable. 
The behavior of the airplane is stable and 
adequately damped to allow acceptable 
target tracking. The amplitudes of motion 
are too slight to result in structural failure 
or loss of control. 
(3) Coupled motion divergent and unacceptable. 
The rate of divergence is too rapid for the 
pilot to recognize the condition and recover 
prior’ to structural failure or complete loss 
of control. 
(4) Coupled. motion divergent but acceptable. 
For such a condition the rate of divergence 
is quite slow and considerable roll displace- 
ment is necessary to produce a critical ampli- 
tude. The condition can be recognized 
easily in time to take corrective action. 
There are available various means to cope 
with the problem of roll coupling. The fol- 
lowing items can be applied to control the 
problem of roll coupling: 
(ZZ) Increase directional stability. 
(b) Reduce dihedral effect. 
(c) M’ h 1‘ mnmze t e mc mation of the inertia 
axis at normal flight conditions. 
(d) Reduce undesirable aerodynamic 
coupling. 
(e) Limit roll rate, roll duration, and 
angle of attack or load factor for performing 
rolling maneuvers. 
NAVWEPS DD-EOT-80 
STABMTY AND CONTROL 
The first four items can be effected,only during 
design or by design changes. Some roll per- 
formance restriction is inevitable since all of 
the desirable characteristics are difficult to 
obtain without serious compromise elsewhere 
in the airplane design. The typical high 
speed airplane will have some sort of roll pet- 
formance limitation provided by flight restric- 
tions or automatic control devices to prevent 
reaching some critical condition from which 
recovery is impossible. Any roll restriction 
provided an airplane must be regarded as a 
principal flight operating limitation since the 
more severe motions can cause complete loss 
of control and structural failure. 
HELICOPTER STABILITY AND CONTROL 
In discussing many of the problems of sta- 
bility and control that occur in high speed 
airplanes, one might be prone to believe that 
the slow flying helicopter does not have any 
such problems. Unfortunately, this is not 
the case. Flying qualities that would be con- 
sidered totally unsatisfactory by fixed-wing 
standards ate normal for helicopters. Heli- 
copter pilots are living evidence that an un- 
stable aircraft ca. k‘.: controlled. Also, they 
are evidence ~a. control without stability 
requires constant attention and results in con- 
siderable pilot fatigue. 
“Inertia coupling” problems are relatively 
new to fixed-wing aircraft but a similar effect 
in the helicopter rotor has resulted in some 
of its most important characteristics. This 
aerodynamic-dynamic coupling effect is so im- 
portant that it must be considered in discussing 
both stability and control. The helicopter 
derives both longitudinal and lateral control 
by tilting the main rotor and thus producing 
a pltchmg or rolling moment as indicated in 
figure 4.35. The magnitude of the rotor thrust 
the angle of tilt, and the height of the rotor 
hub above the c.g. determine the control 
moment produced. It should be noted that 
low control effectiveness would result when 
the rotor thrust is low. Some helicopters 
319