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Old Sep 15, 2014, 11:23 AM
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tblount70's Avatar
United States, TN, Thompson's Station
Joined May 2004
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Redesign Spars?

I'm just starting my first serious scratch build - Elf Senior Bipe. One thing I really planned from the beginning was to redesign the spars a bit. Just about every kit I've built (couple dozen) have had upper and lower main spars. Most have had some sort of shear element - either individual shear webs or some other connecting piece between the spars. At this point I'm comfortable building this design.

With the OT plans and even some not-so-old, I see one piece main spars.

Has anyone seen a thread on the ins and outs of changing the spars around? I did a little digging and didn't find anything.

Thanks in advance...
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Old Sep 16, 2014, 08:48 PM
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Warren B's Avatar
Mt Evelyn, Melbourne, OZ
Joined Dec 2008
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I see the Elf Senior has 3/16 x 3/4 pine spars.

Putting the same amount of pine into top and bottom 3/16 x 3/8 spars with webbing (1/16 sheet balsa with grain vertical) would be a lot stronger (way more than double).

However that is not to say that the original spars are too weak, 60" is not a large wing.

The trouble with top and bottom spars with this wing is that there is no leading edge sheeting, so the top spar will show and look rather ugly (and slightly disrupt the airflow, but probably not noticeable).

If you were also going to sheet the wing LE, then top and bottom spars would be the way to go.

If you are worried I would probably just add a little to the spar width take it to 1/4" X 3/4.
More depth would have a greater strength increase, but there is not enough room in the ribs above the spar.
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Old Sep 16, 2014, 09:01 PM
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eastern pa
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I'm in the waaaay sloooow process of building an 84" span Red Ripper which also shows original large spars.

I am switching to an upper and lower webbed spar for better strength, less weight and most importantly, I would break an unbelievable amount of ribs cutting the large notch for the original spar seeing there is only about 1/8 inch of rib above the spar.
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Old Sep 17, 2014, 01:41 AM
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There is absolutely no doubt that top and bottom spars with graduated webbing is structurally superior to the set up often seen on vintage designs for lower spars only. I suppose it all depends upon your views on maintaining "authenticity" - or whether you are building to fly in SAM contests which prohibit such structural changes. One weakness of the relatively massive bottom spars is that, if sized to be strong enough at the centre they will be heavier than needed at the tips; it is easier to build in tapered strength and tapered weight with the top and bottom spar/graduated webbing system.

A halfway house is to still use the main bottom spar but fit a couple of lighter spars on top as well to produce a more "balanced" structure which will resist warping and upward curving ("cow horn hedral") better.

As Warren says, without LE sheeting the top spar can be a little intrusive, however inserting riblets will help a lot.
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Old Sep 17, 2014, 02:17 AM
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Ilkley, West Yorkshire, UK
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Personally, I have never got on well with LE sheeting and prefer the upper spar & riblets approach with an additional 1/16" sq spar well forward just to prevent covering sag.

But I do hear that SAM has a contract out on my head.....
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Old Sep 17, 2014, 09:36 AM
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Northumberland, England
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If anyone feels that they must use a heavy single main spar to maintain an appearance of authenticity, IMO, a good compromise would be to laminate the spar.
For example the Elf Senior spar mentioned above could be laminated from 3 of 3/16" x 1/4" spruce to help stiffen it against an upward curve & counter any natural curve already present in the wood.
Better still the centre laminate could be a length of 3/16" x 1/4" using spruce for the inner 1/3 spliced to 2/3 of balsa for the outer in order to keep the tips light leaving the strength where it's most needed.
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Old Sep 17, 2014, 12:56 PM
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eastern pa
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A halfway house is to still use the main bottom spar but fit a couple of lighter spars on top as well to produce a more "balanced" structure which will resist warping and upward curving ("cow horn hedral") better.


You are right on this. I forgot to mention I am also adding 3 small spars on top in front of the webbed spar. Maybe 1/8 or 3/16 square, full span.
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