employ an offset flapping hinge to increase the 
control effectiveness by creating a centrifugal 
force couple when the rotor is tilted. This is 
shown in figure 4.35. 
The rotor is tilted by taking advantage of 
the gyroscopic effect of the rotor system. This 
effect causes a rotating mass which is disturbed 
about one axis to respond about another axis, 
as shown in figure 4.35. A forward tilt to the 
rotor is obtained by decreasing the pitch of the 
blade when at the starboard position and in- 
creasing the pitch of the blade when at the 
port position. The lateral dissymmetry of 
lift which results causes the rotor to tilt for- 
ward by the gyroscopic effect. 
A differential blade pitch change like this 
is called a cyclic.pitch change since each blade 
goes thr0ugh.a complete cycle of varying pitch 
angles as it completes one revolution of rota- 
tion about the hub. A cyclic pitch change is 
accomplished by the pilot by the use of the 
cyclic stick. The control arrangement is such 
that the rotor tilts in the same direction that 
the cyclic stick is deflected. 
A variation in rotor thrust is accomplished 
by increasing>sthe pitch of the blades simul- 
taneously or collectively. This type of control 
action is called “collective pitch” and is ac- 
complished by the use of the collective pitch 
stick. In operation, the cyclic stick is an- 
alogous to the control stick of an airplane, 
and the collective stick is analogous to the 
throttle ~of an ,airplane. 
There are several possibilities for longi- 
tudinal control of a tandem-rotor helicopter. 
A pitching moment can be produced by tilting 
both rotors by a cyclic pitch change in each 
rotor, by a differential collective pitch change 
that increases the thrust on one rotor and de- 
creases it on the other, or by some combination 
of these methods. The two basic methods are 
illustrated in figure 4.36. Obviously, a change 
in fuselage attitude must accompany the dif- 
ferential collective method of longitudinal 
control. 
NAVWEPS DO-80T-80 
STABILITY AND CONTROL 
Adequate pitch and lateral control effective- 
ness are easy to obtain in the typical helicopter 
and usually present no problems. The more 
usual problem is an excess of control effective- 
ness which results in an overly sensitive heli- 
copter. The helicopter control specifications 
attempt to assure satisfactory control charac- 
teristics by requiring adequate margins of con- 
trol travel and effectiveness without objection- 
able sensitivity. 
Directional control in a single rotor heli- 
copter is obtained by a tail rotor (antitorque 
rotor) since a conventional aerodynamic sur- 
face would not be effective at low speeds or 
hovering. The directional control require- 
ments of the tail rotor on a typical shaft-driven 
helicopter are quite demanding since it must 
counteract the engine torque being supplied to 
the main rotor as well as provide directional 
control. Being a rotor in every respect, the 
tail rotor requires some of the engine power to 
generate its control forces. Unfortunately, the 
maximum demands of the tail rotor occur at 
conditions when engine power is also in great 
demand. The most critical condition is while 
hovering at maximum gross weight. The tail 
rotor effectiveness is determined by the rotor 
characteristics and the distance the tail rotor 
is behind the c.g. The control specifications 
require the helicopter to be able to turn in the 
most critical direction at some specified rate 
while hovering at maximum gross weight in a 
specified wind condition. Also, it is required 
that the helicopter have sufficient directional 
control to fly sideways up to 30 knots, an 
important requirement for plane guard duties. 
The directional control requirements are 
easily met by a tip-driven helicopter since the 
directional control does not have to counter 
the engine torque. 
Directional control of a tandem-rotor heli- 
copter is accomplished by differential cyclic