Dallas,
There was some discussion earlier about self centring bellcranks, check this link.


 http://stunthanger.com/smf/index.php?topic=3808.0

Cheers,
Jim

Worth noting is that the Yatsenko Shark uses a self centering 'crank.

Rick Campbell

Dallas,

The idea is that when you pull one line, the 'arm' (from bellcrank pivot) gets smaller. As a, result, the arm to the other end of the bellcrank gets longer. When you relax pressure, the longer arm has greater 'leverage', so, at least at first,  has a greater tendency to return the bellcrank to neutral.

HOW and WHY we can provide control inputs  to flap and elevator:

Figger -

If a model is pulling 3g outward in level flight, If the bellcrank arms are pierced with the leadouts, and the...
(Edited to correct the effects of an untimely nap while on-line:)
... controls are in neutral, the pull is equal on each arm. If the arms aim a bit from the pivot toward the flier, this is still true. When you input some control, the arm you are puliing gets shorter - thinking of pull as straight down the length of the lines and leadouts. The other arm gets longer, but you've unloaded some of the pull from it to get the leverage to move the pushrods, thus the control surfaces.

When you return the handle towards neutral, the temporarily "longer" arm helps with greater leverage to restore the equal load condition at neutral.

(I think I fell asleep when I originally tried to post this. I've had some oddly long and stressing days, lately. Mostly behind me now... I hope...)   

Dallas,

Let me try a simple visualization exercise for you...

A 'straight' bellcrank doesn't care what angle it rests at when equal weights are hung from both leadout eyes, right? They stay balanced in whatever position because the distances foreshorten simultaneously on both sides.

A 'self-centering' bellcrank has the arms droop below the pivot, and they are usually tilted that way as well. Hang identical weight off each arm, and it will tend to swing back to "neutral" from any other position you try to put it in. And you can see why: swing it one way, and that arm 'gets longer' while the other 'gets shorter.' Identical weight, remember? The longer arm has more leverage, more turning force, until the other arm reaches the  position where both are equal in length.

And, as Don B mentioned, the control surface airloads feed back through the pushrod system to help restore neutral quite well. Choosing a 'self-centering' bellcrank is basically a matter of preference, and does not usually make a really important difference.

Now, if you mount it backwards, it would become a 'self-destructive' bellcrank that finds it hard to stay at neutral - tending to lock over anywhere else... DON'T do that!

Self-centering has to do with providing a "return to center" force on the bellcrank. First, the aerodynamic force on the control surface exerts a load on the pushrod, generating a moment about the bellcrank pivot. The tension in the control lines exert a balanced force about the pivot.  If the bellcrank pivot is on the line between the leadouts the only force (moment) countering the pushrod moment is from the tension force in the control lines.   By moving the pivot point of the bellcrank to a point toward the pushrod from a line between the leadout ends and the pushrod, a moment is established opposite to that from the pushrod, thus setting up the "self-centering" effect.  Conversly, moving the pivot away from the pushrod is an upsetting moment, that moment being in the same direction as that from the pushrod.

Hope this doesn't muddy the waters. 

Hi Guys,
The S/C bellcranks have been around quite a while and do work as intended.  They were hardly my idea but I was looking for a more positive lock to the bottoms to square maneuvers.   After chatting with Big Art about this I decided to do a little testing to verify the results and/or benefits if any were found.   The Sig 4” version with 10° of forward sweep worked the best of all the different forward sweeps that were tested.  The different angles tested were, 5°, 10°, 15°, and 20°.  There was no difference in the 5° sweep at all that I could tell, the 10° did show some of the positive attributes I was looking for while the 15° and 20° exhibited way to much centering effect.   They were also way too soft around the neutral position and required too much control input to get a positive effect.  One of the benefits from this arrangement was the up line was moved to the front arm of the B/C and this is what I still think is the most important feature of the Sig 4” B/C.  A flapped model hides some of the S/C effect with the feedback from the control surfaces which is a good thing.  Try this B/C on an unflapped model and it become very apparent that the S/C feature works.
Another benefit of the S/C bellcrank is it helps the use of the more rearward CG of the modern big time PA models.  It gives a more “groovy feel” at the neutral position during level flight.  I’ve used the Sig 4” bellcrank on all the models I’ve built for years including Classic & OTS models and found it to work very well on all of them.
It is OK if others disagree with me on this (they are still my friends), but I feel I have verified my findings based on flight testing and careful analysis and intend to keep using them.

Later,
Mikey