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General control line discussion => Open Forum => Topic started by: PJ Rowland on December 02, 2013, 02:30:54 AM
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Ok so I seem to have a serious issue.
My new model has fuse mounted undercarts. There are 2 blocks either side of the pipe that a top plate of ply is glued to. That plate has 3 x 4/40 bolts that secure the carbon undercart.
I have an issue in that this plate has been breaking off the 2 maple blocks. The net result of this failure is that during landings the undercart is under load and is breaking off, Ive had the wheels ( spats ) go through the wing. This has now happened twice, and im getting frustrated becasue its causing damage to the paintwork each time it breaks.
The last time I fixed it - I used JB weld becasue the ply plate is very close to the tuned pipe tunnel. However after 15 flights it refailed.
Im at my wits end to assertain why the plate is failing.
Any suggestions to gluing ply to maple strong enough to hold an undercart plate would be greatly appreciated.
When looking at the damage or where the glue failed it appears to have pulled clean off one half of the plate as if the glue hasnt held. I did a test of the glue and it took alot of force to break the test piece ,not once did the glue break clean off like it did with the section on the model..- so there could be a possibility that the plate wasnt thorough cleaned from any oil, but becasue its tuned pipe, the amount of oil would have been small.
Very frustrated...........
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Perhaps you could post a picture. I have gotten away with a similar plywood-maple joint where my landing gear (undercart) mounts, but the gear mount to maple blocks which are glued to plywood fuselage side doublers. This may be different than your configuration.
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sure thing.
Here is the plate attached to the carbon undercarts - This plate SHOULD be attached to the blocks in the fuse, but its this plate that is pulling off the blocks...
(http://i41.tinypic.com/muyc1v.jpg)
Here are the 2 blocks which are 1/2" long x slightly less than 1/4 " wide. Maple block.
(http://i41.tinypic.com/rtmj5d.jpg)
Glue Part A to Part B.. Sounds simple but I seem to be struggeling with this simple factoid.
JB Weld just pulled clean off, Im not sure if the proximity to the pipe is causing the issues. I dont know If I should pin it with a small bolt, glass it, carbon it.. What I want to do is fit it properly..
(http://i39.tinypic.com/2ebznrb.jpg)
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Howard, I understand what you have done - Similar to my situation but you have drilled through the blocks themselves and mount to that. I thought of doing this but used the plate to add more stability and strength to the structure. Its ironic Im having issues with it pulling off...
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This is the bottom of my Cutlass. Your blocks seem to be failing because they are not long enough forward and aft. Try to clean it up really well and epoxy maple blocks along the sides of the fuse. I like to drill them out for brass inserts rather than the blind nut on the bottom. I always break the blind nuts off.
Derek
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This is a good thread. was thinking about this, 'cause my TG LG goes like that...
I'll drill a hole and pin the maple blocks to the fuse, to have some extra strenght...
Marcus
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yes, dimmensions are too smal ... I do it with 6mm thick balsa fillets with vertical grain connecting to wing ... never failed
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So the source of the failure is not enough surface area on the block holding the plate?
That doesnt make sense becasue the blocks themselves havent pulled off from the side of the fuse.
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PJ I too agree not enough contact area. Also those maple blocks are really not-porous so the epoxy isn't getting much more than a surface hold. If you'd drill a number of small holes or otherwise gouge up the contact surface to allow some 'teeth' for the epoxy it would help. Werwage sometimes cut shallow grooves in motor beams where they were glued to get more bite.
Dave
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So the source of the failure is not enough surface area on the block holding the plate?
That doesnt make sense becasue the blocks themselves havent pulled off from the side of the fuse.
Not enough surface area between the plywood plate and the block. Plus I would bolt directly to the block but the one you have right now is not big enough for that.
Just my .02
Derek
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The failure is the glue, easy to see. Why? Well, I would venture that after a few landings the glue starts get some stress cracks. After that the plate and the gear are stronger than the glue and it lets go.
Look how Derek is going to mount his gear. If I am not mistaken he is going to drill out the maple beams and put inserts into them and mount the gear right to the beams themselves. The pressure from the landing on his gear mounts is vertical along the inside of the fuse where he has alot of surface area glued to his gear mounts. In your case you surface area is very small and in a horizontal postion that will not be able to handle the drag of the landings and over time the glue gives way. Notice how your mounts that on on the inside of the fuse have not let go.
Too solve the current situation I would remove those tiny mounts inside the fuse. Double them in length and make them as wide as possible. You should be able to go as wide as that bulkhead cut out for the pipe and not have any interference. I would add some plywood and block them to that bulkhead as well. That gives you two attachment points. I would recess them down into the fuse enough that the plywood plate you have your gear mounted on can fit between between the fuse sides and on top of the new maple mounts. Making it an extremely tight fit. When the mounting plate is installed it would then be flush with the bottom of the fuse sides. This gives the plate two attachment points as well.
Make a new plate for the gear. I wouldn't hog it out either. Leave it solid. Drill a couple of small 1/16" or .5 mm holes and use counter sunk screws and mount the plate to the new maple with glue and screws.
If there is enough material in your gear you could scrap the gear plywood plate and just mount the gear directly to the maple. That would be the best way. In that case you would make the maple mounts flush with the bottom of the fuse sides like Derek's. Note how large his mounts are compared to yours. I would still set them up to be part of that bulkhead. But by mounting to the maple you have removed the plate and its glue joint as a possible failure spot.
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PJ,
Agree with all! My beams are 2-3 inches long tapering to the rear with the front face of the beams epoxied to F-2 (plywood fuse former # 2). No ply plate. I bolt the gear to the beams directly with 4 4-40's. The extra length of the beams leaves me xtra room for a couple more holes in the beams such that I can move the gear forward or backward depending on whether I'm flying off Pavement or grass.
Good luck! H^^
W.
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How do you get your pipe out with that gear plate mounted on there? Do you have a pic with the engine/pipe installed with the gear in place? No cowl of course.
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Yup, and the full boogie Randy way includes balsa brace posts on 4 corners going to a balsa plate. I think this takes much stress off of the maple glue joint and you are basically closing in a "box", which makes it extra strong for those occasional bouncy landings and rough grass fields.
I've attached a few pic's, one a really bad photocopy of an old drawing, sorry, and a pic of the actual install I did in my Rapier during the build which should help explain the drawing.
Edit: Also note the brass threaded inserts. Makes it easier than dealing with blind nuts, but if you are being weight conscious, there actually is a measurable difference of a couple grams, don't ask me how much, I don't remember, but I evidently thought it was ok.
Hope that helps,
Eric
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Here is the plate attached to the carbon undercarts - This plate SHOULD be attached to the blocks in the fuse, but its this plate that is pulling off the blocks...
Everybody else got it, the blocks are far too small, so even if you bolt the gear straight up to it without the plywood plate, all that will do is cause the fuse side to fail. Need to grind/sand/cut the blocks off, get down to the bare doubler, make them longer and wider, and bolt the gear straight to the blocks. I would extend the blocks all the way up to and glue them to the former, too.
I am however puzzled by the original failure. I would have expected the plywood to delaminate before the glue joint to the blocks failed. After the first time, you can explain the bond line failure as trying to glue epoxy to epoxy, but I would certainly have expected to see the plywood facing still stuck to the block, and chunks taken out of the plywood plate. That may well have been affected by heat in some way. Looks like the pipe must get right up against the plywood plate, and I can see a dark streak on the underside of the wing that may be that epoxy carbonizing a bit, too. Maybe it is running too hot? Mine never liked to have restrictions in the airflow through the tunnel like that former would do.
Brett
p.s. by the way, a possible the reason it failed where it did, rather than at the fuse side, is that the load on the joint between the fuse side and block is in shear, and for the block and the plywood is in peel. After the first crack, it get progressively worse because the angle of load pulling it apart grows and gets more leverage.
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I think everyone here has covered the reason for the failure very well and has also offered some excellent fixes so I'm not really going to get into that but I did want to mention something about your selection of adhesives.
I really don't know what type of epoxy (I assume it was epoxy) that you used in the beginning, but you did mention that the last repair was with JB Weld. In my opinion this is not a good choice for this type of bonding. While JB Weld is a very strong composition due to the fillers in the resin, those same fillers do not allow a good penetration of the adhesive into the components of the structure.
If a compound like JB weld is to be used it should be given a lot of surface area and attempt to use the strength of the compound itself rather than the bond line. In other words... holes or cavities in both materials to be joined so the epoxy can flow into both parts of the structure and then the load force is directed into the epoxy itself rather than just a straight bond line.
Randy Cuberly
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You are getting some good advice.
I use Hysol 9430 for stuff like that.
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Thanks everyone..
When multi Nats winners tell you to start cutting and replacing, Im cutting and replacing.
I dont have enough material in the carbon undercart to mount directly to the maple , shows what I get for trying to be smart with athestics.
Doug, Derek, Howard, Eric, I sincerely appreciate ALL the help, it was starting to frustrate me.
Brett : The cause of the ORIGINAL failure was a wheel jamm during landing. Either a stick or a stone went into the wheel arch, wheel came off the hub and I landed with it skidding to a stop eventually shearing off. Your probably correct in that the glue to glue surfaces were not cleaned or keyed correctly. At the time I thought it was going to be a simple repair.
Alas I was mistake.
Ill post some photos of the repair once Ive done it.
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Just another pic of Randy Smith style LG mounts. Cover plate in fitst photo fitted between maple mounts and LG. Held on by LG screws, cosmetic only. 8)
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Update :
I have now fixed the undercart block using the techniques offered here. I didnt have enough meat to put the carbon undercarts directly into the maple blocks so went with the plate.
Its longer, wider and glued in with Araldite super strength epoxy. The plate was screwed in with counter sunk 2/56 bolts for extra extra extra support ( thanks doug M ! ) It was probably overkill but the idea of repairing any further damage due to this failure was not appealing.
I will test it Saturday weather permitting then off to the Nats in late Dec.