Bellcranks

How do you figure out what hole to use in the bellcrank for your pushrod. I always seem to end up with to much throw on the brodak bellcrank, but I'm afraid to use the inner hole and not have enough? The middle hole seems to be to much throw.

It's a Brodak 3" crank, btw.

If the middle is to much throw on a 3" bellcrank then by all means use the inside hole. Throw ration from the bellcrank increases as you progess outward.

>If the middle is to much throw on a 3" bellcrank then by all
>means use the inside hole. Throw ration from the bellcrank
>increases as you progess outward.
>
>Jim Pollock

You can also use a taller hole on the flap (or elevator) horn while keeping the same hole in the belcrank. The belcrank and the horns being used are a unit. The hole that you use on the one will determine where the hole needs to be on the other. Try to get about 70 degrees throw on the belcrank for the maximum throw that you want on the control surface to which the pushrod is attached. Then also make sure you have clearance for the horn and pushrod. Sometimes when you use the innermost hole on the belcrank (or bottom most hole on the flap horn going to the elevator horn) you can end up with the pushrod "bottoming out" against the stabilizer before you get full deflection. Not good.

This issue of SN has a great article from Tom Morris on figuring out bellcrank, flap horn, and elevator horns which gives dimensions and angles. Very good indeed. Ah the benefit of PAMPA membership.

>How do you figure out what hole to use in the bellcrank for
>your pushrod. I always seem to end up with to much throw on
>the brodak bellcrank, but I'm afraid to use the inner hole
>and not have enough? The middle hole seems to be to much
>throw.

John;

As Leonard has pointed out, the amount of throw we have with most modern ships is in excess of what is necessary for square maneuvers. Although we may have 30 - 45 degrees of movement, we normally need less than that. That, combined with the fact that you approach the "Netzenband Wall" faster with a 3" bellcrank than a 4" bellcrank, most of us use the inner most hole on the bellcrank.

Question - what ship are you planning to use the 3" bellcrank in?