Author Topic: Bell Crank Mounting (Read 5167 times)

Michael Massey · « **on:** January 26, 2014, 04:19:07 PM »

I continue to struggle with “porky” airplanes. One area I think that I tend to “over build” is the bellcrank platform. General Sherman would have nothing on me but on the other hand my bell cranks never come out. So in building a solid bell crank assembly, how much is too much? So a little, quick and dirty, experiment.

I built a jig, bell crank with leadouts, and installed 1/16 very light weight balsa with the grain running the same direction as the pulling force on the bell crank would be. That is assuming you install the balsa planking at the center of the wing with the grain running parallel with the wing span. As you can see, the space between the two balsa “mounts” is about what a rib might represent. I used a 6-32 screw to support the bell crank but the screw was not tight on the balsa mounts.

I clamped that assembly in a vice and using my scales, pull tested the installation. No surprise, it was very weak and began to rip the balsa wood with less than 10#’s pull.

Without even bothering to repair the torn balsa, I CA’ed a piece of 1/32 plywood on the top and bottom of the torn balsa bell crank mount. The plywood was scrap plywood, approximately 1” or so and in roughly a square shape. (Nobody said I was an engineer.) I then mounted the bell crank within that “repaired” mount and again pull tested. I cannot tell you at what point the bell crank will pull out as my scales only go to 50#’s. I pulled to 50#, bottomed out the scales, pulled harder and everything held just fine. I held the 50# pull for a minute or so just to see if that made any difference. It did not. The pictures do not show it well but the bell crank mount held to the point I was bending the screw.

How do some of you mount your bell cranks?

Howard Rush · « **Reply #1 on:** January 26, 2014, 05:29:26 PM »

Very nice experiment.

john e. holliday · « **Reply #2 on:** January 26, 2014, 05:33:36 PM »

I am surprised that with that much unsupported area of the bolt that it didn't fold on you. Goes to show how much gain for structure a little plywood can do. I put ply plates top and bottom now on my shafts that the bell crank is riding on. This is after the wing is in the fuselage and the plates butt up to the fuselage inboard side. If I use rods I now cut some slots on both ends of the rods for the epoxy to grip into along with the ply plates. If you do a search on SSW for Al Rabe construction of his planes you will see a very good and light bell crank mount that will tear up a plane if pulled to enough pounds.

Russell Shaffer · « **Reply #3 on:** January 26, 2014, 05:35:33 PM »

It's more than interesting. Thanks, Mike.

Dennis Toth · « **Reply #4 on:** January 26, 2014, 06:05:14 PM »

Mike,
One thing I have found useful is to take a picture of the actual installation in the wing. The reason is if you ever need to check anything later you know what end to open up to (I had to retighten the nut holding the bellcrank and was lucky to be able to go into the bottom and get it done).

Best, DennisT

Howard Rush · « **Reply #5 on:** January 26, 2014, 06:19:28 PM »

Quote from: Dennis Toth on January 26, 2014, 06:05:14 PM

One thing I have found useful is to take a picture of the actual installation in the wing.

Another good idea. Thanks.

Steve Helmick · « **Reply #6 on:** January 26, 2014, 07:37:48 PM »

Mike....Most are mounting their BC's on a piano wire pin, supported top & bottom, just like on a foam wing. Two 1/16" ply plates epoxied onto the top and bottom surface of the center sheeting, transferring the loads into the fuselage sides. The second plate (top & bottom) keep the pin from working its way out. Spacers keep the BC located. The BC should have a long brass tube for a bearing, to keep the BC from wobbling. There should be a PDF showing this on somebody's website. Maybe!

Steve

wwwarbird · « **Reply #7 on:** January 26, 2014, 07:58:26 PM »

Neat experiment, interesting to see an actual test of this sort too.

FWIW, I always make sure my bellcrank installations are extra sturdy as well, possibly sacrificing a slight bit of weight at times. I've just always figured that if there is any one place in a model to overbuild, it's at the bellcrank mount.

Michael Massey · « **Reply #8 on:** January 26, 2014, 08:34:52 PM »

Although I did not go to a lot of trouble with this little experiment, I use "floating" 4" bell cranks. Those, of course, are supported by a 1/8" piano wire with brass bushings for bearings etc. I use those but for this little experiment, I went with the "quick and dirty" essentials because I was only testing the bell crank mounting process.

I like the floating BC with the 1/8" wire. I think they are stronger and fairly easy to install.

Thanks you all for you inputs however. I am always looking for different ways to do things well.

Mike

Tim Wescott · « **Reply #9 on:** January 26, 2014, 08:41:09 PM »

Quote from: Michael Massey on January 26, 2014, 04:19:07 PM

(Nobody said I was an engineer.)

And your workmanship bears that out -- it needs to be a lot worse before any machinist would believe that an engineer made that.

Chris Wilson · « **Reply #10 on:** January 26, 2014, 09:41:29 PM »

Quote from: john e. holliday on January 26, 2014, 05:33:36 PM

If you do a search on SSW for Al Rabe construction of his planes you will see a very good and light bell crank mount that will tear up a plane if pulled to enough pounds.

Agreed, I too like Al's 'hard point' bell crank mounting system as it seems the most direct way to tie together the centre of the post and the fuselage.
It just makes sense that if you have a vertical mount post then its best supported by a vertical plate along most of its length.

Can't really see the value in just supporting the very ends of the post with a horizontal plate - I know that it is sufficient in most cases but there seems to be a better mouse trap out there.

Thanks.

Paul Walker · « **Reply #11 on:** January 26, 2014, 11:06:05 PM »

What I'm surprised about is the amount of load the balsa only installation sustained.

I use top and bottom 3/32 ply plates with the bell crank on a 1/8" piano wire post. The B-17 had either aluminum or carbon to support the bell crank. It was on a 1/8" piano wire that was used to splice the wing together at the root. It sustained the FAI pull test(10 times plane weight, which was a LOT).

Nice Mike!

pipemakermike · « **Reply #12 on:** January 27, 2014, 12:41:43 AM »

When Al Rabe put his description of his method of bellcrank mounting I captured it and posted a cleaned up description on my website. You can find it here:-
http://www.machineconcepts.co.uk/aeromodelling/rabe_bellcrank/rabe_bellcrank.htm

john e. holliday · « **Reply #13 on:** January 27, 2014, 08:26:01 AM »

Thanks Mike, that's the construction I was trying to find.

James_Mynes · « **Reply #14 on:** January 29, 2014, 02:02:30 PM »

This is my recent installation in a Magician wing.

Randy Cuberly · « **Reply #15 on:** January 29, 2014, 07:17:42 PM »

I also think your testing was interesting and provided some very useful information.
I will say catagorically that I don't really think the bellcrank mount in a competitive stunter is the best place to look for making a light wing. There are of course caveates to that statement in that it certainly is possible to go overboard, but the idea of using balsa, which you probably did not intend to do but merely used that as a base line, which is OK but I would hope no inexperienced folks that see this thread think that it is acceptable. Please understand that I don't say that as a criticism to you just a clarification for less experienced people.

That said I personally believe that the bellcrank mount should be supported by both main wing spars and both above and below the bellcrank. This provides the best transfer of energy into the wing and puts the glue joints in shear and allows a long glue joint as in the photo's below. The photos show a typical bellcrank mount that I've used in many airframes for a lot of years and have never had a problem. No I didn't invent anything with it I picked it up in the 1960's from someone else I can't even remember but it is commonly used today in various iterations. An important thing to note is that it also provides great strength for the wing center section and spars. It also keeps the 1/8 music wire relatively short which adds much resistance to bending and flex of the bellcrank position, which I think is a slight disadvantage to the methods that support the bellcrank on top of the wing sheeting against the fuselage (of course that makes for a strong mount but the longer wire certainly is subjected to more bending stress).
The photos show the mount and bellcrank in my Whitely Shoestring that is under construction (scratch built not the kit).
I used to drill lightening holes in the plywood until I weighed both and realized that the 2 or three grams of weight saved simply wasn't worth the work involved. This wing is 670 Sq in. and weighs 8.7 oz as it sits. Light enough in my opinion.
Balsa selection and geodetic construction that allows the use of fewer ribs and less wood is the answer to light wing construction in my humble opinion...no I didn't invent that either.

Randy Cuberly

Michael Massey · « **Reply #16 on:** January 30, 2014, 05:23:52 PM »

You are right Randy. I would never suggest mounting the bellcrank in a stunter as I did in the experiment. But I never really knew where the breaking point of my bell crank installations were. I pull tested several planes that I had crashed for non related reasons, but was never able to measure a breaking point. The purpose of this experiment was to start at the other end of the spectrum, that being the very weak installation and strengthen it from there until I found a strong, but not overbuilt, installation. I was just very surprised at how strong the seemingly weak installation that I tested actually was. I had no idea that soft 1/16 balsa, supported by two thin plywood lamination's would support that much stress.

With that knowledge, I feel I am a little more flexible in adapting some of my bell crank installations to fit my needs at the time.

Randy Cuberly · « **Reply #17 on:** January 30, 2014, 07:37:50 PM »

Quote from: Michael Massey on January 30, 2014, 05:23:52 PM

You are right Randy. I would never suggest mounting the bellcrank in a stunter as I did in the experiment. But I never really knew where the breaking point of my bell crank installations were. I pull tested several planes that I had crashed for non related reasons, but was never able to measure a breaking point. The purpose of this experiment was to start at the other end of the spectrum, that being the very weak installation and strengthen it from there until I found a strong, but not overbuilt, installation. I was just very surprised at how strong the seemingly weak installation that I tested actually was. I had no idea that soft 1/16 balsa, supported by two thin plywood lamination's would support that much stress.

With that knowledge, I feel I am a little more flexible in adapting some of my bell crank installations to fit my needs at the time.

Yes I understand your methodology and I wasn't trying to marginalize the importance of the testing you did.
I would mention that there is always one problem that any kind of strength testing doesn't really consider and that is fatigue, especially fatigue caused by age, temperature cycling, and vibration. Nearly all materials are subject to fatigue from those three factors. Unfortunately the only practical way for us as modelers to deal with those factors is to insure that our control installations are a lot stronger than they need to be just to survive a pull test, or flight stresses.
Vibration is a big factor in wear and fatigue so as much as I hate electronic "gadgets" I have to admit as a mechanical analyst that there are clear advantages to the lower vibration factors of electric motors.

Of course the other two factors still exist!

Randy Cuberly