Leading Edge Sheeting

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

mstull

Well-Known Member
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
One thing I've always struggled with in ultralight design is leading edge sheeting. The more you use, the less the fabric dips between the ribs. But the sheeting adds precious weight. Then there's the question of what to make the sheeting out of: thin aircraft plywood; aluminum; carbon; or fiberglass? Then there's how to keep the aft edge of it from creating a corner... especially where the fabric dips between the ribs.

I've tried various solutions for this problem, and usually ended up with a reasonable result. But I've always felt it was a compromise. More sheeting and/or more ribs makes a better airfoil, but add weight. More ribs can usually allow thinner sheeting. But the weight you save with the thinner sheeting is lost with the added ribs. Then there's false (partial) ribs, if your structure provides a way to support them. I use heat shrunk Dacron fabric (Stits) that tries real hard to dip between ribs.

I'm posting this to hopefully get some new ideas. I have seen a few designs that don't use any sheeting at all. They usually use more ribs and/or false ribs, and sometimes carry those ribs over the leading edge tubing. But I don't like the way it looks, with the fabric dipping tremendously between ribs. I'm sure it's as reasonable a compromise as other ideas though. And I've looked at all the other common solutions/compromises I could find.

One solution would be if you could make the fabric have much more tension span wise than it does chord wise. That would keep it from dipping between ribs. I've thought about trying to accomplish that by how I glue the fabric on the frame. But it just doesn't seem to work out in the gluing process.

Another idea would be to use a special fabric that shrinks a whole lot more one way than the other. The warp could be made of ordinary Dacron, and the fill could be made of a different kind of thread. I don't know anything about fabric weaving. But it would seem that it should be possible. I don't know how to find companies that weave aircraft grade fabrics... and whether they'd be willing to make a small batch at a reasonable price. And I don't know what kind of thread to ask for in the fill, other than maybe cotton. A small batch to a fabric weaver might be hundreds of yards.

Is it possible to weave the fabric with the fill not pulled tight, and still end up with a reasonably smooth fabric? That way it could use the same Dacron for the warp and fill. And when you shrink it, it (hopefully) wouldn't pull as tight in the warp direction. Again, I would need a fabric weaver to make a special batch. And it would be totally experimental about how lose the fill would need to be, in terms that a fabric weaver uses.

What about temporarily laying a 2 by 4 (or something) span wise on the frame, so when you glue the fabric on the perimeter (over the 2 by 4), there'll be a certain amount of slack when you remove the 2 by 4. But how do you deal with that slack when you glue the fabric at the tip and root ribs? It would seem that you'd have to glue it with tiny pleats every inch or something. And that might be impossible to do evenly, if at all.

What if you heat shrunk the fabric, just along the tip and root glue lines, before gluing the tips and roots, to avoid this problem, still using the 2 by 4 idea? You could glue the leading and trailing edges. Then remove the 2 by 4. Then heat shrink the fabric only where the tip and root glue lines will be. And then glue those.

Removing the 2 by 4 without snagging the fabric could be a problem. So It would be easier to use short segments, or something other than wood. A better material might be those long, brightly colored, tubular shaped, foam floats used as swimming toys. They're several feet long by a few inches in diameter. I think this idea might be worth a try. But it might take some experimenting to find the right diameter toy so you end up with just the right tension but no slack after final shrinking.

Do you creative people have any ideas? If we can find a reasonable solution, we can eliminate the leading edge sheeting in a way that is both aerodynamically and weight efficient.
 

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
12,993
Location
Port Townsend WA
Mark,
I have been thinking about this also.
I had the idea of buying special fabric as you mentioned. Another thought was to mask the fabric so only the spanwise threads get heat shrunk with a heat gun. The mask could be a small sheet with multiple horizontal slots that is held in front of the heat gun. The chordwise thread would shrink some too but that is desired anyway.

As for leading edge sheeting, I need ideas about that also. My plan is to build the D-tube with geodetic strips. I could cover direct on the strips, or better, apply some very light sheet. The sheet would need to span 6" across the strips. I have some .006 aluminum I might try.
The human power aircraft used thin foam, I think. However, it must be something that does not warp over time.
I need some more ideas on light sheet also.
BB
 

Midniteoyl

Well-Known Member
Joined
Sep 3, 2003
Messages
2,406
Location
Indiana
Not having ever done a wing in fabric, I'm wondering if you couldnt glue the tip first then using a 2x4, or similar, with the fabric stapled to it, 'roll' the 2x4 from the top to the bottom of the root, thus pulling the fabric tight. Tighter than by hand. This would give you a spanwise pretention of the fabric and when heat shrunk, should pull tighter in the span than cord.

They never do it anymore, but this is the way screening a door was done so the screen didnt sag.
 

mstull

Well-Known Member
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
BB,

I doubt your slotted mask would really shrink one way more than the other. And I doubt you could control the temperature accurately at all with a heat gun. The book says to use a calibrated iron only... no heat guns. And from my experience, the book is right. The temperature to get a good shrink without weakening the fabric is quite critical.

I have had the best luck with carbon sheeting. I started by carving and sanding a male, balsa mold 3' long, the shape of the area to be sheeted. Then I laid up fiberglass on that to make a female mold. Then I laid up the 3' long carbon sheeting segments in that. I'll attach a photo of how that comes out.

I tried thin (1 mm I think) plywood on my biplane. But some of it warped before I could get it covered. If I use plywood again, I'll be sure and seal it on both sides before attaching it to the frame. But sealing adds weight. And thin sheeting like that needs to be reinforced with some span wise fiberglass beams to keep the fabric from pulling its rear edge down between ribs, unless the ribs are spaced extremely closely. To make those beams, I glued a little triangle of balsa on the inside of the sheeting between each rib, and laid up fiberglass tape over it. Here's a photo of that. The beam is near the bottom of the photo. It works great.

Aluminum sheeting tends to need the same reinforcement beam, unless you can attach it strongly enough at the tip and root to take the tremendous tension that's trying to make it dip between ribs, and make it out of one continuous piece of aluminum... unless you space the ribs extremely closely. It wouldn't be hard to make reinforcement beams between the ribs out of some 1/16" or thinner aluminum angle. But again that would add weight.

Oyl,

I've thought about pre-tensioning the fabric one way as you suggest, but I doubt it would do much if anything. When you shrink the fabric, it tends to pull to a certain tension. If it's already near or at that tension in one direction, it won't make it any/much tighter that way. So it will still tend to come out about the same tension both ways.

But if you start with some slack one way, it will only shrink to a certain percentage of its unshrunk size. Once you find out what that percentage is, it should be possible to get the right amount of slack so it shrinks up with little to no tension one way. Even so, it might be very tricky to provide just the right slack so it ends up smooth, but with no wrinkles of slack. It could take a fair bit of experimenting, wasting expensive fabric.

Keep brainstorming.
 

Attachments

mstull

Well-Known Member
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
One problem with only having fabric tension one way... in a high G maneuver, the fabric will try to lift up off the ribs extremely hard, tearing at the rib stitching. With normally shrunk fabric, much of that lifting force goes cord wise to the leading and trailing edges. It might be necessary to use many more rib stitches or PK screws, which would add a lot of labor... and the ribs would have to be strong enough to take that load.
 

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
12,993
Location
Port Townsend WA
Fiberglass sheet warps over time. The bottom aileron skins on my Grob are noticeably distorted where only single sheet is employed. Sandwich is used to prevent this on the top skin but sandwich is very heavy.
Is thin carbon better than fiberglass for warping?
I assumed it was the epoxy that shrinks and causes the warping.

Airbus uses a laminate of aluminum foil and fiberglass. They call it GLARE.

Perforated aluminum is another idea.

BB
 
Joined
Jan 25, 2003
Messages
430
Location
Shirley airport MA
I do exactly what you described for the very same reason but I don’t stick anything under the fabric, I just lay it on and then create a measured slack. On the minimaxes I built this is a problem because the sunken fabric contacts the spars running a ¼ inch below the rib line in very ugly way.
I start gluing the fabric around the perimeter and just the corners first. Glue down the end rib leading edge and the trailing edge with the fabric in between having about 1.5 inch extra slack follow that with gluing the rest of the leading and trailing edge with maintaining the same slack then shrink the fabric just between at the end ribs to remove that slack and glue it down , once its glued all around I shrink it just enough to smooth out all the wrinkles and then glue the ribs .
This does work very well to lessen the fabric scalloping between the ribs as the fabric gets a lot more tight in the span wise direction the codr vise.
George
 

mstull

Well-Known Member
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
BB,

I use multiple plies of carbon... three or more. One or two plies of layup have almost no stiffness. And it's important that the layup be stabilized by post (heat) curing. So it should always try to spring back to its original shape as long as you keep it from getting too hot. Like it says in Burt's book, paint your plane with white or light colors so it doesn't get too hot in the sun.

A dual wall (with core) layup comes out extremely stiff, and can be heavy, as you say. A trouble with dual wall leading edge sheeting is its thickness. The ribs would need to be notched so there's not a step at the aft edge of the sheeting. I consider this a problem, even with thin sheeting. Often I'll add rib caps to the ribs after the leading edge sheeting is added, and make the rib caps butt up against the sheeting so they come out the same hight with no step.

George,

I'm glad to hear that allowing some slack can help.
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,091
Location
Orange County, California
Mark -

Well, you were looking for off-the-wall thinking, so here goes...

The wing is a total-sum game in terms of weight, so I think you should look beyond just the leading-edge skin and ribs. IIRC, your carbon wing was cantilever, yes? But your newest (lexan sheeted) wing is wire-braced.

If you're comfortable with wire-bracing, there's a great opportunity to pull weight out of the two spar configurations you've used and apply it to the wing leading edge skins/ribs. How about going to more of an I-beam spar, wire-braced? The spar would only need to be stiff enough to take the loads between the wire-bracing points, and would be very light. Your wire bracing takes care of torsion loads, carried with some stronger ribs to the trailing edge. The weight loss over either a full-cantilever tube spar or a ladder frame with smaller tubes should be enough to use full sheeting from the leading edge back to the spar, perhaps in either plywood or carbon, supported by light ribs?

The point being that you should be able to pull enough weight out of other areas of the structure to allow more sheeting on the leading edge without going to something exotic like biaxial fabric.

Some short false ribs (perhaps 0.1c?) aft of the spar could keep the fabric from sagging too much, and not add much weight.
 
Last edited:

mstull

Well-Known Member
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
Thanks Topaz,

Yes, that's what I'm looking for... creative thinking. In answer to your questions... that wing, with the single carbon spar, used a single flying wire for positive G loads, but was cantilevered for negative loads. So I'd call it semi-cantilevered. Yes, I'm comfortable with wire braced wings. I was planning on that for my next wings... king post and all.

As you know, my U/Ls usually come out well below the weight limit. So I'm not hard against a weight problem. I was just hoping for some fresh ideas for leading edge sheeting alternatives.

Here's a brain storm... sort of a compromise between normal sheeting and no sheeting: Instead of using a solid material (like plywood, composite, or aluminum), what if you glue a 1' wide strip of heavier weight Dacron fabric between the root to tip, where the sheeting would normally go, and shrink it tight. Then just cover everything with ordinary light fabric, and shrink it like normal. The strip of heavier fabric would help keep the light fabric from dipping between the ribs. This idea would be very easy and inexpensive, and might be a reasonable compromise. A non-stretchable fabric might even work better for this "sheeting"... for example, a strip of bare fiberglass cloth (that's not filled with epoxy).
 

Midniteoyl

Well-Known Member
Joined
Sep 3, 2003
Messages
2,406
Location
Indiana
Here's a brain storm... sort of a compromise between normal sheeting and no sheeting: Instead of using a solid material (like plywood, composite, or aluminum), what if you glue a 1' wide strip of heavier weight Dacron fabric between the root to tip, where the sheeting would normally go, and shrink it tight. Then just cover everything with ordinary light fabric, and shrink it like normal. The strip of heavier fabric would help keep the light fabric from dipping between the ribs.
Was my next idea.. :) Or, what about a reinforcement stitching spanwise? Fold the fabric back on itself by say 1/2 inch and double stitch together. You could do this 3 or 4 times from LE sheeting to TE.....
 

Dana

Super Moderator
Staff member
Joined
Apr 3, 2007
Messages
9,136
Location
CT, USA
Another possibility is to go ahead and make the carbon leading edge heavier, replace the tube spar with a lighter shear web, and run the carbon leading edge all the way back to it, so the D-tube carries the loads. It may or may not come out heavier, but the leading edge fabric support becomes a non issue.

-Dana

If at first you don't succeed, skydiving is not for you.
 

Jeremy

Well-Known Member
Joined
Feb 23, 2003
Messages
75
Location
Salisbury, England
I experimented with 1/16" thick balsa sheet, covered with thin glass/epoxy. It makes a good leading edge, as the grain of the balsa adds a fair bit to the stiffness between the ribs.

I used the 4" wide, 4' long balsa sheets from my local model shop, that I joined together into useful widths using cyanoacrylate "zap" glue. I then glassed and lightly vac bagged on one side only, while the sheets were still on the bench.

When cured, I found that this sheet was very stiff along the grain, but incredibly flexible across the grain. It seemed to take incredibly tight curves if the glass cloth side was facing outwards, with very little force needed to get it to follow a typical LE profile.

Once the sheet is bonded to the ribs, a thin layer of glass/epoxy on the inside surface turns the LE into a very stiff structure.

Using a lighter cloth, maybe Kevlar or even perhaps carbon, might give an even better result. I used 160g/m² cloth, the very thin surfacing stuff, in a tight plain weave.

Jeremy
 

mstull

Well-Known Member
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
Thanks Dana,

I tried the carbon D section on my first U/L wing. It worked great, came out very light, and resolved the sheeting issue, like you say. But it was way too much work. Unlike the carbon leading edge in the photo above, that D section needs to be laid up in a single piece. So it takes a mold as long as each wing. It took me almost a year to make that wing. I'm sure with my present experience, I could find a couple short cuts to make it much faster. But it's still a relatively huge amount of work.

Jeremy,

I've tried single side composite over balsa. It does flex like you say, but without composite on both sides, the balsa gradually sags under a steady load, like fabric tension. A similar idea would be to bend the balsa into the proper shape, and hold it that way while you cover both sides with composite. That could be done in a mold and vacuum bag, or other ways. It only takes a very thin layer of composite, like one ply of 2 or 3 oz glass, and a pretty thin balsa core. And the composite effectively seals the wood in the process.

I'm leaning towards trying my last idea, of using heavier fabric about a foot wide, instead of solid sheeting. I have a roll of unidirectional glass that's just the right width. But it is a bit heavy. The tip and root of the wing frame will have to be structurally reinforced to take the load. But I was planning for that anyway. The only other trick will be to attach the glass fabric so it comes out even. But that's mostly an issue of careful handling.

Using heavy Dacron would be simpler. You wouldn't have to worry about handling at all, since it would be heat shrunk tight. I'm just not sure how much the light fabric on top will pull it down between ribs. I bet it would stay up pretty well if you went all the way up to the "heavy" 3.4 oz or 3.7 oz cloth. Using Dacron, you'd have to fill that fabric with sealer (Poly Brush), so it wouldn't dry out the light fabric on top in the finishing process. That would add some weight, but still be much lighter than solid sheeting. It would be super quick and easy... and downright failure proof.

But keep brainstorming. We might think of something even better.
 

mstull

Well-Known Member
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
Topaz,

I think the choice between using a relatively unstretchable material like fiberglass, and heavy Dacron, might be worthy of your analysis. When I put it like that, it's obvious that fiberglass will stretch under enough load. If a totally unstretchable material was glued to the root and tip, with no slack at all, and a load was applied between ribs, the pulling (tensile) force trying to break the "sheeting" material, and tear it free of the tip and root, would be infinite. That force would be much less if it was allowed a little slack to dip slightly between ribs, or if it could stretch enough to provide that slight dipping.

So questions loom: Would unidirectional fiberglass cloth stretch enough to lower the forces down to something reasonable? Obviously, the wing structure that supports the tip and root can't be infinitely strong... or even strong enough to withstand 2,000#. It would weigh too much. I'd estimate maybe 1,000# of tip and root strength (in that front foot of the upper surface) might be reasonably possible for a true U/L. It wouldn't make sense to have to add more weight in the structure than ordinary sheeting would add.

Obviously, the number of fiberglass fibers in the unidirectional fiberglass would have a great effect on how much it would stretch under a given load. With half as many fibers, each fiber would take twice the load, stretching it about twice as much. And the more it stretches, the less the load, because it would dip more between the ribs. So going with a unidirectional fabric that has half as many fibers might result in 1/4 the load on the structure. This would apply similarly to heavy Dacron fabric. The lighter the fabric, the more it would be stretched, and the less strength the tip and root structure would need to have.

Even properly supported at the tip and root, that very large force would tend to flex the front spar. And in a normally (strut or wire) supported wing, that flexing force on the spar would add to the normal flight load that's trying to bend the spar the same way. In high positive G loadings, a lot of things would come into play, effecting the load on that front spar. The light fabric would be aerodynamically lifted up, so it's not pulling down on the "sheeting" material, for example. So the bending force on that spar might not be much greater than with ordinary sheeting at high G loads.

All in all, I wonder how much my fabric sheeting idea could be expected to help? Would its aerodynamic benefit, of keeping the light fabric from dipping between ribs, be countered too adversely by the added structural weight to deal with the forces it would create? It would seem that using a heavy Dacron fabric would be much less likely to overload the structure. But how much dipping would that prevent? I realize that without structural values and dimensions, you're not going to be able to come up with a numerical answer. This is one of those times that you just picture the forces and structures, and try to get a feeling for what might work best... a good exercise for a creative mind.

Gluing the light fabric with less tension cord wise, like George suggests, would make a huge difference in how much force the light fabric would be pulling down on the fabric "sheeting". So a combination of the two ideas might be most ideal.

My own feeling would be to use a heavy Dacron fabric for the leading edge "sheeting", just to be certain not to overload the structure... and glue the light fabric with less tension cord wise to further reduce the forces and dipping. But overall, it would still be a good and reasonable alternative to ordinary sheeting. What do you think?

Here's a photo of the carbon D section spar on my first wing.
 

Attachments

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,091
Location
Orange County, California
Mark -

Well, I'll give you my thoughts, certainly - if you've been following my "Wing Structural Concept" thread, you'll know that serious structural analysis is beyond me quite yet, although I'm getting a grasp of the fundamental concepts and getting ready to work it through on my own design.

That said, and given that you're looking for 'out of the box' thinking, I guess I'd go back to first principles here, and ask what precisely is the goal you're trying to achieve? It seems like, ultimately, you're looking to get a better airfoil shape (or at least one that's more true to the airfoil you've chosen), but without an undue weight penalty. You've also mentioned that your airplanes are consistently well under the Part 103 limit, so I'm wondering where the weight problem comes in for this question? A general desire to control weight to maintain performance? Is the goal to generally improve some aspect of performance through the improvement in airfoil shape? You also mention that a full D-cell might take too long to build, so it looks like you've got some conflicting requirements.

Please forgive the "big dumb questions," but I generally find I get the best results when I have a clear and unequivocal understanding of the basic question and goals, rather than focusing upon a particular aspect of design from the beginning. All of your ideas with various fabrics seem plausible to me, but without knowing exactly what goal they're trying to satisfy, I don't have any way to choose one over another.

If the goal really is a better airfoil shape with minimum weight penalty, I've got to go with Dana and suggest a full D-cell structure, then focus upon methods to make that buildable in a reasonable amount of time. Perhaps there's a way to build that structure without having to invest in full-molded construction? Off the top of my head, I could see hot-wiring a foam core per the Rutan method, laying up peel-ply at each point there would be a rib, then laying up carbon fiber over it, hand-layup. The last layer to go on the ply is a thin mylar sheet, then vacuum bag the whole thing. After cure, you could chew out the foam with any variety of tools, rip out the peel-ply, peel off the mylar outer skin, and have a thin carbon skin with a glass-smooth outer surface, just as if it were molded. A series of foam ribs (hotwired as a stack) provide support for the D-cell, and the trailing edge of the D-cell skin bonds to the top and bottom of an I-beam spar - possibly a wood shear web with pultruded carbon rod caps.

It just occurred to me that you're covering the whole thing with fabric - the mylar and vacuum-bagging become redundant if you cover the D-cell with fabric, saving you that much more work. The outer surface of the carbon can likely remain in the hand-layup state when you bond the fabric to it. The fabric will 'smooth out' any small surface roughness in the carbon fiber layup.

If you're simply looking to prevent fabric 'sag' with a given wing structure, no matter what the ultimate goal of the whole thing, layering up the fabric as you say seems logical. I kind of like the method Strojnik advocates for this - build your leading edge in segments from fiberglass, then remove it before it's absolutely cured to apply to the leading edge. Your fabric goes over that. As you say, it'll still sag a little bit, but the only way to get NO sag is to provide either a sandwich structure or full-foam support under the entire area, something that you think incurs too much weight penalty for your purpose.

I don't think using glass without the epoxy is going to prove workable. The stuff is just too stretchy and floppy - I've got a roll of BID in my garage that I'm using to start getting some experience with composites, and the stuff is practically alive. You can stretch it so much in any off-axis direction when there's no epoxy involved that I think it'll be TOO isotropic for your purposes: no strength at all even slightly off-axis. If the wing twists at all, you're likely to get wrinkles as the fiberglass support of the outer skin breaks down from off-axis loads. Even if you have UNI in a strip aft of your spar, you'll have to bond it to the outer fabric skin somehow. I can't see how you could reasonably 'pretension' fiberglass and then bond Stits or other fabric to it.

I hope some of this brainstorming is helpful!
 
Last edited:

Dana

Super Moderator
Staff member
Joined
Apr 3, 2007
Messages
9,136
Location
CT, USA
A couple of random thoughts:

First, I'm not clear why you're so concerned about the fabric dipping between the ribs in the first place? Certainly you lose the optimum airfoil, and on a sleek sailplane, yess, but at ultralight speeds, and given all the other inefficiencies (wire bracing, open cockpit, etc.) is it that big a deal?

You could add false ribs to support the fabric between the main ribs. In carbon, these could be extremely light.

With or without false ribs, you could string some thin carbon filament tape (perhaps 1/4" wide or perhpas even less) spanwise, to better support the fabric. The tapes might be close together near the leading edge, with the intervals increasing as you go farther aft.

This one probably won't help at all, but I'll throw it in for what it's worth: Some years ago on an R/C model I had very good results with a light balsa structure covered with fiberglass. I attached the fiberglass to the balsa not with resin, but with a water based automotive primer... using toilet paper to blot up all the excess primer while pressing the glass down. The result was a very light, very rigid structure (but alas, not strong enough to withstand the crash that resulted from a nasty tipstalling tendency of the design).

-Dana

End rush hour traffic now! Legalize vehicular weaponry!
 

mstull

Well-Known Member
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
Topaz,

I don't mind the questions. My goal is not necessarily to achieve the perfect, true airfoil, although I care more about aerodynamics than many U/L designers/compromisers... with their tubular trailing edges, for example. And I'm not necessarily trying to save every possible pound of weight, although I do believe you can gain more performance with lighter weight, than with any other modification.

I don't expect this new fabric sheeting idea to actually be better than ordinary sheeting. But I feel frustrated when I spend lots of time, weight, and/or money making other types of sheeting (or any other part) that is merely a lesser of evils.

Yes, build time makes a big difference to me. I'd rather a wing be quick to build and cover at just 96% aerodynamically perfect, than one that takes a year to build at 100%. The difference in actual flying performance isn't worth the extra effort. The drag from the open cockpit and other exposed parts is so huge, that making the wing or other part perfect is a waste of time, weight, and/or expense... like Dana says.

U/Ls need an awful lot of wing area to pass the stall speed limit, yet need to be super light to pass the weight limit. So we compromise most everything else.

I have been frustrated with the compromises involved with leading edge sheeting. And it intrigues me that some U/L designers go to the extent of not having any at all, figuring it's not worth the time, weight, or expense, and just let the fabric dip deeply between ribs.

How much more lift does a perfectly smooth airfoil make on an U/L, compared to one that has some aerodynamic compromise? From my experience, the answer is: enough to notice, but probably not enough to be worth spending large amounts of time, weight, or expense to accomplish. So why waste the time and money making perfect sheeting?

An aerodynamically well made wing does have a noticeably better stall speed. But the formulas in AC103-7 don't give us any credit for that. So again, why waste the time and money making perfect sheeting?

So I'm looking for a reasonable compromise. I don't mind spending a little extra time, weight, or expense to get a nice sheeting. The carbon ones I make are a good example of the perfect side. They're expensive, a bit heavy, and take a fair bit of time. I would make those again.

The plywood ones on my biplane are okay too, but nothing to write home about. They can be made fairly perfect with reasonable time, weight, and expense. I would make those again.

The carbon D section is way too much work. Now that I've started making wings that use an aluminum "ladder" frame, I doubt I'll go back to using much carbon. Carbon is so expensive now. And aluminum is so quick and easy, I'm already spoiled.

So what is the ideal sheeting for an U/L? The fact that different solutions are used by different designers/compromisers, proves that we don't have a good answer yet. I'd like to experiment with a new idea if it has a reasonable chance of success. If heavy Dacron fabric is significantly better than nothing, it can't be beat for quick, easy, light, and inexpensive. It's weight penalty might come from having to make the structure stronger, particularly at the tip and root. That's what I'm asking for your impression about.

Dana,

I too have experimented with making composite on R/C planes with a matrix (other than resin) that cures with evaporation. I tried Elmer's glue, and got results that were good enough that I kept making parts that way.

Compared with epoxy, you end up with a very dry layup, which saves a lot of weight. But the matrix didn't really stick to the glass fibers... At best it encapsulated it. I've always felt that if someone could make a synthetic cellulose fiber, there would be huge potential for making light weight composites with an evaporative cured matrix.
 

pwood66889

Well-Known Member
Joined
Feb 10, 2007
Messages
1,576
Location
Sopchoppy, Florida, USA
One leading edge treatement I saw, Mike, was foam from the front tube back 6 or 8 inches. The rigid foam was an inch thick (if I recall correctly) and had Very Light spackle applied to the gaps where the ribs were. The ribs were 1/2 x .035 aluminum tube. I believe it was varnished.
I'll try to get up some drawings, etc., iffn yer interested.
Percy in NM, USA
 

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
12,993
Location
Port Townsend WA
Maybe the dacron fabric could be laid smooth and cured with resin in the area of the D-tube. Then the dacron fabric "skin" is wrapped around the leading edge and glued. Since the fabric is stiff, no shrinking would be done to the D-cell area.
Aft of the main spar the bare fabric would be shrunk in the usual way. The fabric would need support along the main spar.
This would not last for long as some buckles would appear over time. But it might be better than nothing.
BB
 
2
Top