NAVWEPS 00-801-80 
STABILITY AND CONTROL 
hovering and large trim moments upon entering 
autorotation will limit the stabilizer size to 
a relatively small surface. Usually the hori- 
zontal stabilizer is used only to give the fuse- 
lage the desired moment characteristics. 
The angle of attack stability of a tandem- 
rotor helicopter is adversely affected by the 
downwash from the forward rotor reducing 
the angle of attack and thrust of the rear 
rotor. This reduction of thrust behind the 
cg. causes the helicopter to pitch up to a 
higher angle of attack, thereby adding to the 
angle of attack instability. 
As in the airplane, several oscillatory modes 
of motion are characteristic of the dynamic 
stability of a helicopter. The phugoid is the 
most troublesome for the helicopter. The 
phugoid mode is unstable in the majority of 
helicopters which operate without the assist- 
ance of artificial stabilization devices. The 
dynamic instability of the helicopter is given 
evidence by the flying qualities specification for 
helicopters. These specifications essentially 
limit the rate of divergence of the dynamic oscil- 
lations for the ordinary helicopter. Although 
this dynamic instability can be controlled, it 
requires constant attention by the pilot and 
results in pilot fatigue. The elimination of 
the dynamic instability would contribute 
greatly to improving the flying qualities of 
the helicopter. 
This dynamic instability characteristic is 
particularly important if the helicopter is 
expected to be used for instrument flight in 
all-weather operations. In fact, a seriously 
divergent phugoid mode would make instru- 
ment flight impractical. For this reason, the 
flying qualities specification requires that 
helicopters with an instrument capability 
exhibit varying degrees of stability or insta- 
bility depending on the period of the oscilla- 
tion. Long period oscillations (over 20 sec- 
onds) must not double in amplitude in less 
than 15 seconds whereas short period oscil- 
lations (under 10 seconds) must damp to half 
amplitude in two cycles. 
The only immediate solution for the dynamic 
instability is an attitude stabilization system 
which is essentially an autopilot. Other 
solutions to the dynamic instability problem 
involve mechanical, aerodynamic, or elec- 
tronic control feedback of pitch attitude, 
pitch velocity, normal acceleration, or angle 
of attack. The improvement of the heli- 
copter’s stability is mandatory to fully utilize 
its unique capability. As more of the heli- 
copter problems are analyzed and studied, the 
flying qualities of helicopters wiI1 improve 
and be comparable to the fixed wing aircraft. 
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