CG & Center of BellCrank

Hello,
Quick question. Where should the center of the bellcrank be in relationship to the CG?? I know the CG can be changed, but is there a standard to shoot for? Thanks;)

Only the location of the leadout guides matters. The bellcrank can be anywhere at all in relation to the CG, as long as the center point of the leadout guides is behind the CG by a small amount - a 5% angle is the starting point for this.

.
Kiwi

>Only the location of the leadout guides matters. The
>bellcrank can be anywhere at all in relation to the CG, as
>long as the center point of the leadout guides is behind the
>CG by a small amount - a 5% angle is the starting point for
>this.

True. However, the location of the bellcrank affects the length of the control rod from the bellcrank to the flap. The shorter that control rod is, the more distortion in the symmetric operation of the controls with respect to the bellcrank rotation.

Nobody worries much about it. Tilting the bellcrank axis forward so that the control rod is perpendicular to the radius of the flap control horn is one method for improving linearity. A neutral drive offset and bellcrank tilt of about 7 degrees is a typical good setting.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"Room service? Send up a larger room!" -Groucho Marx

Kiwi is of course correct. It makes no difference on the flight and line situation where (fore and aft)the BC is mounted relative to the cg. This is why we like to suggest adjustable position leadouts.

Put it(BC) in the position that makes the most sense structurally and to minimize problems in the geometry of the pusrod.

FWIW

Even though the leadout position is most critical to the way the airplane flies, there is no reason to think that having the leadouts bending (say) 20 degrees at the leadout guides would not have any effect on the way the controls work, respond, and the amount of force required to activate them. Obviously, it would create friction that would have to be overcome to drive the control surfaces. Remember that all the power that runs your controls is line tension. Your hand puts more line tension on one line than the other is all. When you use line tension to drive the controls, you have less line tension, so keeping the controls smooth, silky and free of binding helps. Bending the leadouts doesn't help. Yes, the 20 degrees is exaggerated, to show that the closer you are to having correct bellcrank location can help.

Common sense says to put the BC close to the wing spar for strength, and I would personally mount it forward of the spar if there was adequate room to put a 4" bellcrank there and clear the LE without chopping away the ribs too much. Since the spars will usually be at the maximum thickness, and the CG will always be ahead of that, it therefore puts the BC close to the CG, resulting in minimal leadout bending. Remember that the less the line tension (smaller models/ engines), the more important a big bellcrank, giving more leverage at low line tension levels. Hope this helps you understand all the issues.
Sammy

Sammy, that was the most concise paragraph I ever read concerning bellcrank placement. I am in agreement, as it works for me.
Mike in Atwater