Over 20kg model question

[robbieskipton]

Hi All,

Hope you’re all looking forward to the shows as much as i am

Got a question, Please don’t shoot me down......

I’m designing yet another model at the moment... well 2 actually... 1 a 4 meter katana and a 60% extra

My question relates to wing design...

As most wings have a main spar, I’m planning on getting rid of this for a new technique using 2 spars made from 6 - 8mm Carbon Tube depending on application. (Please see Rough Drawing As easier to understand)
In essence it’s a box spar using the top skin and bottom wing skin to complete box....

So my question is, would this be acceptable for over 20kg models, as i have worked out that this method would be 10x stronger than the conventional Main spar...( Plus would be a lot lighter )

Please let me know thoughts, as need to know before I continue.

[robbieskipton]

Top View

[Andy Boylett]

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[Nick Reeves 3055]

from the drawing there look as though there is very little contact area between the carbon tubes and the webs and skins, unless im missing something that is

[robbieskipton]

Cad Stress Analysis Novelty of 3d ..

10x is a little ott to be honest.... 3 is correct

With carbon, it is bidirectional ( Bends in both directions).. With ply wood, yes it is strong as it is made up of multi grains going in different directions. However... If you consider a 6mm Piece of ply..

what you have in theory is only 3mm of that in the horizontal plane. And 3mm in the Vertical plane..... ( that’s how’s its made up)

Now if you harness carbon correctly with using 2 x Carbon rod on 1 section of wing (Rear for example), then simply web between the 2.... You are in fact (Theory) making 1 piece of 6mm Carbon Rod 12mm..... Then as there are 2 parts ( 4 tubes) you have 2 x 12mm Carbon rods running up wing.. Then ( Long winded)... you bring the 2 together by the top and bottom skin, .. You in theory have a 24mm Carbon box running up the wing ...

you are in theory only making a box... i would even go to stronger than an 'I' Beam made of wood...for less weight

If you require figures...( Measured in G force)

6mm Ply, Lightened wing spar - Breaking point 4.7G on a 60LB MODEL... weight 2.7lbs (wood)
Carbon As Above- Breaking point - 14.1g on a 60lbs model ).. weight.....2lbs

[robbieskipton]

I did say its a rough drawing

[Phil Clark]

1) Limited contact area between carbon tube & the lite ply shear spar is a problem.

2) Changing from lite ply to balsa shear spar/webbing at the same positon as the phenolic ends is a problem, you have a localised stress point there already, so why make it worse by introducing a weaker material as well?

3) You quote you have saved 0.7lb on a 60lb model.......is all this really worth such a small weight saving?......will perfomance really benefit from it?

Looks a bit like trying to reinvent the wheel to me with little or no real benefit other than 'bling' coz it's an aerobatic model with a carbon spar!!!!!!

[Andy Boylett]

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[Phil Clark]

To get a good bond of the tube to the shear web, you'll have to run a 'fillet' of adhesive down both sides to creat a decent gluing footprint......the adhesive suitable for bonding disimilar materials will be a slow cure epoxy of some kind and this is heavy. You say you've only saved 0.7lb anyway.......by the time you've added in a load of epoxy, you probably won't be far different in weight, so again, looks like a lot of extra 'experimental' work for little if any gain to me.

[robbieskipton]

thanks for all your input so far.. However, still has not answered the question...... Would the LMA pass this on an over 20kg model?...

Remember 0.7lbs... is per wing...... So would be 1.4lbs on a complete wing... .. Which every pound counts on an aerobatic.. Especially as trying to get A.U.W less than 90lbs on a 4.7 meter wing

I am aware that exclusive modelbau do this on most there models, and have no issue at all with strength, but it is on a model under 44lbs........ my idea is, to make a stronger wing spar and structure with using composite techniques in places so i can minimise the skin thickness........ thus saving more weight.. I am aiming for only 8 - 9lbs per wing......... (which is 2.1 meters per panel)... very achievable I believe

Idea noted though Andy on the [] tube........ and also noted on webbing Phil.... Probably be best to run 3mm lite ply at least 1/2 way up to even load

[Phil Clark]

The decision whether to pass it or not Robbie will be down to your inspector........and as this is a 'new' structure as far as the majority are concerned, I think the only way to proove whether it's up to the task in hand is to build a spar and statically test it (probably to distruction), then built an 'all timber' version and test that as well....this will give you results to compare.

Phil

[Rob Buckley]

As Phil says, the only way to really say (and to take into account variabilities in the construction) would be to make one, load it up & break it. This would also show how & where it breaks, which is of equal interest to the ultimate strength. With a static load of 192Kg (to get to the 14.1g ultimate load calculated) it should go with an impressive bang at any rate!

Rob, is it possible (just out of interest you understand) to show us how you calculated the ultimate load capability of the spar to be 14.1g?