From: howie@intermec.com (Howard Kaplan x1696)
Newsgroups: rec.models.rc
Subject: Re: Please Explain Helicopters
Date: 14 Dec 92 23:59:20 GMT
Reply-To: howie@intermec.com (Howard Kaplan x1696)
Organization: Intermec Corporation, Everett WA

In article <1992Dec9.144400.13803@mlb.semi.harris.com> mwp@ms9.mis.semi.harris.com writes:
>After browsing through the articles relating to helicopters, you
>folks have stirred my curiosity regarding how the heck they fly.
>More importantly, how do you CONTROL them?
>
>What do the 4 or 5 channels control?  

Fixed pitch helis fly on 4 channels:

Throttle - which controls rotor speed to make the machine go up and down.

Tail Pitch - Like rudder on a planes to control the heading of the nose or
tail depending how you look at it.)

Fore/Aft Cyclic - Controls the tilting forward and backward of the machine.
(Like elevator in an airplane except you don't use it to climb or descend.)
The term cyclic pitch is used because the pitch of the blades vary cyclicly
(i.e. - periodicially in sync with the rotor.). To tilt forward the pitch
raises as the blades passes around to the rear of the machine and is
lowered as it passes the front of the machine. The net effect is the rotor
plane tilts forward (or any direction you desire) and the machine starts
to put its nose down and move forward.

Roll Cyclic - Banks the machine left or right. Work Almost exactly like
aileron on a plane.


For a collective pitch machine the 5th channel (you can use a 4 channel radio
and either mechanical or electrically couple the throttle and collective) is
used to control the pitch of both main blades at the same time (collectively)
and this is what raises and lowers the model.  The throttle and collective
servos are controlled from the same stick. As the collective pitch goes up so
does the throttle. The increased load on the engine tends to keep the rotor
speed constant.  A machnine that is setup properly will not have much
variance in head speed.  

To make a heli fly up and away like a plane you lift off a bit with
collective and push the nose down (forward cyclic or down elevator) and
gradually increase collective and forward cyclic and it's off a running.
Note this is the biggest departure from a plane where you pull back on the
elevator to take off.

>As a helicopter moves faster through the air (suppose in a straight 
>line), how do you account for the needed dynamic changes in pitch 
>of the blades and rotor?  Doesn't it continually want to bank and 
>turn as speed increases? Please explain in less than 10,000 words.

This effect is pretty minor. It varies from machine to machine.  My machines
don't seem to roll much to the right at all when flying fast forward flight.
I may be adding a slight bit of left cyslic to compensate, but it is not like
the effect is uncontrollable. The more obvious effect is the need to add
rudder control to keep the tail straight because the tail gyro is over
compensating the torque effect because the heli tail weather vanes in the
direction of forward flight.  Some guys use two channel gyros and have a
lower sensitivity setting for forward flight.  This is were computer radios
become really nice for doing all the mixing the helps compensate for these
secondary effects.  

This has just scractched the surface. 

Howard Kaplan
Newsgroups: rec.models.rc