F/G box beams

[tk]

A question for the engineering types . . .

Since I have left-over fiberglass cloth and epoxy resin from canoe/kayak projects, I'd like to explore the possibility of constructing a frame from light-weight wood box beams covered in a composite. What I'm thinking of is top and bottom of .5" poplar joined by sides of .25" luaun ply wrapped in 3.5 oz. cloth. Can anyone evaluate this plan for strength and suggest width and height, and/or alternate materials that might be used or needed. The constants I'd like to stick with are the 3.5 oz. cloth and epoxy resin. Additionally, my initial thought is to use torsion axles.

Any input greatly appreciated.

Tom

[Gaston]

don't think 3.5 cloth would be strong enough. I made some 1/4" luan open ended " boxes" then tried to rip them apart. I was useing 2 layers of cloth 6.5 oz and epoxy and the beams (boxes) ripped the glass along the seams ad tore the luan apart at the laminates when i stood on them. I dont think the work building them is worth the extra size frame members would require. I used steel 2" tube. cheap, small, and proven,

[asianflava]

Not saying that it can't be done because I believe Long-EZs use a composite spar in the wings. The problem is, will it be worth the expense in time and materials. Unlike aircraft, you can still use the trailer even when built overweight.

[angib]

OK, you wanted a nerd's answer, so here it is! I reckon that a 5" square box made of the poplar and luan should equal in strength the 2"x2"x1/8" square steel tube that is often used for teardrop frames.

There is a wide varition in the strength of poplar - I have assumed strucutral grade No 2, but if you can get grade No 1, then a 4" box is probably enough. If your poplar is not structurally graded, don't use it. I have some doubt over the luan ply, as that is not really a structural material.

In order to join the corners, I've gone to a stitch-and-glue boatbuilding handbook. Three layers of 2.5oz cloth each 3" wide and staggered 1.5"/1"/2" along each side should do. This is required on both sides of the ply, which leaves you a problem to close the box - I think a 1" poplar square or triangular fillet, bonded with epoxy glue can replace the glass on two internal corners. The diagram below shows the overall layout and the staggering pattern for the cloth - I'm assume in practice you would put at least one layer all round the box to seal it.



According to my calcs, this should produce a beam with a maximum bending strength of 22,000 lb-in - this is about 1.5 times the strength of the steel tube, but that's intentional - wood materials are much more variable than steel, so you need a much bigger material safety factor.

All of this is just a rough estimate based on limited test data for poplar. The only way to safely use this type of construction is to build a test beam and load it up. 22,000 lb-in is a 200 lb person standing in the centre of a 220" (18' 4") beam supported only at its ends - you can alter the ratios to suit, so the same result is obtained by a 400 lb load over a 110" long beam.

I stress - to use this construction without testing a sample beam would be intentionally dangerous.

Sorry for all these warnings but I don't want anyone thinking that any pieces of ply made into a 5" box and held together with a penny nail every 4" is suitable!

Andrew

[mbader]

Here is a place that sell fiberglass beams but I don't see any specs.
http://www.ultrafiberglass.com/Shapes/F ... shapes.htm

And here is a paper on fiberglass beam construction:
http://www.deltacomposites.com/lit_libr ... 20beams%22

[angib]

What Tom is describing is really a wood beam held together with some fibreglass. In this case, the fiberglass isn't really important to the overall strength of the beam.

The finished thickness of three layers of 3.5oz cloth is only about 0.015" or about 3% of the thickness of the poplar so, even though the fiberglass is much stronger than the wood, it still doesn't make much of a contribution.

Andrew

[tk]

Thanks for the replies. Andrew, your explanation as usual clears a lot of fog. In this case it puts my ideas in the category of 'way too much work involved for the gain.'

Thanks again,
Tom

[angib]

In this case it puts my ideas in the category of 'way too much work involved for the gain.'

Yes, I think that will always be true where wood frames are involved - steel is wonderfully simple by comparison!

Andrew