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Building Tips and technical articles. => Building techniques => Topic started by: Chris Gilbert IRL-1638 on April 03, 2011, 02:13:51 PM
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Trailing edge shear webs always give me grief.
I've tried a few different ways of fitting them, and they all work, sort of, but they're not as well/easily fitted as I would like.
If I fit them before closing the trailing edge they are too flexible to sand to an exact fit.
What is the best way of completing this task?
I'm reluctant to cut them 'exactly' to size, as I'm not sure my skill level is up to a perfect wood to wood joint under these circumstances, and I'd end up adding glue (weight) to get the trailing edge 'box' sealed.
Or maybe that is the answer to my question, and I should just accept that this is a slow and exact part of the building process - a bit like fitting hinges.
Help!!
Chris
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I install them after glueing on the trailing edge sheeting. Grain vertical. Cut, Fit, tedious work but that's part of being a modeler. If you use contest balsa, minimum weight build up and for the strength they give. Hint: fit, install, then glue with a drop of thin CA. Us a sharp X-acto sharp blade so the wood won't tear. If the part is too short but fits otherwise, not necessarily a re-do, just fit in a small piece to complete. Did I mention tedious? 8)
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I install them after glueing on the trailing edge sheeting. Grain vertical. Cut, Fit, tedious work but that's part of being a modeler. If you use contest balsa, minimum weight build up and for the strength they give. Hint: fit, install, then glue with a drop of thin CA. Us a sharp X-acto sharp blade so the wood won't tear. If the part is too short but fits otherwise, not necessarily a re-do, just fit in a small piece to complete. Did I mention tedious? 8)
the quick and easy way is to glue them to the FACE of the LE sheets - then you can make them slightly oversized and sand to the contour of the LE sheeting - still gives the same strength -and easier to do - still make sure the grain runs vertically tho.
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My TE has 1/16" top and bottom sheets at 23 degrees to each other. I set the table saw to 11.5 degrees and sawed some strips of vertical-grain stuff. It doesn't have to be too accurate: push it in far enough and it fits.
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It seems that unless you can cut them like Howard, then cutting slightly oversize and lightly sanding them to fit. If you get them a tiny bit undersized, then you can "shove" them in so they are just behind the faces of the TE. No problem unless you are doing a transparent covering. ;D
Big Bear
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Thanks guys, sounds like I need to take a few deep breaths and get on with it, in fact many deep breaths.
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If you get "too short" pieces, put them aside for the next (or some other rib bay). I bet you'll find one it fits!
W.
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You were saved a 5.000.000 word essay . the thumb hit something . H^^
If SPAR grain is ALONG the wing , on a SHEET SPAR , fit the blankety
Trailing edge ones the same way ! To avoid the Notorious Albatros
syndrome from inappropriate torsional ridgidity.
Darn things twisted around the rear spar and snapped of , as it was
more RIDGID , not nessesarily STRONGEr , than the front spat .
Oh Dear . This is relevant in gusts . mw~
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Hey Bob:
The step where you set the height of web using the vernier caliper, do you do that for every cut;
or do you do that one time and some how fix the square at that position for all the cuts ?
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Mr Bob Hunt,
THANK YOU!
Chris
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Thanks Bob, you have saved the old DOC much grief on the new build. H^^
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Since these are supposed to be shear webs, it really isn't necessary to fill in neatly between the ribs, except for your own satisfaction. It isn't even really necessary to web between each rib. As an example, three shear webs on the main spars, one at the root, one about 1/3 of the way out and one in the last bay, add a huge amount of stiffness and strength. Each additional web adds some stiffness, but with added weight and diminishing returns.
If you are really serious, get some thin 1/32 in. balsa and laminate it at 90 deg. with the absolute minimum amount of glue(Sigment or ohter solvent cement saves weight). Cut the webs so the grain is at 45 deg. and fit in place.
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I've used something similar to Bob's method. I've also built trailing edges as a separate structure. Top and bottom sheet, little v-shaped riblets and front sheet. This can be built on the bench absolutely straight. I glue the trailing edge on and put small gussets on the rear of the ribs.
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I do my T.E. shear webs very similar to Bob also with the exception that I install them before the top TE sheet is on.
I dont know if it's the best way for others but it is the easiest way for me.
check out the pics on my website
http://www.builtrightflyright.com/New_Web_Pgs/kits/Stuka/stkabld03.htm
http://www.builtrightflyright.com/New_Web_Pgs/kits/stuntwagon/stntwgnbldg2.htm
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As an aside, it really doesn't matter if you run them vertical grain, horizontal puts the load path a lot closer to the way you want it. The flying loads are diagonally across the web, so putting them with the grain the long way is a lot closer to the right direction - and they stay a lot straighter. The only downside is that if they ever shrink, it will pull a concavity in the box.
Brett
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Brett,
So, diagonal is better? I use vertical grain on the sheer web of the spar and trailing edge sheeting most to help with vertical flex. I do other stuff to try to counteract horizontal twisting and flex.
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Gordan came up with a neat system that we've been using on his Tony, Pathfinder L.E., and others. It uses a horizontally grained, notched trailing edge, shear web as part of the jigging system. It's shown on the PF L.E., and Tony plans. It uses a rib tab just aft of the spar slot, and by using them both as parts of the jigging system, will build a wing as straight as your building surface.
It's possible to frame up the wing, install the top and bottom spars, controls, leading, trailing, and center sheeting, while still jigged up. The wing comes out straight, and strong.
Several who have used this system have been impressed with the results.
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An aircraft engineer friend of mine once told me about WWII Spitfires having a stress analysis test of their spar webbing and they concluded that the very centre of the web had no stress in it what so ever during flight and so was deleted by simply stamping out the sheet with a large circular hole in the middle of it.
The closest parallel to doing this with wooden models that I can think of is to use four diagonally grained triangles in each corner of the rib bays, but this would be horribly tedious and would gain very little I would think.
Cheers.
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I see things have moved on somewhat since my initial question.
I just wanted to add, that having tried Bob (Hunt)s method, that was the easiest and most pleasurable shear web installation I've ever done.
The end result even looks very acceptable, for someone who's only experience of wood working is building (and repairing) model airplanes.
Chris
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The older I get, the more I make use of jigs and fixtures. H^^
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Yes, I did the trailing edge like Bob H. showed. Did not take that long. Soon as I get the pics ready will post them. So far only one pic sent to the designer. H^^
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You can't go wrong following a Bob H. example. H^^ Ron.
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The older I get, the more I make use of jigs and fixtures. H^^
I heard that, I've made cutting fixtures to cut shear webs, but like Bob said, the width will vary along the span. So I also very carefully match the ribs to minimize that variation. I am going to try building the TE as a separate structure on my next build, I think that's a great idea. I may even take it a step further and do the same with the LE "D" boxes.
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I have done the trailing edge like Bobby Hunt showed. Did not take that long with a little sanding here and there. If you go to Walter's site and click on the International Stunt Winner kit. There is a pictoral as well a written section on how he did he trailling edge. Took longer on a smaller plane as I used glue instead of CA to put the TE shear webs in. He has the grain running span wise, not verticle. Sanding down to glue on the top TE sheeting did not take that long.
Decided to quit being lazy and looked up Walter's pic. jeh