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Old Jul 10, 2013, 02:10 PM
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SW-2 Swept Wing prototype

Thought I'd take a stab at a DLG swept wing with ridiculously thin airfoils. The construction method is composite and foam with the intent of testing stressed skin construction for the Horten Sickle (coming up next).

Shooting for lightweight this time but am generally unfamiliar with composites so am just guessing at a workable lay up schedule. Built a mock up with some kevlar at the front 1/3rd with 50K CF tow underneath, which seems workable as a main spar. Pink foam for shear. The remainder of the wing is just 4oz glass which dents easily with just normal handling, so some upgrade is needed there.

Also, need help positioning mylars at LE so Kevlar is held back. Sanding LE with kevlar is a mess.......That's why we do mock ups!

The new Bloggie camera will not focus close so photos will be limited.

Kent
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Old Jul 11, 2013, 08:57 PM
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Cool stuff. Will there be winglets or some kind of vertical fin?

50k tow seems like overkill for a plane like this unless you plan to bungee launch.
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Old Jul 12, 2013, 12:45 AM
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Quote:
Originally Posted by Steve C View Post
Cool stuff. Will there be winglets or some kind of vertical fin?

50k tow seems like overkill for a plane like this unless you plan to bungee launch.
Surely a DLG launch would be at least as stressful as a bungee launch.
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Old Jul 12, 2013, 11:45 AM
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Quote:
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Cool stuff. Will there be winglets or some kind of vertical fin?
50k tow seems like overkill for a plane like this unless you plan to bungee launch.
Yes, there will be winglets. Although I hope to hand launch this exclusively, there will be a took hook too. Both launching techniques appear to be high stress to me. I'm on mock up #4

This wing is very thin....about 5/16" at the tip. That's a thin spar and the idea of swinging it around for the discuss launch has me just a little concerned. 50K tow maybe be overkill, but it is what I have on hand, so it is easy and cheap to do. All kevlar is 5oz. Quite honestly, I thought that it was under designed for finger blade attachment.

I've seen other planes on the Hand Launch forum with highly refined designs with the absolute minimum of reinforcement, but this is prototype and I rather have a plane that is 10% heavier than competition grade and durable enough to survive flight testing, than one that might break on the first launch.

Thanks for your comments. I've never designed, built or flown a DLG so I'm sure I have a big learning curve ahead of me.

Kent
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Old Jul 12, 2013, 01:06 PM
Herk
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Kent, I don't think that this new wing concept of yours is necessarily "ridiculously thin."

This home brew wing of mine has airfoil thicknesses in the range of about 6%. The spars are .005 thick CF laminate and the skin is three quarter ounce fiberglass. The reinforcement at the tip is CF tissue. It holds up fine to launches. At the launch blade the wing is one quarter of an inch thick.

Now I'm not the super launcher that I was at one time, but I can easily launch this high enough to catch thermals. It has shown no signs of weakness.

Your plan sounds as if the wing will be much stronger than this one illustrated.
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Old Jul 12, 2013, 01:36 PM
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Thanks for your comments and the reality check Herk. Your example shows what I consider to be competition grade construction. It's light construction simply blows my mind. Yes, it is clear that my prototype is currently way way more heavily reinforced than the example you posted. Like maybe 6 times more reinforcement.

I started with a mock up that had 4 oz glass as a primary skin ( 6 times as heavy as the standard ) and found that simple handling of the wing caused dents in the skin. I guess this is to be expected, but I choose to up that reinforcement to 1.5 oz plus 4 oz. glass. That approach ( mock up #4 ) passed my handling test.

So from my test, 7 times the standard reinforcement is the minimum design for me. From there I added 5 oz. bias kevlar at the hinge and TE, which again is many times the thickness of the standard material used in DLG ( I've been reading Phil Barnes posts ). Not so much by design, but simply because it is the only material I have in the shop.

By now, all of the real DLG builders have stopped reading this thread because I'm clearly a raw beginner.

I will re-think this and try another mock up. For starters, I think that I will have to adopt some special handling rules for this wing. I'd like to have a primary skin of the 1.5 oz. glass due to the nice finish it provides.

Kent
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Old Jul 12, 2013, 03:37 PM
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I can't figure why you are getting this handling damage issue. Maybe the epoxy isn't really cured completely, or maybe it's the pink foam. I've not had any serious problem with that despite always using lighter skins.

This little model - 36 inch span has no spars, and the entire wing is just blue foam with 1.5 ounce fiberglass skins. These wings that I've pictured are not bagged. The skins are just attached with brushed on Minwax Polycrylic resin. I'd think that bagged epoxy would be tougher.
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Old Jul 12, 2013, 03:59 PM
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Yes, I think that you are right on both counts.

The epoxy is at a 24 hour cure at room temperature. It would be harder if cured at higher temp.

The pink foam is certainly softer than blue. Sadly, blue foam is not longer readily available here.

The real culprit may be my handling when cutting the hinge out and sanding the LE, so I'll clean up my act and build another mock up in a lighter version. see sketch.

Kent
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Old Jul 12, 2013, 05:13 PM
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I think just letting the epoxy cure properly is enough for strength. Some of the more expensive DLG wings are actually hollow - just the skin and some built in d-box type layout and nothing else. You can try it out on another prototype and fly the one you're building in the mean time. Maybe 1 week curing time?

A lot of DLG wings are bagged and cured at room temperatures. Even the molded ones are usually built at room temperatures. Of course, it depends on what "room temperature" means. I'd take longer to cure in the winter.
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Old Jul 12, 2013, 05:49 PM
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I have a primitive oven, driven by a standard space heater, that I use for "real" lay ups, so mock #5 will get the oven treatment.

Even if the glass skin is very dentable, I can simply handle it by the forward placed spar, which is Kevlar and CF tow.
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Old Jul 12, 2013, 08:14 PM
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So this is the lighter version. Probably about twice the reinforcement of a standard DLG.
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Old Jul 12, 2013, 10:00 PM
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I found that placing blue foam derived glass/epoxy parts in direct sunlight for only one day turned the rubbery soft bits ( 2 weeks old) into very hard /stiff almost usable ones. Surprising actually.
Easy enough to try.
I don't understand that with 4 oz cloth you could normally dent these.. without intentional effort .
Something's amiss.
Perhaps an alternate 'brand' epoxy is needed?
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Old Jul 13, 2013, 11:39 AM
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Dentability 101

Quote:
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I don't understand that with 4 oz cloth you could normally dent these.. without intentional effort .
Thanks Bare. You may be on to something there. I'll be the first to confess that I am rough on my toys. Maybe my normal handling equates to intentional effort, but I'm trying to play nice with this one. I still find it hard to believe that an entire industry is built around such delicate wings. Maybe my mock ups are more fragile than the rest for some technical reason, such as incomplete curing. I'll place all of the mock ups out in sun (if it ever comes out), as you suggest. Feel free to post photos of your light skins.

BTW, I'm using US Composites resin and Hexcel 1.5 glass.

Mock up #5
1.5 glass at 90/0 plus my typical red kevlar/50K CF tow spar has cooked for 4 hours in the oven then left over night in the same insulated oven. Is it a complete cure?....not hardly, but much closer to ideal.

Even when I make an effort to handle the work gently, in the process of tearing off the mylars I dented the glass. OK, note to self....stop touching the glass. The finish is great and gentle handling causes no denting. This is certainly a workable method. When I build the wing, I"ll cure the hell out of it.

Then I noticed something quite revealing. After marking up some notes on the skin of the mock up with a sharpie, THE FELT TIPPED PEN (fine point) made a significant dent in the glass. Not surprisingly, the same pen, at the 5 oz Kevlar hinge did no damage.

Will this wing get harder after a full cure? I hope so. But even if it does not, when flying at the slopes, I can handle it by the LE/spar portion and have no problems.

So is all this effort for light weight worth it???....All the mock ups are about the same size so I did a quick check. Mock up #4, the one that I "approved" weighed in at 18 grams and Mock up #5 weighed in at 9 grams....Big difference.

From what I read on line, a common design is 3/4 oz. glass on the bias over either 1.7 oz/ Kevlar or carbon cloth. I can see the wisdom in this approach. I have none of those materials and will go with Mock up #5. My estimate for total wing weight is 250 grams (not including the wires)

Again, sorry for the blurry picces. The Bloggie only begins to come into focus at arms length. Not bad for a pin hole camera.
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Old Jul 13, 2013, 11:13 PM
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well it sounds/reads like you are seriously trying. I can understand the frustration.
But imo.. if.. after oven time and an overnight rest one can mark the glassed surface with a Sharpie !?.
It's clearly time to invest in an 'alternate' and fresh supply of Epoxy goop.
Hell.. try some cheap 1$ store 5 min epoxy as comparison.
I would have had a fit and stomped it to death on the spot .

I'm fairly gentle in handling my toys but clumsy, even brutal in using them
2 layers of 1 oz cloth (sans benefit of that effete bias direction stuff :-) on a p51 type belly scoop has endured ..many... bad belly slider landings with nothing other than paintwork issues.
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Old Jul 13, 2013, 11:22 PM
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Adam made a comment about US Composites 635 resin that it needs a long hot cure, so I'm not concerned. The wing will bake for 24 hours. So far, the resin has been great. I'd be in good shape if I invested in some 1.7 oz. Kevlar. I'll bang this wing out with what I have on hand and see what I think after some field testing. These mock ups have been fun for me. Learn by doing.
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