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Author Topic: Wing planform design  (Read 1383 times)

Offline Randy Powell

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Wing planform design
« on: January 16, 2008, 07:42:10 PM »
I'm considering building a mid-sixties classic plane that has a swept back wing. It shows the leadouts sweeping back pretty severely with the bellcrank on the spar, but based on the position of the CG (pretty far back since the wing is swept so much), the leadouts actually exit in a pretty conventional position. (about 3/4" behind the marked CG)

Anyway, that's not the design issue. One of the reasons for sweeping the trailing edged forward in a more conventional layout is to move the high point out near the tips as far forward as practical to cut down tip turbulence (among other things). What's the effect of sweeping everything back? Pretty severe taper on the leading edge (maybe 30 degrees) and the trailing edge also sweeps back. For that matter, so does the stab ... even more severely.

I'm told this plane flies pretty well. Is the envelope of decent performance that wide? Something to think about anyway.
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Offline Mark Scarborough

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Re: Wing planform design
« Reply #1 on: January 16, 2008, 09:35:09 PM »
Get her built Randy, and we will find out!
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Offline John Miller

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Re: Wing planform design
« Reply #2 on: January 17, 2008, 08:35:32 AM »
Randy,

I'm sure you already are aware of what I'm going to say, but, on the off chance it will help you, or someone else, let me offer this.

From what I remember in my readings of Simmons book "Model Aerodynamics," I believe sweep back adds stability, as it simulates dihydral in a plane. Certainly designs that use a large amount of sweep back, such as the excellent "FireCracker", show good stability in rough or turbulent air. I would hazard a guess that sweep back wouold be something to try in a high AR design, (hint)

I'm guessing that the plane you are planning on building will show such tendencies as well. I believe the tip stall would be delayed as a natural consquence of the further aft location due to the sweep back.

The performance envelope, as you've already experienced, is not a static margin.  D>K H^^
« Last Edit: January 17, 2008, 10:19:20 AM by John Miller »
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Offline don Burke

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Re: Wing planform design
« Reply #3 on: January 17, 2008, 08:57:12 AM »
The sweepback does contribute "dihedral effect", I think the rule of thumb is one degree apparent dihedral for every 6-10 degrees of sweep.  I also remember that it's the LE sweep that's important.  A swept forward LE would give the same effect, re: the german WWII designs and NASA experiments with swept forward designs.
don Burke AMA 843
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Offline Ted Fancher

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Re: Wing planform design
« Reply #4 on: January 17, 2008, 01:26:40 PM »
Hi Randy,

This is exactly the sort of situation that calls for a thorough knowledge of the MAC (Mean Aerodynamic Chord) ... a subject about which I've received some irrational and pointed reaction from certain quarters when I've suggested it be the fundamental basis for our design considerations.

For most all purposes you can think of the wing as "being" at the MAC and make all of your various calculations about that location just as if the wing were a constant chord plank.  Figure you CG at the appropriate position on the MAC and make your leadout exit accordingly ... even if that means it must exit at or even into the leading edge at the tip (based on how much sweep you're dealing with).

The only thing I'd be concerned about aerodynamically is the possibility that a tip stall might occur more readily on such a wing (FWIW, I don't think "tip stalls" are an issue at all for stunt models in a general sense).  Any wing develops a certain amount of "spanwise" flow as the air passes over and under it (caused by the wing tip vortices at the tips).  These are quite modest in a "straight" wing and, again, not something I give serious thought to designwise.  The spanwise flow on a swept wing is exacerbated by that sweep and it is conceivable that, if excessive, the wing might have a tendency to stall at a lower A of A near the tips.  Whether the possibility is a problem or not is primarily based on the wing loading and how close to a stall the ship will be at in hard corners.  The classic era wing loadings are generally so low that with a decent airfoil and anything other than a sharp leading edge it shouldn't be an issue.

Ted Fancher

Offline Dennis Adamisin

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Re: Wing planform design
« Reply #5 on: January 17, 2008, 10:28:22 PM »
THe history of stunt design is filled with a lot of designs that were THE ANSWER: Thin wings, thick wings, 18% root with 9% tips, 18% root with 21% tips, straight wings, double tapered wings, swept forward TE's swept wings.  Heck some Michigander even pushes High Aspect Ratios!   n~

In the end I'm not sure we can definitely "prove" even that a tapered wing will out-fly a constant chord wing.

I reported earlier that I was blown away at how well the 1958(?) Sterling Spifire flies - its designed with everything "wrong" and flies like crazy.  I am really CURIOUS about how well my new swept wing Swinger is going to fly.

Randy - I think you are right, the "sweet spot" for CLPA is a bit broader than we'd expect it to be.  The Super Sonic does not LOOK like a "normal" stunter, but I'll bet it will fly just fine - I really like the long fuselage.

Ted pretty well said it all: PAY ATTENTION to the MAC working around it and relative to it.  Keep the LE blunt, then keep it straight & light with a good engine...

Oh yeah, then go practice some more!

Denny Adamisin
Fort Wayne, IN

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Offline don Burke

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Re: Wing planform design
« Reply #6 on: January 18, 2008, 09:45:04 AM »
In basic aerodynamics everything is referenced to the MACs of both the wing and the stab, that's why it's called that.  It's easy to figure on a constant chord, no brainer, or a straight taper  1/2 span.  There is a graphical method for straight taper, but I don't remember exactly what it is and I don't want to inadvetantly put out erroneous info.  It changes a small amount when tip area of curved tips is included.  I also don't remember what it is for elliptical surfaces, must be old age & CRS.
don Burke AMA 843
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Offline phil c

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Re: Wing planform design
« Reply #7 on: January 18, 2008, 10:51:09 AM »
I've got a few observations from building /watching hundreds of planes.

A straight, constant chord wing will get through the first 90 or so degrees of a turn faster than a tapered wing.  Then the speed bleeds off and it slows down more.

A highly tapered wing with the quarter chord line swept forward slightly(1-3 deg) behaves better in transitions maneuvers and wind.

If you have the MAC of the wing, the CG, and the leadouts positioned properly almost anything will fly pretty well.
phil Cartier

Offline Will Hinton

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Re: Wing planform design
« Reply #8 on: January 21, 2008, 11:54:36 AM »
Randy,
I'm eager to see how well your experiment does because maybe then I'llbuild the swept wing version of the F9F, the Couger!
Good luck with it,
Will
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Offline Randy Powell

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Re: Wing planform design
« Reply #9 on: January 21, 2008, 01:24:25 PM »
Thanks for the input. I've just started working on a Bernie Ash "Supersonic". It has a big more sweep than the Swinger or Freedom 45 that are about the same era. My primary concern was whether I needed to get creative with the leadout adjuster, possibly moving it into the leading edge somewhat. As noted, this plane has a really, really long fuselage and the CG is noted on the plans as being very, very far aft. I was somewhat concerned about the hard angle you'd end up with if the bellcrank were mounted on the spar This would give you a 25% to 30% angle at the wing tip. Apart from drag on the leadout slider, it seems that without some trick leadout adjuster, you could end up with some control feel issues.

My first thought was to just move the bellcrank back in the wing to reduce the angle. Might be tough on bellcrank to flap geometry though. So, I'm trying to consider some options. One might be to raise the bellcrank relative to the flap horn and leadout position. This might allow the thing to be moved back in the wing, reducing the angle without messing up the bellcrank to flap horn geometry.

This project will be fun, but there are some issues to get around, I think. I could probably build it just as it is on the plans and it would fly fine, but I'm afraid that sooner rather than later, I would have problems with sawing into the leadout slider or having problems with control feel due to the drag on the slider. That and the CG were my primary issues.
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Offline John Miller

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Re: Wing planform design
« Reply #10 on: January 21, 2008, 07:03:17 PM »
Randy, locate the MAC, put your BC on a line running tip to tip through the CG at the MAC. If there's a problem with the flap horn, run it to the elevator, and then back to the flaps. On my Sheeks Staggerwing, I tapped into the elevator pushrod about half way to the elevator, and brought the flap pushrod forward to the flaps from therre. It actually helped take care of the geometry problem quite handily.

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Offline Randy Powell

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Re: Wing planform design
« Reply #11 on: January 21, 2008, 10:05:01 PM »
John,

Now, that's a good idea. I figured that I'd need to move the bellcrank darned near to the trailing edge to cut down on the bind at the slider. Good idea to just cut into the pushrod along the way somewhere and route in back to the flap horn. I'm going to have to have some interesting "lucky boxes" to accommodate the arrangement. The plans show dual split horns, but I want to avoid that if I can. But it will take some lucky boxes with a pretty wide sweep. I have a couple of ideas around that.

These kinds of projects can really be fun ... and maddening.
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 Randy Powell


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