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Electric Stunt => Gettin all AMP'ed up! => Topic started by: jose modesto on January 06, 2012, 08:15:20 PM
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Dennis this wing is for an Electric Model Dennis. looking at your "Sweet Pea" and the stress skin sparless Wing,I'm looking for your opinion on using composite balsa wing shells for a stress sparless wing.
the wing panels composite consist of 1.90z Kevlar cloth,fiberglass cloth,balsa and epoxy. The wing is built in the Lost foam jig.
Dennis Take a look at the photos and let me know if this wing construction is adequate for a stress sparless wing.
Can a sparless stress wing have any openings. (open bays)
Jose Modesto
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Just incredible Jose
The sparless wings simply recognize that the skins can provide all of the bending and torsional strength we need, The ribs keep the skins from distorting whic makes it very strong. ANy closed "tubular" structure will be extremely stiff in torsion too. It is very much like a foam wing with ribs performing the role of the foam. On my sparless wings I used rib spacings ranging from 3/4" (with 1/32 skins) to 1.5", with most of the wings using 1" rib spacing. Of course the foam rib eggcrate wing relied on the skins.
Your skins are pre molded and very much stronger bcause of the composite lay-up. Since the skins are doing all the work then stronger skins are a good thing! Because the skins are stiffer fewer ribs will be required too. However, using te ribs will help assure that the wing maintains an airfoil like shape under stress!
Open bays reduce the torsional strength and become stress risers. Large radius corners are a good idea.
Your ribs look like they are already cut for spars...?
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Do you mean "open bays" as in "whacking big holes in the wing"?
If so, then they'll not only weaken the wing torsionally and become stress risers (particularly if they have square edges), but to the extent that they cut across the fabric of the wing skin in the thick part of the wing they'll weaken the wing in bending -- which is a Bad Thing.
Think of your wing skin as being a whole bunch of itty bitty spars, all running from tip to tip. The 'spars' that are at the thickest part of the wing do the most work, those at the trailing edge and right on the leading edge to almost none at all. Cutting a hole in the wing is like putting a notch in your spar.
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Jose,
first off, I ike the concept,,
I am not Dennis, but,,, if I may
In this case the ribs spacing can be looked at another way as well, at some point you need to paint that surface, the less ribs, the less support and the more difficult it will be to effectively sand the surface and paint it. ribs only weigh a small amount, so I wouldnt go less,
again, I love the concept,, I am curious as to what kind of weight you end up with...
are you planning on a joiner stub spar or something to stabilize the center joint of the wing?
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Dennis,Tim And Mark,thanks for the input.
Tim my question as to openings in the wing skin. I wanted to know if you could lighten near the tips and last rib bay. what do you think?
Mark on this wing the number of ribs, are what is used with the Hunt Lost Foam wing. I had the ribs laser cut By Walter Umland. Mark do you think i need more or less ribs. What about half ribs in the leading edge were the gaps are wide. The intent in assembly is to join wings as you would a foam wing with glass cloth at center reinforcement.
The weight of the Kevlar wing panel is 6.5oz the intent is to use the surface as shown as the base coat(clear balsa finish) with trim colors added
the SV-22 wing attached weighs 4.2oz per panel. if you are inclined to do a balsa finish (Kenny Stevens SV-22 clear balsa with trim colores) with trim colors then the wing is extremely light.
One additional weight saving is to convert panel to open bay this reduces the wing panels SV-22 to 3.5oz each. In the solid surface panel, 70% of the finish is already applied.
Mark take a look at photo #2 showing half ribs
Dennis,Tim and Mark take a look at Sv22 wing with spar and half ribs. should i add the half ribs to the sparlees stress wing panels.
some photos of SV-22 wing
Note: I have used the shell wing panels to create plug in wing,solid one piece wings,take apart and open bay wings. Next up is what i call New foam.More on this later
Jose Modesto
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Additional photos
Photo #1 wing panels fitting
#2 Plug in wing SV-22
photo #3 plug in wing half
Photo#4 wing panels as open bay
Photo #5 wing panels solid and open bay
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This may be My first Stress skin,sparlees,ribless wing.
The wing panels were vacuum bagged with a 3/16" one pound foam insert.
This wing method eliminates the ribs and spars. I made 4 clam shells(glass,balsa foam,CF mat,glass) and glued the wing half's first with a 1/8" center rib. The wing half's were then glued as you would a foam wing in the foam cradles.
Motor Plat 25-14 Batt Hyperion 5s 4000,schulze,FM9
Jose Modesto
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Jose:
You certainly don't need as much strength, either torsion or bending, out near the wing tips -- so you can probably get away with opening up the skins at that point. I would make triangular holes, with web that follow the ribs, and with an inch or two of skin left unmolested along the high point of the wing (that's your "spar").
As far as ribs and sanding, no matter what you do your ribs are going to "print through" -- the question is whether (a) you'll notice them at all, (b) you'll only notice them when sanding, or (c), you'll get everything as smooth as glass in your shop, paint, pat yourself on the back, then see everything print through when the temperature changes (look at a shiny black Corvett on a sunny day to see this effect -- unless it's been entirely disassembled and reassembled by a fanatic, you'll be able to see all the factory joints, and any crash repair that's ever been done).
If the kevlar makes the balsa significantly stiffer than when it's raw, you may not need stiffening ribs. If it doesn't, then you should consider stiffening ribs in the back as well as the front. I'd just go with one rib in the back, and maybe just one in the front. Unfortunately, the best way to tell is to put things together without the ribs, then see how much of a pain it is to sand the thing.
And don't paint the plane black.
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Looking at that gorgeous wood grain, how could you stand to cover it with paint?? I love what he is doing with the molded wings. H^^
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That is a beautiful job of using Kevlar. You have to master a lot of technique to use that stuff. I found that .5-oz. carbon mat on top of the Kevlar gives a good surface finish that doesn't get lumpy with time. I presume you are putting glass on the surface for that reason. You can get 1 oz. Kevlar. It costs more (it did in 1990, anyhow) than the heavier stuff.
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Howard do you have a source for 1oz kevlar. The lightest I can get is 1.9oz/sy the .5 glass cloth on the exterior is ther to fill Kevlar cloth for better finish. I prefer carbon fiber for wing skins. The carbon fiber cloth is stiffer than Kevlar. Howard have a source for Carbon fiber cloth under 2oz/sy?
Tim thanks for the direction. Tim Bob Hunt is working on foam inserts. The foam inserts will eliminate any internal structure. Should have them by end of week. I will post these items prior to molding new wing panels.
Jose Modesto
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CST, http://www.cstsales.com/products.html , has 1-oz. Kevlar and .5-oz. carbon mat, which they call tissue. The lightest carbon cloth they have is 2 oz. I don't know of any lighter.
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OMG. Howard, I love your small print. It is so much of what life is. Thank you for sharing. I love your sense of humor. Remember, this is not your money
Jose' - you are doing beautiful work. Looking forward to flying with you this year in the Northeast
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Howard really is the man. We sure missed him when he left the area. H^^
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Jose'
Just don't hit the kevlar with sandpaper! It will turn into a carpet texture and will never lay down. The glass overlay is what you want to finish and sand to. Don't ask me how I know. My guess is you already do and that is why you used it. Watching your posts I'm pretty sure you are familiar with the stuff. But I just didn't want to see you run into that if you don't.
bob branch
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Kind of an echo of Bob's question: Do you need Kevlar at all?
Here's why I ask. Historically it has been shown that balsa covered stress skinned wings (foam or ribbed) are plenty strong, even with a lite silkspan/dope finish on one side. Mark's comments about rib sag when sanding/finishing apply too yet plain balsa works prety well.
However I gotta believe that 1/2oz glass over balsa is far stronger than tissue/dope and you come out of the mold with a paintable finish - no heavy sanding or fillercoat required. Kevlar certainly adds toughness but is it overkill?
Do you also glass the inside surface? I'd think that by doing so your wing skins would be even stiffer and require very little additional support.
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Bob and Dennis.the Kevlar does not sand well.yes it turns into a carpet. Don't ask how I know. Bob the idea was to substitute the 2.4carbon cloth with the 1.9oz Kevlar for an overall weight savings. Also the Kevlar has a closer weave for a better finish. The wing panel starts with the .5 Glass cloth for that perfect finish.
Dennis the last plane shown does not use any carbon fiber cloth or Kevlar. The composite matrix from exterior to interior consist of .5glass cloth ,1/16" balsa, 3/16 foam insert,.2 carbon fiber mat. (NOTE:.5 glass cloth can be subsituted for .2 cfm)The model spans 66" and weighs 72oz.
The use of exotic materials are not needed.
My experiments with stress skin surfaces is that the foam insert allows for the removal of all internal structure.
Jose modesto
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Kind of an echo ... Do you also glass the inside surface? I'd think that by doing so your wing skins would be even stiffer and require very little additional support.
I was gonna ask that question!
Dean P.
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As a point of reference, last year (?) in Flying models, Dee Rice wrote an article about his lost foam wing system, solid foam core,, CF tow rib placements, and glass epoxy exterior. then the foam is cut out from inside the wing leaving virtually no foam excepting the narrow strip for a spar. very interesting idea,,, no molds,,
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Hi Gang,
I just love this stuff!
Bravo and a half.
Just about 10 years ago, the first generation composite wing Pattern ships built in almost exactly this way started to fail at an alarming rate.
Here is the failure mode: the leading edge glue joint would crack, then the wing skins would meet in the middle and fold.
One very good flyer actually heard the leading edge joint crack and he gingerly landed with a floppy wing panel.
The solution(s)?
1) a spar of almost meaningless spanwise strength, just to keep the top and bottom skins apart. Foam and vertical grained balsa have both been used effectively.
2) re-enforcement of the LE joint. How about a 1-piece taco-shell sheet?
3) Ribs ... the F3A guys were trying to eliminate too many stinkin' ribs.
Regards,
Dean P.
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How does one mold such skins > do you cut a pair of wood blocks and need a press ?