This article is from the Radio Control (R/C) Flying FAQ, by Shamim Mohamed email@example.com with numerous contributions by others.
How many channels do you need to control a heli and why?
You need five channels to control a heli. You need one each to control
pitch, roll, and yaw. You need one to control throttle, and you need one
to control collective. You might think that one servo could control both
throttle and collective, since they are related. There are several reasons
this wouldn't work, however. The main rotor disk of a heli is huge and
generates a correspondingly huge amount of drag compared to an airplane
prop. (If you think of the heli rotor disk as a big propellor, its
actually pretty amazing that a tiny little .32 engine can turn it at all.
There's about a 10:1 gear down from the engine to the main rotor, which
makes it possible for the engine to turn the main rotor.) So, you have to
have fairly fine control over the relationship between the collective pitch
(and corresponding drag) and the throttle setting. If you get it wrong,
the engine bogs badly or races wildly. Also, auto-rotation is an important
maneuver, and this entails control of collective pitch while the throttle
is set to idle. Finally, for inverted flight you want to have full
throttle both at maximum up collective and maximum down collective.
What are the radio options?
Pitch curves and throttle curves: You can adjust the amount of servo travel
at 0% stick, 25%, 50%, 75%, and 100%, both for throttle servo and
collective servo. This feature is a must.
Throttle hold: Flip this switch to practice auto-rotation; the throttle is
reduced to idle. All the other controls still work normally.
Idle up: This is an alternate mode, usually used for aerobatics. You can
set throttle and pitch curves, mixes, etc., and change over to the
different setup at altitude or whenever.
Programmable mixing: This neat feature lets you establish a relationship
between channels. One channel is designated as the input or master
channel. As the master channel varies, it causes small changes to the
output channel. This is an advanced feature.
Revolution mixing: This feature causes increases in tail rotor as throttle
and pitch increase. This is useful to compensate for the increased torque
the engine produces. I feel that this is a somewhat over-rated feature,
and that it only really comes into its own when you're doing aerobatics.
Even then, a programmable mix may be better.
Electronic trim adjustment: similar to and augments mechanical trim
End point travel adjustment: sets where servos go at max stick displacement
Exponential: can be used to make cyclic less sensitive in midrange.
Can I use my airplane radio?
It is possible to control a helicopter with a 4-channel airplane radio.
You can master hovering and move into elementary forward flight this way.
For anything beyond that, you will need a helicopter radio. If you do try
to use a 4-channel airplane radio, build a Y-connector, and control two
separate servos (collective and throttle) off the throttle channel. Then
adjust control arms to get a form of mechanical throttle and pitch curve
adjustment. It's not too hard to set a heli up so that it will hover
tolerably well at mid-stick this way, and you can contrive to increase lift
above mid-stick and lose lift below mid-stick.