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Wing spars advice

Posted: 06 Nov 2018, 00:46
by Lawrence D
Hi folks

New to very big glider building and just started work on a third scale Sperber Junior (5.2m) which I've scaled up from a John Watkins plan...it seemed like a good idea until I laid out the tape measure to get an idea of the size of it and now a bit concerned about floppy wings! Any suggestions for the spars please....I am sure scaling up John's spar sizes will not be enough...cubed law etc.

Cheers

Lawrence

Re: Wing spars advice

Posted: 06 Nov 2018, 08:20
by chris williams
I'm not familiar with this plan, Lawrence, although it has been scaled up to 3rd in the past. How does John tackle the gull join; with a full-length spar and fore & aft ribs, or in two panels joined at the gull break?

Re: Wing spars advice

Posted: 06 Nov 2018, 08:23
by Cliff Evans
You could have gone for Jilles's plan at 1/3.25 rather than scale up Johns!
https://scalesoaring.co.uk/schweyer-spe ... ior-plans/

Re: Wing spars advice

Posted: 06 Nov 2018, 22:02
by Lawrence D
Hi Chris,

I first saw plans in a mid 90's QFI, and knew John from flying on the Mynd at the time. I like the design and back story of the Alp crossing and need to get a big glider out of my system!

His original design was 3.9m span, and had 1/2 inch x 1/8 spruce spar top and bottom with 1.5 mm ply webs each side which turned to balsa webs at about half wing span. The spar width tapers to 3/8th at the tips from half span- my gut feeling is the spar system would have been working hard on the original design. Each wing is a single panel and I it looks like the spars had to be steamed to get the shape.

Thanks

Lawrence

Re: Wing spars advice

Posted: 07 Nov 2018, 00:10
by chris williams
OK, next question...do you intend to throw her about, or fly in a more sedate fashion...?

Re: Wing spars advice

Posted: 07 Nov 2018, 04:45
by Jilles
The 1:3.125 (4.8m) Sperber junior from my design used 1/2"x1/4" spars tapered to 1/4"X1/4" past the gull radius to the tip
The 5.2 m version should have the same size in my opinion. It will withstand the occasionally hard arrival. ( not the pilot fault of-course !!!) Three have been built to my knowledge and are still alive. a 1.5mm ply webbing is essential as for most spars.

Re: Wing spars advice

Posted: 07 Nov 2018, 23:51
by Lawrence D
Chris- for this it would be mainly about soaring, and some scale like aero's, but nothing (intentionally) too radical.....

Jilles- many thanks for the recommendation.

Cheers

Lawrence