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29 What is cyclic? Collective? (RC flying - Helicopters)




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This article is from the Radio Control (R/C) Flying FAQ, by Shamim Mohamed shamim@math.isu.edu with numerous contributions by others.

29 What is cyclic? Collective? (RC flying - Helicopters)

On most R/C helis (and full-scale helis for that matter), the main blades
can change their (so-called) pitch angle. What this means is that if you
sit the heli on a table and look at the tip of one of the main blades, the
chordline of the blade can be tilted through a range of angles by the
servos. In this sense, the rotor disk of a heli is a bit like a
variable-pitch prop on an airplane. If the heli is hovering and you wish
to make it climb straight up, you increase the pitch of the main blades,
and increase the throttle so that the engine can overcome the increased
drag and keep the blades turning at the same speed. The increased blade
pitch results in more lift, and so the heli climbs. (With R/C helis,
unlike R/C airplanes, engine RPM's are supposed to stay the same over (most
of) the throttle range. At high throttle the engine puts out more power,
but there is a corresponding increase in the load on the engine due to
increased main rotor blade pitch, and so the engine stays at the same
RPM's.) This overall increase in pitch that makes the heli climb is called
collective control.

To get the heli to pitch forward or back, and to roll left and right, there
are controls that are analogous to airplane elevators and ailerons. These
controls are refered to as cyclic controls. The idea is to set up
asymmetric lift on the rotor disk. (This is similar to what ailerons do to
an airplane-one wing can be made to generate more lift than the other, and
so the airplane rolls.) If there's asymmetric lift on the rotor disk, the
plane of rotation of the rotor disk is going to change. For instance, the
rotor disk (and the heli that is attached to it) might go a bit nose-down.
In that case, the heli will transition out of a hover and start flying
forward. Similarly, the heli can be made to lean back (nose-high), left,
right, or any combination of these. The way this asymmetric lift is set up
is to vary the pitch of each blade as it goes around. For instance, say
you push forward on the cyclic control stick (the right one on your
transmitter, which does the same thing as an aileron/elevator control stick
on an airplane radio). This will make the blade pitch down as it travels
through the forward-moving part of the rotor disk (usually the left side of
the rotor disk), and it will make the blade pitch up as it travels through
the backward-moving part of the rotor disk (usually the right side of the
rotor disk).

 

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