Hey all

I'm assembling a Brodak (plastic 4 inch) bellcrank with .027 steel leadout cable.  This is the first time I have attempted to run the cable through an annealed 1/16 brass tube.  I can't get the cable  to go through the .035 hole.  What is the trick?  Or do you go up to 3/32 size tube?

Also, regarding wrapping, once through the tube and in place, is it a simple single wrap about a 1/2 inch long and then a small amount of epoxy on it?

I have searched and not found exactly what I'm asking here.

Thanks!

David     

That works OK for.  Maybe you need to de-burr the hole with a #11 blade and get a good clean cut on the .027 7-strand cable.

Once the brass tube is bent into the horse shoe shape it is almost enough to secure the cable all by itself.  For sure, it does a good job of minimizing stress on the wrapping and solder/ crimped termination.

Do you use cable cutters or simple wire cutters?  If wire cutters are a bit dull they will flatten the cut and make it too large to fit.  A drop of CA on the ends will hold the strands together and you can sand/file it smooth. As to the wrapping AFTER you bend the loop I just bend it over like a normal eyelet and lightly wrap. Then shrink wrap the joint.

Ken

 A single wrap as you described is asking for trouble. It's either lazy, ignorant of proper procedures, or foolish.

 After doing your first wrap, fold the tag end of the leadout wire back towards the bellcrank, and wrap the fine wire back towards the bellcrank. Wrap up to the end of the brass tubing. give it a pull test before you install it, then cover it with shrink tubing. In the AMA Control Line General rules, this is shown in figure #3. To go back with the second wrap only takes a minute or two. Fixing your crashed airplane when the leadout lets loose will take a bit longer. A very experienced local modeler, who is no longer with us, had this happen more than once, because he didn't think the second wrap was necessary or worth the trouble.

Thank all three of you.  I appreciate it.

My cutting technique was the problem regarding getting the .027 cable thru the hole.  That's resolved.

Bill - thanks - I will double wrap as you described.

I'm glad you asked when you didn't know. Kinda' why we're all here. 

I believe you have to drill the holes in the belcrank slightly bigger too or at least champher the holes. You don't want any binding there. If you double wrap it, like you should as Bill suggests, there's no need for glue or heat shrink tubing. There's a certain size drill shank you can use to bend the tubing after the wire is in place. I'll have to check my notes.

MM

Quote from: Motorman on June 25, 2026, 03:09:05 PMI believe you have to drill the holes in the belcrank slightly bigger too or at least champher the holes. You don't want any binding there. If you double wrap it, like you should as Bill suggests, there's no need for glue or heat shrink tubing. There's a certain size drill shank you can use to bend the tubing after the wire is in place. I'll have to check my notes.

MM

Good points about the drill shank and camper.  The bend is done BEFORE putting it into the bellcrank holes.  He is tight the shrink wrap is unnecessary for strength. I use it to keep everything in place.

Ken

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The leadouts on the bellcrank end turned out good. 

Now I need advice on which control horns to use in both the flaps and elevator.  I suspect most of you fabricate your own, but I would like to buy off the shelf if possible.  Plans show a 3"x 1/8 on flaps.  Only one I can find is a Brodak that has the upright tilted forward (differential) Thoughts on this? I'm concerned about hole placement not being correct.

Plans show elevator to be a 2 1/2 x 3/32. Brodak has one of these and it has 3 adjustment holes.  Go for it?

Also, what about going 3/32 on flap horn to make it easier to tweak?

Thank you in advance for your response.

David

You aren't going to find a horn with a post other that straight from the usual sources.  If you angle a horn to give you 90 degrees to the bellcrank it will be wrong for the flaps.  If you position it to give you 90 degrees to the to the elevator it will be wrong for the bellcrank. You have two major goals in setting up controls. 1) The elevators move the same in relation to the flaps, and 2) that the flaps move the same as your control handle. This is completely opposite of the way must people do things with the conventional horns but it does work if that is all you have.  If you are going to order one get a triangular one with separate holes for the bellcrank pushrod and elevator pushrod.  I am sure one of the vendors here sells them.

Ken 

Quote from: Ken Culbertson on July 12, 2026, 05:24:03 PMYou aren't going to find a horn with a post other that straight from the usual sources.  If you angle a horn to give you 90 degrees to the bellcrank it will be wrong for the flaps.  If you position it to give you 90 degrees to the to the elevator it will be wrong for the bellcrank. You have two major goals in setting up controls. 1) The elevators move the same in relation to the flaps, and 2) that the flaps move the same as your control handle. This is completely opposite of the way must people do things with the conventional horns but it does work if that is all you have.  If you are going to order one get a triangular one with separate holes for the bellcrank pushrod and elevator pushrod.  I am sure one of the vendors here sells them.

Ken 

Thanks Ken.  Maybe a supplier on this board will see this thread.

Here is the Brodak 3"

https://brodak.com/3-1-8-differential-control-horn.html

Quote from: DJohnson on July 12, 2026, 05:40:35 PMThanks Ken.  Maybe a supplier on this board will see this thread.

Here is the Brodak 3"

https://brodak.com/3-1-8-differential-control-horn.html

I agree that it is better that a straight up and down but you still have the same 90 degree issues only smaller.  Whether or not this horn will be any better than a straight one angled forward depends on the height of the tail.  Maybe we are getting ahead of ourselves a bit.  What plane is this going into?  We flew just fine with stock Veco horns for years.  Perhaps that is because the horns are longer today then back then.  It is absolutely a plane by plane issue.  On a high tail design like a Nobler a stock horn bent forward to give the 90 degreed to the bellcrank from the top hole will form a nearly 90 degrees from the second to the elevator which would be bent back to form it's own 90 degrees. 

Not knowing your skill level I can attest totally to the advantage you gain in learning/practicing maneuvers if you have a plane that turns the same inside and out.

Ken

Quote from: Ken Culbertson on July 12, 2026, 07:04:16 PMNot knowing your skill level I can attest totally to the advantage you gain in learning/practicing maneuvers if you have a plane that turns the same inside and out.

Ken

Ken, I'm building an Impact.  My skill level is "sucks".  However, I want it to fly really good when I give you the handle if I ever make it up to Dallas!

Quote from: DJohnson on July 13, 2026, 08:49:28 AMKen, I'm building an Impact.  My skill level is "sucks".  However, I want it to fly really good when I give you the handle if I ever make it up to Dallas!

I am familiar with the Impact. You are getting stunt-tradition advice, which is largely wrong. I can send you Impact control system numbers that work when I get home.

A 3" bellcrank will guarantee that your plane will suck. Per Paul Walker's advice, we use 1/8" rod for flap control horn, 3/32" for elevator, .031" leadout cable.

Bill is giving you good advice.

Quote from: Howard Rush on July 13, 2026, 09:47:51 AMI am familiar with the Impact. You are getting stunt-tradition advice, which is largely wrong. I can send you Impact control system numbers that work when I get home.

A 3" bellcrank will guarantee that your plane will suck. Per Paul Walker's advice, we use 1/8" rod for flap control horn, 3/32" for elevator, .031" leadout cable.

Bill is giving you good advice.

Howard,

Thanks for jumping in.  Bellcrank is a 4 inch with carbon rod on a 440 ball link.  It is buried in the wing so I will leave it in there.  I need the horn for flaps and elevator.  I was planning on using an arrow shaft for the elevator push rod but can go some other way if it is recommended. My flaps are built and will need a 3 inch wide horn that I have not sourced yet.  2 1/2 inch wide is needed for elevator. I also would like your feedback on how much nose extension is needed to help with the inherent aft CG.  I was thinking ~1 inch.

Mucho thanks

David

As I recall the Brodak bellcrank had three holes. Generally speaking, inner hole for steady racing planes, center hole for stunt deflection and outer hole for fast combat reactions.

You have to match the flap horn arm to the bellcrank arm so you get the flap deflection you want with reasonable bellcrank travel. You're on the inside hole of a Brodak 4" bellcrank so you might want an 1-1/4" flap horn. The flap horn is tilted forward to match the tilt of the pushrod for equal travel up and down. 1/8" flap wire is easy enough to tweek.

On a plane that size you may get recomendations for an 1-1/2" flap horn. You may find you're locking out the bellcrank and still not getting enough flap travel. In this case you'll need to go farther out on the bellcrank arm to make it work.

MM

Quote from: Motorman on July 13, 2026, 12:13:37 PMAs I recall the Brodak bellcrank had three holes. Generally speaking, inner hole for steady racing planes, center hole for stunt deflection and outer hole for fast combat reactions.

You have to match the flap horn arm to the bellcrank arm so you get the flap deflection you want with reasonable bellcrank travel. You're on the inside hole of a Brodak 4" bellcrank so you might want an 1-1/4" flap horn. The flap horn is tilted forward to match the tilt of the pushrod for equal travel up and down. 1/8" flap wire is easy enough to tweek.

On a plane that size you may get recomendations for an 1-1/2" flap horn. You may find you're locking out the bellcrank and still not getting enough flap travel. In this case you'll need to go farther out on the bellcrank arm to make it work.

MM

Thanks Motorman.

I am on the outer most hole on the Brodak 4" nylon bellcrank.  I believe it measures ~7/8 inch from pivot point (where vertical rod goes through the bellcrank).  I have a spare and will measure it.

That makes it more challenging, but it can still be made linear and give you the requisite leverage over flap hinge moment. Do you have Excel?

Quote from: Howard Rush on July 13, 2026, 04:00:09 PMThat makes it more challenging, but it can still be made linear and give you the requisite leverage over flap hinge moment. Do you have Excel?

My flap control rod is in the outer hole which is .920 from the pivot axle.  How bad did I screw up?  I do have excel.

I don't know offhand. You can start with an arbitrary bellcrank and usually make a flap control horn that will give a linear response. I'll send you the program when I get home.  The control horn may need to be pretty tall.

In the beginning before you know exactly what you want, most everybody has had to tear into a wing to change something with the bellcrank. Couple of holes so you can get the wrenches in then put it in the middle hole. It's a rite of passage. 

MM

Motorman,

Not a bad idea but I'm not sure the middle hole is ideal either.  I measured my spare bellcrank and the bellcrank as shown on the plans. 

Brodak 4" outer hole = .920  Impact plans = .845
Brodak 4" middle hole = .674

Also here are the horn measurements from the plans:

Flap horn - Bellcrank to Flaps = 1.5", Elevator = 1"

Elevator horn = 1"

Hope this makes sense. 

I'm open to all comments, thoughts, recommendations, etc   

I also want to see what Howard thinks.
 

.845 - .875 is pretty normal for an 1-1/2 flap horn. You're a little out of the box on the good side at .920 but, you won't be hurting for bellcrank travel or leverage. You might have to narrow the spacing at the handle but you'll be fine. 

.674 is about 11/16th which would be good for an 1-1/4" flap horn. That's what all my sub 600 planes use.


MM

I ran the numbers using straight line math, which I appreciate is more than likely an incorrect way to do it.  To summarize, the flap horn needs to be 1/8 inch taller to mimic the geometry on the plans.   The bellcrank hole I'm using is 9% further out from the plan's hole.  If I add 9% to the Plan's Flap horn it would be 1/8 inch taller, or 1 5/8" tall. 

Once it is decided, who can make the horns for me?