Tapered wing tip sweep- forward or back advantages/disadvantages??? - RC Groups
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Mar 12, 2011, 06:28 PM
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Tapered wing tip sweep- forward or back advantages/disadvantages???

Just curious about the aerodynamic advantages and disadvantages of tapered wing outboard sections that taper forward(or forward-ish) like the HOB "2x4" and "2x6" and Sig Riser(I think) or Bird of Time, Skybench bird series Vs. the rearward taper in designs like the Great Planes "Spirit", Ace "Easy Eagle", the "Crysallis".

I've noticed that most HLG/DLG's taper back, more modern freeflight gliders seem to taper back(older ones taper forward).

Is there an advantage in a straight line taper like a P-51 wing Vs. and elliptical taper like a Spitfire wing? Or is elliptical not used as much due to more time spent crafting an elliptical taper?

Is there a total drag reduction issue(like tip vortex?)?

Does one have more or less tip stall tendencies?

Or is it mostly personal asthetic sensabilities?

I'm guessing that for polyhedral Rudder/Elevator thermal floater woody's, a good airfoil, the right polyhedral angles and just having tapering outboard wing sections in and of itself with good tips is what makes the biggest difference?????

Thanks - Paul
Last edited by Pauliwog; Mar 12, 2011 at 06:53 PM.
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Mar 12, 2011, 06:55 PM
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It's been asserted that a bit of sweep back near the tips can reduce induced drag somewhat. You don't want to do a lot of the wing, as I understand it, because you'll get tip stall unless you put in quite a bit of washout. As an extreme example, I think the Horten gliders used up to 8 degrees. A real elliptical, as I understand it, is a bit too narrow out near the tips. Obviously a lot harder to build. Reverse considerations for sweep forward. However, consider what happens to pitch if you get an INBOARD stall with lots of forward sweep. The aircraft will pitch up. However, I don't think these concerns apply too heavily if just a bit of the tip is swept. A double taper can be very close to an ideal elliptical lift distribution. However, that elliptical distribution is ideal for a "planar", unswept wing. There are all sorts of non planar dodges. Try doing a web search on a guy named Ilan Kroo and on "nonplanar lifting systems". Hours of entertainment. If you want to see classics, I think some of Prantdl's (sp?) papers are on NASA web servers. They're pretty interesting, though I'll admit I don't follow the derivations very much. Martin Simons has a good book on model aircraft aerodynamics, though I don't know how much he goes into tip sweep.

Measure sweep from 25 percent of chord. You may find that what looks like forward sweep is really no sweep at all.
Mar 12, 2011, 07:13 PM
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More on taper-n-things

Lincoln - Thanks for the reply, I may be using some terms not as accurately I thought.

So, with that in mind- Would you call a P-51 wing a 'double-taper' shape that does not actually sweep forward? If I'm understanding correctly, then to me that would be a fairly "straight" wing with a 'double-taper' .....?

And, continuing on that theme, that basic shape is seen so many times in so many planes of a large variety of speeds, applications, etc, it seems to be a good basic good handling shape and a good basic shape for outboard sections of a polyhedral design. Or as Thornburg says "if it looks right, it probably is".

Thanks -Paul
Mar 13, 2011, 04:17 AM
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rdeis's Avatar
Straight tapers have the least structural challenges. A swept tip induces a twist in the wing under launch loads that has to be dealt with. This is why the Bubble Dancer and Supra use a straight taper.

An aft swept tip has been said to reduce induced drag in the past-- the scheuman planform was very popular and has a straight trailing edge with a 3-part taper that approximates an ellipse. That was followed by crescents, but as I understand it the supposed advantage of crescents were widely debunked.

Aft swept tips do have some stall behavior and handling advantages, though there seems to be some debate as to why.
Mar 13, 2011, 11:49 AM
Deniable plausibility
Shedofdread's Avatar
Download 'Liftroll' (do a search for 'liftroll jeff hazel' and that should find it) and play about with different planform shapes. You'll soon see the implications of different planforms.

Have fun!

Mar 13, 2011, 03:52 PM
Registered User
As far as "double taper" goes, I should have used a different term, because I've seen this one used in different ways. What I meant is an inner section with a little taper, and an outer section with more taper. This can approximate an elliptical lift distribution quite well.
Here's an example:

Another way to go is a constant chord inner section and tapered outer. Not as good an approximation but easier to build.

If you have a constant taper all the way along, that is superior structurally, as the root chord is deeper and you can put in a deeper spar. The kind of double taper I referrred to also has some of the same structural advantage, and the constant chord center section has less. Of course a constant chord all the way along doesn't have this advantage at all.

Generally, you assume that the aerodynamic center of an airfoil is at the 25 percent chord point. That's where symmetrical airfoils tend to lift at. Other airfoils with pitching moments will have lift there and a pitching torque which increases with speed. Usually nose down. It's kind of fictional but it does a better job explaining how an airfoil still wants to twist down when there's zero lift. The "center of lift" way of thinking assumes that the point where the lift acts moves around. THis works fine at moderate lift coefficients, but when the pitching moment is still there but the lift is nearing zero, one has to assume a "center of lift" as far away as Alpha Centauri. Kind of inconvenient if you don't have anything larger than E size paper, and I imagine even CAD systems would have a problem with it.
Mar 19, 2011, 04:08 AM
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Avaldes's Avatar
The proper design of a wing planform is a science and and art. There are trade-offs to be made in efficiency vs handling. The full process is hard to describe in a single post. You might read about the design of the Pike Perfect wing on the Samba website. Philip Kolb did an excellent write up on how he designed that wing; describing the planform design as well as the airfoil selections. It is a very good read:


Regarding "straight tapered" or "double tapered" wings like those you mentioned, there is some simplicity regarding structural design of those wings. If the quarter chord is straight and you place the spar there, you have effectively de-coupled the bending and torsion loads on the wing. Meaning that when the wing bends under load, you minimize the amount of twisting of the wing tips. This means that you don't end up with unwanted wash out or wash in while launching or turning hard. You have also minimized the change of pitching moment vs angle of attack. Leading to an airplane that doesn't change the required elevator trim during those high load situations.

Sometimes simplicity is not the most desirable thing, and a good designer can manipulate those characteristics to enhance the handling of the airplane during launch, pylon turns, etc. Airplane design is not simple, that is why people spend their lives and careers studying and applying these principles.
Mar 19, 2011, 12:09 PM
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Originally Posted by Avaldes
snip Airplane design is not simple, that is why people spend their lives and careers studying and applying these principles.
I think with models, that's only true for the last 5% or 10% of the performance. Close enough that if you don't win you have to blame the pilot. I think most technically minded guys could design a reasonably competitive glider. Partly by borrowing from others, of course. I suspect that, for instance, an RES woody glider where the 25% chord point is unswept, using a single airfoil, with two wing segments on each side and straight tips, could still be quite competitive if well made.
Mar 19, 2011, 05:51 PM
Marc PUJOL Paris,Fr
You may also tak into account some other aspect than aerodynamic.
For example, for HLG, it is important that the PEG is located behind the CG.

This conduct to foreward concept.

It is also important that Aileron has a reduction in chord % at the tip. This allow the get a better elliptical shape with and without flap. And if you don't want to had other vervo commands, you need to play with géometry (forward swep at the tip).

I have also read a paper that promote the foreward configuration.

For me such aerodynamic consideration is few things compared to others such as has good profils all over the tail (thats work over the critical Re at any angles of attack and flap deflectioon), yawing stability (dynamic ones)...

I never succeed to find any difference between foreward or backward configurations except for talles aircrafts.
Mar 22, 2011, 03:30 PM
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rdeis's Avatar
Originally Posted by lincoln
As far as "double taper" goes, I should have used a different term, because I've seen this one used in different ways. What I meant is an inner section with a little taper, and an outer section with more taper.
Exact opposite of the P-51 mentioned above, yes? And may or may not be combined with sweep in either section?
Mar 23, 2011, 10:01 AM
agony sweetns the victory
atjurhs's Avatar
LiftRoll is great and so is XFLR5 and Peter Kolb's write up is an excellent read. So stand on the shoulders of giants? What I mean follows what lincoln advocates in "partly borrowing from others" You would be very hard pressed to outperform Mark Drela's designs published on the Charles River webpages. Of course they all have superb performance in LiftRoll and XFLR5, and it wouldn't surprise me if Peter Kolb gained much of his thinking from Mark's designs as well.
Mar 23, 2011, 04:41 PM
Registered User

I can comment on the question regarding the P-51 straight taper versus the Spitfire ellipse.

The elliptical plan form of the Spitfire puts more lift inboard (where the wing chord is wide) than outboard (where the wing chord, and therefore lift, is less), and that reduces bending at the root. Well and good. A stronger wing results at the expense of forming compound curves from flat sheets of aluminum. The taper of the P-51 was chosen as it approximated the lift distribution of an ellipse, and so reduced bending at the root. The straight taper is easier to form from flat sheets.

Wings with constant-chord center sections and tapered outer sections are that way for the same reason: move lift inboard, but they are even easier to make because now, there are a bunch of identical ribs to build.

At least, that's the tale told to me, and the rationale contained in my undergraduate aerodynamics text.

Yours, Greg
Mar 23, 2011, 10:04 PM
Registered User
Elliptical lift distribution is also minimum induced drag for "planar" wings. (Ok, I don't want to explain "planar" just now, so look up Ilan Kroo and non planar wings. But don't blame me if you disappear into the net for hours.

I heard that the Spitfire's taper wasn't ideal but was exaggerated to make room for the guns. Certainly the early ones were kind of pointy.
I'm not sure what you mean by opposite of the mustang. As an example of what I was talking about, maybe the root chord is 1 foot, 10 inches at the panel joint, and 5 inches at the tip. (Not necessarily ideal, just an example.)
The ugly Mantis had the kind of wing I meant:
The Windsong had the other kind:
Mar 24, 2011, 07:20 PM
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rdeis's Avatar
> I'm not sure what you mean by opposite of the mustang.

One of the P-51's distinctive features is its double taper wing, where the inner section has a steeper taper than the outer section, making the leading edge of the planform concave.

The elipse approximation you mentioned has the steeper taper in the outer section, making the LE convex.
Mar 26, 2011, 11:32 AM
Registered User
Oh, now I get it. I think that's mostly to make room for the landing gear when it retracts.

Check out what they did to the AT-6 for the same reason:

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