"Buck style" bellcrank failed after 300 flights

[Chris_Rud]

It has been a hard season for me and this years season was cut short due to another control system failure. I'm posting this so that other people will know what happened since I would absolutely hate to hear of another person having this issue. About a month ago I was flying with Billy and when I had him fly my orange Typhoon he said that it was not wanting to fly level. We checked the controls and there was slop from the bellcrank to the flaps. We cut the top off and looked inside and found that the ball link had worn the hole in the aluminum and the 4-40 bolt from the balllink was rocking. I took off the ball link and laminated the top and bottom with phenolic and JB welded it all back together (you can see this in the picture). It worked great for another 50 flights when on the bottom of an hourglass the aluminum delaminated from the balsa and buckled. It looks like it only buckled about a half inch but when you are turning at the bottom of the hourglass you don't have a lot of room to loose when you don't have full control movement. Like I said earlier I'm posting just to let other people know that they can wear out. I was recommended this style of crank but from now on I will stick with phenolic bellcranks.

So after 670 flights this year I'm going to have to pass on the team trials and get back to the workbench. I don't have the energy to do another 2 week build like before this years NATS. I wish everyone luck and happy flying!

[Brett Buck]

It has been a hard season for me and this years season was cut short due to another control system failure. I'm posting this so that other people will know what happened since I would absolutely hate to hear of another person having this issue. About a month ago I was flying with Billy and when I had him fly my orange Typhoon he said that it was not wanting to fly level. We checked the controls and there was slop from the bellcrank to the flaps. We cut the top off and looked inside and found that the ball link had worn the hole in the aluminum and the 4-40 bolt from the balllink was rocking. I took off the ball link and laminated the top and bottom with phenolic and JB welded it all back together (you can see this in the picture). It worked great for another 50 flights when on the bottom of an hourglass the aluminum delaminated from the balsa and buckled. It looks like it only buckled about a half inch but when you are turning at the bottom of the hourglass you don't have a lot of room to loose when you don't have full control movement. Like I said earlier I'm posting just to let other people know that they can wear out. I was recommended this style of crank but from now on I will stick with phenolic bellcranks.

So after 670 flights this year I'm going to have to pass on the team trials and get back to the workbench. I don't have the energy to do another 2 week build like before this years NATS. I wish everyone luck and happy flying!

   I am very sorry to hear about that.  Where did you get that? To be clear, I *did not* make this bellcrank. I want to know because I want to talk to them.

    The grain appears to go the wrong way. It's supposed to be end grain balsa. With it flat and with no compression fitting I can see how the ball would work loose. Also, when I tested the prototype to destruction, it took a tremendous amount of force to cause that sort of buckling, even after I intentionally delaminated by prying on the forked end. I wonder of the facing material is the specified 6061-T6 aluminum, or (more likely) something much softer.

    Brett

 I blew this picture up and note that the aluminum peeled away very cleanly from the balsa, suggesting that the glue bond was not correct, either.

  I would add that the buckling in my test part was similar but I had to pick myself all the way off the ground and then bounce to get it to fail that way.

[Crist Rigotti]

Sorry to see that happen Chris.  I'll miss seeing you at the TT!

[Randy Powell]

That counts as a splat. Man, that it horrible. Sorry to hear about the failure.

[Crist Rigotti]

I'm wondering if it came from Stunt Hangar Hobbies?

http://stunthanger.com/hobby/product_info.php?cPath=24_29&products_id=54

[Steve Fitton]

Very sorry to hear about that Chris, that was a good flying plane.

[Chris_Rud]

I'm wondering if it came from Stunt Hangar Hobbies?

http://stunthanger.com/hobby/product_info.php?cPath=24_29&products_id=54

Yes I got it from Stunt Hangar Hobby. I believe Tom Morris makes it. Tom makes great products and I'm surprised that it failed.

-Chris

[Tim Wescott]

My condolences -- it's a ratty thing to have happen.

+1 on all the mechanical details of what Brett said.  I don't use a Brett-style bellcrank because I'm too lazy and because it's easy to whack a decent bellcrank out of phenolic, not because I don't think it's at least a little bit better than phenolic.

On top of what Brett said:

Even assuming that you were using exactly the right epoxy, I'm not sure that gluing the wood in side-grain wouldn't contribute to the delamination: at one point my dad did a lot of experimenting with gluing wood to fiberglass, and basically no matter what he did, eventually the glue joint breaks.  Usually it breaks right at the glue-wood interface, leaving a thin layer of wood behind on the glue (in other words, it's the wood that fails, not the glue).  It can seem strong as houses, then one day it just falls off with barely a "pop".  Our theory was that because of differential expansion between wood and glass, both due to temperature and due to humidity, the joint is continually stressed.  Then because wood is not strong in shear, particularly shear in parallel with the grain.  So, eventually, the joint just lets go.

If the wood-aluminum joint was made with CA -- well, CA is noted for being very weak in shear, and brittle to boot.

Brett's method of gluing the wood end-grain not only gives you compression strength at the bolt-holes but also, in my humble opinion, isn't going to come unglued as readily.  Both the conclusion and the opinion stem from wood being strong in compression in end-grain, but very compliant in both compression and tension across the grain.  The compression strength across the bolt-holes is obvious.  The lack of delamination is (again in my opinion) because the wood is so compliant across the grain that as the aluminum grows and shrinks with temperature it just carries the wood with it -- the wood just doesn't have enough fight in it to put any stress on the glue joint.

Brett, assuming I ever get up the gumption to make a Brett bellcrank, do you use any old goo from the hobby shop to glue things together, or do you use JB weld, or do you use something even fancier than that?

[Derek Barry]

Sorry Chris.... I feel your pain.

Derek

[Brett Buck]

Brett, assuming I ever get up the gumption to make a Brett bellcrank, do you use any old goo from the hobby shop to glue things together, or do you use JB weld, or do you use something even fancier than that?

   Not fancier, I use West Systems 2-hour epoxy I got from Jim Aron, who got it from Aerospace composites. It is much tougher than the usual finishing epoxy. Another issue is the potential use of a conversion coating on the aluminum to ensure you aren't gluing to oxidised metal. I apply the glue and then sand it with 240 wet/dry so the bare metal is not exposed to air. If you don't then some sort of conversion coating is a good idea.

    Brett

[Steve Fitton]

Did the motor survive?

[Chris_Rud]

Did the motor survive?

It's crazy but the prop survived. It hit straight in. I think the case is ok and it will turn over but there's ruff spots. Not going to do anything but send it to randy and have him fix it up. I think it will live to fight another day.

-chris

[Bill Johnson]

  I am very sorry to hear about that.  Where did you get that? To be clear, I *did not* make this bellcrank. I want to know because I want to talk to them.

    The grain appears to go the wrong way. It's supposed to be end grain balsa. With it flat and with no compression fitting I can see how the ball would work loose. Also, when I tested the prototype to destruction, it took a tremendous amount of force to cause that sort of buckling, even after I intentionally delaminated by prying on the forked end. I wonder of the facing material is the specified 6061-T6 aluminum, or (more likely) something much softer.

    Brett

 I blew this picture up and note that the aluminum peeled away very cleanly from the balsa, suggesting that the glue bond was not correct, either.

  I would add that the buckling in my test part was similar but I had to pick myself all the way off the ground and then bounce to get it to fail that way.

It appears the grain is the wrong way, and there's no adhesion to the wood that I can see. The wood surface looks untouched. Cleanliness and good surface prep is key to good bonds. I prefer Hysol 9309 for this application. It's used in high strength titanium/composite rotor blade repairs and, if prepped, mixed and used correctly, withstands stresses far greater then you'll see in this bellcrank application. Also, 6061T6 is significantly stronger then 6061T4.

[Motorman]

Sorry to see that. I didn't want to say anything but, I would never have a sandwich bellcrank in one of my planes. Maybe you can make it less likely to delaminate with special glue or whatever but why risk it, a solid piece will never peal apart like that.

MM    

[Matt Colan]

Very sorry to see that Chris! That plane flew great at the NATS.

[Dan McEntee]

I think there was a previous use of this type of construction. Weren't the cabin floors of most airliners made of a composite sandwich of aluminum and balsa, with the balsa grain running vertical between the aluminum? The construction method is sound but needs to be executed properly.
   Sorry about the loss of the model.
   Type at you later,
  Dan McEntee

[RC Storick]

I don't have the energy to do another 2 week build like before this years NATS. I wish everyone luck and happy flying!

You don't need to build one for this contest just borrow one.

[Jared Hays]

Holy Crap 
 

[Steve Riebe]

Sorry for your loss Chris.  Everything seemed to be working quite well for you on Saturday. I'm confident you will be back next season with something even better.  One thing is for certain, the worlds are going to miss one world class competitor.

[Bill Johnson]

I think there was a previous use of this type of construction. Weren't the cabin floors of most airliners made of a composite sandwich of aluminum and balsa, with the balsa grain running vertical between the aluminum? The construction method is sound but needs to be executed properly.
   Sorry about the loss of the model.
   Type at you later,
  Dan McEntee

It's quite common, Dan, going back a long time, and still is. You are exactly right on the grain orientation for floors. Now we use honeycomb in between the skin layers. That method was also used on helicopter rotor blades aft of the spar.

As I said, the key is surface prep. On the aluminum, Brett referred to a conversion coating. That is typically alodine which is a corrosion preventive measure normally used on 2024 and 7075 aluminum, both of which tend to corrode much easier then 6061. On the other hand, prior to alodine, the aluminum is typically acid etched with a material called AlumiPrep 33. After etching, the aluminum takes the alodine (or adhesive) much better.

Sufficient surface etching can easily be achieved by using a maroon (medium grade) ScotchBrite 7447 pad. No green pads! Those usually have a cleaning agent in them for pots and pans which is corrosive to aluminum. ScotchBrite 7447s have aluminum oxide as the cutting agent. After scarfing up the surface to a dull finish, clean with alcohol. In critical applications, you then do a "water break". Spray water over the surface and it should sheet out over the piece evenly. Any beading up and the surface is not completely clean. Once it's clean and dry, it's ready for assembly. Don't worry about exposure to air. It takes at least a week or so for aluminum oxide to form on 2024, a couple days for 7075, and will not typically occur for years on bare 6061 if ever. in fact, 6061 is used with no coatings on secondary engine exhaust components because of it's corrosion resistance.

P.S. Post revised per advice of Bill Byles. Incorrect etching solution called out initially.

[Joseph Patterson]

      What Robert S. said!
        Doug

[john ohnimus]

I think Bill has nailed it. It looks as though there was no adhesion between the balsa and the aluminum. Surface prep is everything!!

[Brett Buck]

I don't anything about this, but have a question.  What is wrong with the old Fox or Perfect solid aluminum bellcrank with the bronze bushing?  Are they not strong enough?

   Not strong enough, wear out, but the real reason in this case is to make a better way for the leadouts to go to the bellcrank. All the methods that loop the leadout through the hole are notoriously prone to failure. including bushing the leadout with tubing. And they tend to tilt on the shaft when used with ball links.

  The two bellcranks I tested to destruction took nearly my entire considerable weight to cause any damage. I cannot conceive of the failure that happened to Chris under the ~10-12 lb load that it sees in flight, or under occasional 45-ish lb pull tests.

     Brett

[Jim Carter]

   Not strong enough, wear out, but the real reason in this case is to make a better way for the leadouts to go to the bellcrank. All the methods that loop the leadout through the hole are notoriously prone to failure.

Have mercy!    Y'all must be flying bricks!!  I've yet to have one fail me in all my years of "planting trees".  As I write this, I have a bird with 510 documented flights on it, 90% of which have been witnessed by my flying buddies over the years) and although it has made a number of dents in the earth I can honestly say not one has been due to the failure of the bellcrank   .  Nearly every one of mine (especially the older ones) has either a Fox or Perfect bellcrank in it.  Other old birds in my "squadron" have 136, 107 and 72 flights respectively, all with either Fox or Perfect bellcranks (I may have to step back because the one with 107 might have been replaced with a Brodak bellcrank, I just don't remember and I can't see inside).  Am I doing something wrong?  Am I missing something something?  Just askin'   

[Brett Buck]

Have mercy!    Y'all must be flying bricks!!  I've yet to have one fail me in all my years of "planting trees".  As I write this, I have a bird with 510 documented flights on it, 90% of which have been witnessed by my flying buddies over the years) and although it has made a number of dents in the earth I can honestly say not one has been due to the failure of the bellcrank   .  Nearly every one of mine (especially the older ones) has either a Fox or Perfect bellcrank in it.  Other old birds in my "squadron" have 136, 107 and 72 flights respectively, all with either Fox or Perfect bellcranks (I may have to step back because the one with 107 might have been replaced with a Brodak bellcrank, I just don't remember and I can't see inside).  Am I doing something wrong?  Am I missing something something?  Just askin'   

   The difference between flying competitive corners and sizes in all sorts of conditions, with large models, for maybe thousands of flights, is a bit different story. The flat aluminum cranks will not stand up to that, nor will the leadouts, for long.

    I don't know what exactly caused this failure. But just imagine the kind of force required to buckle a piece of 6061T6 through that kind of angle over the space of 1". In fact, you probably can't bend a single sheet like that without clamping it into a vise and pounding on it with a hammer.

   Brett

[Lauri Malila]

Chris,

Sorry for your loss.
An easy fix for the buckling problem would be to wrap the outside with kevlar thread and epoxy/cyano. We do it routinely in our wing spars that have similar construction principle. Except that carbon is used on surfaces, not aluminium. Of course, the top view of bellcranc has to be such that the thread does not slide when wrapping. Maybe a small step every 1/4" or so.
I have my doubts that you can ever trust just the balsa core/aluminium skin adhesion in this case, some wrapping would secure it 100%.
Also, maybe a simple aluminium rivet in the critical place would secure the skins in place.
Vertical grain core is of course nice, but personally I think it's not absolutely necessary as the up/down bending forces are quite small. If I used horizontal grain balsa I would, after spreading the glue, stick the wood full of pin holes to spread the glue and shear stresses deeper into the wood.

Lauri

[Avaiojet]

I think there was a previous use of this type of construction. Weren't the cabin floors of most airliners made of a composite sandwich of aluminum and balsa, with the balsa grain running vertical between the aluminum? The construction method is sound but needs to be executed properly.
   Sorry about the loss of the model.
   Type at you later,
  Dan McEntee

That balsa wood end grain thing, it's done this way with the construction of some boats for sub-flooring also.

[RC Storick]

    I don't know what exactly caused this failure. But just imagine the kind of force required to buckle a piece of 6061T6 through that kind of angle over the space of 1". In fact, you probably can't bend a single sheet like that without clamping it into a vise and pounding on it with a hammer.

   Brett

This buckling to me looks like impact damage. I am no reconstructive engineer but I can't see the force our planes fly at bending the aluminum. I can see the impact bending this in this direction. I use that bellcrank and will continue to do so. I'm not sure on the number of cranks sold you would have to ask Tom that question but this is the first I have seen do this.

[Doug Moon]

Chris,

This sucks!  I am sorry to see you lost your plane.

[Chris_Rud]

This buckling to me looks like impact damage. I am no reconstructive engineer but I can't see the force our planes fly at bending the aluminum. I can see the impact bending this in this direction. I use that bellcrank and will continue to do so. I'm not sure on the number of cranks sold you would have to ask Tom that question but this is the first I have seen do this.

I'm not trying to start a debate... I was simply posting a message about what happen to my plane. People can take it or leave it. I put a lot of flights on planes and it lasted for 300. My guess is that most people who use the crank don't fly that much. Maybe I'm wrong... I wore out a handle that everyone else hasn't had an issue with earlier in the year... Maybe I just fly to much 

I have checked all other parts of the control system and everything else is in tact. Clearly something gave when I started that bottom corner. It has to be the culprit. And since the buckling is that exact same thing that happened when Brett was trying to make them fail it seems very clear that it was the reason for the failure.

-chris

[Brett Buck]

Vertical grain core is of course nice, but personally I think it's not absolutely necessary as the up/down bending forces are quite small.

    The purpose was more to prevent something like what happened here. In this case the glue joint failed somehow. If the glue joint doesn't fail, the next failure mode would be the buckling force splitting the wood horizontally along the grain. You cannot pull it apart with tension along the grain lines.

   In any case, with the specified materials it easily beats the pull test *with no balsa at all*, just the two aluminum facing sheets and the standoffs and bolts.

   An earlier version that Tom made also cut huge triangular lightening holes in the center. I recommended that they stop doing that, and also, that any existing bellcranks be wrapped with kevlar thread and glue along the inner arms.

  If anyone is concerned with the solid version, I suggest a series of 1/16" holes about 3/16 from the inner edge, and about 3/32 apart, into which you lace Kevlar and epoxy, keeping substantial tension on the string until the glue dries. Sullivan Tuff-strand lines, for example.

   Again, I tested two of the cranks I made to destruction and it had no issue with this, and I also intentionally peeled the facing sheets apart, and that was also with great difficulty.

   There seems to be a fundamental misunderstanding of the purpose of the design. It was to get a forked end to keep the leadouts alive, and to be stronger/stiffer than the typical 1/4" Delrin/GRP/phenolic. It was never intended to save any weight as weight doesn't matter any more, within reason, and 10 extra grams is worth it to keep the leadouts together. It actually did come out reasonably light but that's just a coincidence.

    Brett

    

[Doug Moon]

    I don't know what exactly caused this failure. But just imagine the kind of force required to buckle a piece of 6061T6 through that kind of angle over the space of 1". In fact, you probably can't bend a single sheet like that without clamping it into a vise and pounding on it with a hammer.

   Brett

Yep, and if this indeed happened in flight then we know the system is seeing a lot more pressure than just 10-12 lbs pull in flight.

I know on Friday morning at the nats during the first round I had to walk towards my plane several times to ease up the pull/pressure. The pull was at least 2x if not more than I normally see in flight. During the round loops I feared it was going to fold.  I know Chris flew during that time as well, the stress point could have started there, who knows...

There are times in flight where the model is seeing severe pressure from all different angles. In flight our planes are in a constant state of "tension" across the entire frame. The pressure will find the weak spot and beat on it until it lets go.  Normal CL fliers might fly a plane 50-75 times a year, this plane would have lasted 4-6 years with those numbers. Long enough to move to another plane. But Chris is still in his early "IronMan" days of his stunt career and still beating his models to death day in and day out.  There is no telling where it all started on this one.

I think it was Bob Barron that said something to the effect of he would build the model using simple techniques, CA, and 5 minute epoxy and expect it to last one year, if you get more than that its a bonus.

CLPA is hard on models.  It may not look like it on the outside because they are so pretty.  But if you open up an old stunter and get a look inside there is typically lots of wear and tear....at least that's what I have seen...

[Brett Buck]

I have checked all other parts of the control system and everything else is in tact. Clearly something gave when I started that bottom corner. It has to be the culprit. And since the buckling is that exact same thing that happened when Brett was trying to make them fail it seems very clear that it was the reason for the failure.

  Yes, but that was at something around 200 lbs of force. That's why I think the one you got has some issues different from mine.

   There's no problem posting your results. It makes me very upset to see this happen with something associated with me and that should have a factor of about of 20 margin over normal flying loads. I didn't cause this and the design is still sound as far as I know. I feel indirectly responsible because I let the design out of my direct control and I had concerns over some of what I saw.

     I would be interested in whether or not the ball came loose due to compression of the wood (i.e lacked a standoff between the sheets where the bolt goes through) or from the face sheet wearing. I can see also sorts of black stuff around the ball. and once the wood compresses a little, the ball will come loose, then pound out the hole.

   Also, when you installed the standoffs for the leadouts, did you have to force them?

    I ask because based on the degree of buckling shown I suspect that the face sheets are not actually 6061T6 but something much softer, and that the spacing was likely not correct, causing a peeling force to be applied. Until I found the electronic standoffs that were machined to a consistent dimension, I spent hours and hours getting standoffs to fit correctly.

    Brett

[Bill Johnson]

It really is as simple as a bond failure. Boeing, Airbus, Douglas, McDonnell-Douglas, Bell, Eurocopter, etc, etc ALL use bonded composites for applications just like this. No threads, no rivet, screws, etc. It's just a matter of proper surface prep and an appropriate adhesive. Carbon fiber is no different. You cannot take carbon fiber laminates straight out of a box/bag with shiney smooth surfaces and expect a reliable bond. Knowing how to bond composites is the key and it's not that difficult to do properly.

[john e. holliday]

How about a picture of the bell crank out in the open.  The picture I see does not look like a typical bell crank failure.  

[Tim Wescott]

How about a picture of the bell crank out in the open.  The picture I see does not look like a typical bell crank failure.  

It may not be a typical bell crank failure, but then it's not a typical bellcrank construction.  Given the way that the plates are loaded, the way the aluminum bent is exactly what I would expect to see from line tension that exceeded the material's ability to stand up.  I'm not saying anything about whether 6061-T6 would stand up here, 'cause Brett already has.

[Randy Cuberly]

Have mercy!    Y'all must be flying bricks!!  I've yet to have one fail me in all my years of "planting trees".  As I write this, I have a bird with 510 documented flights on it, 90% of which have been witnessed by my flying buddies over the years) and although it has made a number of dents in the earth I can honestly say not one has been due to the failure of the bellcrank   .  Nearly every one of mine (especially the older ones) has either a Fox or Perfect bellcrank in it.  Other old birds in my "squadron" have 136, 107 and 72 flights respectively, all with either Fox or Perfect bellcranks (I may have to step back because the one with 107 might have been replaced with a Brodak bellcrank, I just don't remember and I can't see inside).  Am I doing something wrong?  Am I missing something something?  Just askin'   

I know Brett already mentioned something about this but I'm going to anyway to emphasize the fact!
My guess would be that you don't have a problem because you don't fly hard corners with 60+ oz airplanes in windy conditions...very windy conditions sometimes.  The loads go up very significantly when you're flying in a 15 mph wind doing square eights downwind.  Or for that matter even in a 8-10 mph wind like nearly all contests!!

Randy Cuberly

[Gerald Arana]

Wrapping the bell crank with "Kevlar" is a waste of time and energy.

I say this for the following reason: Several years ago Dr. Bayard (Now deceased) a SBSS (south bay soaring society) did an extensive study on wing spars to see if he could come up with a strong/light system to with stand the "ZOOM" launch with out folding the 100" +  wings.

The spar system was: Spruce/balsa end grain web/spruce.
Several were used (Stressed to the breaking point) with different reinforcements being used.

The one with "Kevlar" was an abysmal failure. When load tested the 1/8" X 3/8" spruce spar simple "CUT" right through it like it wasn't even there!

The spar that was the strongest was the one with CF tow & slow cure CA with the spar over it (Top spar) to prevent "oil" canning and CF on the outside of the bottom spar. I don't recommend CA for anything other than liquid pins.

These statements can be verified by SBSS if you can find them. (I've been out of it for 20 or so years)

Cheers, Jerry

[billbyles]

<snip>
<snip> On the other hand, prior to alodine, the aluminum is typically acid etched with a material called Prep Sol. <snip
<snip

Hi Bill,

"Prep-sol" is a naphtha-based paint surface cleaner that was made by Du Pont for use in automotive refinish applications to clean the surface to be painted just prior to painting.  I used it for years in my full-scale aircraft restoration/painting business and it is a very mild solvent based cleaner and not acid-based.  There may be another product with a similar name that is used to etching aluminum but for that purpose I use acid-based brightener followed by alodine.

[Brett Buck]

Wrapping the bell crank with "Kevlar" is a waste of time and energy.

   No, it isn't. The threading of Kevlar around the inner edge directly addresses the issue you see with buckling of the material, by applying the strength of the kevlar (primarily strong in tension) in direct opposition to the forces trying to buckle the aluminum.  I can't say whether the spar application is similar or not, but I suspect the issue with the spar is that the strength and toughness of the material is not used in the correct way. For example, if you just wrap it around the spar, of course it's not going to do anything useful. I guess the spar fails in tension or compression, so you need to run it spanwise, and the wrap, in this case, would do nothing.

     I still imagine that if the face sheets are not as specified, it might just give you a failure in a different spot. Again, I will make up another test piece soon using *no filler material at all* and verify my prior results. I knew I could make it even tougher, but it just didn't seem necessary as it was already gross overkill. Mine have been working for 10 years+ and many more than 300 flights and dozens of pull tests with no detectable degradation at all. I am dumbfounded at Chris' failure as the load necessary to break my version would destroy the rest of the airplane first.

    Brett

[Lauri Malila]

[quote author=Gerald Arana

I say this for the following reason: Several years ago Dr. Bayard (Now deceased) a SBSS (south bay soaring society) did an extensive study on wing spars to see if he could come up with a strong/light system to with stand the "ZOOM" launch with out folding the 100" +  wings.

The spar system was: Spruce/balsa end grain web/spruce.
Several were used (Stressed to the breaking point) with different reinforcements being used...

[/quote]

Yes, maybe several years (decades?) ago with wood spars but not anymore. I understand you're talking about free flight wings?
In time of zoom launches, the max. line tension at release was maybe 10kp. Today the towhook does not even open with such small force. I'd guestimate that the force at moment of line release is between 30 and 50kp. That makes kevlar wrapping absolutely crusial. L

[john ohnimus]

Looks like something soft maybe "0". T6 if bent in that manner would probably crack depending on the grain.  

[Steve Fitton]

Chris what did it feel like when it failed?  When my first Dreadnought had the flap horn fail in the reverse wing over pullout, it felt like a bomb went off in the plane and I instantly knew something had broken and I was doomed.  I'm curious if you felt a bang or if it suddenly just would not turn....

[john ohnimus]

You can also see the delam, the balsa stuck to the lower piece in the pic. The top appears to be a clean separation. And judging from the impact damage to the surronding structure, that may be what buckled the aluminum.

[dale gleason]

My new stunt plane weighed 74 oz at the NATs, and is powered by the ROJett 76. From early indications, it is a supergood flyer, I'd hate to lose it to a mechanical. It has the Brett Buck style bellcrank purchased from Tom Morris. I don't fly nearly as often as a lot of us,  200 flights a year would be my average, usually in pretty substantial wind conditions. I'm wondering if this is an isolated case or can we expect this to be a real threat? Tough question, I know. Maybe Tom can post something to help make the decision on whether or not to perform pre-emptive surgery....

Tough break, Chris. I've got an Impcat and a Mule at your disposal,
dale g

[Gerald Arana]

   No, it isn't. The threading of Kevlar around the inner edge directly addresses the issue you see with buckling of the material, by applying the strength of the kevlar (primarily strong in tension) in direct opposition to the forces trying to buckle the aluminum.  I can't say whether the spar application is similar or not, but I suspect the issue with the spar is that the strength and toughness of the material is not used in the correct way. For example, if you just wrap it around the spar, of course it's not going to do anything useful. I guess the spar fails in tension or compression, so you need to run it spanwise, and the wrap, in this case, would do nothing.

     I still imagine that if the face sheets are not as specified, it might just give you a failure in a different spot. Again, I will make up another test piece soon using *no filler material at all* and verify my prior results. I knew I could make it even tougher, but it just didn't seem necessary as it was already gross overkill. Mine have been working for 10 years+ and many more than 300 flights and dozens of pull tests with no detectable degradation at all. I am dumbfounded at Chris' failure as the load necessary to break my version would destroy the rest of the airplane first.

    Brett

Yes, Brett. It was wrapped "around" the spar. And was totally useless. Dr. Bayard didn't use it span wise. The failures were in compression since that's the way most gliders are stressed.

Kind of like pulling a rope or pushing one! Ever try pushing a rope? 

Jerry

[Gerald Arana]

Yes, maybe several years (decades?) ago with wood spars but not anymore. I understand you're talking about free flight wings?
In time of zoom launches, the max. line tension at release was maybe 10kp. Today the towhook does not even open with such small force. I'd guestimate that the force at moment of line release is between 30 and 50kp. That makes kevlar wrapping absolutely crusial. L
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Are you kidding me!?  My 100" glider could break a 200 LB tow easily in the wind. (that is; if I were towing in the wind) Also I was the only guy I ever saw dive from a few 1000 feet and do a snap roll. That's right! Let me see you do it with your ship!

OBTW, I didn't use the spar system mentioned...........

It was my "Chameleon" that I used in competition for several years.

Cheers, Jerry

Chris, Very sorry about your ship.  I didn't mean to hi jack this thread.

[Ward Van Duzer]

Back to the 50's and 3 inch BC's...There was a reason Fox and Veco Aluminum BC's cost 5 bucks and Perfect BC's cost 80 cents! NEVER use any perfect BC's. NEVER!

Fast forward to the millenium...Windy and Hunt sold thousands of bellcranks. What were the odds of failure with them? Bet most of the failures were improper
lead outs, not bellcranks.

Yes, and toward the end Windy was splitting the ends of his 5/16 in. thick BC's.

And...The more complex the machine, the greater the odds of failure.

Therefore the more parts, the more parts to break.

Nothing can gowrong, gowrong, gowrong....

Just an old phart thinking out loud.


W.

[Richard Oliver]

Several planes, 1000s of flights, no failures. One of mine has 4000 flights over 5 years. I often fly in 15 mph wind, like today.
Will continue to use as is from Tom Morris.

RO

[Crist Rigotti]

Has anybody notified Tom?