Nov 29, 2003, 11:22 AM
Registered User

# Bell-Hiller Explained

This is an article I wrote for our local club. I thought some of you might find it useful.

It's fairly obvious that helicopters develop lift from their spinning rotor blades, but how are helicopters controlled? The pilot has four basic controls, much like an airplane; aileron and elevator (often called cyclic on a helicopter), rudder, and throttle/collective.

The cyclic controls (aileron and elevator) cause the helicopter to tilt left, right, forward and backward. This is achieved by changing the pitch of the blades as they rotate. Imagine increasing the blade pitch (and therefore lift) when the blade is at the rear of the helicopter, and decreasing it when it's toward the front of the helicopter, constantly changing pitch through each rotation. You might imagine that the increased lift in the rear and decreased lift in the front would cause the machine to tip forward. Although this is good for conceptual purposes, it's not what really happens. The spinning rotorblades actually create a gyroscope, and when you put a force on a gyroscope, it tilts in a direction 90 degrees from the direction the force was applied. Because of this, the blade pitch needs to be increased and decreased 90 degrees in advance of where you might initially think. In the example above where the pitch in increased when at the rear of the helicopter, the machine would actually tilt left or right depending on whether the blades rotated clockwise or counterclockwise.

The blade pitch is controlled by the swashplate. The swashplate is actually two plates connected by a bearing that the main shaft goes through. The bottom of the swashplate is tilted by pilot controls (or servos in a model). The top plate rotates at the same speed as the blades and in a simple system, is attached to the leading or trailing edge of each main blade with a linkage. As the plate is tipped forward, this linkage increases and decreases the blade pitch when the blade is at the left and right sides of the helicopter and through the magic of the gyroscopic effect, the helicopter pitches forward.

Some models are built using this simple system, but is has it's drawbacks. The main drawback is that a small amount of pitch change can pitch the machine very quickly. For model helicopters, it is often too quickly, makingthe model difficult or impossible to fly.

Bell (of Bell Helicopters) solved this stabalization problem by adding a stabilizer bar perpendicular to the main rotor blades with weights at each end. When spinning with the main blades, this weighted bar created another gyroscope that wanted to stay spinning in it's own independent plane. The tilt of this stabilizer bar was mechanically mixed with the output from the swashplate (the pilot's control input) to control the pitch of the main blades. Now, when the pilot gave an input to pitch forward, the input would go through the swashplate to the main blades and immediately pitch the main rotorblade plane forward. The stabalizer bar, however, would prefer to stay in the plane it was spinning in, so it would lag behind the main rotorblade plane. This difference between the main rotorblade plane and the stabilizer bar plane would, through the mechanical linkage, decrease the input to the main rotorblade pitch counteracting the input provided directly from the pilot. Thus the roll rate was stabilized.

 Nov 29, 2003, 01:22 PM Registered User pretty good explanation, brad. here's some more good info, with diagrams: http://www.w3mh.co.uk/articles/html/csm9-11.htm see the bottom of this page for more good colin mill articles: http://www.w3mh.co.uk/articles/articles.htm
 Sep 14, 2008, 09:35 PM martyracenis A moderator felt this post violated the following rule: Offensive content (Offsite link). It is temporarily hidden while martyracenis edits it.
 Sep 15, 2008, 02:42 PM Registered User Erm...resurrected zombie thread... Toshi
 Sep 15, 2008, 04:18 PM Registered User Holy old thread batman! Still a good read none the less
 Sep 15, 2008, 10:04 PM Bush Pilots Bounce Higher Wow somebody dug out a 5 year old thread ! Amazed it is still in the data base !
 Sep 20, 2008, 10:46 PM Registered User I was just researching the topic a bit and happened to come across the thread. When I put in the post, I hadn't looked at the date and had no clue it was from 5 years ago...oh well.
 May 07, 2009, 05:08 PM Did you get it on video? good info, worth a bump!
 May 08, 2009, 12:24 AM Fly Runaway Fans The data is all out there if you spend ~a week googling and reading. This is very concise. Now you know why CP heads are so gawldang complicated to rebuild.
May 08, 2009, 01:11 AM
Quote:
 Originally Posted by arbilab Now you know why CP heads are so gawldang complicated to rebuild.
Once you get a few rebuilds under your belt they really aren't that bad. That first one is kind of daunting, but when you get into it, it's kind of fun.
 May 08, 2009, 01:44 AM Fly Runaway Fans Know whatcha mean Norcal. In the late 80s, I rebuilt the head assemblies of \$500,000 video library machines. Daunting the first time, even with good old American documentation to go by. There's no such thing as that anymore. But then, the income stream of a top-40-market network TV affiliate doesn't depend on heli rebuilding either. Comes to the point I can't put it back together, I can still fly the ones I CAN until I figure it out.
 Jan 05, 2010, 09:17 AM Registered User thank you very much learn a lots
 Mar 04, 2011, 01:42 PM Registered User Awesome information. Keep the beast alive.
 Mar 04, 2011, 01:51 PM I love HAM Can we make this sticky or is some FAQ thread already linking to this post?
 Mar 04, 2011, 11:33 PM Registered User Great for us beginners!