On a few ARF's I've put together recently, I've noticed that the shear webs are glued in with the grain running spanwise, raather than the way I was always taught, bottom of wing to top of wing .

These were all different brands, one a Phoenix Extra 330S, one a GP Kaos 40, a GP Big Stick 40, and they all had the shear webs wrong. But if the webs are glued on all 4 sides, where they meet the 2 ribs and the 2 spars, would they be as strong as grain up and down?

In almost all designs the webbing is overdone. First of all, it should be vertical. It does not have to be on both sides of the spars, just have it on the back. It does not have to be continuous from rib to rib. All it has to do is supplement the ribs to prevent buckling. For example, if you are using 1/8" balsa ribs on an airfoil that is 2" thick, a 1/2" wide strip of 1/8" balsa glued next to each rib will be enough to do the job. The amount of work and weight gain will be negligable and you will be able to smirk at the poor schmucks who painstakingly do their webbing like they are building intergalactic spaceships.

I noticed the same thing during the autopsy of a buddy's ARF awhile back. Truth is, I can't remember being able to see the insides of many ARF wings to know whether or not it's a common practice with the ARF factories.

But what I noticed was that the wood also happened to be quarter grain. That's a perfectly good choice for webbing and the type of grain in this case made the choice perfectly acceptable. BTW, that wing didn't have webbing full span and most planes don't need full span anyway.

I wouldn't worry too much that the webbing grain is or isn't oriented the optimum direction. ARFs have other wood selection problems that're significant. The direction of the grain in webbing really depends on the wood itself more than the grain direction as to whether or not the piece will be up to the task. Because if the web touches at all 4 sides, it'll probably be strong enough to resist the compression at mid-bay. That is, if the web is appropriately glued.

There are lots of choices that matter as much as the direction of the grain. Is it attached all around. Is it thick enough. Is it flawed wood. It's certainly true that if we were building our own models, lot's of choices would be made with better understanding. But not all of them have fatal consequences. I'm worried a lot more by the design choice of liteply in firewalls and wing hold-down tabs/bulkheads.

This is an old argument that always gathers much comment. If you check out your Strength and Materials handbook, you will see that the stresses in any web is at a 45 degree angle to the span so it really makes no difference in whether the grain is vertical or horizontal in the webbing, both are equally strong in shear loads. Now, if you are worried about crushing (say the cat or dog steps on your wing) then vertical grain is the strongest. If you want maximum shear strength, you need the grain at 45 Degrees to the span but; if you go inverted the shear stresses reverse by 90 degrees so now that same 45 degree is the weakest direction. That is why the specifications for shear webbing on homebuilt planes usually calls out plywood with the grain at a 45 degree angle to the span to give maximum shear strength for both positive and negative G loads. In our models, don't worry, either spanwise grain or vertical grain is equally effective.

I recently built a Juno Tsunami which utilized a style of shear web I'd not seen before. I was quite impressed with the strength and rigidity this approach provided. I think it addresses both spanwise and vertical support very well, plus very little weight gain and simple to implement. I'll substitute this approach on any built up wing I build from now on.

ChiefK

ChiefK,

I like it.

Dave Olson

> All it has to do is supplement the ribs to prevent buckling. <

Hmm. I am no engineer but that has never been mentioned in my reading over many years. I have read that the purpose of the shear webs is to tie the two spars together so that they act as one beam and the ribs don't even have to touch the webs, even if many modelers think that is necessary. Another myth is extending the webs out to the very tip when there is no bending load to speak of there.

Dave, try building a .60 sized 3D model with 1/16" balsa ribs and you will agree with a top designer like Tom Stryker that a vertical strip of 1/16" balsa that connects to the spar / rib intersection is beneficial.
Stryker is a designer who knows how to make 2 pounds worth of wood do 10 pounds worth of work.

Most of the load IS in the 45 degree direction when it comes to the actual primary function of webbing. However there's a second load direction that doesn't seem to receive the same attention. This is the tendency for the upper spar to want to buckle inwards when put under a lot of compression. For this aspect vertical grain is by far a better option.

Even better is where the webbing is located between the two spar caps and has a good fit as opposed to being glued to the front or back edges of the spar caps. And the thinner the wing the more important this aspect becomes. On a big aerobatic ship with a thick section you can easily get away with glueing to the faces. But on a thin sailplane wing the shallow spar cap spacing multiplies the effect of the same amount of G load and glue or balsa will easily fail if you're relying on just the joint to hold. Hence why sailplanes all go with vertical webbing located between the spars.

Diagonal web "truss" bracing that I mentioned earlier supports that 45 degree stress as well as the vertical and span wise. The size and density of the brace material can be matched to the level of support required, and glue is only required at the end points. I've used 1/16" sheet webbing many times before... never again. Faster, stronger, lighter. Works for me. The Tsunami wing I mentioned earlier is a high aspect very thin wing (for a sport pattern model) that I fly very fast. No problemo.....

ChiefK

You sound very confident like you've done a lot of analysis in this area but I'm a skeptic. I'd like to see your work to be properly convinced. Using your knowledge of typical RC flight loads and beams in pure bending I'd like to see proof that maximum shear stress occurs at 45 degrees. As I'm sure you already know, there are several methods for showing stress transformation so just use the method your most comfortable with.

The only analysis that counts for anything is to build the same wing over and over to the point of failure, then back off a notch.

I don't think anybody can make a plane on paper that is optimized and ready to produce without building a plane or three and burning some fuel. But preliminary analysis cuts out perhaps 80% of the cut n' try by getting you in the ballpark before you ever start cutting wood. Best of all, it can quickly eliminate all those "cool and sexy" ideas that we often fall in love with but are DOA by exposing them to Newtons world. Frankly, I think you need a lot of both.

That kind of analysis was already done back in Hal Debolt's day, no need to reinvent the wheel. The properties of wood, especially balsa are too variable for elaborate mathematic analysis to be of much value. The simple answer to the original question is that in most typical applications, horizontal webs are a waste of wood, might as well just go with a continuous full depth spar from a single piece of wood. Vertical grain stationed against each rib keeps the spars in their original, as designed positioms and also gives the ribs more structural value.

You are correct in always asking for proof. Since I am not a registered mechanical engineer, I referenced any mechanical engineering handbook. Just look up cantilevered beams (that is what a wing spar/web combination is) and accept the standard engineering proof, no need for any of us as individuals to try to prove the point when it has been done by the experts. You can also check the government requirements as set up with the FAA and/or other safety organizations that are directly involved with such specifications involving home built aircraft. If you do a search on the forums, I'm sure you will find reams of discussion both pro and con relative to grain orientation which you can sift to your satisfaction. Personally, I tend to believe the reference and engineering books.

Using the standards that are written for man carrying conveyances with all the huge safety factors, [like bridges and full scale planes] will result in a model that is guaranteed to have 10 pounds of wood doing 2 pounds worth of work.

A safety factor of 1 for a model takes the most skill , experience and wisdom to acheive that no engineering book can give you.

Well, nobody (except maybe Berkley and Scientific) used those standards that I can remember, but a lot of them used the principles, which is what I think was being suggested.

Well....... actually............ for the properties to be of value doesn't take too much effort. At least any experienced builder back in the day had little trouble rejecting bad wood in kits and had no trouble selecting not only the right density and strength of wood for scratch building, but also knew how to select wood by it's grain. It is really quite easy to pick the wood that'd suit each job in your model. It was also not much problem adding extra strength by using the right wood grain for the job. However, I agree that mathematical analysis isn't of much value as almost none of the values and formulas were known, much less used, by anyone. Almost every successful competitor I knew back in the day who designed his own stuff could work out CG formulas in their sleep, but wouldn't have been able to identify a single structural formula if it was taped to his forehead. I don't remember a soul who even thought about using them. But you've never seen a more avid bunch of crash autopsiers. We learned by crash testing as it were. I still have mixed emotions about crashed models. Yeah, it's sad, BUT THINK ABOUT WHAT IS ABOUT TO BE LEARNED!!! [][]

Hey Combatpigg,
Looks like a scratch combat wing. Nice work. Reminds me of a Granderson design (I think it was) that I campaigned for a couple of years. High AR with a chubby front 1/3 of the chord. Turned pretty good. Didn't have much straightline speed but that didn't matter, did it.

DaRock, I'll go with careful wood selection, too. I've also got engineering books, with all the vectorial stress analysis tables...but their value is dwarfed by looking over just ONE proven model aircraft blueprint for whatever your interested in. Neither book of mine has a chapter devoted to balsa though.

The previous photo is of the SuDoKoi designed by Tom Stryker for Morris Hobbies, a .45 to .60 sized 3D profile. Awesome plane, a pioneer of 3D flight for the masses.

The plane pictured here is closer to the "Granderdog" you speak of. It was designed by Norm McFadden and campaigned for several years by Norm, Bob Carver and myself.
10 ozs without engine, AMA fast Combat with a 2HP .36.

BTW, these planes did have horizontal grain webbing, but it's main purpose was to give the foam LE gluing surface. These planes exploded if anything didn't go as planned, so there wasn't much point in any heroic efforts as far as shear webbing goes. The average combat match lasts about 20 seconds. I built them in batches of 6 to make it through a 2 day contest, because win or lose your plane could be toast.

Excuse me for butting in but it seems to me that what we are talking about here is a truss. The whole purpose of the webbing is to keep the top and bottom spars apart. In order for the wing to bend the spars must collapse together, if they can not do this because of the shear webbing then the tensile strength of the spars comes into effect. In other words the spars would have to break. The ribs alone are not strong enough to keep the spars apart because the grain is horizontal, so adding more horizontal grained webs is not going to help much.
It is easy to prove this point. Take two sticks of equal length and put them together side by side then bend them both together and observe what happens to the length. The one on the outside of the curve having a longer circumference to cover will end up shorter. Now hold them apart a little and attempt to bend them keeping the ends at equal length. At some point they will have to come together because the inside one will have to form itself to a smaller radious.

Most of what you say in the quote is correct, you are just slightly in error in the way you interpret the results. What happens is that the upper spar is put in tension while the lower spar is put in compression, the resulting forces between the upper and lower spars is a shear force at a 45 degree angle to the spars where the upper spar is trying to elongate while the lower spar goes into compression (assuming a positive G load). Now what happens if no shear web exists and you exceed the breaking stress of the lower spar is that it will snap and move upward toward the upper spar at the break. Reverse the load (negative G load) and the forces shift 90 degrees with the upper spar in compression and the lower on in tension and the shear forces shift that 90 degrees.

You are absolutely right and I should have gone into the tension and compression thing but I was getting writers cramp.

Are you sure about this? I would assume that with positive G load, the upper spar would be in compression. What am I missing?

My bad, you are correct, my error, interchange the two or assume a negative G load. The shear forces are still at 45 degrees to the span.

Try looking at it this way. Take a 1/2" sq stick of soft balsa 3 feet long and draw lines on it every inch ina direction perpendicular to the length. Now bend it into a curve. You'll discover the lines you drew are still straight BUT no longer parallel. Instead, they now all point toward the center of the radius of curvature. Therefore on the outside surface of the stick the material has stretched outward due to tension and the inner surface of the stick has compressed.

Axial stress occurs when you try to pull on the ends of a bolt. Shear stress occurs when you use the bolt to clamp two plates together and then try pulling the plates apart. The bolt is said to be loaded "in shear". Thats why big scissors are called "shears"

Back to the 1/2" stick. If the outer surface is stretched and the inner surface is compressed, then we have internal shear. The shear is tangent to the arc of curvature in pure bending. No g-loads, webs or balsa heterogeneity to clutter up the concept either.

What Rodney failed to understand in all of his study in this area is that the shear in a cantilevered beam varies from zero at the tip all the way up to the full bending moment divided by the second moment of inertia at the root yet the vertical load is constant. Therefore at some point in the wing the shear will be at 45 degrees but it's different everywhere else. Furthermore, the point of maximum stress in web shear is dominated by the pure bending shear at the root where the worst case occurs. Shear in pure bending is horizontal as illustrated at the beginning with the 1/2" stick example.