Hooked-On-RC

My Feedback: (16)

I am in the process of building the wing of the kit I am building. It requires the installation of shear webs on the front and rear of the spars, however it also includes a shear web in between the spars, like an "I" beam. What I cannot tell is whether the shear web comes in contact with the wing ribs. Should the shear webs contact the wings ribs? If not, how much clearance should I provide?
Any assistance in greatly appreciated.

CafeenMan

Yes, they should contact the ribs if possible to make a solid unit. Gaps in the webs will weaken them, but the wing will probably be plenty strong even with some gaps.

Here's some info about webs and how I install them. I groove the spar slightly, but didn't used to do that until I had a table saw. If the wing is constant chord, make a plywood pattern and cut one web from it. Adjust the pattern until the web is a perfect fit. Once you have it, cut all the webs.

http://www.airfieldmodels.com/inform...shear_webs.htm

http://www.airfieldmodels.com/inform...ruction/02.htm

Hope all this helps.

- Paul

CafeenMan

PS. Shear webs also help prevent the wing from twisting. That's not their purpose, but wing with no sheeting except in the center are very flexible and can be warped easily by the covering. Shear webs will significantly stiffen a wing. It won't totally prevent it, but it will take a lot more force to twist a wing with webs than one without. That's just a neat side-benefit to them.

joetsunami-RCU

Hooked,
I hope I'm mis-reading your post. 3 shear webs is overkill.

Given that I am misreading your post, then here's my answer, and why: I place them
'close'. Within an 1/8" or so of the rib on each side is good enough. They are there to stop shear, which is one spar moving horizontally relative to the other. They do this well without completely filling the space.

I don't place them any closer, because I cut them all at the same time for width and height. If I slid the shear web tight against a rib, then slid the next rib tight to the shear web, then slid the next shear web tight to the rib, etc, etc, till the end of the wing, and I were off in width by 1/16th inch in each web, by the time I reached the end of the wing, it could be a LARGE amount off. 9 ribs would get you 1/2 inch!

It's not worth the hassle.
Joe

CafeenMan

Joe - Design your own planes, slide the webs and ribs in so they all touch each other and that's the wingspan you meant it to be.

J_R

I am with CafeenMan. The shear webs add a trememdous amount of strength to the wing. They must be against the ribs for max strength.

What kind of plane is this? Sailplanes tend to use more shear webs because of the stress of the wings on a winch launch.

If you really want to get techincal, the best shear webs are ones that are made from a lamination of two pieces of balsa with the grain in an x pattern. The I beam webs better distribute compression while the ones on the face of the spar better distribute tension. The whole point is distribute the load between the upper and lower spars equally.

In some cases, the webs will decrease in thickness at you go from the root to the tip. If it has very thick shear webs in the center, be sure they taper to conform to the shape of the ribs and do not stick up holding the sheeting higher than it should be in front of the spar. Only sailplane guys seem to worry about things like that.

If it is, indeed, a sailplane, think about using aliphatic resin (YELLOW wood glue, like Titebond) instead of CA. It flexes more on launch and the joint won't break from the stress.

JR

CafeenMan

This is new information to me and it makes sense, but do you have a link or something where I can read more about it. I don't mind laminating wood together. I already laminate any plywood going into my planes so that it's flat instead of warped like it always is when I buy it.

- Paul

Rodney

The comment that the strongest web is made with two layers at 90 degrees to each other is correct. If you look up I beams in a strength and materials book, you will see that the forces under bending are at 45 degrees to the span; one direction for upward bending and shifts 90 degrees for downward bending. however, in our use, either vertical or spanwise grain works equally well and will definately strengthen the wing for bending moments.

BillHarris

Paul's articles on shear webs are dead-on, and JR is correct about shear webs in sailplane wings. The inner spars are sometimes wrapped in Kevlar thread to further strengthen the shear webs for agressive winch launches!

Additional articles on wing construction can be found at:

http://www.charlesriverrc.org/articles.htm

--Bill