Our hobby is full of terms that are incorrect if the true definition of the words are followed. For instance, CG (Center of Gravity) is an incorrect term. We are not seeking the Center of Gravity, we are seeking the balance point, but the term CG sounds unique and savvy, so we continue to use the term improperly.

Ditto shear webs. The wing is trying to crush itself as it bends, it is therefore important that the grain runs vertically, which is the strongest orientation in order to compensate for the destructive forces. Yes, there is a slight amount of shear involved, but I doubt nearly as much as the force that is bending the wing.

Shear web is the incorrect term, but in this instance, I do not know what the correct term would be. Any mechanical engineers out there that do know?

I don't read the entire thread when RCU sends me a message and a URL in order to reach a thread that the software thinks that I am interested in, so I do miss a lot of the comments that preceded mine. Most of us form opinions based upon the concensus of opinions that we encounter. Even though I may be repeating someone elses comments (unknowingly), it never the less adds to the concensus, one way or another.


Ed Cregger

Crate Cruncher-

This wing is for the Great Planes BLT Park Flyer. It has to be as light as possible. The wing has sheeting only over the 2 center bays. The webs use a lot less material than if I sheeted the entire wing. Also, since this is an easy-flying park trainer, it won't be engaging in many balsa-splitting maneuvers anyway.

As you can see in my photos above, I simply glued the shear webs to the back of the spars. I could have saved a little weight by making smaller webs and putting them in the middle of spars, but as for the strength, my impression is that it wouldn't make too much difference.

James

This is probably one of the more misunderstood structural topics. In reallity, the strongest shear webs are those that have the grain at 45 degrees to the wingspan. However, when the stress reverses (flying upside down instead of right side up) this switches 90 degrees, still 45 degrees to the span but 90 degrees from before. On real planes, using ply shear webs, there is a definate requirement to always have the ply grain at 45 degrees to the vertical for this very reason. Any one who has studied beam design can confirm this. Two places you can check are http://www.vishay.com/brands/measure.../sgbt/lclb.htm and http://www.silentflight.net/images/s.../shearwebs.pdf. To make a long story short, vertical grain webbing and horizontal grain webbing are equally strong. Just because it is usually easier to do in models, I put the grain vertically unless I'm using a ply shear web then it is at 45 degrees.

Funny, while looking for a link to an explanation for this on the internet I uncovered the same exact thread topic on another R/C website that quickly devolved into a 6 way debate too! It seems everyone has heard about ONE or TWO of the stress modes and figures THAT is the only possible answer. End of story!

There is no simple answer to James question. There's a lot more going on inside a spar than most (including me) realized.

1). There is horizontal shear due to flight loads best illustrated by Minnflyer's graphic.
2). There is additional horizontal shear caused by wing twist, or torsion.
3). There is localized compression due to the flight load creating a bending moment near the root where the wing and fuselage connect. (Johnson's website described this mode only.) Note: Whenever there is compression on a beam there is also shear in both the vertical and horizontal direction and they are always equal. That is why the total shear due to compression is at a 45 degree angle. I think this is what Rodney is referring to.

In addition, there are other modes causing stress including: induced and parasitic drag loads forcing the wing backward relative to the fuselage, engine vibration in the wings natural frequency, localized landing gear shock loads, wind buffeting, and many other things that have to be considered in order to FULLY describe a complete picture of whats going on deep inside an infinitesimal section of a spar web.

Rather than spending a lot of time in FULLY describing a problem, design engineers often rely on their experience to ignore the smaller loads, calculate the worse case effect of the big ones, and then multiply the calculated minimum design strength requirement by some multiple called the Safety Factor (I call it the CYA factor).

The most important loads in an R/C sport plane's wing as best I can tell are the first three modes described above and possibly landing shock. The worse case magnitudes of these loads are the maximum g-loads sustainable which is related to maximum speed and aircraft weight. Safety factor for aircraft is low because of weight penalty, usually 3 or 4. A column in a building might be as high as 10!

I haven't actually calculated the loads and shear stresses involved in a "typical" .60 size sport plane spar but given the first three modes described above it seems to me that orienting the grain vertically benefits the structure in two important ways: it is stronger against localized compression at the junction with the fuselage or landing gear, AND presents the strongest direction (cross-grain) to horizontal shear which is much higher than the vertical shear.

Sorry for such a long boring post[8D]

CrateCruncher

P.S. James, I understand for a small park flyer a fully sheeted wing is probably overkill. But I think from now on I will be fully sheeting my .60 and bigger sport planes. Open-bay wing design is more work and results in a weaker wing.

Oh what are us engineers to do?????

I'm one aeronautical structures engineer that usually stays way clear of these debates!

Funny, I'm doing a structural analysis of a full scale aluminum, general aviation wing spar right now. By and large, I'd say that this analysis is probably simpler than that of our models!

Vertical or horizontal you ask? Well, I can't say, and won't say, because every model and situation is different. I happen to believe that one rule doesn't fit all. Honestly, and this is a big point, the shear stresses in most of our models is very low. With plenty of ribs and the usual sized spar caps, we could probably do without shear webs for the outer 3/4 wing span of a sport model. To really help your wing: spend plenty of time getting that center wing joint right. Oh, don't let the kids or dog step on the wing - that's what really brings those spar caps together.

Gary

Some of the profile 3D planes intentionally leave the last couple of bays open with no shear webs to allow some tip flex in case you stuff one into the ground. Sometimes a bit of compliance is a good thing.

Mark

Check out the discussion of aircraft spars for home-builders here http://www.auf.asn.au/scratchbuilder/beams.html

It talks about laminations of DIAGONAL grain ply as optimal for shear webs. It also show the shearing forces as diagonal across the web, which makes sense as I think about it. The spars are both compressing and sliding, resulting in a net diagonal force.

Does anyone use diagonal grain webs in model aircraft?

James

EDIT - I just read Rodney's post after I posted this... Seems diagonal ply is used in real planes, maybe the ideal solution is to make balsa ply from thin balsa sheets then cut diagonal grain webs from it. If you don't want to go to the trouble of laminating sheets of balsa, then the single vertical grain web seems like the best compromise.

Also, it is not only a question of how strong the spar needs to be, but how stiff. Strength will prevent it from breaking, but stiffness will make the wing perform better.

James

Well James, your first clue should have been "Great Planes". I think the correct term is oxy-moron. For instance, I got a kit of their 60 sized "Stick". One of the major design flaws was in the fuselage. The fuselage has ply doublers that stop just short of the firewall. So instead of having a plywood to plywood joint which is fairly strong on the fuselage sides, you have the firewall glued to balsa fuselage sides, butting up to the plywood doubler. Just dumb design. Of course this is in a kit that featured 18 pound balsa in the wing ribs. This was impressive, since I had never seen wood above 12 pound stock. It will handle the Kansas wind.

As to shear webs, most models are overdesigned. One kit design used 1/8" square sticks in an X pattern to act as shear webs between ribs. The ribs should provide enough compression resistance for most sport models, even though the grain is wrong.

Well said - A lot of the stuff we build is over-designed, so you can get away with a lot.

wanna build a very strong spar on any wing, build a Tri Beam spar...picture below. This spar features shear webs on both sides of the horizontal rails, but also one in between them. This increases the strength of the spar dramatically. Be sure all three shears touch the wing ribs on either side of them and glue them at all points. I built a spar like this for the outer halves of a Corsair wing and I stood on it to test it. 230 pounds and not one sign of crushing!

My youngest student and I are building the 4*60 kit right now and one of the things I noticed was the shear webs are built in and part of the spars, front and rear. The built in shear webs are cut out of Hortizontal grained balsa and each is A seperate one piece part running the length of the spars. I didn't say anything to him about shear webs at all, the build is hard enough on his young mind and it can wait until his next build.

No problem, this will be as good as having the grain vertical. If you check the regulations for shear webs on full scale aircraft, you will find that they recommend the grain to be at 45 degrees to the span, actually a ply with one having the grain 90 degrees to the other but both grains being 45 degrees to the span. The shear forces are always at a 45 degree angle and shifts 90 degrees depending on the direction of the load (upright or inverted).