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Design => Stunt design => Topic started by: Dennis Toth on September 13, 2010, 05:06:34 AM
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Guys,
Seems like stunt wing design flip flops on hard covering (sheeted) vs soft cover (polyspan, plastic). Since the thing that changes most between these two approaches is the weight and as has been said "nothing is lighter then air" the benefits of a true airfoil shape seem to be secondary. If the weight were close would there be any significant difference in the flight characteristics? With the true airfoil could you get more lift from a smaller wing area?
Best, DennisT
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The sheeted wing would flex less. Stiffness is a big deal.
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The sheeting wing would flex less. Stiffness is a big deal.
Yup. Plus, a sheeted wing can have smaller spars, or in the extreme (think foam wing) no 'spars' at all, but with the load carried entirely by the sheeting.
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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.
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Well, weight can make a big difference. I think a design has a "design weight envelope" that, as long as you stay in it, the plane flies fine. You get toward either end and create some problems and get outside of it and the plane won't fly as well. If it's lighter than the design envelope, you get problems with balancing lift and penetration. You can cut down the flaps or reduce throw to reduce lift and that can help, but you still end up with some problems. Too heavy and the wing can't carry the load. But inside the envelope, the wing preforms fine. This came home to me when I built a plane that flew great at 58oz (probably on the top end of the design envelope) but it's successor was a dog at 65oz. The same design didn't fly very well when I built a test project at 48oz either. So I suspect that the design envelope is probably about 54 to at most 59oz. The 48oz version improved markedly when 6oz of ballast over the CG.
Now, there may have been variations of stiffness or structure that contributed to the problems, but that's the overall impression.
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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
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Hmmmmmmm........ sheeted vs, open bays. Yep....... I think Randy has given the answers. I received some plans, courtesy of Denny Adamisin of his Classic vintage Typhoon. The Adamisins used planked wings as a strength measure he said. Good wood selection and I think the weight would not be a deal breaker, plus the wing is stronger.
I built a Profile of my design years ago. The first had a sheeted wing, which was a very good model. I built another and it had open bays. I still have it and the flying characteristics were about the same between the two. The WS was 50" and used a reduced Impact wing.
Mongo
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Bill, what was the difference in weight? If you can remember ;D 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
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Bill, what was the difference in weight? If you can remember ;D 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
Hi Neville,
I got an email (it was missing some words! LOL!!) and *thought* I replied? I will try again.
I do not remember the exact difference, but I *think* it was in the 1-2 oz. range or so. Not over 2 oz. The design was very *light* to begin with, and an *Impact* airfoil (reduced). Lots of lift. ;D (I should build another!)
Thanks!
Bill
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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
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I don't think weight should be a problem if you carefully select your sheeting, glue shouldn't be an issue as capstrips are stuck on to the bit of rib the sheeting goes. The reason you get hills and valleys on sheeting is sanding too hard when the sheeting is attached, so its best to join and sand the sheet to a really smooth surface before attaching! You can then use non-shrinking dope, fewer coats required than silk, most enamels or polyurethane's don't shrink? Al Rabe uses sheeting on his wings and gets a superb finish. ;D And as Bill says you may only add an ounce or two, or less if you are really careful!! I don't think I'd like to use 1/32 sheet, I'd end up putting a finger through it :(
Cheers
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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.
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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.
Hi Paul,
Most all the 20 pointers I know of were silkspanned wings, not planked or foam cores.. Windy's Cardinal was the last one, correct? Did Howard get 20 points??
Mongo
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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.
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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
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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. ;D
Bill
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Hey Dennis....is this for your 4stroke diesel?
Pretty much the sheeted wings are less likely to get hanger rash...
Leave some paint off and they weigh about the same....
Have fun!
And come on up for the Swap Meet November 21st!
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Hi Neville,
Over here, it would probably cause problems. ;D
Bill
No It isn't any problem, you can sheet the wings if you like and be totally within the rules
Randy
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I really didn't think it would be a problem, as you are not altering the shape or area of the model, just putting on a better surface to finish, and making a stiffer wing, less prone to warping?
Cheers Neville
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.......... 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?
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No It isn't any problem, you can sheet the wings if you like and be totally within the rules
Randy
LOL!! You are correct, Randy, but then, again............ even a plane with a picture dated 1956 is under scrutiny....... ;D
Bill
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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
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With the true airfoil could you get more lift from a smaller wing area?
I think so. There is usually a ridge at the back of the sheeting on a D-tube wing that can cause trouble. Some built-up wings are better than others about this. You could see for yourself by using Profili or JavaFoil. They're both free.
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.
You could take advantage of the smoother airfoil by making the wing thinner for the same lift, but I don't think that would achieve your objective. You'd do better by keeping the optimal airfoil (whatever thickness that is) and span and reducing the chord. Weight would probably have a bigger effect on reacceleration out of a corner. That shouldn't be a problem with electrics, though.
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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.
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LOL!! You are correct, Randy, but then, again............ even a plane with a picture dated 1956 is under scrutiny....... ;D
Bill
The solution to that problem is simply to bring your own copy of the Classic rules which will quickly inform them that their only authorized means to downgrade the sheeted wing is to deduct Fidelity Points or, if they opt not to use Fidelity Points, to "...consider..." any deviation from the original when determining the number of appearance points to reflect the differences. I'll include the current iteration of those rules. Note in particular the underlined (by me) sentence in Rule 4.0 which, I believe, is pretty germane to the current discussion. In case you haven't heard about Fidelity Points I've also included the references to same in the rules.
Ted
"4.0 Material and Modifications: Models which more closely reflect the construction and finish of the
original airplane will receive superior awards during appearance judging. Although a foam core-winged
model of an aircraft which originally was of I-Beam construction is acceptable it could be expected to
receive fewer appearance/fidelity points than an equally constructed and finished model of the same
aircraft using materials more closely reflecting those of the original.
5.0 Fidelity Points: Fidelity Points from 0 to 20 will be awarded for fidelity to the concept of the original
design and to the spirit of the event. Obvious distortions of the original design of any eligible model to
gain an actual or perceived performance advantage over the original design will also be subject to
reduction of fidelity points. Again, the decision as to the level of distortion and the penalty appropriate
for such will be at the discretion of the on-site officials and not subject to dispute.
5.1. Fidelity points may or may not be awarded at any given event, at the Contest Director’s discretion.
Advertising for such contests should note whether or not such fidelity will be judged and awarded.
6.0 Appearance Points: Appearance points, from 0 to 20, per current AMA Precision Aerobatics rules, will
be awarded prior to the contestant’s first flight. It is suggested that all models be judged together as the
first official act of competition. This allows the greatest possible accuracy of comparison for judging and
also will showcase these attractive models for photo opportunities. Appearance will be assigned by
appearance judges based on the level of craftsmanship, finish, and overall beauty of the models.
6.1. If Fidelity Points (Para 5.0 above) are not awarded separately (Para 5.1), then the awarding of
Appearance Points should include some consideration for the fidelity of the model."
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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
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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?
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.
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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?
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Yes...much thicker ribs. Here's a profile (first pic):
Also ribs need to be wider-spaced than typical practice, to allow the covering space to droop. The cross-section at the lowest (narrowest) point of the droop falls into a typical rounded airfoil, with the diamond-shaped ribs serving as wing fences to control air flow. A very interesting exercise; I expected the droop to be unpredictable but found it is very consistent from one application to the next. Rib spacing controls how much droop you get.
The last series of pix = the stunter that uses the same principle.
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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.
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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
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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
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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
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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.
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Getting back to the original question of flight characteristics between soft and hard covering, with soft covering there will be a series of 'fences' inhibiting span wise airflow and although the 'correctness' of your lovingly chosen section will vary along the wing through a series of hills and valleys I have never seen this as a detriment.
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I have never seen this as a detriment.
That makes one of us.
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That makes one of us.
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?
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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.
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I am now confronting the covering and sanding of two more built-up wings. I'm not looking forward to it.
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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. HB~>
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I am now confronting the covering and sanding of two more built-up wings. I'm not looking forward to it.
Howard, two wings,, a biplane version of the Impact perhaps?
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... 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.
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Howard, do you mean that you sand through silk or balsa covering? I always join the planks together cut them to fit, sand them thoroughly then fix them to the framework, all the hard sanding is then done!
Cheers
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I sand through silkspan. I'm going to try Polyspan.
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Old but fun and maybe relevant...
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Another.....
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Probably is relevant :)
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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
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Pete Peterson has an open-bay Rondinelli Venus. The original was sheeted.
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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