Tapering the flaps and ailerons on a constant chord wing??

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Monty

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The ultralight foot launched sailplane thread inspired me to ask this question.

If you take a constant chord wing with one of the standard turbulent NACA sections and then add a slight taper to the flaps and ailerons, what is the effect on the wing lift distribution and other characteristics?

It strikes me as an ingenious way to give the tapered effect while keeping the construction of the wing relatively simple.
 

Dana

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What you will have, in effect, is a slightly thicker tip airfoil section with more aft curvature and (I would think) a lower critical AOA since the steeper aft surface would tend to separate sooner. I'm not sure, though, if you could get enough taper to have any significant effect.

Actually, my Kolb Ultrastar is build just the way you describe (no flaps though, just full span ailerons). I don't know the reason for it in this design.

-Dana

Dullard: someone who can open an encyclopedia or dictionary and only read what they'd planned to.
 

orion

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I've seen this presented as sort of a poor man's way of getting better Aspect Ratio - it essentially allows for a simpler main wing structure in trade for an improved yet still less than optimal aerodynamic configuration. The way it needs to be done is that the section is kept true at the tip, with the surface extension occurring in the aileron and flap as you move inboard. If you do it the other way the tip section comes out sort of weird. Numerically it works however there seem to be practical limits - if you extend the root flap section too much that shape also becomes a bit on the weird side so it's usually a good idea to model said section and look at it in XFoil or equivalent, to make sure you're not introducing something that will give you unexpected behavior.
 

Aircar

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I intially took this to mean having a tapered flap on a CONSTANT chord wing --I think you mean having a constant chord "extended D nose" as it were with tapered trailing edge addition (so it is no longer constant chord )

You can modify the aft section of an airfoil by changing the closure angle to change the chord if that is the idea and it of course done on sails and sailfoils and there is some literature also around wind turbines and even propellers (hovercraft,airboats,VTOLs mainly) where the idea is to use a standard extrusion for the spar and change the chord via the aft section .

It can be made to work after a fashion but I wonder how far this can be taken to the smaller chord/same depth end of things (with rotary airfoils there are centrifugal effects that tend to mask separation effects at the root end --also recently studied in more detail.
Just about every bird has some form of extended chord without a matching increase in depth (there are no 'thick airfoils' on birds at the root end and relatively 'constant' size of bones spanwise )

Taking the other interpretation --a CONSTANT CHORD wing with tapering moving surface aft --you can get interesting changes of effective twist without having the basic distribution affected and can thereby shift the lateral lift centre as well which is handy for some configurations (eg tailless).

NOT tapering a trailing edge control surface to match the chord can result in very poor behaviour --several examples in gliders include the Schweizer 1-23 (highly tapered outer wings with constant chord ailerons) and Caproni Ca21 . the 1-23 had nasty tip stalling if thermalled too slow and could be made to flick roll out of a loop and continue what was somewhat like a 45degree nose down spin --some aerobatic aircraft also deliberately have 'mismatched' aileron chords to enhance the flick rolling 'ability' .
-The 1-23 aileron chord is nearly 50% at the tip compared to about 20% at the inner end so that deflection stalls the tip early even though it is more effective (some old aircraft --inc WW2 fighters, actually run the aileron chord to zero at the inner end and have a 'semi elliptical' planform sometimes with the control axis very skewed to the flow direction -- there may be some advantage to this set up as it was quite common but it is not clear why.
 

orion

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I took this to mean a constant chord wing with tapering control surfaces. This allows the main wing panel to be built of a constant section (spars and ribs) and get a bit of a tapering benefit from the ailerons and flaps. I've played with the concept about fifteen years ago but was never really satisfied with the aesthetic result. I also did not see a significant benefit (calculated) over a simple, constant chord wing. I suppose that with a bit of tweaking and a nice tip (rounded maybe) one could come up with something attractive - it would be interesting to see some hard numbers as to whether this does anything or not.
 

Aircar

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The benefit of a constant chord wing from folded spars and identical ribs together with a tapered planform is featured in the tail surfaces of the Sonnex (and probably other Monnet recent designs) --the leading edge is a conventional nose shape and the tip airfoil is normal but inbetween a progressively widening flat connects the front and rear spars --the ruddervators are themselves tapered I think from memory .

What this does to airfoil properties is anybody's guess --obsessing over the airfoil sections for the tiny tailplanes on gliders seems to pale in comparison --refer back to the advice of David Thurston on the effectiveness of 'flat plate' airfoils as on most Cub type aircraft (and I have a vague idea that the Tiger Moth horizontal tail was also substantially flat between the front and rear spars but that might easily be wrong )

Having an even slightly skewed axis on a straight tapered wing does create some curved hinge line difficulties as does not following constant thickness lines (even if the airfoil section changes from tip to root ) --the Schreder HP 16 to 18 sailplanes had a small discrepency in profile from this effect and it was apparently enough to really degrade the aileron effectiveness as shown by small modifications made by builders to correct it. One composite kit aircraft made here (Cobra) had a constant depth fin acheived by the manufacturer pulling the thicker root end skin down to a parallel rear /rudder spar --the bit between was actually concave --the one and only prototype was uncontrollable and speared off the strip before lift off ( some time later I got called in to sort it all out having had nothing to do with the aircraft prior --needed a fuselage stretch new tail tooling ,rudder, nosewheel gearing etc --the initial idea to put a simple constant chord rudder on the aircraft turned out to be a bad one.)
 

Monty

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I took this to mean a constant chord wing with tapering control surfaces. This allows the main wing panel to be built of a constant section (spars and ribs) and get a bit of a tapering benefit from the ailerons and flaps.
This is what I had in mind. I also intended to taper from the tip so that the tip airfoil is "correct". The root would be strange. Essentially an increasing % flap/aileron chord ratio as you move inboard.

The amount of taper would have to be small, to keep things reasonable, and it would probably look best on higher aspect ratio wings. As to the effect...your guess is as good as mine (well...probably better). Within, reason, I would say not a whole lot of difference.

I can't really see it making things worse.
 

fly2kads

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There were a bunch of control line stunt models that were built this way, with tapered, full-span flaps attached to a constant chord wing. The modest taper seemed to be more for looks than anything else. The practice carried over to some early radio control models as well. Abundant power absolves all sorts of sins at that scale, so no telling what it really did aerodynamically.
 

Autodidact

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And how about doing a sort of inverse variation on this theme: have a straight tapered D-tube leading edge and from about mid semi-span outward progressively reducing the chord aft of the spar so that you end up with a poor-mans elliptical planform? You'd have a slightly thicker version (but not exactly) of the root section and with fabric covering aft of the spar it would simple to build - would this have tip stall problems?
 

Aircar

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Autod--quite possibly the reverse (better non tip stalling with a thicker tip section) --Jim Bede justified the constant depth but tapering chord BD5 wing on the basis that thich airfoils stall later and more gently than thin airfoils . The tubular spar of the BD 5 imposed this anyway but it is opposite to the usual set up where tip airfoils are thinner than the root end ( on gliders almost always this way but the structure benefits from a thick root ) -- didn't seem to hurt the BD5 and might even be a good thing.
 

orion

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Historically it seems that the thicker root sections evolved due to the need for more fuel and structure. I remember discussing this with a few folks who were in on several developments where the designers came up with an ideal wing but the mission analysis revealed that the particular size and volume resulted in too many penalties. The "quick" fix was to build the wing thicker at the root - the subsequent penalties were just accepted.
 
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