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Old Aug 15, 2012, 03:26 AM
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Aileron chord on multi-taper wings.

Hi chaps,

Just idle curiosity I suppose, but lets say we have a wing with multiple taper, like a schuemann (sic?) planform.

If one were to use a full span constant chord flapperon for simplicity, then evidently if one chose that to be, for example, 20% of wing chord at the root, then at the tip where the taper is very sharp, that % would substantially increase.

However, if the % chord were kept the same (root and tip, ignoring the intermediate panel breaks), then assuming the TE is unswept, that would put a small degree of sweepback on the aileron hinge line. As per the diagram.

If the hingeline is swept back, my gut instinct, (which is based on very limited brain I admit), wants to tell me that there would be a spanwise flow from root-tip along the aileron. Is that correct, and if so, would that yield a reduction in control response?. If it IS correct and would have a negative effect, then could that effect be mitigated by using a small vertical "fence" at the outboard end of the aileron?

I'm sure that in reality it wouldn't make much difference, not measurable by me anyway, but I just wondered.

Cheers

Matt
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Old Aug 15, 2012, 08:25 AM
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The real issue if the model is othe than a slow floater , is that on a rectangular aileron the narrower chord has less torsional resistance
-leading to vague responses .
Ratios of aileron size really can be confusing
The more area toward the tip -increases response but torsional problems rise quickly

whatever you do try to make a geodetic or similar diagonal structure and keep it light .
Every exspurt has an opinion of correct aileron size (zzzzz)

Whichever you choose -do not ignore structural integrity- and weight reduction practices -- more important than size.
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Old Aug 15, 2012, 10:00 AM
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First off any effects a swept aileron line in cases such as you're showing is really is a "tempest in a teapot" sort of thing.

Such concern also misses the point that the Schuemann planform itself is a swept wing that is intended to force the production of SOME outward spanwise flow in the air over the wing to limit or eliminate span wise flow INWARDS as occurs on many conventional wings. The idea being that a little outward flow is supposed to be less of a penalty than the other factors that it aids in lowering or preventing.

The "wings" you show are not really equal either. WIth the constant chord ailerons the % of chord out at the tips, where the ailerons are most effective, is far higher on the first case than the second. To be equivalent the constant chord example would need to have the ailerons be a LOT more narrow.

The diagrams you show do not look very glider like in terms of aspect ratio and considering the full span nature of the ailerons. If it IS for a sailplane sort of application then it's not a bad idea to worry about such things. If it's for a sport or general fly around task then the Schuemann wing planform is really far more trouble than it's worth. And if this IS for a soaring application then I'd be more concerned about the hinge line location more for how it affects the airfoil than any concern over spanwise flow due to any sweep in the hinge line. A lot of the airfoils we modelers are using for soaring applications are intended to work best when the hinge line for flaps and ailerons used in camber adjusting is set at a specific chord point. A constant chord aileron setup on a tapered wing such as this would certainly mess that up.
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Old Aug 15, 2012, 11:22 AM
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And structurally - it sucks -
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Old Aug 15, 2012, 02:01 PM
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Quote:
The diagrams you show do not look very glider like in terms of aspect ratio
Not to pick nits, but the wings are pretty high AR for the intended application as a fast slope glider, and full span, constant chord ailerons are fairly "de rigeur" in that application for example, on this Baudis Banana (60" span)

http://www.f3f.co.uk/knewt/images/banana/d91g2153.htm

You can see the ailerons are nigh on 50% of the wing chord near the tips and the AR is pretty low, lower than in the sketch I have shown.

Similarly, on this Stiletto (60in Slope)

http://www.phoenixmp.com/acatalog/Stiletto.pdf

Both these models are pretty decent performers, so I think I am not too far away. And note, both these use a constant chord aileron on a tapered and an elliptical wing.

But, I am quite willing to bow to greater wisdom than I profess to have, and maybe some rethinking is in order, I was just wondering which would be the better option, as I was unsure of the effect of sweeping back the hingeline. I have always designed on the TLAR basis before, and had some success and some failure, so I wanted to have a more informed approach this time around.

Yes, I always thought that specific sections were intended for camber changing flaps at a specific % chord, but I have seen so many examples that fly in the face of this, that I was unsure of the facts, hence I turn to people who know a great deal more than I do about these things.
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Old Aug 15, 2012, 03:23 PM
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OK, for a compact span slope racer I can see the wings you showed working out nicely.

As for the aileron chord and airfoils on these models I suspect they want lots of control surface chord to enhance their maneuvering rate. Even so it seems like a LOT of control surface area.

If you truly want to try to make a model which is that little extra better than the rank and file I'd suggest go with the constant chord ailerons you show but make them extend only over the outer 50% of the wing span. By getting rid of the inner half of the hinge line you'll clean up the design by more than the effect of using a tapered aileron for a little less drag than the full span ailerons. The only thing the full span option has going for it is that the servos can be located further in and simple linkages used. Aerodynamically it's not as clean as conventional outer portion control surfaces.
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Old Aug 15, 2012, 06:15 PM
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Having looked at it again, I think you are right on with the outboard only ailerons. I don't need the expense of 2 extra servo's for a 4 servo wing, and with the smaller ailerons, I can get away with a reasonably light servo in each wing, driving the aileron at its root.

I see where you are going with the semi span ailerons, leaving a nice clean chunk of wing inboard.

Revised plan shown below. Looks pretty neat. Thanks Bruce.
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Old Aug 15, 2012, 06:55 PM
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There might be some advantage in having inboard control surfaces too, to mix them as flaperons and for "crow brake" setups. It might help in achieving precise spot landings, and the added aerodynamic washout could help at slow speeds and high AOA. Also, reflexing the trailing edge at high speed would unload the tail and reduce drag slightly. The price to pay is added complexity and weight
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Old Aug 15, 2012, 07:15 PM
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My 2 meter has inboard flaps and ailerons

I run the ailerons slightly reflexed most of the time (up) and for really ast descent
the flaps go down 75 degrees - I manually change elevator as desired-
total weight change is pretty minor
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Old Aug 16, 2012, 12:52 AM
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Quote:
There might be some advantage in having inboard control surfaces too, to mix them as flaperons and for "crow brake" setups
That would probably be my ideal,but practicality and dictates that for the moment, cheap, cheerful and simple is the way to go. I can set up the ailerons as spoilers for landing (both up) but I don't *think* I'll need that on a slope model, but it remains to be seen, the main thing is to just build the thing. Its been a long time since I did an OD model through to completion, so keeping it simple is the key I think.

The high(ish) AR wing is a slight departure that I'm a little concerned about as this will be a 60" or thereabouts span, so root chord is only 7", but should be ok if I keep the wing loading down to a reasonable figure, say 10-12oz.
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Old Aug 16, 2012, 01:17 AM
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The flight task for a 60 inch size range of slope model is such that I don't see adding the complexity of a 4 surface wing as being worthwhile. On a bigger model I can see where the ability to use crow even on a sloper could aid in making the "arrival" a little less strenuous. But the smaller 60 inch models are pretty tough.

MCarlton, the new plan looks pretty slick. And it would be what I'd go for. As a bonus the "solid" inner areas are going to be a little bit more tough then the full span aileron version. So it may be able to resist a somewhat more energetic "arrival" at the end of the flight and laugh it off than the other models.
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Old Aug 16, 2012, 02:46 AM
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Many thanks Bruce, the more I look at it, the more right it looks.

Finished it up anyway, nose moment looks a trifle long, but I'd rather that than have to add a bunch of nose weight, I don't think its long enough to be a problem, so I'm happy with how it is so I'll go with it.

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Old Aug 16, 2012, 12:43 PM
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What is your construction method for the wing going to be? My thinking is that if it's going to be 1/16 balsa over ribs or foam core that the taper transitions can be "softened" quite a bit with sanding and the loss in skin stregth at the joints made up for by using 1/2 glass cloth and either resin or one of the polyurethane varnishes as a skin.
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Old Aug 16, 2012, 01:09 PM
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I was contemplating a foam core wing, with a 0.25gm "veil cloth" of glass underneath a veneer of 0.8mm Obechi. The cloth is there for a bit of strength, and to give a sharp TE to sand to. I'm not good enough with composites to have the glass as the finish, so it'll most likely just be film covered.

Given the centre section is constant chord, I was thinking of making that as one complete panel to avoid joining and so on at the centre section, unless of course I can get the cores cut by someone who can CNC mill them out without needing seperate panels for the tips.

Either way, I don't think the tip panels would need much in the way of reinforcement at the joints, possibly a 1/16" ply brace for the larger tip panel joint to the centre section, as the cloth and veneer would be continuous over the joint.
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Old Aug 16, 2012, 09:35 PM
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If you're using the thin skin of glass then you will likely not need any other reinforcement. Especially if you laid a ribbon of similar cloth or slightly thicker inside under the skins and lightly rebate the cores with a stick and some sandpaper to remove just enough to allow for the cloth.

On the exterior if you apply and resin up the skins on a sheet of shiney mylar and then slap them on and stick the whole mess back in the beds with a thin layer of foam rubber for pressure you could then weight the whole mess and leave it to cure. Once hard but still "green" peel off the mylar sheets for a mirror like surface finish. The 1/4 inch foam rubber pressure sheets and lots of weights is a "sort of poor man's vacuum bag" setup.
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