Stability and Control

CHAPTER 3

HELICOPTER AERODYNAMICS WORKBOOK

STABILITY AND CONTROL

From our discussion so far, it may seem that in a hover, all forces balance out, and once a

stable position has been set (collective setting to produce enough power, cyclic position to

maintain a position over the ground, and enough antitorque compensation to offset torque effect),

no further control inputs are required to maintain a hover. It will become readily apparent as you

embark on a mission to hover this is not the case. Helicopters are inherently unstable in a hover,

response to control inputs are not immediate, and the rotor systems produce their own gusty air,

all of which must be corrected for constantly by the pilot.

CENTER OF GRAVITY

Because the fuselage of the aircraft is suspended beneath the rotor system, it reacts to

changes in attitude of the rotor disk like a pendulum. When the tip-path-plane shifts, the total

aerodynamic force and virtual axis (the apparent axis of rotation) will shift, but the mechanical

axis (the actual axis of rotation) and the center of gravity, which is ideally aligned with the

mechanical axis, lag behind. As the center of gravity attempts to align itself with the virtual axis,

the mechanical axis (which is rigidly connected to the fuselage) also shifts, and the aircraft

accelerates (see figure 3-9).

In the case of high-speed forward flight, the nose of the aircraft would be low due to the tilt

of the rotor disk and moment due to fuselage drag. To compensate for this, a cambered

horizontal stabilizer is incorporated to provide a downward lifting force on the tail of the aircraft.

Therefore, the aircraft fuselage maintains a near level attitude during cruise flight.

3-8 HELICOPTER POWERED FLIGHT ANALYSIS