belcrank best parameters

[dave siegler]

When designing a bellcrank what are the parameters relating to pivot point, arm length and drive arm length?   

1) is the pivot ahead, behind or on cent of the line attach points?
2) How far should the pivot from the drive arm? 

I fool with little foam airplanes and printed some recently.

[Motorman]

The bellcranks that work good for me have 4" spacing with an 11/16th push rod arm so, you can figure the ratio and scale it to your plane. Depends on your control horn length too. The bellcrank pivot/attach holes can be in a straight line with no problems.

Don't know how small your planes are but I'm using a 3"x 1/2" in my .061 powered 1/2A Path Finder.

MM

[Jim Svitko]

I suppose it depends on how you like your controls to be set up.  Do you prefer fast controls, or slow?  Fast or slow control preference will be something to consider when determining the distance from bellcrank pivot to pushrod hole.

And, as far as where the bellcrank pivot should be:

I am trying to find a thread about that.  One was posted here, some time ago, and it explained the geometry. 

[Jim Svitko]

This thread might provide some information.  Maybe more than some would need, or even care about.   Nevertheless, it shows some of the intricacies of our control set-ups.

https://stunthanger.com/smf/open-forum/carbon-bellcranks/

[Howard Rush]

I fool with little foam airplanes and printed some recently.

Printed some airplanes?  Cool.  Please elaborate. 

[Howard Rush]

This thread might provide some information.  Maybe more than some would need, or even care about.   Nevertheless, it shows some of the intricacies of our control set-ups.

https://stunthanger.com/smf/open-forum/carbon-bellcranks/

I liked Monty's method of making a forked-bellcrank mold.  Mike Haverly also showed some good ideas there. 

Some quantitative stuff on bellcrank hole alignment is at https://stunthanger.com/smf/engineering-board/self-centering-ciphering/ .

Bellcrank input arm length:  You are making a control input with your handle to react (balance) airplane control surface hinge moment.  Between the handle and control surfaces are some springs: control line elasticity, control line shape caused by wind, and maybe pushrod springiness.
  The longer the bellcrank input arms and the farther the bellcrank rotates, the more leverage you have over control surface hinge moment.  How much is enough? It depends on some stuff that's easy to calculate and control surface hinge moment, which isn't.  There is probably a point of diminishing returns.  I use a 4" bellcrank moving through almost +/- 90 degrees to move the flaps a little less than 30 degrees.  That works OK with my airplane, but requires a big line spacing at the handle.

Bellcrank output arm length: The more you have, the less force on the pushrod and the less the effect of control linkage slop.  Neither is a big deal with carbon tube pushrods and ball links.   I pick the bellcrank output arm length by first choosing max flap and bellcrank deflections, then doing a 3D analysis of the control kinematics (I think that's the word) to make the relationship between leadout travel and control deflection linear.  That gives a rough ratio between flap control horn length and bellcrank output arm length.  I generally pick the latter based on the flap control horns I have in my drawer.  Maybe more important than the bellcrank output arm length is the angle of the bellcrank output hole relative to the line between the input holes.  This has a big influence on control system linearity. 

[dave siegler]

Printed some airplanes?  Cool.  Please elaborate.

No sorry, 3d printed bell cranks for indoor foamies.  The flight loads are very low. 

It is cold in the upper midwest and Its fun to be able to do the pattern on 15 foot lines.