Author Topic: Trailing Edge Shear webs - best way? (Read 2631 times)

Chris Gilbert IRL-1638 · « **on:** April 03, 2011, 02:13:51 PM »

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

Balsa Butcher · « **Reply #1 on:** April 03, 2011, 02:38:42 PM »

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?

Wynn Robins · « **Reply #2 on:** April 03, 2011, 04:37:32 PM »

Quote from: Pete Cunha on April 03, 2011, 02:38:42 PM

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?

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.

Howard Rush · « **Reply #3 on:** April 03, 2011, 05:59:24 PM »

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.

Bill Little · « **Reply #4 on:** April 03, 2011, 11:00:06 PM »

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.

Big Bear

Chris Gilbert IRL-1638 · « **Reply #5 on:** April 04, 2011, 06:06:02 AM »

Thanks guys, sounds like I need to take a few deep breaths and get on with it, in fact many deep breaths.

Ward Van Duzer · « **Reply #6 on:** April 04, 2011, 12:18:07 PM »

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.

Air Ministry . · « **Reply #7 on:** April 05, 2011, 03:54:34 AM »

You were saved a 5.000.000 word essay . the thumb hit something .

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 .

Bob Hunt · « **Reply #8 on:** April 05, 2011, 07:31:08 AM »

Here's a photo essay on how I do it. Keep in mind that I build wings professionally and had to come up with a quick and accurate method.

I use a simple fixture that allows me to cut parallel shear web pieces. The fixture has a basswood "stop" that is glued at 90 degrees to the edge of the fixture base board. I use a vernier caliper to measure the gap at the front of the TE pieces and then lock the vernier at that dimension. I use a combination square to hold the 1/16-inch shear web balsa down on the fixture and use the vernier to space the edge of the square the required distance from the basswood stop. I then cut the shear web blank. I position the blank on the wing and mark the amount I need using a #11 blade. I then cut the shear web from the blank and use the edge of the knife to make it fit flush with the front of the TE pieces. I then feed in some thin CA glue and let it flow using capillary action. Voila, a perfectly fit shear web!

Later - Bob Hunt

Allan Perret · « **Reply #9 on:** April 05, 2011, 07:55:37 AM »

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 ?

Bob Hunt · « **Reply #10 on:** April 05, 2011, 08:00:10 AM »

Hi Allan:

In some wings the depth of the web stays very constant and in some wings it deminishes or even increases, so you have to adjust by measuring as you go. More often than not I find that one setting works all the way out.

Later - Bob

Chris Gilbert IRL-1638 · « **Reply #11 on:** April 05, 2011, 12:18:17 PM »

Mr Bob Hunt,

THANK YOU!

Chris

Bob Hunt · « **Reply #12 on:** April 05, 2011, 12:50:42 PM »

10-4 guys, my pleasure!

Bob

john e. holliday · « **Reply #13 on:** April 06, 2011, 05:37:10 PM »

Thanks Bob, you have saved the old DOC much grief on the new build.

Bob Hunt · « **Reply #14 on:** April 07, 2011, 12:29:14 PM »

Hi Doc:

I never want you to experience grief!

Bob

phil c · « **Reply #15 on:** April 09, 2011, 08:14:52 PM »

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.

Bob Hunt · « **Reply #16 on:** April 10, 2011, 08:39:47 AM »

Hi Phil:

What you say is true! However these serve as not only shear webs, but also as "Anti-TE Crush" supports. They add very little to the overall weight of the ship and add a lot of strength both in shear and in anti-crush.

Later - Bob

Jim Thomerson · « **Reply #17 on:** April 10, 2011, 09:44:58 AM »

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.

builditright · « **Reply #18 on:** April 10, 2011, 08:31:32 PM »

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

Brett Buck · « **Reply #19 on:** April 11, 2011, 10:23:53 AM »

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

Randy Powell · « **Reply #20 on:** April 11, 2011, 01:58:19 PM »

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.

John Miller · « **Reply #21 on:** April 11, 2011, 03:36:31 PM »

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.

Chris Wilson · « **Reply #22 on:** April 11, 2011, 07:07:01 PM »

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.

Chris Gilbert IRL-1638 · « **Reply #23 on:** April 12, 2011, 06:14:25 AM »

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

Jim Thomerson · « **Reply #24 on:** April 12, 2011, 09:11:10 AM »

The older I get, the more I make use of jigs and fixtures.

john e. holliday · « **Reply #25 on:** April 12, 2011, 08:55:11 PM »

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.

Ron Merrill · « **Reply #26 on:** April 13, 2011, 08:24:12 AM »

You can't go wrong following a Bob H. example.

Ron.

Randy Ryan · « **Reply #27 on:** April 14, 2011, 09:54:01 AM »

Quote from: Jim Thomerson on April 12, 2011, 09:11:10 AM

The older I get, the more I make use of jigs and fixtures.

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.

john e. holliday · « **Reply #28 on:** April 20, 2011, 08:46:16 AM »

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