Forgive me if you’ve read this already, I posted the question on rec.models as well. I’m building my 1st kit and got a bit stumped when the instructions called for gluing the shear webs between the wing spars. There were not pre-cut shear webs in the kit so I cut some out of what I believe is the appropriate balsa and glued them on. I know I have the grain running in the right direction based on what I’ve read. The material was not wide enough to cover the entire space from one rib to another so I ended up gluing another piece to fill up the gap. It was either that or center up the shear web and have a small gap leading to the wing rib on either side.

What I have now are shear webs glued in each bay (area between ribs) down the entire length of the wing. I’m second guessing myself now and wondering if this is going to be enough strength. The material it called for was 1/16 X 3” balsa and that is what I used but I’m getting paranoid.

Is it correct to say that if shear webs are good, then more & sturdier ones are better? The last thing I want is a catastrophic wing failure on its maiden flight. I’m thinking of taking some light plywood or more balsa and gluing shear webs on the backside of the spars as well.

FYI I’m building a tower fun 51 profile plane. This is not my 1st plane but it is my first kit.

Any advice would be appreciated.

"Is it correct to say that if shear webs are good, then more & sturdier ones are better? "

I would say the answer is no, remember weight is an important part of aerodynamics. I am no expert on kits, since I am in the process of building my first one as well. However, with my wing it called for the same construction as what you are descibing. With just the Ribs, Spars and Shear webs things still seemed a little flimsy, but once I started putting the balsa sheeting on things firmed up nicely.

Hope that helps.

Rick

Makes sense. I guess I'm just paranoid. I have visions of the wing folding in two on the first snap roll. I'm much more prepared to accept a crash based on pilot error than from faulty construction.

I keep thinking of Apollo 13 and how they say the fault was built into the spacecraft years before the launch. I can't help but think that I've somehow already built in a fatal flaw.

If it cruches, it crunches. I'll live. I just don't want it to be due to something simple that I forgot to do.

What you have done is quite sufficient, I would add no more. It is not particularly necessary for the shear webs to butt flush up against the ribs. The important part is a secure bond to the upper and lower wing spars. The grain of the shear webs should be vertical in respect to the spars.

Vince

A little reinforcement, within reason, is a good idea. If for no other reason than to make you sleep better at night. Just don't go nuts with it. Also, just adding addition shear webs will be beneficial only if the spars are already strong enough to match the added webs. If not, spar doublers may be in order. It's like adding more concrete without beefing up the steel rebars. They must be matched in property for the additional bulk to make sense.

I've sometimes glued additional shear webs on the backside up to about 1/3 ~1/2 wing span. Just because the original designer spec'ed it one way doesn't mean that is the optimum setup. It is usually what he/she believes to be the ideal compromise between weight, strength and ease of construction, among other considerations.

One of the joys of building a kit, for me anyways, is the flexibility to alter the design a little. However, if you are not sure whether a particular mod is a good idea and how much strength it would add to the structure, given the weight penalty, it is better to leave well enough alone and build it stock.

What Vince said holds true in every respect.

Regarding sheer webs... I like about 3/32 in balsa, and I like 'em to FIT. Fit rib to rib and covering the spar top to bottom. The comments about weight are also true. Build 'em to fly, not crash! Also, webs all the way out to the tips are next to useless. Off the top of my head, I'd say about 1/2 of wingspan is sufficient. I mean, it's there to beef up high stress points...and that isn't at the tip. Good luck...

Yes, But we're not talking "Stress" here... We're talking about "Shear"

You are better off having gaps in between the ribs and the shear webs, but having the webs go the ENTIRE length of the wing, then you are by filling the gaps and stopping the webs half way out.

Filling the gaps (with the webs going the full length of the wing) is like screwing your brick BBQ grill to the ground so it won't fall over.

Does it help? Yes.

Is it needed? No.

The shear webbing will increase the stiffness and strength of the wing. Many people don't understand the mechanics. The idea that the spar size needs to be increased if shear webbing is added is blatantly false.....The spars will always fracture given a certain stress. The shear webbing joins the lower and upper spar into one beam (mostly carrying a shear loading, thus shear web). The moment of inertia is greatly increased. It kind of transforms it into an I-beam. The stress caused by the bending moment is inversely proportional to the moment of inertia. Therefore, the higher the moment of inertia, the lower the stress on the spars. Just look at the web on an I-beam, or engineered lumber. It is very thin. The further out the material is from the center the higher the moment of inertia.

Basically, the shear webbing will decrease the stress on the spars given a certain loading. The wing will be stronger given the same SIZE spars. The shear webbing can be thin and very light. Like was mentioned earlier the maximum bending moment occurs at the center of the wing and shear webbing may not be needed out near the tip. The outer edge of the wing does not create the same amount of lift as the inner section. But that's another subject.

Baitshack, I wouldn't worry. Trust the designers. I don't think you had to add extra pieces. I built a plane last year with the identical configuration, but the instructions were clear that the pieces wouldn't fill the gap, and you should just butt them up to one rib. The plane has proved to be VERY durable (don't ask how I know!)

I've seen a lot of crashes (quite a few of them my own), and in my experiences, wings are very strong between the tip and the fuse (if a wing breaks, it is almost always at the fuse). Don't lose any sleep on this one, when there are so many more interesting dilemmas to worry about, such as nylon vs metal clevises, and z-bends vs solder, etc.

I agree. I also fiberglass the center section of all planes I build.

By the way, you are 100% correct Volfy. At a certain point the shear web will be overkill. I hope I didn't tick anyone off. I misread what was stated above.

I have seen numerous planes fold wings at the field. If you want, go ahead and reinforce the wing a little. I believe that most kits are built strong enough and that others aren't. I wouldn't put all of my faith in the manufacturer. The small added weight will possibly payoff in the future with a non-crashed palne. Better safe than sorry?

AQ500, I don't believe you offended anyone here, certainly not me. We are here to try to help the original poster, now we may have confused the issue more than we're helping, but at least the spirit is a good one.

Wow! I didn't know there was going to be that much confusion about the function of shearwebs. As with any bending stress on a beam structure, there are two components to it: shear stress and normal stress. The shear webs withstand the shear stress, while the spars take care of the normal stress along the length of the wing. I see in many kits, the designer put in some monsterous square spruce stock for spars but with 1/16" thin balsa for shearwebs - that's a mismatch. As I stated, in order to determine whether additional shearwebs would help or not, one must know the stock configuration.

It is imperative that you don't leave gaps in the shear webbing, otherwise stress risers will occur right at where you have open gaps - especially near the wing root. For this reason, it is better for full length shearwebs to cut through the ribs, than for the ribs to cut through shearwebs. Unfortunately, latter method is generally easier to build, so most kit designers specify it that way. Then, it is up to the kit builder to follow the instructions and make sure the job is done right.

Again, if the builder is not sure, it is best to follow the kit instructions and not try to overengineer.

Baitshack,

I agree with AQ500.

Harry Higley in his book, "Building Bipes", page 25 states "if you ever buy a kit that does not provide shear webs, add them yourself - no exception."

The shear webs should butt up against the ribs to eliminate the shearing tendency of where spars connect to the ribs. Shear webs will also reduce your up and down flexing and give the wing compressive strength.

Harry goes on to say, The webs and leading edge planking form a "D" shaped tube. The shear webs comprise an important element of the "D" tube. Without them, the planking forms a much less rigid "C" shape.

I have added shear webs to my Sig Smith Mini-plane and they have made a big difference.


You did fine.

An additional comment regarding shear webs from a glider guider:
As AQ500 said, the idea is to make kind of an I-beam. In sailplanes with framed up wings, shear webs are added BETWEEN the spars maiking a real I-beam. It's a little more trouble, but not much. They are added before the second spar is glued on. Some sailplanes I have built use webbing material that is the same width as the spars. Others use 1/16th sheeting. They can be of light balsa so they add very little weight and a LOT of strength.

I remember reading articles about this years ago suggesting that the webs run tightly from rib to rib with no gaps for maximum strength. I've always installed them this way and have yet to have a wing fold. I always use them in my powered planes too, even if they were not included in the design. Cheap insurance!

WHAT BOKUDA SAID!!! I said it too, earlier...if you're going to fit a sheer web in place, fit it tightly...(maybe snug is the word) between the ribs WITH contact. 2 reasons: snug means NO PLAY. Snug means LESS GLUE (weight). You snug it in there, 2-3 drops of CA and you're in business. I've been guilty in the past, but there really is no substitute for having good wood joints on a plane. It's the difference between being a hack and a craftsman. If it doesn't fit, we have the power to make another, right??? I struggle to be a craftsman, but I think the hangar is worth it....

I've seen a lot of kits that add the shear webs to the SIDES of the spars instead of between them. As long as they still touch the ribs, is this method any worse than the between-the-spar method?

Sides are fine, if not preferred. There's much more adhesive area to absorb shear loads on the sides of the spars and webs than the edge of the web to the spars.

While I-beams represent an ideal cross section in bending (short of a box spar), I would argue a C-spar is best in our case. In our implementations, we have to rely on adhesive to provide the shear strength, and the more surface area for adhesive the better for strength. If I had an extruded aluminum spar, by all means give me an I-beam, but for a glued spar structure make it a C-beam.

I'd have to disagree. The advantage of close fitting webs between the spars is precisely that they DO NOT rely on adhesives for the shear strength. The adhesive just stops the webs from moving around, the strength is obtained because the webs are a close fit between the spars.

Don't get me wrong I sometimes, perhaps even most frequently, use the easier to build C section but most glues are nowhere near as strong in shear as you might think. Fortunately we always overbuild anyway. Do you know, some people even take the shear webs all the way out to the wingtips despite there being almost no load out there ?

Steve

Just when you think you're making progress, some bloke comes along with his own theories.

Steve, we are talking about SHEAR....

SHEAR!!!

Take two pieces of 3/8 x 3/8 stock, lay them parallel on the table 1 inch apart so they run from left to right. Now slide one to the left, and one to the right.

Did the tips move less than the rest of the sticks? No??? Then what makes you think that the shear is less at the wingtips???? Shear webs do NOT need to come in contact with the ribs. They need to keep the upper and lower spars from shearing (moving in opposite directions from on another) That is why the grain goes vertical, if it went horizontal, the webs themselves would shear. (see pic)

MinnFlyer, I disagree completely. Shear stress does decrease toward the wingtips.

What you said might be true if it were a concentrated load at the far end of a cantilever beam. However, the aerodynamic load on a wing (and to a lesser degree the weigh of the wing itself) is a not a concentrated point load, but a distributed one along the span of the wing. As such, there is practically no load at all at the wing tip. This distributed load causes a bending moment on the wing (i.e. cantilever bean) and it increases toward the wing root. The bend moment consists of two components:

1. Normal stress (tensile and compression) along the span of the wing. This is carried by the spars.

2. Shear stress, which translates to a force which seeks to move the top and bottom spars relative to each other. This is carried by the shear webs.

(BTW, there might also be a third stress component - torsion - if the loading is not even between LE and TE along the span, but we can ignore that for this discussion.)

Because this bending moment decreases progressively from wing root to tip, these two components, normal and shear, also decrease accordingly. Therefore, it is perfectly acceptable to use progressively thinner shear webs, or none at all, toward the wing tips.

I have to disagree back at you...

Close fitting shear webs are fine, but with a weak adhesive bond at the edges due to a tiny bond area, any failure in the adhesive to a spar would allow the shear web to collapse. Shear webs require complete constraint of the web to be effective, which I would argue is more difficult to achieve in practice with an assembled I-beam.

FWIW, most adhesives are actually very strong in shear...they are generally weaker in tension and poor in bending/peel. This is why scarf joints are used to joint two flat plates, to maximize the shearing forces in the adhesive joint which the adhesive is better equipped to withstand. Similarly, the vastly increased surface area of adhesive as in a C-spar allows a smaller shear web to be used (if desired) and I think have a greater degree of "operator independence".

MinnFlyer, Volfy is correct...shear stress does decrease in a cantilever arm which a wing half approximates. Point loads or distributed loads, doesn't matter. The part of the equation your interpretation is missing is the sum of the moments along the beam. For example, using your illustration the bending moments are maximum on the right end of the spar and zero at the wingtip due to the point load on the left end. At any infinitessimal point on a beam in equilibrium, the sum of the forces and moments must equal 0.

I guess we have to agree to disagree . I think in part that's because the name "shear web" blinds some people to the fact that, as well as operating in shear, these elements of the spar structure also carry (distribute might be a better word) some of the normal tension/compression loads. This they do better as an I-beam (and that's also where the part about fitting between the ribs comes in). It's easy to see why people don't bother though. As you correctly say that structure is much more difficult to build well and really does rely on completely accurate fits everywhere.

I'm relieved that I'm not the only one who recognises the variable loading along the wing though. Many years ago we built FF models with tapered spars and spars that only covered part span out from the root. I can't remember the wings ever failing anywhere but at the root .

Steve

Forgot to answer the question about placement of the shear webs...

I remember reading an article in RCM some 10yrs ago about a study they did on the relative strength of the wing with various shear web placements:

1. "I" beam
2. Outside of D tube
3. Inside of D tube

I don't remember exactly, but either 2 or 3 came out marginally stronger than the rest according to their test results.

In any case, I agree with their conclusion that in actuall practice, they should all work very well. So ease of construction can weigh in heavier in one's decision to use one type or another. Personally, when I have the time to do it, I use very lightweight vertical grain balsa to fill in the area between the top and bottom spars completely - no gaps between ribs and maximum glue surface for the shear webs. Ounce for ounce of shear web weight added, this is the strongest method.

BTW, if your top and bottom spars are not parallel to each other and you want to use side shear webs, be sure to sand/shape the spars so that the spar surfaces to which the shear webs will attach become parrallel. OTherwise, there will not be adequate glue surface.

Right on Minnflyer. A picture is worth a thousand words.