Perhaps Steve wouldn't mind sharing the details on his leading edge – I think that would be great if he did!

Your points on the use of the carbon are exactly as I pointed out earlier. Occasionally, though we just LOVE to save weight, there are often other things that weigh in the decision of what to use. There is: corrosion, cost, availability, galvanic compatibility, strength, and environmental exposure. On the commercial side we also worry about whether a material is on an approved list or not, and whether there are B basis allowables available. It's important to understand the picture, before making any rash judgments.

I frankly don't know why Steve would use Kevlar – again I'm sure hoping he decides to share a little of his thinking on that. Normally the only place I've seen Kevlar is on the inside surface of a graphite cockpit (like the Sparrow hawk) and that's because it stretches a lot and is good in tension and is used to prevent shards of graphite from seriously poking you should you go down with the ship. Same reason it's good for bullet resistant vests (nothing is bullet proof!). The problem with Kevlar is that it's really bad in compression, so I`ve personally never used it structurally (glass yes, carbon yes, Kevlar, no). The trick is keeping the thin carbon or glass from buckling. That can either be done by adding plies or adding some foam or vertical stiffeners. My buddy originally did his spar that way, but had a problem with the glass adhering to the foam due really to the process, but decided that because the process wasn't robust, he would change it. This is actually one place where the plywood would probably be good at the root transitioning to lighter material outboard to save weight where the cumulative shear stress in the web drops off. The warping of the shear web in this location doesn't affect the airfoil so I wouldn't have a problem using it there. Of course, as you mentioned the plies should be in the 45 deg direction – same for the composite if it is used. Again it's likely a combination of materials will be needed to optimize weight. The plywood will obviously be heavier outboard where the loads are low.

The beauty of the finite element method is that we can get a pretty good guess before we spend all the time and money building something – that could get pretty expensive doing it trial and error, and would make optimization difficult. I'm working on getting some plans (have electronic version, but want to get some print copies and get a version of an FE package to build a model of the CD. That should be a bit revealing! I got a lot of irons in th fire, but hope to get to it before too long!

--- In This email address is being protected from spambots. You need JavaScript enabled to view it., "Philip Lardner" wrote:
>
> Um... no!
>
> Steve Arndt substituted moulded carbon for the original ply when making the
> D-skins for his Magic Dragon, which he covered with 1.8oz Dacron and Super
> Flight System 2-part paint. Watching Steve's videos, where he did use 1/32"
> ply D-skins, in the horizontal stabiliser, it has very clearly deformed due
> to moisture - something I really want to avoid. Though he doesn't specify
> exactly how many layers (or the weight or orientation) of carbon cloth he
> used, Steve does mention that his carbon leading edges "significantly
> enhance the strength of the glider."
>
> D-skins aside... watching his 'Magic Dragon uncovered' video I don't
> understand his use of mixed materials in the main wing spar shear-web. Steve
> used 1.2oz Kevlar from the root to the centre, and then, outboard of that,
> 1.8oz Fiberglass for 2 sections, then 3/4oz fiberglass for 4 sections and
> finally 1/2oz fiberglass for the remaining sections. Why does he use this
> mixture of Kevlar and fiberglass, and why not simply use carbon to save
> weight, or a single material throughout?
>
> The original Maupin design calls for the main spar shear web to be made from
> just a single 1/32" ply with the grain running 45deg to its length.
>
> Ultimately there is only one way to discover the truth - and that is to
> build comparison pieces (ply, carbon, carbon-foam sandwich) and test them to
> destruction before incorporating the stronger elements into the final
> build... and then load test (not quite to destruction!)
>
> Although only mentioned in passing while making his videos, Steve shows the
> mounting holes for his own-design winglets. Does anyone know is these proved
> to be effective or beneficial to the overall performance of his Magic
> Dargon, and are there any designs or plans for them floating around?
>
> Phil.
> (in soggy Ireland)