Bellcrank Test

[Motorman]

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[Brett Buck]

I'm about to pull test a couple of bellcranks I made for a 575 Sq. In. stunt ship with an OS LA 46. One is made from 1/8" G10 and the other is phenolic/linen. I don't think I can make them any lighter. The G10 is 10 grams and the phenolic is 7 grams. I don't want to destroy them but, how much should they take? 75 Lb. pull test enough?

     I would do 2x the projected pull test. 75 lbs is 2x for a 60 ounce airplane.

    Brett

[Ken Culbertson]

I'm about to pull test a couple of bellcranks I made for a 575 Sq. In. stunt ship with an OS LA 46. One is made from 1/8" G10 and the other is phenolic/linen. I don't think I can make them any lighter. The G10 is 10 grams and the phenolic is 7 grams. I don't want to destroy them but, how much should they take? 75 Lb. pull test enough?

Thanks,
Motorman

Brett beat me to the strength answer.  2x is what I use but I do it more to test my connections before I fly and take out the slack in the wrappings.  I do it just before finishing.  I hook up a handle and about 10' of cable and pull.  I also work the controls under stress including letting each line take near full load.  Ever since I had a BC overcenter on me a couple of years ago I have been quite "anal" about bellcrank strength.

Ken

[john e. holliday]

I want to see some one fly a plane that pulls the same as the pull test.

[Air Ministry .]

20 G.  or double that. with a ' jump/bounce' at the full load . 

[Dan McEntee]

   Well, if you keep getting obsessed with this weight thing on a part as critical as a bell crank you will get the disaster that you are trying to avoid. The few grams that you are obsessing over can surely be eliminated collectively from the rest of the model and/or finish. Spend the time related to it on something else. I think this is the last part that you should be trying to save weight on when it doesn't really matter.
  Type at you later,
   Dan McEntee

[Ken Culbertson]

 

   Well, if you keep getting obsessed with this weight thing on a part as critical as a bell crank you will get the disaster that you are trying to avoid. The few grams that you are obsessing over can surely be eliminated collectively from the rest of the model and/or finish. Spend the time related to it on something else. I think this is the last part that you should be trying to save weight on when it doesn't really matter.
  Type at you later,
   Dan McEntee

[Brett Buck]

Well, they both withstood a 75 Lb. pull test but, I didn't like the way they were flexing. Stay tuned.

    It's 75 lb on a thin, 10-gram, fiberglass plate, of course it flexed. The failure mode will not be with the load in-plane,  it will be when there is a slight twist, and then it pulls out-of-plane.

      Take two thinner plates, separate them with some lightweight filler material like phenolic honeycomb, or balsa, you get much more cross-section for nearly the same weight, but much stiffer and not likely to pull out of plane.
   
     Brett

[Dave Hull]

Brett,

If he beefs up the bellcrank but saves the weight in other places as some have suggested, it likely will "pull out of plane."

The Divot

PS--An unnamed club member borrowed an old-time plane to round out his entries for a contest. During the pull test, the bellcrank mount failed. I think it zipped thru all the ribs out to the wingtip. Dunno how that turned out. I can just see him handing back the plane with a limp inboard wing....   He was scarred for life by that experience.

[Brett Buck]

If he beefs up the bellcrank but saves the weight in other places as some have suggested, it likely will "pull out of plane."

   I am not sure why we are so concerned about bellcrank weight (on the order of grams) or strength - the larger problem is the leadout failing, which is completely inevitable using a single plate bellcrank, no matter how you think you are protecting it.

  Most people don't appreciate the kind of beating that the control systems in these planes take -apparently FAR worse than the biggest RC models, because the "extra heavy duty" RC hardware tends to fail or wear out, sometimes in short order, on stunt planes. It almost seems as if the vibration may be a bigger problem - I have worn out a metal quik-link on the *rudder*, where it is never intended to move, it is running on *plywood*, and it continually bathed in oil. But it is firmly attached to a long, very stiff beam with a big-boy piped 61 at the other end.

      Brett

[Air Ministry .]

As with most drivers at the time, Ritchie was also an experienced sports car driver, and he drove one of the new Lotus 40 cars () for the Los Angeles Times Grand Prix for Sports Cars at Riverside, California on October 31, 1965.

It was at this race that when Ginther was asked what the difference was between the Lotus 30 and 40. He answered that they took the Lotus 30, made ten more mistakes and called it the Lotus 40.

C. A. B. C. , Colin Chapman's theory was if the car disintegrated as it crossed the finish line , it was designed to the optimum limits . Some did this previous to the finish line .
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Some ' Ulta Lite ' F2B ships, in the 80s , were sucessfull if they lated one season .
The Nose Ive just repaired on a light 60 size  ship , mainly  sufferd from fuel soak . AND less than sufficient adheshive in the light Tri Lam nose sides . !/32 ply, 1/32 balsa , 3/32 balsa .
Think itd been flown a few seasons though .

A plane built to last DECADES will necesarrilly have a touch more glue throughout. Particularly in strategic places .

1/. Belcrank Mount .

2/. Spar Joiners ( Likely IS 1/. also . Dual Purpose )

3/. U. C. mounts .

4/. Wing & Stab to fuse - doublers .  Glued Inside - fillet . wing to fuse . Some even F Glassed the inside join / fillet . Stab. L E & Spar likewise - maintains integredy .

4/. A .  Stooge Tailwheel Leg . A S. Hand plane with stooge loop on tailwheel wire , I caght the tailwheel leg on a seat getting it in & out of the car . Releved it broke free there , rather'n when I was halfway
     to the handle . Sloting & Keying a few bits there wot hurt none .

SO we pick up a few ounces . Even four or five . Super heavy duty bushed controls may ad a few Oz. , of that total . But the Fatigue Life is greatly enhanced .

F. S. Aircraft components are lifed , and replaceable . A D C 3 was somewhat durable . Horses for courses . A lite wt. specifically for a seasons use ( hopefully ) will be lighter than a Sherman Tank ,
But may not accept that much miss use .

Vital Components should not aproach opitmum failure loading in use , around a 200% saftey margin would be a reasonable datum . Unless its a one season wonder , at say 150% . Maybe thereabouts .

 

[Alan Resinger]

Here is the bellcrank that I  designed about 7 years ago along with Ark Gorodetsky.   It is expertly CNC machined by Ark from high strength aircraft aluminum. Besides Ark and I, Chris Cox, Paul Walker, Jim Hoffman  and some others use it.  We normally fly 300 or more flights a year so these things are durable.  Weight as shown with ball link and case hardened 1/8" high strength steel pivot wire
In delrin bushings is 21.5g.  I have pulled this thing at over 150lbs.  I suported my weight on it.
As can be seen, it is a forked arm crankwith brass eyelets on the ends od the Sullivan C-D leadout cable.  I agree with Brett in that the cable is the weakest point.  I don't know how many times in the past 40 years l have performed  contest pull tests, but know that a true 45 pull test is a lot and even in the worst wind I have ever flown in has never reached 45 pounds.of feel at the handle.

[Brett Buck]

I didn't make this one, I was just a consultant:

[Craig Beswick]

What material is the filling?

[Tim Wescott]

If you're pulling them at 20g and they're getting distorted but aren't getting damaged -- who cares?  The 10g pull test is designed to be a "that's never going to happen" pull already, and you're doing twice that.  So what's going to happen?  The plane undergoes some event where it bounces 20g at the ends of the lines and at that moment the control geometry is messed up?  So at that moment you can't steer the plane as well as you normally would -- but you had no control whatsoever up to that moment, the pilot is probably freaking out anyway, and as soon as the plane settles on the ends of the lines then you'll have full control.

Stop worrying.  Ship it.

[Brett Buck]

What material is the filling?

  End-grain balsa.

     Brett

[Brett Buck]

In what?  It is shaped to give neutral controls on an Infinity, so the arms are angled at the complementary angle for the leadouts, and the ball link connection is angled to give a 90 degree projected angle when viewed from above.

    This prevents it from being "self-centering", or "self-uncentering", that is, neutral loads.

     Brett

[Steve Fitton]

Where do the phenolic sheets come from? Can you cut them with a bandsaw?