Bellcrank Location

I'm building the wing up for my Humongous and am having a problem with the bellcrank location. If I install the 4" bellcrank in the "stock" location, the forward lead out is in front of the spar. Can I move the bellcrank aft between 1 and 2 inches? Or should I leave it where it is and run the forward lead out in front of the spar.

Thanks -

The location of the leadouts at the exit point (leadout guide) is the only critical point that matters. otherwise the location of the belcrank makes no difference except for the need to keep the leadouts reasonably straight so they don't bind at the exit. Most of the time, the spar location is placed on the plans so it is approximately centered at the leadout exit. Then one can be ahead of the spar and one behind. However, to give you adequate movement, you may need to slot the spar slightly at the inboard tip.

Full depth spars are a problem in another way. If the belcrank is not centered on the spar in the center section of the wing, then the spar may get in the way of full movement of the belcrank as well.

So:

You need to give yourself enough clearance to be able to adjust the location of the leadouts (with an adjustable guide)

You need to give yourself enough clearance at the belcrank so the belcrank can have full movement both ways without the spar getting in the way.

If your design does not call for full depth spars, then you have a lot more freedom in setting the location for the belcrank.

Well, not ABSOLUTELY (no effects due to moving the bellcrank). Moving the bellcrank changes the geometry of the control system, specifically the length of the control rod to the flap horn, and this in turn affects its linearity. The shorter the control rod and/or the larger the horn arm drive radius, the more pronounced the distortion is.

You can tilt the bellcrank axis so that it is parallel with the neutral position angle of the of the control rod plus offset the neutral angle on the horn arm accordingly. This will improve linearity. But the comments about geometry still hold to a degree.

That said, most folks go through their entire CL existence without noticing or being adversely affected by "non-linearities" of the control system. (We simply adjust our human control system to compensate!)

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"Nothing so needs reforming as other people's habits." -Mark Twain

Mine has the front leadout (at the tip), right at the back edge of the spar, the rear leadout about 3/4" back from there. Basically, the bellcrank can be anyplace, tho bending the leadouts at the tip is never going to be a good thing. I would put the BC in the stock location.

On the other hand, my last crash with this Humongus pretty neatly split the wing down the CL of the airfoil, and I have no idea how to fix it. It will never be as pretty again, that's for certain. Vertical sheer webs should prevent a lot of that, and is recommended. Warren trussing the spars is a bad idea, IMHO. Also, this design needs *plenty* of power! Mine has a good .46LA, but I'd consider a ST G.51 (or Orwick .64 for a sparky). Steve

Leonard/Steve - thanks for the quick reply. Steve - I had planned to use "X" bracing on the spar. Is this Warren trussing? I have an adjustable leadout with 1" spacing so I believe I will mount the bellcrank so the forward leadout is just aft of the spar and mount the leadout so its forward position is also just aft of the spar. Will it work out OK?

Thanks again -

I have an adjustable leadout with 1" spacing so I
>believe I will mount the bellcrank so the forward leadout is
>just aft of the spar and mount the leadout so its forward
>position is also just aft of the spar. Will it work out OK?
>
>Thanks again -

Where is the optimum leadout location going to be on your airplane? If it needs to be forward of the spar, then you are going to limit yourself by the location unless you notch the spar accordingly to allow the leadout to move forward somewhat.

>Leonard/Steve - thanks for the quick reply. Steve - I had
>planned to use "X" bracing on the spar. Is this Warren
>trussing? I have an adjustable leadout with 1" spacing so I
>believe I will mount the bellcrank so the forward leadout is
>just aft of the spar and mount the leadout so its forward
>position is also just aft of the spar. Will it work out OK?
>
>Thanks again -

A Warren Truss has both diagonal and orthogonal braces.
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"If each one of us could make just one other happy, the whole world would know happiness." -Georges Simenon

I built in a little rudder offset just to look authentic. If I build another I'll set the rudder straight. My front lead comes out in front of the spar. I had thought it might. I did a vertical sheet web to make a D-tube. Toward the inside tip I cut the web into two pieces, with a 3/16 gap between, to make two T-shaped spars rather than an I-beam. Ted Snow remarked that he had never had his leadouts so far forward. The airplane still had a little yaw, but not bad enough to worry about. That Humongous was a favorite airplane.