Author Topic: Why is my wing cracking? (Read 10180 times)

Matt Colan · « **on:** July 21, 2011, 10:45:15 AM »

I noticed when I was doing some maintenence on the Trivial Pursuit that there was a stress crack going around the leading edge of the inboard wing. I followed the crack and it went through the bottom of the fuselage and was working its way down the centerline of the wing. We sealed the crack with epoxy and glass cloth thinking that would solve it. I flew the plane two more times on Saturday to show it off on my birthday, and when I went to clean it, I noticed that the crack we thought we sealed continued traveling down the wing at the fillet, and was just starting to turn towards the fuselage. I put a little pressure on the sheeting to see if it was soft and I noticed a 2nd crack appeared where we sealed the first one. I then looked on the outboard wing to see if it was cracking there, and sure enough it started cracking at the leading edge and was working it's way down the wing, almost exactly like the inboard wing. I went to see if the sheeting was soft and sure was also soft.

When I was at the NATS, at the beginning of the week, the plane made no creaking sounds on a pull test. By the end of the week, it sounded like an old house (maybe an exaggeration, but when you walk down the stairs of an old house, it creaks, much like what I could hear with this plane).

Here's some pictures of the cracks, does anybody know what might be causing them?

Matt Colan · « **Reply #1 on:** July 21, 2011, 10:46:30 AM »

And here's a few more of the pipe tunnel where we laid the glass cloth in just to show how far it traveled.

Derek Barry · « **Reply #2 on:** July 21, 2011, 11:00:07 AM »

a few questions....

what kind of glue did you use to put the wing in?

What did you use for fillets?

Is it just the paint cracking or is it all the way through the Balsa?

I have seen paint become very brittle before, we tried a new paint one time and after every flight I notice new cracks. Even the open bays were cracking.
Another thing is, with that fancy removable bottom block you are making the structure weaker that it has to be. I have done it that way before and I didn't have any problems. Now I just make the bottom open and leave the fuse sides full. Stress cracks do happen sometimes but I would say that yours seem to be pretty bad.

Derek

Peter Nevai · « **Reply #3 on:** July 21, 2011, 11:23:30 AM »

My gut feeling? An inferior grade of epoxy, A fast cure epoxy or a improperly mixed epoxy joints are failing. Especially considering the locations of the cracks. Many quick set epoxies tend to not to harden solid many are down right rubbery after cured. Improperly mixed or a inferior grade epoxy can produce similar results. I also let epoxy joints cure a couple of days before painting or applying any type of solvent based product.

I have learned to shun any fast cure Big box store brand epoxy, except for at the field fast repairs. I use the slowest curing best quality marine grade epoxy adhesives I can find that are available in small quantities. My experience is that Devcon, Loctite, and other consumer grade epoxy adhesives are not suitable for our use. Which is why I really miss the Hobby Poxy II product. It was a great slow epoxy that had a 3 hour working time and an 24 hour cure. Stuff was tough as nails.

Heat from the pipe can also soften low quality epoxy joints. Actually heat will soften any epoxy that is not mixed properly or not 100% cured.

This is asuming that you used epoxy at these locations and had good and tight fitting wing to fuse joints.

You asked so that's my guess. Could be the epoxy joints are flexing and perhaps failing at some points.

Chris McMillin · « **Reply #4 on:** July 21, 2011, 11:23:35 AM »

Typically one wants to make the wing mounting area as solidily boxed together as possible. Since you seem to respect Windy, I point to his admonitions against large, open spaces on fuselages covered by cowlings with their associated fasteners. Where you are having these fractures it looks as if the fuselage is constructed with a shallow attached fuselage remnant below the wing. This leaves the fuselage wide open across the bottom. The pipe tunnel cover is a very non structural cowling. Using these to cover the bottom of the pipe tunnel you've installed cap screws as fasteners and they have much more doubling than the fuselage sides for the wing. These steps from one thickness to another are stress risers, but the small amount of fuselage side isn't enough material to keep the twisting of the fuselage to wing from cracking the structure.

To repair this and make the model last, remove the cowlings and permanently install a pipe tunnel cover and reduce the size of your engine cowling. To do this I would Dremel all of the fastener mounts out of the pipe tunnel to make a uniform surface and epoxy a medium balsa doubler from as far forward of the wing as possible to as far aft of the wing as possible. This piece should be made to conform to the exact shape of the wing bottom and make contact all the way with no gaps, front to back. Finish the interior of the pipe tunnel. Then install permanent bottom to the pipe tunnel. It should be epoxied to the sides carefully, a round cross section will be better than a straight piece, I think. Make your smaller engine cowling mount up to this new pipe tunnel as far forward of the wing leading edge as possible with an 1/8th ply bulkhead opened up into a ring like a real airplane has, the pipe tunnel should have one at it's forward end and the engine cowling at it's aft end. They should mate up like a mirror to one another, to save most your present engine cowling it could sweep aft from top to bottom.

This way the model has no flexing across the pipe tunnel where the wing attaches to the fuselage. It is now boxed together, the box carrying the load of the fuselage twisting against the wing surface.

Hope this helps, you could do this in a week, minimal finish repairs, all on the bottom except for the fillets around the front. Better this than having the wing separate in flight, which is what it'll do. Good luck, nice save, and we'll be looking forward to your repair photos.

Chris...

James Mills · « **Reply #5 on:** July 21, 2011, 11:24:31 AM »

Matt,

As far as the soft sheeting, did you dope and then overlap the tissue/polyspan (not sure what you used) in the center? I also wrap the center of the wing with fiberglass as insurance, I know it may add a little weight but just look at it as insurance. Hope you get it solved, nice looking plane.

James

Peter Nevai · « **Reply #6 on:** July 21, 2011, 11:35:07 AM »

I wonder what if any departures from Teds design there are and or if Teds TP had similar flaws. Is allmost the entire bottom of the airframe open? If the fuse is technically only connected to the top of the wing as suggested above then there's your answer. The wing has to be in 4 sided fuselage box. there has to be a full length pipe tunnel floor epoxied in running from at least the leading edge to trailing edge. Where ever there is no solid bottm fuse block or planking.

Tim Wescott · « **Reply #7 on:** July 21, 2011, 12:02:32 PM »

Matt:

Don't give up unless you've really analyzed the situation
This could be trivial, this could be way bad

Cracks on the surface don't always reflect what's going on underneath in the structure. Normally I'm on top of this, but I recently had to repair a plane three times, because the actual damage to the plane was nearly a foot away from the cracks in the external sheeting. In my case the spar had cracked about half way out on the wing. It took out the glue joints between spar and shear web all the way to the center section, and forced the wing sheeting to take all the load of the spar. This broke at the wing root, and because the shear webbing was broken I could see the spar moving at the center section -- which convinced me that the spar was broken at the wing root. The last time I fixed it started out with "oh what the hell, it's trash anyway, cutting it up won't hurt" and led to an accurate diagnosis, which in turn led to a correct fix. So now I have a slightly rattier, but much stronger, plane.

So any time I see cracks in a structure like this I ask myself what's really damaged. To answer that question, I try to find out how the break moves -- what stress on the airframe makes the cracks open up or close? Does it happen when you twist the fuselage along its length, restrained by the wing? Does it happen when you load the wing (put the fuselage on your thigh and push down on the tips, or rest the wing roots on a pair of supports and push down on the tips). Does it happen when you try to pull the wing out of the fuselage? Does it happen when you try to bend the fuselage along it's length? Up and down, or side to side? Does it happen if you try to bend the wings straight back, or straight forward?

I also try to think of the history of the plane. Have you crashed it? (Didn't you crash this one in practice before the nats?) You may have damaged some of the internal structure, and put all the load on the external sheeting. If that's the case, think about the direction of the impact in that crash, and try to replicate the forces from it -- see if the plane tends to flex more in that direction than others.

I don't know whether your problem is that the plane just isn't strong enough, or if there's hidden damage that's not getting fixed when you ~~goober up~~ repair the outside with epoxy. It's got to be one or the other, but what you do about it (fix the underlying structure or fix the design problems) is different depending on what the problem is.

If worse comes to worst (assuming you don't know anyone with an x-ray machine on which you can slap your plane) you're going to have to do some exploratory surgery. You're going to have to refinish around the cracked sheeting on the leading edge wing root anyway -- it may be a good idea to cut away some sheeting and take a good hard look inside to see if there's any parts that are moving that shouldn't. If there are, it's time to grit your teeth and keep cutting until you find the real problem.

Brett Buck · « **Reply #8 on:** July 21, 2011, 12:03:22 PM »

Quote from: Peter Nevai on July 21, 2011, 11:35:07 AM

I wonder what if any departures from Teds design there are and or if Teds TP had similar flaws.

The original has been flying since 1991 or so, so I would say that it was not intrinsically prone to failure.

Brett

frank williams · « **Reply #9 on:** July 21, 2011, 12:44:37 PM »

The 10 g pull at the NATS this year was horrendous. I'm not certian that the bearings in the tester wheels wern't possibly rusted a bit. I have never felt a pull like the one on the circle 3-4 pit. The handle holder finially broke off on Thursday. I wonder if 10 g's doesn't do more damage to the plane than it protects for a 3+ times nominal load. Note that Matts cracks are on the inboard wing. I know we are trying to protect for a "fly across the circle pop" type senario, but if you factor in the probability of such occuring, I think we are way overdoing the test.

Mark Scarborough · « **Reply #10 on:** July 21, 2011, 12:47:41 PM »

Matt,
I also ask, what is actually cracking, is it just in the finish? the cracks on the side look suspicious to me, and make me think of laquers inherant problems with shrinkage. You may have two seperate issues. If the fillets are cracking, this allows fuel in( although with a pipe its unlikely) then the fuel would work under the dope.
If its just the finish cracking, which to me it looks like, then my thought is that perhaps you put the coats of dope on to dry and they did not "melt" into the base substrate, this would allow them to shrink away from the substrate causing craking like what I see on the fuse sides.
Now if the wing is moving in the fuse joint, thats a seperate issue.
You really need to dig deeper, sadly I mean just that, flake off the edges of the finish to see what is actually going on under the finish. Thats the only way in MY opinion that you can tell whats happening structurally.

John Lindberg · « **Reply #11 on:** July 21, 2011, 01:01:29 PM »

Peter, I, too, miss that Hobby Poxy II three hour glue, I still look for it at the hobby shops, it was great stuff, the longer the curing time, the better, I believe. Some epoxy goes bad after a certain time on the shelf, I have heard.

Peter Nevai · « **Reply #12 on:** July 21, 2011, 01:46:26 PM »

Quote from: Brett Buck on July 21, 2011, 12:03:22 PM

The original has been flying since 1991 or so, so I would say that it was not intrinsically prone to failure.

Brett

Thanks Brett,

That's why I posed the question, IF Teds TP is still solid and Mat's is constructed according to Teds design without changes then we can rule out any structural deficiency, like pipe tunnel design or inadequate wing support etc.

Dick Pacini · « **Reply #13 on:** July 21, 2011, 02:46:40 PM »

When I first saw those pix, I mistook the heavy black lines for your cracks. My thoughts were that a blind man could see that with his cane. However, I read the thread and then went back to the photos. Only after I enlarged the shot did I see what the cracks actually looked like and where they are.

To me, it looks like fillet cracks which we have all fought at one time or another. The structure may be sound but the fillet material is showing fatigue. Of course, the only sure way is to do a bit of surgery.

I would follow the advice of other posters here who listed the type of stress tests you could perform to confirm or dispel the amount of damage. Beef it up as needed. Good luck with it. BTW, have you started the new plane for the W/C yet?

jose modesto · « **Reply #14 on:** July 21, 2011, 03:15:28 PM »

Teds original trivial per suit is not built like Mats model. The fuse below wing after cowl is one piece without the removable belly pan although the plans show it the original was not built as per published plans. Final edition is much closer to pampa published plans.
Let's help mat but understand original was not built as per plans.
Jose Modesto

Bill Little · « **Reply #15 on:** July 21, 2011, 03:30:08 PM »

From the pictures and my smallish monitor, I cannot tell if the cracking is through the structure or only contained in the finish. Also, it appears to be at the edge of the fillets.

Yes, it "could" be simply a problem with the finish, or the fillet material, or........

Big Bear

sleepy gomez · « **Reply #16 on:** July 21, 2011, 03:39:11 PM »

Flexible structure / hard rigid surface

Matt Colan · « **Reply #17 on:** July 21, 2011, 04:24:17 PM »

Here's how I put the wing in the plane:

I made the cutouts in the plane the normal way, cutting off the bottom of the fuselage and then putting the wing in. The wing fit in perfectly and we glued it in with brodak 30 minute epoxy. The fillets are brodak super-fill (the blue stuff). I have fillet cracks on other planes and these Arent fillet cracks, because I can push down on the wing and see the crack separate from the fillet.

The plane was built per the RSM final edition plans which is the same design Ted used in 2004.

Matt Colan · « **Reply #18 on:** July 21, 2011, 04:37:19 PM »

Quote from: frank williams on July 21, 2011, 12:44:37 PM

The 10 g pull at the NATS this year was horrendous. I'm not certian that the bearings in the tester wheels wern't possibly rusted a bit. I have never felt a pull like the one on the circle 3-4 pit. The handle holder finially broke off on Thursday. I wonder if 10 g's doesn't do more damage to the plane than it protects for a 3+ times nominal load. Note that Matts cracks are on the inboard wing. I know we are trying to protect for a "fly across the circle pop" type senario, but if you factor in the probability of such occuring, I think we are way overdoing the test.

That is exactly what my grandfather thinks the cracks came from. Also at the beginning of the week the plane wouldn't be groaning or making any weird noises. By the end of the week it sounded like you were walking down the steps of an old house

Brett Buck · « **Reply #19 on:** July 21, 2011, 05:12:56 PM »

Quote from: frank williams on July 21, 2011, 12:44:37 PM

The 10 g pull at the NATS this year was horrendous. I'm not certian that the bearings in the tester wheels wern't possibly rusted a bit. I have never felt a pull like the one on the circle 3-4 pit. The handle holder finially broke off on Thursday. I wonder if 10 g's doesn't do more damage to the plane than it protects for a 3+ times nominal load. Note that Matts cracks are on the inboard wing. I know we are trying to protect for a "fly across the circle pop" type senario, but if you factor in the probability of such occuring, I think we are way overdoing the test.

It's about a factor of 4, so that when you break one line, you still have a 2x margin.

I am not sure why its' different, but I thought, if anything, I was getting under-pulled on the 3-4 tester. Seemed awfully light and it was my thought that maybe only two of the 4 switches were opening, giving me a partial weight.

Brett

Crist Rigotti · « **Reply #20 on:** July 21, 2011, 08:20:17 PM »

I didn't notice anything abnormal on either of the testers. I just checked my plane and nope, no cracks.

Matt Colan · « **Reply #21 on:** July 21, 2011, 08:43:01 PM »

Quote from: Crist Rigotti on July 21, 2011, 08:20:17 PM

I didn't notice anything abnormal on either of the testers. I just checked my plane and nope, no cracks.

That's what I was telling grandpa, if they were overpulling the planes, then I wouldn't be the only one with cracks in the fillet area of the wing

Monty Summach · « **Reply #22 on:** July 21, 2011, 08:56:35 PM »

Hi Matt - sorry about your plane - it is a lot of work to build a model like yours.
A couple of things- Do you think the wing is still sound? If you support it near the tips and push in the middle, does it flex? Do the cracks open up? If for some reason the spar splice has let go, then all the flights loads are going into the sheeting (although a lot of it goes there anyway). If you think the wing is still sound, then you have a chance of fixing things. If the internal structure of the wing is broken - then I don't think any external patching you try will last for long - sorry.
As for boxing in the bottom of the fuselage - it is hard to argue that that is not the ideal structure to have, but lots of piped models do not have this feature and do not fail (see photo of my SV for example). One thing you can see in the picture is the ply doubler that the bellcrank post is sticking out of. That is there (top and bottom) to transfer loads into the sheeting, but it also helps to support the sides of the fuse during the pull test. During the pull test, it is best to get your finders as close to this supported area as possible in order to avoid flexing the sides of the fuselage.
Regards,

Paul Walker · « **Reply #23 on:** July 21, 2011, 09:15:23 PM »

Quote from: Matt Colan on July 21, 2011, 08:43:01 PM

That's what I was telling grandpa, if they were overpulling the planes, then I wouldn't be the only one with cracks in the fillet area of the wing

Matt,
How do you hold your plane during the pull test?

Brett Buck · « **Reply #24 on:** July 21, 2011, 09:29:47 PM »

Quote from: Matt Colan on July 21, 2011, 08:43:01 PM

That's what I was telling grandpa, if they were overpulling the planes, then I wouldn't be the only one with cracks in the fillet area of the wing

I am pretty sure they weren't routinely overpulling them, maybe Frank had some misfortune (or they wrote the pull test down incorrectly).

I had no issue with the pull test but I am also spreading the load to the extent permitted by the rules (about half or more applied to the wingtip, not just pulling the fuselage).

Brett

Mike Palko · « **Reply #25 on:** July 21, 2011, 09:37:47 PM »

Matt,

Did you cut the leading edge of the wing for tank clearance? If you did that leaves the center sheeting unsupported and you need to add a support rib at the fuse/center sheeting joint. If nothing is supporting the sheeting at this joint (butt joint against the fuse), the joint will almost certainly fail and the crack(s) will continue to spread.

Did you glass the center section of the wing?

What type of wood was used to sheet the wing? Punk soft wood? Slightly heavier wood? What was the grain structure like?

Mike

Matt Colan · « **Reply #26 on:** July 21, 2011, 09:44:38 PM »

Quote from: Paul Walker on July 21, 2011, 09:15:23 PM

Matt,
How do you hold your plane during the pull test?

I held it by putting my right hand at the wing tip and the fuselage is on the left Side of my body and my left hand on the outboard side of the fuse. Then I pull!

John Sunderland · « **Reply #27 on:** July 21, 2011, 10:58:29 PM »

When I pull test, I hold my plane with both hands on the fuse. One in front of the LE and the other behind the flap on the fuse. I have pulled using a hand on the wing tip but I think you may be on to something here. I cant believe the pull test from 3&4 is still the same with that " hump" ... it has had a " kind over the top" feel years ago. If you had "leaning grip" at the tip this could be the culprit. Sorry you had this problem.

Dennis Moritz · « **Reply #28 on:** July 22, 2011, 06:27:15 AM »

Thinking about this for a minute. Wings are not structured to take compression loads, which would happen when some of the pull test weight is a compression load, collapsing the tip towards the fues.

john e. holliday · « **Reply #29 on:** July 22, 2011, 07:54:52 AM »

I still cringe when I see people pull test their planes with a hand on the inboard wing tip. I have actually witnessed a couple of planes in which the bellcrank wound up in the person hand. I alway try to put my hands in front and back ofthe wing on the fuselage. Also I have witnessed people not pulling straight with the pull testing equipment. I hope Matt can find the problem and let us know without too much work.

Brett Buck · « **Reply #30 on:** July 22, 2011, 09:21:49 AM »

Quote from: john e. holliday on July 22, 2011, 07:54:52 AM

I still cringe when I see people pull test their planes with a hand on the inboard wing tip. I have actually witnessed a couple of planes in which the bellcrank wound up in the person hand.

Why does pulling differently make the bellcrank any more likely to come out?

Brett

Tim Wescott · « **Reply #31 on:** July 22, 2011, 09:49:53 AM »

Quote from: Dennis Moritz on July 22, 2011, 06:27:15 AM

Thinking about this for a minute. Wings are not structured to take compression loads, which would happen when some of the pull test weight is a compression load, collapsing the tip towards the fues.

Wings are structured to take compression loads. Whenever the wing is supporting weight (whether from gravity or acceleration in a maneuver), the 'top' side of the wing is experiencing compression all along its length, while the 'bottom' side is experiencing nearly equal amounts of tension. When you pull test, you just happen to be applying that compression to both spars at once, instead of push/pull.

In a wing where the builder/designer took great care in reducing weight you might find enough structure missing that you wouldn't want to put much stress on it in a pull test -- but except for I-beam wings, I don't see many wing designs that I think would have a problem.

Quote from: Brett Buck on July 22, 2011, 09:21:49 AM

Why does pulling differently make the bellcrank any more likely to come out?

If you put that on a quiz in a 2^nd year mechanical engineering class you'd be accused of asking trick questions. Rightly. Because the correct answer is "it does not".

Matt Colan · « **Reply #32 on:** July 22, 2011, 10:06:49 AM »

Quote from: Mike Palko on July 21, 2011, 09:37:47 PM

Matt,
Did you cut the leading edge of the wing for tank clearance? If you did that leaves the center sheeting unsupported and you need to add a support rib at the fuse/center sheeting joint. If nothing is supporting the sheeting at this joint (butt joint against the fuse), the joint will almost certainly fail and the crack(s) will continue to spread.
Did you glass the center section of the wing?

What type of wood was used to sheet the wing? Punk soft wood? Slightly heavier wood? What was the grain structure like?

Mike

MIke,

I did cut out the leading edge of the wing for more tank space. I then put a 1/4" box around the cutout I made. The sheeting was heavier wood I'd I remember correctly and we didn't glass the center section of the wing.

Matt,
How do you hold your plane during the pull test?

[/quote]

Mark Scarborough · « **Reply #33 on:** July 22, 2011, 10:31:15 AM »

Quote from: Brett Buck on July 22, 2011, 09:21:49 AM

Why does pulling differently make the bellcrank any more likely to come out?

Brett

Brett, I dont THINK his point is that it makes it more likely, but instead, IF the bellcrank comes out, it will end up in your hand instead of flopping across the ground.

The reasoning I heard for not holding the wingtip was that if a line broke, the resulting broken line could come back and whip across your hand injuring you,, thats the legand and logic I heard, I personally have no valid opinion either way. I tend to build keeping in mind that I hold the fuse fore and aft of the wing. Now I wonder if I may be doing it wrong,,

dale gleason · « **Reply #34 on:** July 22, 2011, 11:08:19 AM »

With the tremendous power available with my RoJetts and PAs, I don't give a second thought to adding some weight in stressed areas of the airframe. The "lost foam" wing of Bob Hunt's design has 1/8th" basswood spars top and bottom, inletted into the ribs under the sheeting to spread loads in tension and compression. The bellcrank post goes through both these spars. For added security, I also add 1/16th" aircraft birch plywood between the fuselage sides, top and bottom, glued to the sheeting and fuselage sides, through which the bellcrank post passes, too. When pull testing, I make sure I've got my fingers on the area where those plywood plates are located, trying to keep the load as much as possible on the bellcrank post itself. I've learned to cover the wings with polyspan, top and bottom, and run each panel of fabric completly past the sheeting top and bottom, so that assists in compression/tension load spreading. The wing is covered before installing in the fuselage. There's a small rectangle of 1 oz. glass cloth at the joints of the two wing halves leading and trailing edges. All this extra material adds about an ounce. I don't have any concerns about our pulltesting 10G, which comes to about a 45 lb. tug in my case.

Linda Bob and Shelley Gordon, those two unheralded NATs Pit Bosses, noted a few flyers who "yanked" their planes rather abruptly, and they were counseled accordingly. Randy Smith lubed the pulley axles and kept an eye on the machines. Those machines need a little TLC occassionally, I think Allen Goff provides that.

My twelve year old Class II MO1 (foam wing) has endured its 98 lb. pull yet another year, its bellcrank is mounted on a 1/8th birch ply plate keyed through both fuselage sides, a' la Bill Bischoff's design. Carrier uses the same pulley setup, but we also have to drive a huge spike into the ground to keep the pulley apparatus from sliding.

Sorry for the ramble, but these works of art are too precious to be ripped apart by a pulltest, as was my Veco Thunderbird when I was 13 years of age......dg

Mike Palko · « **Reply #35 on:** July 22, 2011, 11:48:22 AM »

Quote from: Matt Colan on July 22, 2011, 10:06:49 AM

MIke,

I did cut out the leading edge of the wing for more tank space. I then put a 1/4" box around the cutout I made. The sheeting was heavier wood I'd I remember correctly and we didn't glass the center section of the wing.

Matt,
How do you hold your plane during the pull test?

I'm not sure I understand what a 1/4" box is, but it sounds like you have a good grasp on the construction techniques required.

As for the pull test I use the same procedure you use. Right hand on the wing tip, left hand on the fuse. My body is positioned so the fuse is on my left side. I've never had a crack develop using this technique.

I assume you are not having any vibration problems? Did you nose over or ding a prop early in the airplanes life that may have caused unnoticeable damage and it is showing up now?

Mike

Dennis Moritz · « **Reply #36 on:** July 22, 2011, 12:09:23 PM »

I don't know the engineering math, but I imagine the load stresses are differently distributed when a wing is in flight compared to the concentration of a load on the end of a wingtip pulling it towards the fues. The load vectors (?) would be entirely different.

Steve Fitton · « **Reply #37 on:** July 22, 2011, 12:31:50 PM »

How does Matt fix the problems at this point? Can he just load up all the cracks with thin CA?

Tim Wescott · « **Reply #38 on:** July 22, 2011, 12:55:27 PM »

Quote from: Dennis Moritz on July 22, 2011, 12:09:23 PM

I don't know the engineering math, but I imagine the load stresses are differently distributed when a wing is in flight compared to the concentration of a load on the end of a wingtip pulling it towards the fues. The load vectors (?) would be entirely different.

Differently distributed, yes. At the spar, at least, different from what the thing sees in flight, at one point or another -- not really.

If the plane is built so the wings won't fold up in the first square maneuver, then the spar has plenty of compression strength and shear resistance at the root to not collapse when you do a pull test at the tip. If it's built so that ordinary hanger bumps and thumps don't break a wing off six inches from the tip, then it won't collapse when you do a pull test at the tip. You might be able to build a plane that'll stand up to the ordinary bumps and thumps of life, and would still crush a wing tip during a 'tip supported' pull test -- but you'd have to work at it.

Tim Wescott · « **Reply #39 on:** July 22, 2011, 12:57:22 PM »

Quote from: Steve Fitton on July 22, 2011, 12:31:50 PM

How does Matt fix the problems at this point? Can he just load up all the cracks with thin CA?

I don't think we've heard back from Matt on the results of any looking around for further problems. If the problem is that something is broken inside (and "creaks during pull test" sure sounds like it) then loading up the cracks with thin CA -- or epoxy, as he's already done -- is just going to lead to re-broken cracks, or more cracks in parallel with the existing ones.

Edit:

Sorry, I didn't read your whole question. Matt needs to find the underlying problem, and fix that. Consensus from those of us who don't have to actually refinish his plane when he's done holds that he needs to look behind all that really good looking paint and see if anything is broken. Then he needs to decide if it's a construction problem, a design problem, or a result of an incident. Then he needs to rip into it, fix the problem inside it so it won't happen again, splice in new sheeting in a seamless manner, and then 'just' refinish that nice paint job so that no one will know the difference.

Douglas Ames · « **Reply #40 on:** July 22, 2011, 01:13:19 PM »

FWIW- I bought some no-name brand of CA at Hobby Lobby. Inferior to Zap-a-Gap... Too brittle, didn't set as consistent either. Maybe the stuff was old?

Peter Nevai · « **Reply #41 on:** July 22, 2011, 02:58:01 PM »

Ripping into it is pretty much the only recourse. You could do a test in advance though. Put your flaps and elevator at neutral, take some tape and stick it to the up and down leadout just at where they exit the wing tip. Attach your handle directly to the leadouts and tug equally on the up and down leadouts. See if the tape moves out of position away from the wing. This will indicate if the bellcrank platform or supporting structure has failed allowing the bellcrank to move. If it does not then you can probably safely stay out of the interior of the wing. If not then you will have to open up the wing cause something in there failed.

Bill Little · « **Reply #42 on:** July 22, 2011, 03:09:19 PM »

HI Matt,

I've read through this a couple times and do not see where you mentioned "how" you mounted the bell crank.

(if you have, I'm sorry I missed it)

Bill

Tim Wescott · « **Reply #43 on:** July 22, 2011, 03:31:36 PM »

Peter:

A spar failure could cause those cracks and leave the plane in a condition to pass your pull test. But if the plane did fail that test I'd want to fix things, pronto!

Matt Colan · « **Reply #44 on:** July 22, 2011, 04:18:35 PM »

Quote from: Mike Palko on July 22, 2011, 11:48:22 AM

I'm not sure I understand what a 1/4" box is, but it sounds like you have a good grasp on the construction techniques required.

As for the pull test I use the same procedure you use. Right hand on the wing tip, left hand on the fuse. My body is positioned so the fuse is on my left side. I've never had a crack develop using this technique.

I assume you are not having any vibration problems? Did you nose over or ding a prop early in the airplanes life that may have caused unnoticeable damage and it is showing up now?

Mike

Hi Mike,

Here is a picture of what I mean by a 1/4" box. I'm also attaching a picture of how I joined the wing into the fuse.

The bellcrank post hasn't moved because I can see it in the pipe tunnel. I mounted the bellcrank so it's "floating" on an 1/8" rod with wheel collars on both sides of it to keep it in place (pretty standard technology).

Quote from: Tim Wescott on July 22, 2011, 12:57:22 PM

I don't think we've heard back from Matt on the results of any looking around for further problems. If the problem is that something is broken inside (and "creaks during pull test" sure sounds like it) then loading up the cracks with thin CA -- or epoxy, as he's already done -- is just going to lead to re-broken cracks, or more cracks in parallel with the existing ones.

Edit:

Sorry, I didn't read your whole question. Matt needs to find the underlying problem, and fix that. Consensus from those of us who don't have to actually refinish his plane when he's done holds that he needs to look behind all that really good looking paint and see if anything is broken. Then he needs to decide if it's a construction problem, a design problem, or a result of an incident. Then he needs to rip into it, fix the problem inside it so it won't happen again, splice in new sheeting in a seamless manner, and then 'just' refinish that nice paint job so that no one will know the difference.

I haven't done exploratory surgery on the plane, but it almost seems like it is glue joints that are failing because, it seems like the cracks are starting where the seam would be when I made the box to help box in the tank compartment after I cut some of the wing out.

The 3rd picture I attached is to show how the bellcrank is mounted. It's not that great a picture, but it will give you an idea.

Mike Palko · « **Reply #45 on:** July 22, 2011, 04:38:52 PM »

Matt,

Is the sheeting a butt joint against the 1/4" balsa? Is there something under the sheeting to support it? In the very first picture you posted it looks like the sheeting broke free of the 1/4" balsa. If it is a butt joint that is exactly what I would expect. If there is a half rib or any type of support under the sheeting that shouldn't be the problem. Maybe I can find a picture to illustrate what I am describing.

Mike

Mike Palko · « **Reply #46 on:** July 22, 2011, 04:52:12 PM »

I found a picture of what I am trying to describe. I use a 1/8" support rib "inside" the sheeting.

Matt Colan · « **Reply #47 on:** July 22, 2011, 05:52:50 PM »

Quote from: Mike Palko on July 22, 2011, 04:38:52 PM

Matt,

Is the sheeting a butt joint against the 1/4" balsa? Is there something under the sheeting to support it? In the very first picture you posted it looks like the sheeting broke free of the 1/4" balsa. If it is a butt joint that is exactly what I would expect. If there is a half rib or any type of support under the sheeting that shouldn't be the problem. Maybe I can find a picture to illustrate what I am describing.

Mike

Mike,

I'm not positive, but I believe when we made the cutout, there was one of the full ribs there, and we glued the 1/4" to that rib. It seems the only way that would work rather than gluing it to 1/16" balsa.

I checked the wing to see if it was flexing with a test Ted Fancher told me how to do. The wing isn't flexing so that would lead me to believe that there is a bad glue joint at the spot where it's cracking.

Tim Wescott · « **Reply #48 on:** July 22, 2011, 06:20:49 PM »

(I just read Matt's response to Mike. I'm still going to post this, but then respond to Matt's comment in a separate post)

The reinforcement around that LE notch does look like it's more cosmetic than anything else (sorry, Matt). And it looks like the cracks are right around where the sheeting meets the reinforcement.

I agree with Mike that some inner structure would have been better -- butted joints just don't have much strength. I would line the inside of such a notch the way Mike did. In fact, I would probably double up the sheet around the notch, then put in the backing plate and the 1/2-ribs. I may even go so far as to double up the sheet with 1/64" or 1/32" plywood to beef up the stress risers at the corners of the notches -- one the one hand I'm a recovering RC guy, so maybe that's extreme, but on the other hand you could make the half-ribs from 1/8 or 3/32 balsa, make up the weight in strategically placed plywood bits, and probably have a stronger model for it.

The reason is that that sheeting is a stressed member of the airframe -- a D-tube like that is stiff because the sheeting is resisting shear forces. If you go and whack a big hole in the sheeting then that nice square hole is going to want to go diamond-shaped when you stress the wing in twist. It'll want to pull at (for instance) the top left & bottom right corners of the hole, and push at the other two. Since balsa isn't very strong across the grain, if you exert enough of that sort of twist on the wing then those box sides will just pull apart, or the butt joint will fail, and that'll be all she wrote.

Putting in half ribs back to the spar will force all of the shear to be in the remaining hole -- the piece that you fill the hole with then essentially becomes a shear web, preventing the hole from distorting. The shear forces will still be concentrated at the corners of the cut-out in the sheeting, which is why I would be inclined to beef up that area with something, but just using Mike's method is probably sufficient unless your name is "Sir Crashalot, Knight of the Ragged Table".

Such a structure as you have may well crack out just from normal engine vibration or from normal flight loads, and it might be limited to just the cut-out and its reinforcement. If that were the case it may well show the "creaks when pull tested" symptom that you're experiencing. If that's all true, then a careful inspection should show two things: one, the cracks are limited to the splices in the fuselage and the box that you lined the notch with, and two, the cracks do not move when you flex the airframe "straight", but do when you twist the wings. So if you're lucky that's all it is.

If you can twist the wings and see the sides of the cracks trying to move against one another, then you done messed up when you made that notch, and you need the "Mike fix". If you can put stress on the airframe "straight" and see the cracks open or close, then chances are that the sheeting is bearing more than its fair share of the strain because something inside is broken. If it's the former then you need to get in there and rebuild the box. If it's the latter, then you need to fix whatever is broke, and rebuild the box. In either case, you probably need to rebuild the box, and when you have the box ripped out of there, you'll have a dandy inspection hole.

Note that if you're really lucky you'll be able to finesse the whole thing, and approach the whole job from inside the fuselage. If that's the case then when you're done the only part of the finish that'll be ruined from your work won't be visible with cowling and pipe cover in place, and you'll still have a nice-looking airplane to fly. It'll require a lot of fiddly work, but it may be less effort in the end than tearing up all that nice paint.

Tim Wescott · « **Reply #49 on:** July 22, 2011, 06:27:04 PM »

Quote from: Matt Colan on July 22, 2011, 05:52:50 PM

I'm not positive, but I believe when we made the cutout, there was one of the full ribs there, and we glued the 1/4" to that rib. It seems the only way that would work rather than gluing it to 1/16" balsa.

Even if the 1/4" is glued to ribs (which is good), your back reinforcement only extends to the sides of the box. So when the wing tries to flex in shear, the stress is going to hit that opening in the D-tube, it's going to try to pull opposite corners of the opening apart, and the only thing it's going to find to resist it is the cross grain of the 1/4" sheet you've got on the sides of the box. Had you butted that 1/4" back sheeting up against the ribs in question, then the stress would hit the open hole, it would transfer to the back sheeting, and it would have all the area of the back sheeting butted up against the rib, which is reinforced by the wing sheeting. It's still not as strong as if you had fitted the back sheeting inside the wing, but it'd be stronger than what you have.

Of course, doubling up the wing sheeting right there would have been even more reinforcement.

Quote

I checked the wing to see if it was flexing with a test Ted Fancher told me how to do. The wing isn't flexing so that would lead me to believe that there is a bad glue joint at the spot where it's cracking.

Did you try twisting the wing? If my current theory is correct then that's what would make the crack move.