wing spar design .  
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wing spar design . - 10/7/2007 10:49:48 PM   
trysforscale


 

Posts: 8
Joined: 11/12/2006
From: royal oak, MI, USA
Status: offline
I have a BUSA 1/4 scale kit that I want to start on soon . I would like to thin the wing thickness down a bit and not have to resort to useing functional flying wires . That means building a very rigid set of wing spars . Wing span is 80 " . I'm thinking a spar that is a triange shape in cross section is probably the strongest , or would I be better off with a round tube . I have read a lot about carbon fiber , but I haven't used it yet .

Any and all thoughts on this are welcomed .............

Guy.
       Post #: 1

RE: wing spar design . - 10/8/2007 3:45:46 AM   
Ron S



Posts: 1061
Joined: 12/12/2001
From: Fort Worth, TX, USA
Status: offline 		Actually, an I-beam shape is probably the best as far as strength to weight is concerned. An I-beam can also be tailored in strength by reducing its dimension (width for example) as you go outboard. A tubular shape isn't bad though, especially if it is just an off-the-shelf purchasable item. You won't want to run it full length though, and somewhere you'll want to transition the tube to some other spar shape.

_____________________________

-Ron S.

(in reply to trysforscale)
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RE: wing spar design . - 10/8/2007 4:01:51 AM   
rmenke


 

Posts: 1772
Joined: 6/25/2004
From: Merced, Ca., CA, USA
Status: offline 		TfS:

For years, the industry has used the "D"tube approach to wing strengthdesign. The leading edge is the center of the D, Sheeting runs from the LE along wing ribs, the upper and lower spars are connected by webbing forming the D. With a little additional strength added by CF at the top and bottom of the spars, you have a reasonablely strong wing. Most use solid webbing with the wood grain runnung up and down. Have never had a wing fail in flight, so I must build a little heavy. Note the current crop of ARFS connect two wing half with a piect of 1/4" ply about 6" long into a cradle inside the D tube, they somehow stay togeather, even on the Sonic 500's (slow Quickees). You should be fine with something like this. You can also substitute spruce for balsa spars. You may re-thinking cutting the wing down unless you are looking for speed. It will change the flight character, likely in a unfavorable manner. If you are going to "kit bash", be willing to accept a few failures along with way to a lot of fun, along with education at the school of hard knocks. ENJOY

(in reply to trysforscale)
       Post #: 3

RE: wing spar design . - 10/8/2007 5:32:20 AM   
dreadnaut



Posts: 645
Joined: 3/13/2004
From: Vista, CA, USA
Status: offline
quote:

ORIGINAL: Ron S

Actually, an I-beam shape is probably the best as far as strength to weight is concerned. An I-beam can also be tailored in strength by reducing its dimension (width for example) as you go outboard. A tubular shape isn't bad though, especially if it is just an off-the-shelf purchasable item. You won't want to run it full length though, and somewhere you'll want to transition the tube to some other spar shape.


The measure of how an I beam resist deflection is the Area Moment of Inertia. This is determined by measureing the area of the flange, and multiplying it by the square of it's distance from the neuteral axis. If you reduce the distance, as in making the wing thinner, you can maintain the Area Moment by increasing the area of the flange. Note that the area of the web does contribute also, but it is minor when compeared to the flange, so I left it out of the calculation to make the math simpler.

The two spars shown have rooughly equivilant Area Monents. I include a snapshot of the excel spread sheet I use. If you have access to this program, or one like it, you will never use a pocket calculator again. I can play with the numbers, and try different flange widths until I come up with one that works. As I am writing this I just reduced the width to 1 (half inch to neutral axis, and changed the number in the width cell to 7/16 (.4375) and get an I of .0316. Good enough.

If you have a D-tube structure you can get a rough estimate of the distance to an aproximate center of the area as 2/3*D. To get an equivilent I, you will probably have to increase the thickness of the sheet.

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< Message edited by dreadnaut -- 10/8/2007 5:56:02 AM >

(in reply to Ron S)
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RE: wing spar design . - 5/19/2008 1:59:55 PM   
lavi rider


 

Posts: 197
Joined: 1/27/2003
From: Givat Zeev, ISRAEL
Status: offline 		Can you please give me better example my wing is 5m long 75mm thick at root and I would like to calculate the spar for it for a weight of 4*85kg...85kg at 4G.

Regards
Peter.

(in reply to dreadnaut)
       Post #: 5

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