Report of the Second Piloted Aircraft Flight Control System - Acgsc.org

Returning to the discussion of the azimuth control where the tip
path plane is tilted by means of the tilt of the swash plate, we can
appreciate the problems resulting from the f ollotdng considerations.
The friction resulting from the feathering bearings, which are required
to support a centrifugal force of approhately 5,000 lbs. is appreciable.
The need for a fine balance is necessary between the aerodynamic thrust
on each blade and the centrifugal force component projected along the line .
of thrust which prevents the rotor from llconinglt too much. Also, the
variety of vibratory and gust disturbances transmitted into the control
system from the aerodynamic surfaces which support the fulJ. weight of the
helicopter and which move as a complicated piece of machinery miqfit be
d
disconcerting. On top of this a helicopter is inherently unstable in
hovering having an unstable mode with a period of the order of magnitude
of 10 seconds or more. mile the pilot can cope with this unstable oscillation
because of the long period, it is desirable that he make frequent
corrections without undue effort. The problem was not insurmountable on
helicopters of the 5,000 lbs. pose weight category, but did limit the size
of the helicopter unless suitable potjer controls were developed. Various
aerodynamic and gyroscopic devices have been incorporated on hellcopters. .
Three of particular interest are the Bell stabiliqer bar, the Hiller control
rotor and the Kaman blade tab. The Bell stabilizing bar is a large rate
gyro rotating at rotor speed (its undamped natural frequency ie hence one
times rotor speed) with viscous damping, which can be adjusted to optMze
the stabilization obtainable with a raze gyro alone. The Hiller control
rotor is a similar device which uses the blades of the control rotor to
obtain aerodynamic damping. Another feature of this control rotor is that
the feathering produces flapping on the small control rotor which introduces
feathering on the large rotor. In short, there is an extra stage of an
aerodpadc .servo in the control loop. The Kainan rotor, the tab is located
on the blade near the tip. A preloaded cable control regulates the angle of
the tab which introduces a twisting moment on the blade, which in turn twists
the very flexible blade to the desired angle without the use of featheflng
bearings. *Each of these devices solve the problem in part, but have certain
disadvantages as well. Power requirements for aerodynamic actuated torques
should be considered, and tests run at Sikorsky Aircraft with tab controlled
rotors wlth featherlne bearings indicated that the additional drag, due to
this type of aerodynamic servo, resulted in power loss fran 5 to 1% of total
power absorbed by the rotor.
The use of hydraulic power appeared to be the most economical method of *
improving the control of the helicopter. In 1949 a hydraulic pmr control
for the three inputs of the swash plate was installed on the S-51 model
helicopter (gross wei@t 5500 lbs.) The power control had infinite booet
Q
ratio, that is no feedback from the output to the input, operated at a
pressure of 800 to 1,000 psi obtained from a constant displacement pump
and unloading valve charging an accumulator. The awragle power requirement
for this system was practically nil, being less than a quarter horeepaser.
Although the original purpoeb of the control was to prove that the hellcopter
could be flown with this type of control in order to pave the way for the