I was reading through an older RCM the other day and looking at the column from Chuck Cunningham. He was discussing shear webs and attachment to spars. I know common practice today is to create an "I-beam" placing the web between the top and bottom spar and flush with the rib sides. He had a diagram with the shear web side (top and bottom) against the spar (top and bottom) and also touching the ribs as done today. He stated this was stronger than palacing the web between the spars and ribs. When did this change?
Mike

I wasn't aware it had ever been any different. If your shear webs are glued to the top and bottom spars, and to the ribs to the left and right of the shear web, you have secured a "box" of attachments so that will be the strongest arrangement.
By the way, the grain of the shear webs should run vertically for the greatest amount of stiffness. That is because the shear webs will be rectangular in shape, and you want the wood fibers to be cut in the short direction.

I don't think it "changed". Both methods have been in use for quite a few years.

I usually instal my sheer webs gluing them to the front (or back ) of the upper and lower spars and to the ribs on each side.

I can see where this may be stronger. If the web is between the spars you only have the thickness of the web to glue to (typically 1/16" ) where if you glue to the front (or back ) of the spars you will typically have 1/4" (or more ) to glue to.

The weakest point on shear webs is the glue joint. It is much easier to make a strong joint by gluing the shear webs on either the back or front of the spars. If you make the perfect glue joint, either method will be equally strong. As to which direction the grain goes, it makes little differance as the maximum sheer forces are always at 45 degrees to the span switching 90 degrees as the bending forces change from up (inside loop or normal 1G flying) to down (inverted or outside loop). The only time vertical grain strength is a best direction is if you try to crush the wing from either the top or bottom. Bottom line, either vertical grain or horizontal grain will be equally effective, do it the easiest way or whatever best utilizes your scrap material.

Another thought is fit. You need to get a pretty tight fit, if you want it to fit in-between the spars & ribs.
If you glue to the sides of the spars, the shear webs can be as tall as you want, and any shape. You jsut need to sand to shape once glued (and you'll typically do a sanding pass at some point...) But, the width needs to be right, to fit between the ribs..

Ask owners of the Hangar 9 33% Sukhoi about I-beam shear webbing. They used this arrangement and it failed rather catastrophically. I recently got a new one ( first one lost the left wing panel after the webbing let loose ) and went in and added shear webbing to the backside of the spars.

I set my little table saw to make a kerf cut 1/16" deep. I set my saw fence to allow the kerf cut to be midway in the spars. When assembled, the shear web and spars become an actual I-beam. The shear webs contact the ribs to form the box that was mentioned above. This requires careful fitting of the shear webbing, but the strength is worth the effort. I too, run the grain of the web material oriented vertically. I also make the spar joiners run out to at least the 3rd rib in the wing bay. Never had a wing failure in flight (anywhere else for that matter).

Bill, AMA 4720
WACO Brotherhood #1

http://www.angelfire.com/indie/aerostuff/dtubes.htm

hmmmm interesting... both kits i've built so far after getting back into it (tower trainer .40 and four star .40) state to put the web "between" the spar I-beam style. My gliders in the past as well called for this. Attaching to front would be much easier and I will start doing that as the webbing is the part I really don't care when building wings. Cut, test, adjust, cut, test, adjust, and on and on for each one.
Mike

Check www.auf.asn.au/scratchbuilder/beams.html so see why grain at 45 degrees to span is the stronger. If you check the homebuilt builders manual for real aircraft, you will also see that they say set the web grain at 45 degrees.

I think you may be misinterpreting the information there. As the paper points out, when a wing is under a lifting load the top spar will be in compression and the bottom spar will be in tension. The opposite opposing forces are called shear. The shear load in the web will always be parallel to the spars not at 45 degrees. The information about placing the web at 45 degrees applies only to plywood since the individual plies are laid up with the grain 90 degrees to each other so that only half of the plys would actually be resisting the shear. By installing the plywood web at 45 degrees you spread the load more evenly throughout the plies.

A simple experiment is to take a stack of playing cards and bend the stack back and forth. You can easily see that there is not much resistance because the cards slide against each other. Now glue the cards together and try to flex them. There will be much more resistance because the glue joints are resisting the shear force. A sheet balsa shear web installed with grain longitudinally will be weaker because the wood wants to split along the grain. The strongest orientation for a balsa shear web is a vertical grain.