Looking for info on controlling airfoil shape in moldless composite wing

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enderw88

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I have looked through all of my references (Strojnik, Rutan, Arnold, Smith, et. al.) and cannot find indication of how to make really accurate wings with moldless construction. Strojnik talks about hot wire cutting foam then laying up skins that are prepared on a piece of plexi (great surface finish). Rutan talks about cutting the foam and glassing over it, then sanding it for surface quality. Arnold never really mentions it. If you assume the foam core is cut correctly (even perfectly) then even a really good laminate layup is going to have waviness and surface irregularity. What is the best technique to shape and verify the airfoil is accurate to +/- 0.010" (0.25mm) and the appropriate twist has been built in? Another way to ask is, can you build a moldless wing with the same quality as a molded sailplane wing? Just getting good surface finish isn't enough it needs to be accurate. I run into real problems trying to think through setting up some external reference system to control the dimensions. I am probably overthinking it.
 

TFF

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One step and done, no. You would first need CNC cut cores. That will get a good base. The layups are tough. Unless you go end to end with the same number of plies, you will induce lumps. A small bit of luck on laying down the layups too. In the end you will still have a fill session or five. You will need accurate multi station rig boards to check; how many stations is up to you. How complicated the wing is makes a difference. If you spend more time sanding than making a mold, time management was not good.
I wonder if anyone has made a one time mold out of hot wire shucks?
 

kent Ashton

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The great sailplane pilot Richard H. Johnson measured wing waviness using a small trolley about 2" long with a dial indicator in the center. I could not find a picture but I have an article in my files. He would slide the trolley along the chord line and plot the dial reading. The plot would be a curve but waviness of the curve-plot would show waviness in the wing which can be filled and sanded. Search "RICHARD H. JOHNSON" WAVINESS and it will bring up some sailplane tests and waviness curves. One example here https://pdfs.semanticscholar.org/4bb1/f5d6169d5270c2944f0e2ed64193af4e5267.pdf

A moldless (Rutan style) wing can be sanded to a very smooth condition. I use a stiff aluminum bar and mark it up with a wide Sharpie. Rub it along the wing perpendicular to the chord and it will leave smudges on any high spots which can then be lightly sanded. I have never used Johnson's trolley device but this is about what the sailplane pilots do. The Mosquito in the PDF above was finished to a waviness of .001" peak to trough!

These days I imagine digital dial guage readings could be recorded on a computer
 

Royal

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Way overkill guys. I just tried finding a good video of how classic car guys refinish cars to a mirror. Couldn't find a good one. So I'll make one.
 

Norman

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Way overkill guys. I just tried finding a good video of how classic car guys refinish cars to a mirror. Couldn't find a good one. So I'll make one.
Checking with a dial indicator after block sanding is not overkill. It depends on the Reynolds number (size and speed). If your wing operates at Re<2,000,000 then just feeling for waves and troughs with your hand may be OK but the wave tolerance gets pretty tight as Re increases and the human hand will not be good enough somewhere between 2 and 5 million. There have been several threads on HBA about how to finish a wing for laminar flow. The site search engine can find them.
 

Royal

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Checking with a dial indicator after block sanding is not overkill. It depends on the Reynolds number (size and speed). If your wing operates at Re<2,000,000 then just feeling for waves and troughs with your hand may be OK but the wave tolerance gets pretty tight as Re increases and the human hand will not be good enough somewhere between 2 and 5 million. There have been several threads on HBA about how to finish a wing for laminar flow. The site search engine can find them.
Yeah that's why I use aluminum straight edges and yard sticks with guide coat. You dont need a dial indicator to achieve a perfect airfoil. Just need the template of the airfoil to make sure you are headed in the right direction.
 

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Jay Kempf

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Way overkill guys. I just tried finding a good video of how classic car guys refinish cars to a mirror. Couldn't find a good one. So I'll make one.
Polishing is part of it. Getting it to the right geometry is another. It ain't overkill if you are going for laminar flow on a large portion of the upper surface.
 

Royal

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No. Polishing isn't what I'm referring to. I'm talking about straight. Light doesn't not bend or anything. Ill just make a video.
 

Royal

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I also think for a wing there are easier ways to get the perfect airfoil without filler. Just build the mold first like an inside out wing with ribs. Get a laser cut profile of the airfoil and make sure you have it perfect. Use Coroplast or a huge sheet of lexan or plexiglass that has a shiny surface for the mold surface. Wax it but it most likey wont need to be waxed. Obviously you cant make the leading edge. Make sure you go past the main spar and rear spar. This will make one side of the skin. Use the correct fiberglass and resin but use the infusion method. It will be stronger, lighter and straighter than any other foam wing. Wet layups are kinda dated and it cost more in materials and time. Don't forget to allow for the thickness of the plexiglass. You can build the mold out of pink foam and mdf board. You can also buy aluminum extruision and bond that on the back side of the mold so its perfectly straight at the spar. I'd do three of those. front, middle and rear.
Best part about this is once you made one side you have all the materials to make the other side.
 

mcrae0104

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Way overkill guys.
It really depends what your goal is. My bet is that most KR-2 (or similar) guys don't bother, but the OP asked how to match the quality of a molded sailplane wing when making a moldless wing. The wave gauge is the way to go for this. Watch Why it Goes So Fast if you want to see this in action (skip to 32:00 for discussion of lamiar flow and to see the wave gauge). This is way beyond the capabilities of a straightedge and backlight.

Your suggestion on making molds is fine, but that's not what the OP asked about. We do have other threads on that topic.
 

Royal

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It really depends what your goal is. My bet is that most KR-2 (or similar) guys don't bother, but the OP asked how to match the quality of a molded sailplane wing when making a moldless wing. The wave gauge is the way to go for this. Watch Why it Goes So Fast if you want to see this in action (skip to 32:00 for discussion of lamiar flow and to see the wave gauge). This is way beyond the capabilities of a straightedge and backlight.

Your suggestion on making molds is fine, but that's not what the OP asked about. We do have other threads on that topic.
That video should be a requirement to be a member of this forum. I have been looking at the fastest planes with little hp and checking out their features. lots of things in the video confirmed my ideas and explained things in so much detail that I didn't understand. thank you
 

Norman

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the OP asked how to match the quality of a molded sailplane wing when making a moldless wing. The wave gauge is the way to go for this. Watch Why it Goes So Fast if you want to see this in action (skip to 32:00 for discussion of lamiar flow and to see the wave gauge). This is way beyond the capabilities of a straightedge and backlight.
Actually that video proves the point that you can get within medium Re laminar tolerance with good block sanding technique. The wave gauge was brought out after the plane was finished not during construction.
 

Royal

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Actually that video proves the point that you can get within medium Re laminar tolerance with good block sanding technique. The wave gauge was brought out after the plane was finished not during construction.
I agree.
 

enderw88

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The great sailplane pilot Richard H. Johnson measured wing waviness using a small trolley about 2" long with a dial indicator in the center. I could not find a picture but I have an article in my files. He would slide the trolley along the chord line and plot the dial reading. The plot would be a curve but waviness of the curve-plot would show waviness in the wing which can be filled and sanded. Search "RICHARD H. JOHNSON" WAVINESS and it will bring up some sailplane tests and waviness curves. One example here https://pdfs.semanticscholar.org/4bb1/f5d6169d5270c2944f0e2ed64193af4e5267.pdf

A moldless (Rutan style) wing can be sanded to a very smooth condition. I use a stiff aluminum bar and mark it up with a wide Sharpie. Rub it along the wing perpendicular to the chord and it will leave smudges on any high spots which can then be lightly sanded. I have never used Johnson's trolley device but this is about what the sailplane pilots do. The Mosquito in the PDF above was finished to a waviness of .001" peak to trough!

These days I imagine digital dial guage readings could be recorded on a computer
I have read that many times. Getting an individual airfoil profile sanded using a template isn't really hard, assuming the wing started out well. Making sure the many airfoil templates you have are correctly oriented is a much harder task if you have a specified twist and the wing is not out of a mold.

Polishing is part of it. Getting it to the right geometry is another. It ain't overkill if you are going for laminar flow on a large portion of the upper surface.
Laminar flow baby, that's what this is about.

It really depends what your goal is. My bet is that most KR-2 (or similar) guys don't bother, but the OP asked how to match the quality of a molded sailplane wing when making a moldless wing. The wave gauge is the way to go for this. Watch Why it Goes So Fast if you want to see this in action (skip to 32:00 for discussion of laminar flow and to see the wave gauge). This is way beyond the capabilities of a straightedge and backlight.

Your suggestion on making molds is fine, but that's not what the OP asked about. We do have other threads on that topic.
I haven't watched that in a while. I will dig it out (I turned my VHS copies digital some time ago but don't remember where I stashed the DVDs. The earlier comment that this may take longer than making a proper mold may be the right answer.

Actually that video proves the point that you can get within medium Re laminar tolerance with good block sanding technique. The wave gauge was brought out after the plane was finished not during construction.
Thanks to all for the insightful replies.
 

D_limiter

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3 months of sanding, he said in that video... :eek:

Edit: on just the wings
 

Norman

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3 months of sanding, he said in that video... :eek:

Edit: on just the wings
Yep, it's tedious but not hard. When I contoured a very badly wavy wing I made the mistake of working on small areas. We felt the wing and marked low spots with pencil then mixed small (Dixie cup size) batches of dry micro. We had a lot of troughs to fill and it took a long time. Should have mixed the micro in a salad bowl and slathered it on the whole wing in one pass.
 

stanislavz

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I was dreaming on same topic for a long time - but in eu, all was stop due to problem of finding proper foam.

But lesson learned - i was not able to find a way to buid solid core wings in fast, cheap and light way having proper geometry.

But - i was lucky, to find, that foil on hot wire cnc cutted foam in female mold shape make as good as possible mold with no effort.

Yes, it will need some sanding, but only for surface finish. You have proper geometry from mold. And you are limited to 2d shapes.
 

BoKu

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I have looked through all of my references (Strojnik, Rutan, Arnold, Smith, et. al.) and cannot find indication of how to make really accurate wings with moldless construction...
It's important to understand that professionals working at first-rate competition sailplane manufacturers traditionally couldn't make accurate wings either, but made great sailplanes and won lots of contests regardless. Do template checks on any LS6 or LS8 and you'll come away wondering if the left and right wings were made in the same factory. And yet these are fine sailplanes that win Nationals level comps and fly straight as arrows.

Here's the thing: Unless you are working with very aggressively laminar airfoils like the middle Wortmans or the current IF1 front runners, once you get aft of about 10% chord, absolute fidelity to contour takes a distant second priority to freedom from waviness. If your airfoil is in the ballpark and you get the waviness below 0.004" per the Johnson gauge (lower is better, of course), it'll be fine.

I've seen regionals and even nationals won with some incredibly sketchy-looking wings. Their secret was getting the waviness down to a dull roar, getting good sealing and ventilation exhaust, tuning their instruments well, and then concentrating on strategy and tactics. And I've also seen as backmarkers the guys (and, yes, it seems to be a guy thing) who put hundreds of hours into profiling to laser-cut templates. If you want to fly, fly. If you want to sand, well, via con Dios, amigo.
 
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