Dec 12, 2004, 05:01 PM
Registered User
Joined Jun 2003
445 Posts
Question
Wing help: spar location with respect to airfoil and planform.

Hi there,

Given a particular wing planform, and a particular airfoil, where do I locate the spar? The planform I'm trying to replicate is shown in the diagram, and the airfoil I would like to use is the Eppler 479. The red lines represent 25% of chord at the ribs.

I'm guessing the spar should be on the 25% chord line?

What if I want to use a straight spar (say, to facilitate the use of a carbon tube).

Thanks.

Images

 Dec 12, 2004, 05:21 PM Registered User Pueblo, CO Joined Oct 2004 16 Posts The wings I have seen have had the main spar located at the high point of the airfoil. Merry Christmas. Ron
 Dec 12, 2004, 07:01 PM B for Bruce The 'Wack, BC, Canada Joined Oct 2002 12,803 Posts At the thickest point is much more common but it's not a rule. But if you try to put it back TOO far then the spar won't be at the point of maximum load on the wing and the wing will try to twist. But if you're using a tubular spar in the middle of the ribs it doesn't really matter where you put it within reason. Anywhere between about 25 and 35% would be acceptable. Bear in mind though that a carbon tube spar is not going to be as strong or rigid as a proper outer surface spar and webbing combination. Carbon is great stuff but it needs to be used in the correct manner to get the most out of it and a tubular spar that is much smaller than the wing thickness is not using the material to it's best.
Dec 12, 2004, 08:49 PM
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Joined Jun 2003
445 Posts
Thanks for the input.

I'm planning for this is to be a built up Depron wing. 37" wingspan. I guess I'll go with 2 carbon tubes through 25%, and "foam sheer webs" around the tube to join the upper and lower surfaces of the wing. Should be okay?

I ran some number through this online cg calculator. and resulted in the CG position in the picture below.

Images

 Dec 12, 2004, 10:11 PM B for Bruce The 'Wack, BC, Canada Joined Oct 2002 12,803 Posts The tubular spar would offer a degree of flexibility if that is important to you. A lighter and stiffer spar that would better control the structure would be an I beam made from a couple of 050 carbon rods with vertical grain balsa between them. I'd use 1/16 balsa and then wrap the whole mess with a double criss crossing wrap of kevlar or carbon thread to bind the whole lot together. The thread forming an XXXXXXX pattern on the webbing. The whole thing would then be painted with something to tie the kevlar thread to the spars.
 Dec 13, 2004, 03:55 AM Registered User East Anglia, UK Joined Sep 2002 29,709 Posts The spar is preuly structural, and has no bearing on aerodynamics, so hgh4est point and deep4est spar is where it does teh most good. Having said that, it ought to be near the center of pressure to minimise twisting on the wingtips. If its to be straight you will have to just eyeball it. won't make a huge amount of difference.
Dec 14, 2004, 01:31 AM
Registered User
Joined Jun 2003
445 Posts
That spar construction method sounds interesting, but I wouldn't want to go too overkill for a parkflier. ...or maybe I do.

Another question if I may. What's the generally accepted principle with regards to dihedral and differing rib heights. What I mean is, that the height of the ribs decrease from root to tip, so, do I centre all my ribs, or do I arrange them to form a flat wing top, but "dihedral" wing bottom? Please refer to my diagram.

Thanks.