Bell Cranks - RC Groups
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Mar 01, 2013, 06:06 PM
Registered User

Bell Cranks

I began 35 sized planes with metal bell cranks. Control feel was like dragging a chain across a rocky road, the lead outs sawed through the bell crank and when I followed Bark Klapinski's great advice and soldered brass bushings into the crank eyelets, it proved fatal to my modifed Magician when the acid that wasn't removed adequately, ate through the lead outs causing the Magician's disintegration. I lost control as the first lead out broke, at the top of the 5th or 6th loop, then the Magician went into a steep dive when the second lead out broke. Nice bird. Then I took Bart's advice again and started using Sig 3 inch nylon bellcranks. I bush the eyelets with brass thimbles. They feel like a Hurst Competetion Plus linkage on a primo Borg Warner T-10 transmission. They require 1 brief modification, don't rust, don't wear out, are virtually indestructable and are the single best improvement I ever made in building model airplanes. I realize that technology is 40 years old, but is there a better way? And remember, simplicity = genius
Last edited by tigreflyer; Mar 01, 2013 at 06:22 PM.
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Mar 02, 2013, 03:52 AM
greggles47's Avatar

Regardless of what the bell crank is made of, it's important that its fulcrum is bushed and that the leadouts where they attach to the bc are solid.
If you want flexible leadouts they need to be soldered to the solid. As you have found it's really important that anything you use as a solder flux is rinsed off the LO.

Mar 02, 2013, 08:31 AM
Old Timer
The Kiwi's Avatar
The copper "end loop" bushing has long been popular with phenolic and metal bellcranks. Two lengths of small diameter tubing -- about 1/16" size, are used at the ends of heavy C-D flexible cable leadouts. Pass the cable through the tubing, drill out the bellcrank holes, Put a small radius bend in the middle of each tube (use a quarter inch dowel as a mandrel), feed it into the B/C and wrap, then solder, the free end of the cable to the main part of the leadout.

I seldom used silver solder / Acid Flux on any joints I couldn't see easily, using resin core solder for internal connectors.
Mar 02, 2013, 01:33 PM
Registered User
I use nylon covered cable fishing leader material for lead outs. With a plastic bellcrank, I cover the leader with the teflon tubing for thick CA. I have a set ,heavily flown for seven years, which shows absolutely no wear on the bellcrank or the CA tubing.
Mar 02, 2013, 08:29 PM
Registered User
I used solid 1/32nd piano wire folded over, sleeved through a brass tube and soldered.
I hope it lasts more than fifty flights.
Mar 02, 2013, 08:43 PM
greggles47's Avatar
Looks good John, the only addition I make to that is to run a 1/4" support from the spar to the TE to fully support the BC mount. A 1/16" rib doesn't provide enough support in a high stress situation.

Mar 02, 2013, 09:35 PM
Registered User
Thanks Greg. I'll add the brace.

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