The sheeted wing would flex less. Stiffness is a big deal.

Dennis:

I think the real answer is that nothing is simple.  It could be that the better airfoil in a sheeted wing pays for the extra weight, it could be that a bit more or less weight makes a difference, but not as much as the normal variability in a contest, it could really be that different pilots are going to get more or less out of a really lightweight plane, etc.

I suspect that once you get back an inch or two from the leading edge the fine details of the airfoil aren't going to make a whole lot of difference, so because of this the sheeting vs. covered won't show in the flight characteristics.  Then it becomes more of an issue of ease of finishing vs. weight, and robustness vs. weight, and all those other secondary considerations that don't matter once the plane is in the air, but have a very large bearing on whether or not the plane gets into the air in the first place, and whether it makes a good impression while it's on the ground.

I'm seriously thinking of sheeting the next wing I build. I always end up sanding through silk on an open bay, then you need about 10 coats of dope to seal the silk!! plus primer and colour? With good wood selection, careful sanding and good preparation, you could probably get away with less than half of that on a sheeted wing, using non-shrinking dope. I refer you to Don Bambricks article in Flying Models on finishing his Fury, that also, had a sheeted wing.

Cheers    Neville

Bill, what was the difference in weight? If you can remember By the way, I have a copy of the Yak 9 coming!    I PM'ed you, but it obviously didn't get through again!

Cheers    Neville

Guys,
OK seems that weight is a key factor, one thought I have is if you use a buildup structure and use non-shrink paint or film then you could use 1/32" balsa covering. In the past you with shrink dopes you had to use at least 1/16" wood to avoid the peaks and valley effect between ribs. With the 1/32" sheeting you could have a smooth surface for film or use a non-shrink paint (I don't know of any non shrink paints, maybe epoxy or Rustoleum, but they are a little heavy), this would work well with electrics. Could be that in order to take full advantage of the hard sheeted wing we need to look at some different airfoil shapes other than the typical NACA 00xx with the 25% high point? Any ideas?

Best,       DennisT

It has a lot to do with finish.  
Guys who go for the 20 points and Concourse De Excellence want something solid to paint, mask, sand, and polish.

In the early 70's in Detroit we (Adamisin's, Banbrick, Gunther) built spar less fully planked wings. The planking was home-cut .080" (to leave room to sand it) and was applied using #2 HobbyPoxie spread over the entire inside surface. Faster drying (6 hour) epoxy on the mating surfaces of the leading and trailing edges. They were VERY stiff and we never had a wing failure. The lightest one ever built was an 8.5 oz wonder by Don Bambrick. The weight included the flaps, controls and lg. They usually ended up inn the 10 - 12 oz region. I'm telling you this because it only shows that if you decide to plank the wing think about the rest of the structure. Planking a wing doesn't make it heavy. The wing skins were assembled into single pieces (upper and lower) before they were applied.

Is it allowed in the rules, to sheet the wing of a Classic model that was published with an open bay silk covered wing?

Cheers 

Hi Neville,

Over here, it would probably cause problems.

Bill

Hi Neville,

Over here, it would probably cause problems.

Bill

No It isn't any problem, you can sheet the wings if you like and be totally within the rules

Randy

.......... the benefits of a true airfoil shape seem to be secondary...............

I have often wondered about this and the comparison of constant wing section provided by a sheeted wing against the hills and valleys of a purely tissue covered wing.

I highly suspect that that the modulation along the span with a soft covered wing would slow down span wise airflow, and thus reduce tip losses, and provide a more chaotic airflow that could well resist stall all the better.

The ribs protrusions providing fences that direct the air to where it should go, straight back over the wing.

Have I got this one wrong?

Guys,
I have been looking at the Olympic article and it seemed like Bob G thought that there could be some aero advantage to the true airfoil shape and thickness that the sheeted wing brings. I'm wondering if by having the true thickness if you could get better lift and lower drag from a thinner section with the sheeted wing? This would mean quicker re-acceleration out of a corner. This could work real well with electrics.

Best,                DennisT

Did Howard get 20 points??

No, but I carried a 20-pointer in my car once.  I got 19 with a sheeted foam wing, and 19 with a built-up wing.  The foam wing was a lot easier to sand and finish. 

Guys,
I have been looking at the Olympic article and it seemed like Bob G thought that there could be some aero advantage to the true airfoil shape and thickness that the sheeted wing brings. I'm wondering if by having the true thickness if you could get better lift and lower drag from a thinner section with the sheeted wing? This would mean quicker re-acceleration out of a corner. This could work real well with electrics.

Best,                DennisT

HI Dennis,

Not sure exactly what "better" lift is.  The bottom line is simply that you need enough lift (better or worse) to support the airplane as it tracks the radius the pilot demands of it.  The "thickness" of a wing isn't a huge issue in terms of drag versus lift.  It does, of course, have an effect on parasitic drag because it increases the "form" that must be pulled through the air.  It does not, however, increase induced drag (drag due to producing lift) thus, whatever damage it does in the drag department doesn't necessarily increase when "G" loads increase in maneuvers.  The bottom line is that, much more important than lift over drag, is a power train the minimizes the loss of airspeed (and lift which increases/drops with airspeed) during maneuvers.

As Al Rabe made pretty clear with his fabulous semi-scale Nats winners (drop tanks and all), form drag isn't in and of itself altogether bad.  It allows (demands) greater thrust at all points which...in the classic 4-2-4 engine run...allows the engine to be run harder without excessive speed and, therefore, makes it easier to haul the increased load during maneuvers.

This is a large part of the reason that stunt ships with wing loadings 50% or more higher than thought ideal in the early days of stunt are now winning pretty much everything.  If the power train keeps the speed up the lift comes right along with it!

Ted

Here's the principle pushed to an extreme, highly succcessfully I might add...I've built several combat wings and one stunt plane using the technique, all outstanding fliers.

The lower pix show the same construction using different weights iron-on, and the last pic, silkspan.  The droop works with most any covering material.  

Ah hah! That confirms my suspicions but leaves reason to believe that the rib shape may have to be over exaggerated to get an average usable thickness - in other words a  thicker rib shape combined with soft cover valleys on a 'starved horse' design may not work on a hard cover wing as it would indeed become too thick.

Food for thought?

Fascinating stuff indeed! Kinda reminds me of the experimental soft skin covered BMW car.
http://www.myconfinedspace.com/2008/06/14/soft-skin-bmw/

And taken to extremes on the USA 1 that has more bones than a garfish in the wing.

Guys

The question is with the hard "true" surface would you get less drag then the soft "valley" covering? If so it should allow a little quicker re-acceleration from the corners.

For IC setups this may not help because they sometimes like the extra drag to hold speed. For electrics it means less amp draw which would allow smaller and lighter battery packs (the big enemy).

Best,       DennisT

HI Dennis,

A good question about the drag, and one I have never given thought to.  It will take someone with a lot more aerodynamic knowledge than I have to give a decent answer to it.

Big Bear

I've made only one model with fully sheeted wings.  It wasn't successful because of the weight.

Strength (torsion) is best with a stiff outer surface.  So, I've had good luck with double-covering with silkspan.  I don't bother criss-crossing grain, just apply the second sheet after 3 or 4 coats of clear on the first sheet.  The result is really sturdy!

Floyd

You might try the second layer using ultralight glass cloth with the strands 45deg. to the span.  The glass would only weigh 3/4 of an ounce or less for a 650 sq.in. airplane.

I have never seen this as a detriment.

That makes one of us.

I don't like the ridge at the back of the sheeted leading edge.  I don't know what the scallopy surface would do.  Frank Williams had a really scallopy wing at the Nats this year.  It had, as I remember, no structure that the covering touched except the ribs.  It looked like it flew great.  Frank always shows up with something cool.

Hi Howard,
                Do you mean that you have identified what is a aerodynamically detrimental with the hills and valleys with the use of soft covered wings on stunt models?

The mechanical strength with a sheeted leading edge, or indeed the entire wing is indisputable but the aerodynamics?

The thing is that you can not ignore mechanical strength, at least not at the size of modern CLPA models. A open rib design as shown in the photos above is fine for short span, low square wings but using the same technique on a 60" or larger wing with significantly more loading is not going to work unless you can fabricate the internal structure out of Carbon Fiber. The subject makes for some interesting conversation but ultimately for CLPA where the G loading is rather high and the wing spans rather large you have to use construction that will not turn your wing into a letter V at the 4th corner of the hourglass.

And on a different note. Sheeting the wing saves you from breaking ribs during handling. Seems like every time I touch the wing on my pathfinder LE as I am putting it together I end up busting a rib or two. 

... unless you can fabricate the internal structure out of Carbon Fiber.

No problem there, but I still sand through the dadgum covering on the rib edges.

I sand through silkspan.  I'm going to try Polyspan.

Another.....

There was a question about sheeting a classic model wing, where the original was open bay.  I guess this is legal, but I wouldn't try that stunt with an OTS plane!  I showed up at a contest with a Ringmaster that didn't have the same number of ribs as the Sterling S-1 kit!  Got nailed for that!

Maybe Classic rules/people are a bit more lenient?

Floyd

Floyd, I posed that question a while back! Like Howard, I'm one of those individuals that sands through silk and tissue, then I have to patch it up, then I get annoyed with myself for not sheeting the wing in the first place
I can't see that it would be a problem on a classic model, as it is only a choice of covering . And if done correctly, it could turn out the same weight as a silk-ed wing?

Cheers