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Old Jun 01, 2012, 01:24 PM
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Canada, QC, Sainte-Julie
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1/4" pine dowel for wing spar - strong enough?

Hey guys,

Help a newbie scratch builder!

I have only 1 carbon spar left (.021) but have a bunch of 1/4" pine dowels - weight is exactly the same as carbon for a 48" length although the dowel is slightly more flexible.

Do you guys think a pine dowel would be strong enough to be used as a wing spar?

The project is a 50" ws Cessna 172 trainer using a Kfm2 airfoil.

AUW should be around 10 to 12 oz by the way - and the plan is to NOT use this for any kind of aerobatics.

Thanks !

Fred
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Old Jun 01, 2012, 01:56 PM
12th Pursuit Squadron
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Garland, Tx.
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I have considered doing almost the same thing - wood dowel in KFm airfoil and about 48" span. At 10-12 oz I think it would be OK for sport flying with not much more than a gentle loop. If you were to use functional wing struts then the wood spar would be more than enough. My project development led me to drop the spar altogether. I will be using a foam 'spar' as a shear web/spreader to get the wing thickness that I want and use functional wing struts to carry the load. The wing will now be two piece with 24" panels attached to either side of the cabin. This project should hit the building board in the next couple of weeks. I hope to present it on this forum in the not too distant future.

J.P.
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Old Jun 01, 2012, 01:59 PM
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I think I'd be worried about pine. It does tend to be bendy. Can you find some bass wood dowels?

What type foam are you using?

You could always use two 3/16 inch dowels and glue them together. I don't think I'd trust the pine alone though.
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Old Jun 01, 2012, 02:01 PM
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Quote:
Originally Posted by dedStik View Post
I think I'd be worried about pine. It does tend to be bendy. Can you find some bass wood dowels?

What type foam are you using?
You're absolutely right - I retested the "rigidity" of both spars and the pine one I can make quite a large arc with it - I thought once epoxied to the wing it would be quite stiff though.

Foam - I'm using dollar store foamboard (3/16" without paper) - both sides with peeled off paper - not the stiffiest material I agree...
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Old Jun 01, 2012, 03:15 PM
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When sammiching a spar I won't use epoxy on it. I've only used epoxy on spars that are exposed. Epoxy will work great, but it adheres to the spar and gives it some rigidity. I found this out the hard way, crashed a small 3D plane I built last week. I was mad about having to buy another cf spar, till I realized it was only encased in the epoxy. Two sets of pliers one either hand and twisting opposite directions the epoxy gave and I could slide the spar from the epoxy like a straw. I have since put it in the rebuild.

Spruce is always a good spar choice, nice hard wood. Bass wood works too if you can't find spruce.
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Old Jun 02, 2012, 07:54 AM
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United States, MI, Houghton Lake
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In my experience 1/4 dowel is not good for planes with a span more then 30-36".
For a 50" aerobatic plane with a KFm2 I would want at least a 1/2" dowel.
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Old Jun 02, 2012, 09:06 AM
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Pine Spar

1/4" pine spar should be adequate for your use. The spar, coupled with the KFm doubler should hold up OK.
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Old Jun 02, 2012, 03:06 PM
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1/4" pine dowel should work fine. Also add another 1/4" dowel in the wing center section, about 1/3 of total span and glued to the full-span dowel. The additional spar will assist in handling the higher center section loads.

Jim R.
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Old Jun 02, 2012, 05:20 PM
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Quote:
Originally Posted by Fred Methot View Post
Foam - I'm using dollar store foamboard (3/16" without paper) - both sides with peeled off paper - not the stiffiest material I agree...
You can make that wing very strong by using the newspaper glue technique
and adding 3/16" dowels to the leading edge and trailing edges.. No spar needed.
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Old Jun 03, 2012, 05:35 PM
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Thanks a lot everyone for all the input!

I will look at all the options suggested and make some tests.

This is very interesting:
Quote:
You can make that wing very strong by using the newspaper glue technique
and adding 3/16" dowels to the leading edge and trailing edges.. No spar needed.
Cheers!

Fred
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Last edited by Fred Methot; Jun 03, 2012 at 05:44 PM.
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