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Building Tips and technical articles. => Building techniques => Topic started by: Perry Rose on August 24, 2016, 02:43:41 PM
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Every kit I've ever built had vertical grain shear webs connecting the spars. Wouldn't horizontal grain shear webs be stiffer?
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No, the purpose of shear webs is to take shear loads between top and bottom spars. For compression forces that want to move spars closer to each other, vertical grain balsa is really good. Even better would be +/- diagonal grain or fiber orientation but in cl stunt wings the gain is not really worth the trouble. L
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Thank you.
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Out of practicality vertical grain proves to be the best (45º two ply is the ultimate but a pain to make.)
Shear forces always exist at 45º to the direction of the applied force, so you could imagine that either of the 90º variations of vertical or horizontal would equally suffice, but, as it was pointed out to me a while ago, NON inflight force direction is always vertical and so the grain should be.
Imagine an "Oops moment, I just dropped, lent on, packed something on top of (etc) on the wing." Now, which grain direction would you prefer here to avoid crush damage?
But, after thinking about the inflight forces logically I think its more about the aspect ratio of the bay that the webbing occupies.
All wing bays are 'landscape' in orientation, in that they are longer horizontally than vertically and so its better to strengthen between the two shortest points, and that's vertical.
Now picture a fuselage bay being 'portrait' and now the two shortest points are horizontal.
Picture two parallel engine bearers, the two shortest points are ....... well I am sure that you get the picture.
I would rather run the grain between the two shortest points than not every time.
Just to cover this off again, the two 'shortest' points in shear are always at 45º and wood is less compressible along the grain - so think simply join the two shortest points!
Hope that helps.
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I did this a few years back. I had built several I beam style spar wing kits. The spar was laser cut from one long sheet and the grain was horizontal. So I scratch built a rm800 (laser cut rib set) and cut the shear webs horizontal. It worked fine, but after researching I do them vertical now
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Perry, how about 1/16" x 1/4" strips laid in a diagional on 45 degrees one way on the front spar then 45degrees in the other way direction on the back of the spar..
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Just as an aside here, stress tests dating back to WWII tell us that spar webs carry very little force in the centre of them and can easily be hollowed out for little loss of strength.
So triangular corner gusseting rather than diagonal stripping to each of the corners of the bay is more effective.
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Just as an aside here, stress tests dating back to WWII tell us that spar webs carry very little force in the centre of them and can easily be hollowed out for little loss of strength.
So triangular corner gusseting rather than diagonal stripping to each of the corners of the bay is more effective.
I kind of disagree here, I think you need to stabilize the center of the spar between ribs too,,
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although, I use full height shear webs on the first third of each panel roughly, at least past the gear ( if its in the wing) and then crossed cap strips on the outer bays,, but how you fit the ends matter in my opinion too
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I kind of disagree here, I think you need to stabilize the center of the spar between ribs too,,
Page 209/10 Mark, but I think that we are splitting hairs here.
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Page 209/10 Mark, but I think that we are splitting hairs here.
OK, oK, i CONCEDE lol, I must confess most of my time in the book was repairing aluminum structures, I only spent a minute or two searching for the spar scarf joint. I think it would be good for modelers to at the least be aware of it
H^^ ;D
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That is for sure the case when you have a massive aliminium wing skin as primary structure. Then, you need less material (or perhaps same material spread to bigger area?) to keep the skin in shape.
But when talking of a spar only, don't hollow it.
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Hi All, Would shear webs be useful if the leading edge wasn't sheeted as in the case of the Twister and Banshee wings?
Phil Spillman
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Hi All, Would shear webs be useful if the leading edge wasn't sheeted as in the case of the Twister and Banshee wings?
Phil Spillman
While LE sheeting has a purpose, a shear web is not a function of a LE being sheeted or not. The shear web is to join top and bottom spars.
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Hi All, Would shear webs be useful if the leading edge wasn't sheeted as in the case of the Twister and Banshee wings?
Phil Spillman
In my opinion they are more important on a non sheeted leading edge, like, say a Twister , because the leading edge sheeting closes the box just like the shear webs do. so without leading edge sheeting the shear webs are pretty imporant
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Of course, planes like Twister and Banshee tend to get crashed more often. Maybe some elasticity saves them from more serious damage.
So, in that light I'd choose D-box if I had to. L
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In regards to hollowing out a spar or the flange part of an I beam, I would never do it either but the webbing between is always overdone if used as solid construction. But it hardly matters in the end.
The purpose in a wing of spars or flanges is to deal with spanwise compression and tension.
The purpose of webbing between the flanges is to limit the shear movement between.
The purpose of the curved section of the D box ( or any exoskeleton) is to limit torsion.
Each component has a distinct purpose and work together to make an effective and light structure.
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In regards to hollowing out a spar or the flange part of an I beam, I would never do it either but the webbing between is always overdone if used as solid construction. But it hardly matters in the end.
The purpose in a wing of spars or flanges is to deal with spanwise compression and tension.
The purpose of webbing between the flanges is to limit the shear movement between.
The purpose of the curved section of the D box ( or any exoskeleton) is to limit torsion.
Each component has a distinct purpose and work together to make an effective and light structure.
yup,right on, Bro!
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Here's a good read on shear webs.
http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/rc_aircraft_design/shear_webs_in_model_aircraft_wings.htm
This is one of my go-to sites for learning how and why things work.
My last model, an Umland F. Twister kit came with thicker(1/16") horizontal grain webs than the plan called for(1/32) vertical webs. I called Walter about it, and he said after an "discussion" with the person cutting the kits, it had to do with conservation of wood. And that the thickness compensated for he loss of compression strength of doing it the right way with the thinner vertical grain webs. I face-mounted them so they'd be scrificial joints in the event of a wreck, and just pop off rather than destroying the wing. These were mounted between the TE sheets.
Rusty
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As Mark S. pointed out, the shear web closes the box structure if the LE is sheeted. After that, it is nearly impossible to straighten a warped wing! Therefore, make sure your wing is perfectly straight before installing shear webs!
p.s. I sometimes use 1/16 X 1/4" strips installed diagonally in an "X" fashion between rib bays. I don't think it saves much weight, and it is a pain to do. I do it only when I'm bored and have nothing better to do.
Floyd
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I agree with Chris- 45deg. is best. The best, best would be to laminate 1/32 sheets at 45deg with a thin coat of urea/formaldehyde resin glue. It gives a stiff, cheap joint as long as you use a large enough backing board for clamping it. I've found that a single web at the root, one in the middle span, and one at the tip is adequate. Maybe two webs evenly distributed if it's a long wing spar- over 48in.
While it's important to build the wing straight it really isn't locked down until at least the leading edge sheeting is on. Shear webs add very little torsional stiffness.
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As Mark S. pointed out, the shear web closes the box structure if the LE is sheeted. After that, it is nearly impossible to straighten a warped wing!
Floyd
Razor slit the sheeting, twist to correct, massage glue into the slits and do not release the twisting pressure until the glue dries.