Because the pilot’s controls are attached to the flybar, and not directly to the main
rotor blades, Hiller control systems naturally exhibit a slight control delay. A hybrid
stabilization system referred to as the Bell/Hiller system incorporates additional
linkages to mix direct rotor blade control with flybar stabilization. The Bell/Hiller
system responds quickly because pilot control commands are transmitted directly to
the main rotor blades, while the system is stabilized by a Hiller-type flybar and
paddles.
A major drawback of flybars and paddles is increased aerodynamic drag. The
circular cross-section flybar wire supporting Hiller paddles can produce more drag
than the paddles. Moreover, since Hiller paddles are typically configured to operate
at a zero angle of attack relative to the rotor head, and since air passing through the
rotor is almost always flowing downward, Hiller paddles can actually operate at a
negative angle of attack with respect to the incoming airflow. In this way, Hiller
paddles actually contribute negative lift.
The unusual
Arlton Subrotor
stabilizer blades on Lite Machines helicopters serve a
triple purpose. As part of the main rotor control system, they amplify pilot control
commands to the main rotor blades. As part of the stability system, they act to keep
the main rotor spinning in a constant plane in space. As rotor blades, they can
produce lift that reduces or eliminates the reversed airflow commonly found near the
rotor hub.
Retreating-Blade Stall
Retreating-blade stall (also referred to as “asymmetric lift”) affects helicopter rotors
that are moving forward (translating). As A Lite Machines helicopter moves forward,
the blade swinging forward over the right side of the helicopter (the advancing blade)
experiences a higher air speed than the blade swinging backward over the left side of
the helicopter (the retreating blade). At high airspeeds the advancing blade
generates high lift, while the inner portions of the retreating blade actually move
backward relative to the oncoming wind, and much of the retreating blade is stalled.
The airflow around the stalled retreating blade is very turbulent and the blade does
not generate much useful lift.
Following the rule-of-thumb for gyroscopes, the high lift generated by the advancing
blade, and the low lift produced by the retreating blade will cause the main rotor disk
to tilt backward away from the oncoming wind. In order to keep a helicopter moving
forward at high speed, the pilot has to maintain forward pressure on the transmitter
fore-aft cyclic control stick to reduce asymmetric lift and tilt the rotor disk forward.
Retreating blade stall can also be induced by changes in rotor speed. Since Lite
Machines helicopters have a fixed-pitch (variable-speed) main rotor, the speed of the
rotor must change to control altitude. When descending from altitude the speed of the
main rotor is reduced substantially, but forward flight speed does not change.
Without pilot intervention, the Lite Machines helicopters will pitch up and fly
backwards due to retreating blade stall. It should be noted that helicopters with
Op er a tor's Guide
How Helicopters Work
LITE MA CHINES
8-7
