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Aug 13, 2017, 10:47 AM
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EdSoars's Avatar
Discussion

Wing skins


I was cruising the net looking for materials for wing skins. While I love 1/64" baltic birch plywood, it is getting expensive at $4/sq ft. I looked at different plastics, but none of them were thin enough, except for the familiar mylar laminating films. I wanted something more rigid and stronger in bending loads.

The black poplar veneer wings from Arthobby or Aloft clearly are strong and light. So I looked at wood veneers. A possible solution is 1/42" (.024", 0.6mm) hickory veneer. Hickory is what baseball bats and shovel handles are made of, because it is strong and resilient. But it would only resist bending loads, and be vulnerable to splitting. It is less dense than birch so would weigh about the same as 1/64 plywood.

So it would be interesting to laminate hickory over foam cores with a layer of 2 oz FG, either inside or outside the veneer. A pack of 4 sheets, 8" x 32", would make up a 64 inch wing. Price without shipping is $16, or $2.30/sq ft. FG is another $0.70/sq ft, so the total would be $3.00/sq ft, still 1/3 less expensive than 1/64" plywood, a little heavier and hopefully stronger and more rigid.

Has anyone else tried something like this?

Ed
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Aug 13, 2017, 04:18 PM
You know nothing....
Stuart A's Avatar
I never followed it up,but I did look into using .6 bamboo veneer(cos I thought it looked cool,and I'd laid a couple of solid bamboo floors)
http://www.plyboo.co.uk/products/fla...bamboo-plywood
Aug 13, 2017, 07:29 PM
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Bare's Avatar
Why in heck would you Bother with "veeners" When Laminating 2!! oz cloth over foam cores?
Reads as Silly strange IMO.
Aug 14, 2017, 12:31 AM
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EdSoars's Avatar
Bare: 2 oz is what I have in stock, or I'd use lighter weight. And 2 oz cloth wouldn't hold the compression in bending deformation without using 3 layers. Been there, folded that.
Aug 14, 2017, 11:46 AM
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Knoll53's Avatar
US composites has 5 oz. Kevlar for $1.84 /s.f. (plus shipping). It's a bargain. A layer of this Kevlar plus a finishing layer of your favorite light glass (for me 3 oz glass) could make a thin and strong replacement for 1/32" plywood, especially with a bias cut. It may be a very thin laminate, and thus too flexible for open framework. Don't know yet. Since it is epoxy based, it's rigidity varies with temperature, so no black paint.

Currently I am thinking of vacuum bagging this combo on a waxed melamine board to create prefab rigid skin for open framework. If removed from the flat mold in the B stage, it would cut easily with a knife or scissors and it would be bendable to the curvy top surface of a D tube. It sounds complex but with the use of a hot box, the skin could be fabricated and installed on open framework in one day.

For your project, when attached to a foam core, this skin would be stronger than 1/32" plywood let alone 1/64" plywood. As a added benefit, the skin serves as a live hinge at control surfaces.

Would this Kevlar skin be heavier than 1/64" plywood? Don't know. Maybe. Could your plane tolerate an extra ounce of AUW?
Aug 14, 2017, 02:51 PM
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EdSoars's Avatar
Somewhere I have the weight per square foot of 1/64 plywood, but it doesn't matter because I know that ounces of AUW, especially in kevlar, have never hurt ANY of my planes ('cause I've tested lots of ounces on lots of planes), and it would save a lot of repairs. It might even be stronger than scrub juniper.

The only tricky part is to trim everything without using sandpaper. Sandpaper and kevlar do not work together.

Do you think the B stage kevlar-glass could be wrapped around a leading edge curve, say 1/8 to 3/16 inch diameter? That would make the method irresistible. On foam, with a hardwood dowel LE, it could be applied in pretty soft condition. It's worth a test panel.
Aug 14, 2017, 07:49 PM
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Knoll53's Avatar
early B stage is like "fruit leather"............gummie and completely moldable. It would bend around a LE. BTW, a sharp bend in 5oz. Kevlar, when fully cured, would be very strong. The LE could have foam backing. The epoxy needs to be hard enough so that the finish surface is not damaged by handling. Use gloves for handling B stage laminates. You could vary the B stage with a strip of foam insulation down the LE, then vac. bag in a hot box so that majority of the skin gets hot and cures quickly. this would leave a gummie strip at the LE tight bend. Timing is critical for B stage.

Yes, all cuts with a scissors. Never sand Kevlar. In the B stage you could even hammer the cut edges to taper them.

You could attach skins to foam with PU glue. The trick would be how to pull the skins tight to the LE. Possibly flat thumb tacks thru the skin could work, but would leave dents after a vac. bag.

A separate top and bottom skin is a lot easier. With a bass wood LE, it would be quite strong.

You could even squeegee on a spread tow cf spar cap to the B stage Kevlar if you laid down a light layer of 90/0 glass on the Kevlar so the the squeegee did not disturb the bias Kevlar. If it was a stiff B stage, you wouldn't need the glass.

You know that I'm going to try this for my open framework model..........
Aug 15, 2017, 09:34 AM
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EdSoars's Avatar
I'll have to ponder that wrapped LE. If the adhesion problem could be solved, it would be SO strong and simple.

I don't see a thread for your open framework model. Whassup?
Aug 15, 2017, 01:19 PM
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Knoll53's Avatar
it is in the "just thinking stage" right now.
Aug 15, 2017, 01:27 PM
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Knoll53's Avatar
Quote:
Originally Posted by EdSoars
If the adhesion problem could be solved,
I have 2 words for you...........contact cement.
Aug 15, 2017, 08:02 PM
B for Bruce
BMatthews's Avatar
I also have two words for you as an alternate..... "vacuum bagging".

Vac bagging cloth and resin is well tested and well proven. Use glass kevlar or carbon or a mixture of them all. And frankly if you're going to set up to bag flat skins over glass or whatever then you may as well bag them right to the cores.
Aug 15, 2017, 08:03 PM
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EdSoars's Avatar
Another two words: transfer tape.
Aug 15, 2017, 10:10 PM
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Knoll53's Avatar
Quote:
Originally Posted by BMatthews
if you're going to set up to bag flat skins over glass or whatever then you may as well bag them right to the cores.
Conventional vac bagging uses a top mylar and bottom mylar. This requires some sort of gooey LE treatment to connect those two skins. It is doable. Yes, well tested, but not without it's problems.

I use stiff thick mylars for vac bagging but still I get a somewhat wavy or lumpy surface. A smooth and glassy surface thanks to the waxed mylars, but wavy. Every time. Maybe my vac pressure is too high. Maybe my foam is too soft. Don't know. Also, mylars slide around when under vacuum sometimes.

On the other hand, every time I vac bag a plywood skin on, it is smooth. There is a benefit to adding in the extra step that allows a stiff fully cured skin to be vac bagged on.

What Ed and I are looking at is a prefabbed continuous skin running around the LE. It sound like a super clean way to build a wing, if it can be done. So far, the steps involved appear to be easier that conventional vac bagging. There may be more steps, but easier steps.

So the Kevlar skin is stronger and cleaner than the 1/64" plywood skin, at less costs.

The epoxy to fabric ratio will be easier to control when layed up on glass or melamine. Conventional vac bagging drives epoxy into the foam, to some degree, and this just adds excess weight without added strength.

The Kevlar skins can be vac bagged to the cores with PU glue which expands and is lighter than epoxy, but at least as strong as the weaker material, the foam.

In my mind the only remaining challenge is how to push the cores into the the LE bend of the Kevlar during the vac bag. If the Kevlar skin did not perfectly conform to the cores at the LE, it would not be worth doing.
Aug 16, 2017, 11:13 PM
Registered User
tspeer's Avatar
Alex Strojnik, in his book "Laminar Aircraft Technologies" describes the method he used to create skins for his full-scale motorglider. The technique might be adaptable to models. Strojnik used an aluminum C channel for his spar, with wide box ribs and fiberglass skins that were molded just the portion of the wing forward of the spar. The cured skins were bent to the shape of the wing behind the spar.

Here's Strojnik's description of the process, "The fiberglass skin is made by pouring and uniformly spreading epoxy on plexiglass and laying up a 50 in wide bidirectional fiberglass cloth and squeegeeing epoxy through this first layer. After a while a second (and third, if so required) sheet of fiberglass are added and expoxied. Before the first ply is completely cured, the skin is covered with a very thin plastic sheet and peeled off the plexiglass plate (plexiglass is one of the few materials that a normal epoxy will not stick to). The skin is gently bent in the middle and deposited over the positive template with the smooth side on the outside. After a proper centering and alignment the skin is weighted on both sides (only one shown on the sketch previous page) by clamping wooden 1 x 2 boards on both sides and loading them with approximately 20 lb each side. A sheet of fabric is laid over the skin and weighted on each side by another 20 lb. After the cure, the skin is removed from the template, the thin plastic sheet peeled off the inner layer, trimmed at edges and is ready to be used."

I think something similar could be used for models, with the core forward of the spar taking the place of the template. The plastic layer wouldn't be needed unless it was useful for supporting the soft B-cured cloth while taking it off the plexiglass, because the skin would be glued to the core in any event. If the plastic was used, then the skin would be molded as Strojnik describes, the plastic peeled, and then the skin placed back on the core and weighted the same way to clamp it while the glue hardened.

The wing would be completed by putting the skinned forward part back into the hotwired bed, the aft core inserted, and the skins vacuum bagged to glue them to the core.

I see the advantage of this method as being able to create an accurate leading edge without any rework to dress the junction between upper and lower skins. The leading edge is the most critical part of an airfoil, and unfortunately much of the time it's the area where the most hand work has to be done to shape it. Another advantage of this method is it would be easy to add a strip of Kevlar or other reinforcement to the leading edge before putting on the outer skin.
Aug 17, 2017, 11:31 AM
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Knoll53's Avatar
Your technique could work Tspeer. It is complex, but it could work. If I have got it clear you ...
  • cut the core at the D tube
  • prefab a full size skin for top and bottom in one piece
  • bend the skin at the LE and attach it to the D tube with hanging weights, cure
  • insert the aft portion of the core and vac bag.

Splitting the skin prep into two stages, as Alex suggests, is a good option. Lay up the first thin glass layer and let it nearly fully cure THEN add additional layer of bias Kevlar and spar caps. While that resin is wet, apply it to the cores or framework. This should be flexible enough to conform to any curve, thus reducing the needed clamping pressure. Possibly no vac. bag needed, only weights on foam beds.

Good food for thought.


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