Broken Bellcrank

[Brian Massey]

This happened to a Gary Anderson yesterday. His first flight of the day with his still new SV-11 was a bit slow, around 6 second laps. Gary attempted his reverse wingover because he felt he still had good line tension. When he pulled over the top from the inverted flight the lines sagged and the wind caught the plane. He back peddled and caught it with room to pull out, but instead the bellcrank broke and the lines pulled out of the plane; at that point the plane "free-flighted" straight into the ground.

The bellcrank looks like a phenolic type, similar to the Tom Morris units. Is this a known problem with phenolic bellcranks, or only with cheap TM knockoffs (This was an SV-11 ARF, so I'm assuming components are not TM originals)?

I'm just curious as I have several planes with TM bellcranks installed, and several more TM bellcranks still in their bags waiting for a new home. So far I've had no issues with them, and I hope no one thinks I'm trying to impugn them.

Thanks,

Brian

[Tim Wescott]

Here's what little I know.  Some of it comes from hanging with mechanical engineering types, some of it comes from growing up in a shop that did fairly highly technical fiberglass work.

Any sort of composite material (which is what filled phenolic is) is going to have strength characteristics that depend on all of the stuff thats put in.  Most composites are built up of a resin and at least one filler.  Resins (phenolic, polyester, epoxy, nylon, etc.) are usually brittle.  Fillers are usually some fibrous material that has (at least comparatively) great tensile strength, but is "floppy" in its natural state.  Typical fillers are things like cloth (linen or canvas), carbon-fiber, glass, Kevlar, etc.  The fibers strengthen the resin by providing tensile strength, the resin strengthens the fibers by providing strength in shear (which is what turns those floppy fibers into something rigid).

So I would expect that cheap phenolic, particularly if it's chosen for looks or for price, is going to be many times weaker than good engineered phenolic from a reputable supplier.

That bellcrank looks like it's made from a poor excuse for linen filled phenolic.  The stuff that I've seen is linen filled throughout, and would break with a very ragged edge.  That stuff looks like it's got 'ragged' right at the outsides of the sheet, and fairly smooth in the middle.  This leads me to believe that it's paper filled (or maybe wood dust!) in the middle, and only linen filled on the outside where you'll see it and say "ooh!  high tech!".  If you have access to the pieces, you can sort of tell: file the end flat (or look at the edges).  You should see the ends of fibers everywhere.  If the center looks smooth, and you only see fiber ends on the edges, then it's fairly cheap stuff.  It'll have strength against bending up or down in the thin direction, but not much more strength than unreinforced in the thick direction.

If Tom Morris is using the good stuff, and hasn't been having problems, and isn't getting screwed over by a supplier (which happens to anyone in the manufacturing business, from time to time), then I wouldn't let that event turn you away from phenolic bellcranks.  Cheap piece of @#$% bellcranks, yes -- but you can make one of those in just about any material on earth, not just phenolic.

Edit

In addition to Brian's question, which I only answered partially and with a lot of extra words, I have a question for the more technical minded:  Does anyone know just how much force gets exerted on a plane in one of those "slack off and bang against the end of the line" events?  Clearly it can depend on circumstances to a huge degree (stiff-arming it vs, letting it ease into tension, etc.) -- but some "typical" number would be nice.

More edit

The only other thing that I can add is that a steel or aluminum bellcrank is going to be more likely to fail gracefully, i.e. get bent to a pretzel without breaking, than a phenolic or carbon fiber, or etc., bellcrank.  Usually the end result will still be a crashed plane, so I don't think we should all run out and change.  But it's something to consider when you're deciding how much extra strength to build into your bellcrank.

[Bill Little]

HI Brian,

Sorry to hear of Gary's mishap!

As far as Tom's actually bellcranks, (not the copies) I have never heard of a failure.  Plenty of models out there with them installed, and they have been around for a pretty long time so I would not be concerned about the ones that come straight from Tom.  He does make them in two different thickness so I always use the thicker ones for large models.

(if you ever want to let some of yours go, let me know! )

Big Bear

[Brian Massey]

In addition to Brian's question, which I only answered partially and with a lot of extra words, I have a question for the more technical minded:  Does anyone know just how much force gets exerted on a plane in one of those "slack off and bang against the end of the line" events?  Clearly it can depend on circumstances to a huge degree (stiff-arming it vs, letting it ease into tension, etc.) -- but some "typical" number would be nice.

Hi Tim;

Thanks for your very good response. In looking at the pieces after the crash, it looked like just cheap fiber board with a plastic coating; it did not look anywhere as good as the TM bellcranks I have. Your explanation just confirmed what I thought. And your edit . . . while standing around helping Gary pick up pieces, we all posed the same question. Gary felt the "jerk" (not him!) on the lines was only moderate in comparison to other instances of catching a plane. From outside the circle it looked like he caught it early enough, and without having to "run" backwards, so we were all surprised with the outcome. If a plane pull tests to 35-40 pounds, what is the amount of force exerted in a "moderate" jerk??? What would be an average multipling factor???

Sorry Big Bear; I have no plans to let go of my stash; I have complete confidence in TM stuff. 

Brian

[Howard Rush]

I have a question for the more technical minded:  Does anyone know just how much force gets exerted on a plane in one of those "slack off and bang against the end of the line" events?

How much energy the lines can absorb before getting to their breaking point is an issue for picking lines to survive one of those events, but the bellcrank need only be stronger than the static breaking strength of the lines.

Good point on the phenolic, Tim.  My guess is that, rather than specifying the strength of the two-sigma bellcrank, the spec was, "Make it look like this."  

[Howard Rush]

About 100 lb. for a pair of .018" lines

[Tim Wescott]

"Make it look like this."  

Or just "Make this".  Back in the early 1980's, at the beginning of the made in China wave, a company in the antique car industry would get old Ford parts and send them to Taiwan for copying.  Then they'd sell them cheap here.  At one point they were selling a cowl vent hinge for a 1932 Ford that was an almost exact copy -- not of what Henry made, but of the cowl vent hinge that my dad's company sold.  My dad's hinge was in no way a visual duplicate: it was made from steel that was flame cut (10 gauge, because that's the smallest that Cliff the Welder would do), with bits of tubing, all welded in a jig.

We teased them about it, and they insisted the hinge had come off of a rusty hulk behind a barn.  Apparently it was a rusty old hulk that someone had worked on a bit!  But the lesson we learned is that you can send anything to China and get a cheap knock off.  Getting the quality up is your task -- so is keeping it up.

[Douglas Ames]

1. "He back peddled and caught it with room to pull out, but instead the bellcrank broke..."

2. "Made in China"

When he caught it, the lines tensioned and snapped at the weakest link...Your ARF-ie bellcrank. That Sucks.

[Mike Greb]

I never have been a fan of big cutouts on load bearing parts. A little bubble in the layup, or a deep scratch and you can go from an 8X safety factor to a 1.5 in a hurry. That being said, I have been flying Strega arc models for the past four years that probably have the same bellcrank.

[John Stiles]

Older guys don't go backwards well....they tend to fall on their rump. At least....crippled up old guys like me, anyhow. But I made it a rule here, never to run backwards, if you don't snatch the bell crank out....you interrupt the attitude of the plane and crash anyway. Since I made this rule, we discovered that in many instances, the plane will recover on it's own. And in most cases that it doesn't, the resulting crash landings aren't all that bad. Too bad about the broken phenolic BC, I have several T.Morris BCs in my planes....never had that happen.....yet.

[Larry Cunningham]

Wow, that almost looks like bakelite, it's certainly some type of phenolic. And that material is tough and hard to break.

I guess it's absolutely certain that it was broken in flight and not in the crash?

For things like this to occur, there is invariably a flaw or a nick or some damage that induced it. You didn't show the other piece of the bellcrank, but it would be worth sticking the pieces back together and looking closely for the flaw - it broke there for a reason.

One thing we may not always appreciate is just how hard the ship can hit the end of the lines when it gets a chance to free flight directly away from the pilot and pick up speed. (When it happened to me, the line cut my finger..) Still, I would have expected a line or even leadout failure before a phenolic bellcrank.

I know there is an emphasis on keeping weight down, and bakelite is a dense material, but I notice the phenolic bellcranks I have on hand have multiple large round holes for the cutouts. The one in the photo has a pretty sharp radius corner on the inner cutout near the leadout hold.

Another comment, perhaps without merit. Perhaps it is the photograph, but the (unbroken) edges of the bellcrank look a bit fuzzy, and the material looks "coarser" to me that the smoother, denser phenolic I've seen before. Even in a composite material like this, a sharp edge is an invitation for development of a stress riser and subsequent failure. It might be worthwhile to smooth the corners with sandpaper; it certainly wouldn't be harmful. But you really should not have to do this (nor should the manufacturer).

I am guessing the phenolic material is at fault, or there was some hidden damage.

Good luck, very sorry to hear about the loss of a good ship.

L.

"Traffic is very heavy at the moment, so if you are thinking of leaving now, you'd better
 set off a few minutes earlier." -Anonymous Traffic Report

[Steve Helmick]

Older guys don't go backwards well....they tend to fall on their rump. At least....crippled up old guys like me, anyhow. But I made it a rule here, never to run backwards, if you don't snatch the bell crank out....you interrupt the attitude of the plane and crash anyway. Since I made this rule, we discovered that in many instances, the plane will recover on it's own. And in most cases that it doesn't, the resulting crash landings aren't all that bad. Too bad about the broken phenolic BC, I have several T.Morris BCs in my planes....never had that happen.....yet.

My policy is not to backpeddle, but to turn around and run like the wind, while holding up elevator. It's saved a couple of planes. So will a couple of degrees of right thrust.  Steve

[Brett Buck]

Wow, that almost looks like bakelite, it's certainly some type of phenolic. And that material is tough and hard to break.

     Tim, in this case, is right - it all depends on the filler material. It it has woven linen it's moderately tough. It if it has sawdust (like bakelite) it's extremely brittle. This looks more like classic chopped-fiber filling (maybe linen, maybe whatever they happened to have laying around that day) like they used to make circuit boards out of, and it's not usually very tough at all.

   But the real kicker is, tough or not - it's 1/4" thick! Thats about 10x as thick as it would need to take the static load, so you would expect even a pretty poor material would be sufficient. I suppose it maybe have been overstressed while being machined, and then the crack traveled. You aren't likely to overstress it using it as a bellcrank, even in this pretty extreme condition.

    I also wonder if the cutout had sharp corners.

    Brett

[Steve Helmick]

I'd like to know if the BC in question was router cut or water jetted, or whatever. In either case, the internal cuts would be made first, then the outside. To start the cutouts, a dopey programmer might have made the plunge cut at the radius, leaving a weak spot. I saw this often with water jetted parts...steps, bumps, "draft angle", etc.  For programming a CNC mill, I always made a little extra effort to ramp in and ramp out so that there was no sign of mismatch, giving a much better looking part. Some appreciated it, but maybe some thought it was a waste of time.

Phenolic is one of the least favorite things to machine ever created (yellow dust), so I always avoided it as much as possible. It does seem to me that there is a grain to the stuff, and this looks like the grain was going the wrong direction. I could be wrong on the grain issue, but the break sure looks like it. I'm not in favor of making cutouts in any bellcrank, but I'm not an engineer. I think that when the loads get serious, the cutouts at least could be tapered, not parallel to the outside as shown. I also wonder whether this BC was installed "backwards" or not, because I think that changes the stresses relative to the grain direction(s) of the phenolic to be at least a potential problem.  Steve   

[William DeMauro]

Send the pctures and a note to John Brodak. I bet he makes it right!!!!

[Gary Anderson]

Send the pctures and a note to John Brodak. I bet he makes it right!!!!

Hi guys,
The SV-11 is a very nice flying plane. I'm glad to let ya know that the Brett Version Ro Jett wasn't hurt in the crash. I had her to rich but line tension seemed fine on the first leg of the wing over when I hit the second leg of the wing over the lines sagged and I caught her before she would have hit the ground, the next moment my flying line where wrapped around me. I couldn't believe that the bellcrank let go. She didn't even hit that hard on the end of the lines?? I don't know how to forward this article to John??? I glad that there is ARF's but I have had my share of bad luck with them and not sure I'll be purchasing any. I have crashed many plane through out my flying years and have never broke a bellcrank??? I have reused many of them, just have never broke one?? The lost of the three blade prop was a hard pill but I'll get over it. Just lucky my sweet running baby is okay. There is a old saying, If Ya Don't Want To Crash-Don't Fly? Brian thank you for posting this article for me and I don't really know how ya can test a bellcrank, just a regular pull test?

[Tim Wescott]

Gary, just send him an email with Brian's pictures.  There's a "contact us" page on the Brodak's site.

Once it's in a plane I don't know a good way to test a bellcrank other than a pull test.  I'm planning on making some phenolic bellcranks, but I think before I do I'm going to make some test pieces and whack them with a hammer, to make sure I understand the ways they can break.

[Randy Powell]

I make CF bellcranks and never had one fail if I kept it as it comes out of the mold. I did saw one in half and use a chunk that was only about an 1/8" thick. Probably would still have been fine if there hadn't been a bubble right around the post. As it was, they thing went in with the bubble split the bellcrank, This was more an idiocy failure than a materials one.

[Perry Rose]

I had a plane hit the lines hard enough to break the lines at three termination points and bend the bell crank post, 1/8" music wire. The homemade bell crank and the 1/16" ply post supports on the wing center sheeting were not damaged.