Aerodynamics of flat plate control surfaces behind normal airfoils - RC Groups
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Apr 07, 2012, 01:21 AM
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Aerodynamics of flat plate control surfaces behind normal airfoils

I really want to build an ultra spad stick: but with a symmetrical airfoil. I've been toying with the idea of a foam core more traditional wing because I can't see if there's any benefit to a symmetrical airfoil shape if you're ending it with a flat plate control surface. I'd rather have the build speed of coro though.

Other than to hide a spar, what good is an airfoil in this case? I'm coming from a background in traditional spar/rib construction where the control surfaces follow the profile of the airfoil. Can anyone help me understand this better? What pros and cons are there with construction like this? Would I even notice with the usual sport aerobatic use of a stik?
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Apr 07, 2012, 02:34 AM
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rockyabq's Avatar
That's actually an interesting question, Chris. I know that there are airfoil analysis programs that should be able to address such questions, but I don't have any experience with them. Perhaps someone on the Sailplane - Thermal forum would be more knowledgeable.
Apr 07, 2012, 10:04 PM
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earlwb's Avatar
Well there are mountains of data about it all over so there isn't much one can say about it.
yes a tapered aileron control surface offers the least drag and turbulence of the airflow over it. But since our model airplane don't go fast enough to really take advantage of it, it winds up being a minor issue that doesn't affect the wing all that much. Ideally you want a tapered control surface, but you really don't have to do it.

If you look t a lot of the built up wings on model planes, they are using tapered control surfaces.

It is tricky but one could do, more or less, tapered control surfaces if they want to. But they would likely have to use something else instead of coroplast for it. Maybe a coroplast base and tapered foam and plastic covering on top of that. That way you can keep the coroplast hinge if you are running the flutes lengthwise.
Apr 08, 2012, 01:12 PM
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Spaderman's Avatar
aerodinamic and spad is two things not really compatible…. the spad is a different approach for construction and the flight, and one needs a little " forget" the rules. The Spad general rules, is: fast and simple constuction and if that to be too heavy! To put a larger engine…. the " finesse" of flight in spad, not very important!
just mine opinion
Apr 08, 2012, 03:50 PM
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so, what does approximating an airfoil shape with a spad get you then? why not just flat plate?
Apr 08, 2012, 05:50 PM
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IMHO, discounting the loss of "airfoilness" of the "flat plate" ailerons, an airfoiled wing yields greater lift, less drag (possibly), and certainly better stall characteristics than a flat plate. Plus, it gives you somewhere to hide a nice, strong spar.
Apr 09, 2012, 03:49 AM
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Spaderman's Avatar
A flat wing with only one sheet of coro not to be very good
With the coro you can obtain a correct wings. Only the ailerons are flat.
Apr 09, 2012, 11:40 AM
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earlwb's Avatar
A flat wing works as a airfoil as it uses the angle of attack to generate lift. But a flat wing has a lot of drag to it and is thus suitable for lower speed planes like we fly in RC. Improvements are using Kline-Fogleman airfoils as the next step up before going with more aerodynamic airfoil shapes.
Usually with a flat plate airfoil, as long as the engine is running it works fairly well, but when the engine stops, the drag forces it to have a steep glide angle more like the Space Shuttle glide rate.
Apr 09, 2012, 11:41 AM
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what about a symmetrical airfoil with a flat aileron behind it? Wouldn't it act much like a flat plate, except better stall characteristics?
Apr 09, 2012, 04:09 PM
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earlwb's Avatar
I have had a few 3D planes with thick wing airfoils and flat control surfaces and they flew just fine like that. So it is a tough call. If I remember right the air flow is smooth over the wing to the ailerons and then it is turbulent off the end as it flows off the trailing edge. So due to the tubulence the ailerons act like they aren't big enough and need to be larger. So it depends on the plane and wing design. If you have larger control surfaces the loss of control surface area due to turbulence is less of a problem than if the ailerons were smaller and had less area.

So the problem is turbulence off the trailing edge reduces the surface area of the ailerons that give you effective control over the wing. A tapered aileron has less turbulence on the trailing edge than a flat surface aileron would have, thus you have more effective control surface area.

Now some 3D planes had a effect where the control surface would get shadowed by the wing if you flew it too fast, thus the ailerons would quit working on you. Sort of a scary feeling until the plane slows back down. Hopefully you have some altitude to recover from it.
Apr 09, 2012, 10:05 PM
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rockyabq's Avatar
Chris, a few years back, I built a plank flying wing exactly as you described in your initial post. It is a symmetrical airfoil that terminates in single sheet strips of coroplast (correx) that function as the elevons. It flies great--much better than it has any right to.

We're talking SPAD here. Build it. It'll take a couple of hours tops. Fly it. Test it. I'm guessing you'll be perfectly satisfied.

Take care,
Apr 09, 2012, 11:52 PM
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I have several coro planes with symmetrical wings with flat control surfaces, and I really like the way they fly. The USS (version II) pictured is symmetrical the other pic is "Version I" after meeting the concrete in high speed vertical landing .....
Apr 10, 2012, 12:10 AM
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alright, you've got me convinced. Biggest spad I've built, here I come. Thanks all!
Apr 11, 2012, 12:53 PM
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Originally Posted by digiex_chris
what about a symmetrical airfoil with a flat aileron behind it? Wouldn't it act much like a flat plate, except better stall characteristics?
First off, Reynolds numbers. With small airplanes Reynolds numbers are too small to make big differences. However, you can really start feeling the difference in just marginal sized airplanes. For instance, my USS has a higher wing loading then my DPS BUT it's a larger wing and handles like a big old trainer, So the size of the plane that you're working with in comparison to the Mark 1 Air Molecule has some deciding factors.

Now onto the flat plate vs airfoil. Both create lift with an applied angle of attack to the airflow. However, an airfoil will create more lift then a flat plate. Part of this is because of the distance the air molecules have to travel when they are presented and obstacle, like a fat airfoil instead of a straight shot plate. Introduce an angle of attack and the air has to travel up and down a hill, the fatter the airfoil the more lift, up to the point where you develop separation. This can be optimized as well and I'll reference flat bottomed or Semi Symmetrical airfoils for this. Your trainer, and my USS, makes just a ton of lift upright, but fly tat puppy upside down and look at the angle of attack you must use to maintain altitude at the same airspeed. Quite drastic I tell ya. A Symmetrical airfoil will produce the same amount of lift upside down as right side up, all things being equal, with the same AoA. Some airfoils, like the Clark Y, have a neutral lift point at a slight negative AoA. That's all airfoil stuff going on there.

Now you also need to factor in rigidity of the airfoiled wing to the flat plate. A SPAD wing is a monocoque wing as well. The spar holds the wing skins apart and the skins make up quite a bit of the structural rigidity. The thinner the wing the less forces it can hold. Now obviously there is a bunch of strength in the spar but I'm talking the rest of the loads not supported directly by the spar.

We can go on and on, but basically, yes you can fly a flat plane. But it gets floppy at speed, isn't rigid and tends not to be very fun or long lasting, at speed. Works really good for low speed, high alpha flight. If you are going to fly upright most of the time or make tight inside loops as in combat, well you might do better with a flat or semi-symmetrical (really it either is or it isn't symmetrical but that's a pet peeve for another time) to optimize for those conditons. Or a Symmetrical for even control upright or inverted. The diversion of airflow from laminar to disrupted going to flat ailerons is a little more abrupt then with a contoured, but it's negligible and easily compensated for in airfoil size.

NOW, if you go over to the Sailplane forums or design forums they will tell you just how much blasphemy I have laid upon your head. But while the low Reynolds numbers that they are playing with are the same as ours, We're talking SPADs here and they're talking molded, matching airfoils. I'm guessing they don't make Symmetrical airfoils by twisting a yardstick to stand on it's end between wing skins and hoping the the skins are as stiff on one side as on the other. Then again, I've seen one guy tout his 1% different airfoil as better then another guys while sanding at least 3% out of the shape thanks to being 3 inches long at the tip. There can be a lot of talk at the end of the day. I won't deny the fact that a 2 meter ship can produce a lot more lift with a lot less Parasitic drag then our SPAD ships do. Just letting you in on SPADworld design characteristics.

If you have a motor and just want to go fly, shoot touch n gos, drop some flour bombs, haul a camera around, Build yourself a USS or a SPADstick and go enjoy yourself. The higher lift of the flat bottom wing will help make up for the higher weight of the Coro bird. That being said my next big plane on the table now will be symmetrical so it will fly right side up as well as right side down.

In the end it's not rocket science, even if it actually is aerodynamics. At SPAD we're not so worried about the theory as we are in the real life characteristics AND those facts have always been proven that if it doesn't have enough lift, it needs a bigger motor, case closed.

Apr 11, 2012, 01:56 PM
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That's excellent info! That's pretty much what I was looking for. At our sizes, it has as much to do with what's happening on the airfoil side as what's happening at the TE.

Big spad, here we come! Just gotta figure out how to get 2 8'x4' sheets home.

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