figure 4.36. The directional control require- 
ments are easily met in a tandem-rotor heli- 
copter because the engine torque from one 
rotor is opposed by the torque of the other 
rotor thereby eliminating one directional mo- 
ment. Of course, some net unbalance of torque 
may have to be overcome if the engine torque 
on the two rotors is different. 
When a tandem-rotor helicopter is rotated 
rapidly about one of the rotors rather than 
about the cg., the other rotor picks up 
“translational lift” as a result of the velocity 
due to rotation and an increase in rotor thrust 
results. This causes pitch-up or pitch-down 
depending on which rotor the helicopter is 
being rotated about. Rotation about the 
forward rotor, which is more common, re- 
sults in pitch-down. 
The overall stability of a helicopter results 
from the individual stability contributions of 
the various components just as in the case of 
the fixed-wing airplane. The stability con- 
tributions can be divided as follows: 
(1) Rotor 
(2) Fuselage 
(3) Stabilizers 
(4) Mechanical devices 
The destabilizing contribution of the fuselage 
and the stabilizing contribution of a stabilizing 
surface are similar in effect to an airplane and 
will not be discussed here. The principal 
stability characteristics that make the heli- 
copter different from an airplane are those of 
the rotor. 
Two types of stability are important in the 
rotor: (1) angle of attack stability and (2) 
velocity stability. In hovering flight the 
relative wind velocity, angle of attack, and 
lift on each blade of the rotor is the same. If 
the rotor is displaced through some angle, no 
changes in forces result. Therefore, the rotor 
has neutral angle of attack stability when 
hovering. However, in forward flight, an 
increase in rotor angle of attack increases the 
lift on the advancing blade more than on the 
NAVWEPS 00-801-80 
STABILITY AND CONTROL 
retreating blade since the relative wind veloci- 
ties are greater on the advancing blade. This 
lateral dissymmetry of lift causes the rotor to 
tilt back due to the gyroscopic effect of the 
rotor, further increasing the rotor angle of 
attack. Thus, the rotor is unstable with 
changes in angle of attack at forward flight 
speeds. Since the magnitude of the unstable 
moment is affected by the magnitude of 
the rotor thrust as well as the tilt of 
the thrust force, a greater instability exists 
for increases in angle of attack than for 
decreases in angle of attack. In addition, the 
instability is greater for increases in angle of 
attack when the rotor thrust also increases. 
If the rotor angle of attack is held constant 
and the rotor is given a translational velocity, 
a dissymmetry of lift results since the velocity 
of the advancing blade is increased while the 
velocity of the retreating blade is decreased. 
This dissymmetry of lift causes the rotor to 
tilt in a direction to oppose the change in 
velocity due to the gyroscopic effect of the 
rotor. Hence, the rotor has velocity stability. 
A hovering helicopter exhibits some degree 
of apparent stability by virtue of its velocity 
stability although it has neutral angle of 
attack stability. This type of hovering sta- 
bility is analogous to the apparent lateral- 
directional stability an airplane exhibits due 
to dihedral effect. Additional hovering sta- 
bility can be obtained by the use of mechanical 
stabilizers such as th,e Bell stabilizer bar, by 
the use of offset flapping hinges, or by syn- 
thetic or artificial stabilization devices. 
The total static stability of a helicopter is 
determined by combining the stability con- 
tributions of all the components. The usual 
result for a typical helicopter is instability 
with angle of attack and a variable velocity 
stability which becomes neutral or unstable 
at high speeds. Of course, the helicopter 
could be made stable with angle of attack by 
providing a large enough horizontal stabilizer.