Am building a kit and have some 1/8 ply that is labeled "Shear Web"...I have no idea what this means? Any help would be greatly appreciated.
Thanks
Pat

Bart,

Shear Webs are structural pieces that strengthen the spars in a wing. Traditionally, they attach to the front or rear faces of the spars and form a "C" channel. If the plans call for them, USE THEM!

If you are up to the drill, cut them to just fit between the spars to for an "I" beam. It'll be lighter and stronger. I've actually taken to making a dado joint in the spars if I can to seat and lock the webs and spars together. It's no heavier, but gives you alot more gluing surface.

Here's a cross-section picture of the two types I mentioned. Hope it helps!

Bart,

As a follow-up, here's a link to an article on our club website showing the construction on a biplane I built 2 years ago. It wasn't supposed to go on the site as it was only a fun project for me, but someone got hold of the CD and oh, well.. The pages on building the wings show the shear webs being made and installed well enough to see...

[link]http://www.pinebarrensmodelers.org/how2/index.htm[/link]

follow Woodpiles advice. I've seen 1st hand what happens when they get left out!

I strongly believe in installing shear webs on any model that flies!!

While I have used the "between the spars" method for shear webs, I get warmer and fuzzier when I attach the web either on the forward or aft face of the spar because it allows clamping of the web while the glue sets, provides more surface area for gluing and is more tolerant of "dimensional inaccuracies" (e.g.; my cutting sloppiness ). While placing the webs in between the spars reduces weight, the overall difference is small (less than half an ounce on a .40 size model), and knowing I have clamped the web to the spar while it was drying ensures the shear webs will function as intended.

Initially, I inserted the webs between the spar caps because of my work in stress analysis on wind tunnel models. There is some theoretical advantage to inserting the shear web between the spars, in terms of even load distribution on the spar caps, but that advantage is relatively minor, and could be lost if one or more of the webs is cut too small and don't mate and adhere adequately to the spar cap. My $.02

Cheers!

Jim

Shear disaster!!!!!!!!!!!!!!!!!!!!!!

Doesn't anybody read the instructions any more?

Manufacturers go to a lot of trouble writing all those words, some of them even are meaningful.

If the manual doesn't mention where to put the "shear webs", only then maybe it's time to ask.

Have a look here:

http://airfieldmodels.com/informatio...raft_wings.htm

gus

I read somewhere though that shear webs are not necessary on models 20" Wingspan and below.

BTW, the grain on the shear webs goes up and down, not side to side.

BrownKnows

Brownknows, it makes no difference which way the grain goes as the stress is at a 45 degree angle to the span and switches 90 degrees as you change from positive G's to negative G's. If you don't believe this just check out any strength and materials book on beam design. It is usually easier to put them in with the grain vertical and will work just fine in that manner and is a good compromise; however if the grain ran spanwise they will be just as strong when it comes to resisting G loads.

There are arguments for both sides when it comes to grain orientation -- horizontal grain resists bending loads better than vertical grain & vertical grain resists compression loads better than horizontal grain. Since there are both stress components present, & since models are typically overbuilt, both orientations work about the same. Just put them in & don't worry about it.

I totally disagree about the grain direction. It should be veritcal, no question about it.

The biggest loads on a wing are in bending. Both inverted and upright. When the wing bends, the spars will tend to try to get closer together, creating a compression load between them. It can be enough to cause ribs to actually crush. I've had wing spars in foam wings crush all the foam between the spars and "come together" a couple of times as well. To handle that kind of load, you need the grain vertical.

Sure, there are other loads involved, but by far the biggest, and the one you really need the webs for is that compression when the wing tries to bend.

Oh, as for them not being necessary beyond a certain wingspan, I'd say "it depends". If you use overly-large structural members for things like the wing spar, then you may not need them. But if you are using the same size wood for spars in a 20" wing as you are on a 40" wing, you're using too much lumber to begin with. Also, most 20" planes that I've seen have been indoor or park flyer models that don't encounter high g-forces, meaning you can use a lighter structure, possibly leaving out shear webs.

My old Supra-fly .25 size (designed by Hano Pretner, back in the 80's) was built really light, and flew great. 48" wing. The outter 3 bays didn't have shear webbing. Out there, the bending loads are very light, so it wasn't necessary. The spars were also very thin, but were doubled up for the inner third of the wing, basically creating a tapered spar. Good design puts structure where you need it and nothing where you don't.

When the spar is under compression (before the wing bend actually happens) it can go in any direction. That includes away from the other spar. So the web can be under compression or tension hence glueing the web to the front or back is better than in the center of the spars IMO. The key is when the spar is deforming from compression but before an actual bending or catastrophic failure of the wing occurs.

Let me add a gallon of gasoline (or nitro) to the fire...

I think the grain direction is a second or third order consideration, because the function of the shear web is to structurally tie the top and bottom spar caps togehter, making a beam... Properly cut and attched, either grain orientation will create a beam, and provide adequate strength.... (and if the leading edge is sheeted, making a "D" section, so much the better!) Upon excessive loading, by the time grain orientation becomes an issue, I would expect the wing is long gone

Cheers!

Jim

Screw it. I'm using ply from now on.

Jim,
If I hadn't seen wings land with broken ribs from flight loads, I'd agree. (ribs were ok at take off, nothign was hit, and found broken on return to the pits). But it can happen. Granted, it's far more common for a wing to totally fail than to have some small failures and still survive. Now, I've not seen broken shear webs with out the rest of the wing failing, but then I've never seen anyone put the webs in with the grain horizontal, every kit or plan I've seen all had it vertical.

Take a couple of square scaps of 1/16" balsa sheet. pinch the wood between your thumb and finger, as if your fingers were the wing spars. You can easily see that the wood is much stronger with the grain running vertical.

I do agree that if you have enough other structure, such as a D-box, then it might not matter. But not all wings are D-box structure or dramatically over built.

Just for the record, Montague....

I have buit a number of kits (3 of my 4 planes are kits, one is an ARF) and all of the call for horizontal shear-webs.

Great Planes PT-40 trainer has a horizontal grain.
Great Planes Spitfire .40 is horizontal.
Great Planes UltraSport+ is horizontal

Go figure.

I imagine that horizontl is easier to build with (fewer pieces...)

gus

i've built planes with weds between the spar. i think it was 4* and an eagle2
most if not all had verticle grain and they all state to have the grain
go verticle.

on 4* .60. the shear wed is one big long piece that's been slotted already and
fit between the spar. obviously the grain is going horizantal. but it's a perfect fit
and a secondary spar is added

only plane i've built without any mention of webs was the rekasha..
i slapp webs in for wieght any how .lol

my lastest kit... a GP p51 verticle grains for front and back out to the retracts.
i think it has a modified ultrasport's wing.

weds in front and behind the spar is so much easier to install than trying to
make a perfect wed between the spar

Gus,

Thanks for the info, that's intersting. I'm surprised. Obviously it works fine in those planes.

The Great Planes Cap 21 .40 size that I built years ago had vertical webs, as did the Top Flight P-51 I had (really old version. The old pre-GP buy out "sand balsa log to airplane shape" TF kits, not the new re-designed ones). I've also built or help build a handful of others, and I don't recall ever seeing horizontal grain webs, but obviously some kits are that way. (It's been a while since I've built a kit, I don't have the time I used to). Go figure.

I agree that it makes little difference in our models on which direction the grain goes. It just goes against my nature to not comment on the errors in some of the above statements. Find yourself a good mechanical engineer or good engineering text book and you will see that (when a beam is under a bending moment) the shear forces in the web are at a 45 degree angle. True, if you step on a wing laying on a flat surface, a vertical grain will be stronger; but, that is not what happens in flight. In flight, you create a bending moment, not direct compression. As you go from upright flight to inverted flight, the direction of sheer forces changes 90 degrees. However, in upright flight the stresses in the upper spar are span wise compression loads and in the lower spar tension loads. These also reverse for inverted flight. The maximum stresses in the spar is on the very outside surfaces so thicking the spars does little to improve strength although widening them does help in direct proportion to the width increase.

The main reason that some mfgr's are going to horizontal grain is that they are employing a new building method.

Now, they use a single piece that spans the entire wing which is sandwiched between the two spars in an "I" beam fashion.

This one-piece "Shear Web" also acts as a jig to align the ribs and spars to help ensure a straighter wing.

I'm just curious...why are they called "shear" webs? I thought the main goal of the shear web was to restrict the opposing lateral movements of the top and bottom spars under stress to add stiffness to the wing, to resist the bending of the wing when subjected to positive or negative G's.

That's what I thought.

Edit: Actually I think I'll retract my statements about why it's called shear web and how/why it works blah blah blah because we're going to get 10 different answers again and it really doesn't matter anyway. Go read the revisions if you really want to know what I said but don't ask or complain about what you see.