Just wondering, my kit calls for the wing shear webs to be glued to the front of both upper and lower spars. Is it worth it structually, to cut shear webs to fit between the spars? Both spars are already fixed!

Older kit but proven, CG Sukhoi SU-26MX, my guess is to run the extra effort!

Thanks for any input!

Steve

Hughes500E,
If the plans call for it then it is definitely worth the extra effort. Shear webs give the wing strength. Especially since this is an acrobatic plane you are going to need that strenght. Without the shear webs the wing is likely to fold up in flight.

Ken

YES! Shear webs prevent the wing from flexing and skewing in flight. They are VERY strong for very little effort to install.

Dr.1

As mentioned, use the webs. Make sure the grain of the web is vertical to the spar.

I think everyone is misunderstanding the question...

If you are asking if it's worth the extra effort to cut the Shear Webs to fit BETWEEN the upper and lower spars (Like an "I" beam) instead of gluing them to the front of the spars, then the answer is -No.

Just glue them in like the instructions state and you'll be plenty strong enough.

Mike,
I guess that will teach me to try and answer questions late at night when I'm tired!! You are absolutely correct.

Ken

Thanks Minnflyer,
Yes, I meant like an I-beam

Was not looking forward to the tedious work involved!

Will glue to the front of the spars!

Thanks everyone

I would do it like and I-beam, because:1) it would be lighter 2) it would not depend on the bond of the glue, but rather on the structural strenght of crossgrain balsa, to support the spars. But that's just me.

I'll 3rd that one..

Glue to the front..

If you were in put them between the spars you greatly reduce your contact area for the glue.

By going on the front you get about 10x the surface area for bonding.

The only way to do a proper I beam with good bonding would be to turn it into a tiny mortise and tennon project.

As stated, glue to the front, and make sure the contact with the ribs are good. Don't leave large gaps.

Cool, I must say I liked the idea of I-beaming myself but I'll take your guys' word for it

Should I use epoxy or is Sig's aliphatic acceptable?

Thanks again!

AR glue is fine. Glue works better in shear, which is what you get when you glue the webs to the front of the spars. An I-beam would be structurally weaker.

Yes the webbing will help a great deal which ever way it is applied, either to the front, rear or in between. However, the grain direction does not make much difference. If you check your strength and materials handbook on beams, you will see that the forces in the web when the spar is under stress are at 45 degrees to the span and will switch 180 degrees as you go from a positive load to a negative load. Therefore, either horizontal or vertical grain will result in the same relative strength as far as the web material goes.

True, Rodney, except when your 3-year-old steps on the wing.

You may be correct, but my gut feeling is that perpendicular to the spars is stronger. I wouldn't do it any other way.

I tend to agee with this view. As the wing tries to flex spanwise from g-loading (the most severe stress it will experience), the upper and lower spars will try to "slide" relative to each other, and shear web with grain parallel to the spar would tend to shear along the grain under this stress, whereas the shear web's grain running perpendicular to this "sliding" movement would resist the motion, permitting less flex.

Unless it's a fairly small (5 pounds or less) model, I use lite ply for my shear webs, with the 2 outer plies perpendicular to the spars. The more snug the fit between the ribs, the more the stress loads are carried by the material and not the adhesive.

Wouldn't, under positive g's, the top spar be compressed and the bottom one stretched? I heard the one that fails is usually the compressed one. Perpendicuar grain will therefore give the spar support so that it doesn't "cave in" (bad english?).

Upon further reasoning I think you guys are correct, I-beam would be weaker (but not that much) but it would also be lighter (less balsa and less glue) But the most important advantage of I-beam is that it looks awesome under transperent covering.

OK, I've seen a lot of posts on shear web grain direction....on several websites. Virtually all of the model building sites say to orient the grain vertically, but what really have to be looked at are the loads applied to a wing spar in flight.

In the case of a channel beam spar (top and bottom spar connected by a shear web at either the front or back), during positive load factors (level flight, inside loops, etc), the top spar is under compression and the bottom spar is under tension. That is due to the bending moments acting on the wing. The top and bottom spars really don't move spanwise in relation to each other very much. In aerospace engineering, we usually assume that deflections of the wing are very small and neglect them. In the case of model airplanes, this is definitely the case.

When the shear stresses in the shear web are calculated, it is found that they are at 45-degree angles to the span of the wing (one direction for positive load factors, the other for negative).

Because of this, the optimum grain direction for a shear web would be BOTH 45-degree directions.

In full-scale (ie "real" airplanes that you build, get in, and fly) composite (fiberglass, carbon fiber, etc) aircraft, shear webs are built with the plies oriented at 45-degree angles to the span. In fact, they make fiberglass and carbon fiber cloth that is bi-directional (50% of the fibers go in one direction, 50% in a perpendicular direction). This is OK to use for relatively low-load shear webs, but main wing spars usually require many plies for strength, so usually several uni-directional plies are used, in perpendicular directions (still at 45 degrees), on top of one another.

But, balsa is unidirectional (all grain in one direction). So, which direction do you pick?

Guessing from the several posts that I've seen, it seems apparent that many people think that the primary loads on the shear web are those that would "pull" the top spar away from the bottom spar (or push them together), so I think this is the reason behind everyone wanting to put the grain in the vertical direction.

BUT, I'd ask you to cut 2 rectangular pieces of 1/16" balsa, maybe about 1" by about 4" each, one with the grain going in the direction of the short side, the other along the long side. Now, hold the one with the grain running along the short side with both hands, as if it would be attached to the spar, and try to bend it upward. The tensile stresses at the bottom edge pull apart the balsa between grains VERY easily.

Now, try it with the other piece of wood, with the grain running along the long edge and repeat the test. Not too easy to break, is it?

Besides bending, there is also another load applied to the spar....that of shear. Imagine the joint at the wing root....where the wing meets the fuselage. The wing lifts upward, but the fuselage is pushing downward (due to its own weight). So, at the wing root, there is a vertical shear similar to the load that a pair of scissors would have on paper (hence the name "shears"). This load actually exists all down the wing out to the tip, but it decreases in magnitude as it goes out (it is highest at the root of the wing).

Cut two more pieces of balsa exactly like the last two. Now, hold one up and try to pull up one end while pushing down the other. The one with the grain along the short side will shear apart along the grain very easily, whereas the other one takes a lot of effort.

My $0.02 is that if you have to use a unidirectional material (like balsa or ply), install the shear web with the grain running spanwise (horizontally, not vertically). Lite ply would be great if weight isn't an issue.

Now, to the other question about whether to glue it to the front or back of the spars....

If the spar is in front of the 1/4 chord point (1/4 of the distance from the leading edge to the training edge), then it would be best to glue it to the back of the spars. If it is behind the quarter chord point, it would be best to glue it to the front of the spars. If it is right on the quarter chord point, then it would be best to glue it in between the spars. The 1/4-chord point is where the resultant lift load acts. In other words, the vast majority of the lift load is being applied along a line running spanwise at about 1/4 chord. The shear center (the point through which a load on a beam produces no torsion (twisting) loads) is actually BEHIND a c-channel. That is, it is about like this + C (where the + is the shear center and the C is the channel beam (top and bottom spars with the shear web on the left)). Putting the shear web on the correct side will reduce and possibly eliminate any torsional loads on the main wing spar. There is a lot of structural interation in the entire wing frame....leading edges, trailing edges, sometimes a front and rear spar, etc....so this is not easy to calculate, but it kind of gives you an idea....in an engineering "geek speak" kind of way.

There are a lot of books on aircraft design (not usually model aircraft....but rather full-scale ones).

Anyway, chew away at this. I don't know what the backgrounds of all of the folks on here are, but mine is in aerospace engineering, with a major in aerospace structures, and I have a few textbooks on aircraft structural analysis that go into a lot more detail than this. That's not to brag, just showing where I'm coming from.

If anyone has another opinion, please include details of the loads you're applying to the wing spar and how the balsa grain works to counter them. Also, try the experiments above.

I looked into this on here because my GP Christen Eagle II lower wing had a vertical crack in the main spar's shear web, and the lower spar failed in tension (basically a postive-g bending failure). After examination of the spar, the shear web was found with the grain running vertical, and the crack in the web was right along the grain, from the bottom up. Fortunately I found it before a flight and can fix it, and I now will be adding a lite ply shear web with the grain running horizontally from the wing root to about the 2nd rib from the center (the crack was just outboard of the 1st rib).

Rich,

I agree that the strength when supporting spars the grain should run horizontal to the spars. But I think that in most model aircraft the main weakness of the wing is not in the tension and compression of the spars, but in the compression of the weakest point in most wing builds which are the ribs. Shear webs on most every model that I have built or reviewed, they are always perpendicular to the spars. Most ribs are 1/32nd and shear webs on the first 4-5 rib sections. I am not an engineer, but the area of highest structural stress is on the wings near the fuselage. I think that 1/4 or 3/8 spruce spars transfer this stress along their length and transfer a large amount of that stress to the ribs. The shear web is designed to transfer loads evenly from both spars and to keep the ribs from bending or crushing and therefore if unidirectional, then they should be in compression between top and bottom spars with grain perpendicular to them.

JM2C

mcarleno,

Yeah, I typically think of full-scale aircraft when I think of structural design, and their ribs are beefier.

I also looked over my post again and neglected to mention that there is a shear stress in the shear web that runs spanwise, and at the center it's as much as the vertical shear stress.

But, I do agree that the ribs in a model are typically cut with the grain running horizontal and are very weak in the vertical axis. There shouldn't be a huge load in that direction, though....not with the primary load being that of bending of the wing.

In the case of my Eagle wing, the shear web didn't do a thing in the bending failure because it was vertically oriented.

I'll look into it further.

Maybe I'll just put some bi-directional fiberglass on the existing shear web at 45 degrees.

--Rich

Beausol, thanks for the detailed explanation. I think the reason most kits and DIY wings use the vertical grain is just for the ease in cutting them and most economical use of the wood. Since the stregth is still usually adequate (definately an improvement over no webbing) either way will be a plus, it is just that, technically, grain oriented at 45 degrees results in the strongest web for forces in one direction (weakest in the other) while either vertical or horizontal grain is a compromise that works and helps in both directions of force.

Rich, I think you're closing in on the reason for the vertical grain preference... column strength between the spars. The 3 year old stepping on the wing is a humorous but accurate comment from above.

Gary
(BSAE, MSAE, DER, PE.... none of which matters to my toys )

As to grain direction on shear webs, in principle it makes no difference as the shear stress is actually at a 45 degree angle to the span of the wing and shifts 90 degrees depending on the direction of the bending force. Either way you instal them, they do help a lot on maintaing the stiffness of the wing. To resist buckling failure of the spar, a vertical grain direction is preferable over a spanwise grain direction.

/Red B.

I guess I've just never heard of the primary design load for a shear web being the vertical compressive load between the upper and lower spar. Maybe it is for models.

Is this because of things like 3-yr-olds stepping on them? I just don't see it as being a major player during flight, but rather in between flights.....not a condition I would design a plane around.

Heck, I have a big enough engine on my Eagle.....I'll just add so much wood in every direction that it will take an 800-lb gorilla to break it.

I was also amazed at the "quality" of craftmanship in the GP Christen Eagle II ARF. The shear webs didn't even touch the ribs, just were glued to the spars. I don't even know HOW my particular break happened (in flight, storage, transport, etc). All I know is that the more covering and sheeting I peeled back, the more damage I saw.

I'm just very lucky I found it before I flew it.

Thanks for the feedback, guys.

--Rich

after reading all the above posts, here is my solution, which works. install shear plates on both sides of the spar. one grain direction up and other length wise. you then have a super beam. try it on a sample on the bench-- not with the slide rule. dick