Why do elliptical wings get a bad rap?

[Joel K. Scholz]

I have read several post stating that elliptical wings are a poor choice for modeling. Some of the better flying planes I have seen have elliptical wings, and everything I read on aerodynamics states that an elliptical wing has about the best stall characteristics of just about any planeform . This is due to the fact that the wing stalls evenly across the entire wing, as opposed say to a tip stall? :confused:

[Ollie]

A purely elliptical planform is an attempt to produce an elliptical lift distribution. It is the elliptical lift distribution that has minimum induced drag for its aspect ratio and planform alone isn't the only way to shape the lift distribution. The elliptical planform would produce an elliptical lift distribution if the flow over the wing were two dimensional. Unfortunately the flow near the tips has a spanwise component that violates the simple assumption. The Spitfire is a famous example of the purely elliptical planform. Even the later models of the Spitfire clipped the extreme tip because spanwise flow and reduced reynolds numbers due to narrow chord near the tip degraded both the profile drag of the airfoil and the elliptical lift distribution. The degradation of performance at model sizes and speeds is even greater with chord reduction in the planform. That is why the planforms of high performance model sailplanes tend to aproximate a trunkated ellipse rather than a full ellipse.

See:
http://aero.stanford.edu/WingCalc.html
This on line wing analysis program calculates and displays lift distribution based on a simple tapered planform, aspect ratio, twist and sweep with changes in angle of attack. The "e" number is the Oswald efficiency factor and is a measure of the degree to which the lift distribution approximates an ellipse. When e=1 the lift distribution is a pure ellipse.

BTW, wings with the most forgiving stall characteristics stall at the root first rather than stalling all over at the same time. Wings that stall at the root first give warning of the impending stall by mushing first instead of abruptly falling.

[Sparky Paul]

They're difficult to build straight!
3 dimensional curves don't accept sheet balsa easily, without the chance of inducing warps as the wood beds itself into the structure.
And washout? How could you measure it? Where?

[competitior]

Joel;
Elliptical planform wings do not work well in the real world..... This is most plainly demostrated in the indoor hand launch glider contest models. A glider with elliptical outlines is at a disadvantage of about 8-10 seconds on a flight of around 1:20.
The best planform seems to be that having the tralling edge perpendicular to the airflow, an elliptical sweep on the leading edge is fine, just keep the tips rather blunt in planform and very thin in profile.
Just for reference, a simple rectangle planform will out glide an elliptical planform everytime when you do not have any outside influnences. Allways keep the tips light. Any chord less that about 3-1/2 inches is just drag. Adjustment and flying ability is more important that theory every time, and I can prove that.
Now having said all that let me also say this only applies to low speed model airplanes, I know nothing about transonic dreams.

The bull**** stops when the clock starts

Competitor, for over 60 years of contest flying

[Mike Rolls]

The Spitfire is a famous example of the purely elliptical planform. Even the later models of the Spitfire clipped the extreme tip because spanwise flow and reduced reynolds numbers due to narrow chord near the tip degraded both the profile drag of the airfoil and the elliptical lift distribution<<

Ollie

The reason for the clipped wings on some Spitfires was to improve the roll rate in low altitude combat. If you look at the versions which depended upon aerodynamic efficiency above all else - the high altitude photo recce marks - you will see that none of them had clipped tips.

Mike

[Sail 'n Soar]

Elliptical lift distribution is great, but for models elliptical wings are a disaster waiting to happen. In addition to Sparky's comments relative to building a straight wing, the basic problem is that for a constant airfoil, the tips will have a lower max CL due to the lower Reynolds numbers going to very low numbers out near that pretty tip. That translates to a guaranteed tip stall if you push max CL accidently or intentionally. About the only wing with worse stall characteristics is a too agresssive straight taper wing with no washout.

Sparky also speaks of measuring washout. If you are trying to get an elliptical lift distrubition you are defeating your original intent by adding washout to correct a tip stall tendency. You can get VERY close to an elliptical lift/minimum induced drag condition and with good stall characteristics with a reasonably straight tapered or double tapered wing with moderate washout without all the complexities of building an elliptical wing.

[Sparky Paul]

Somewhere around here I have a photo of a clipped MKXIV Spit tip. There's almost no thickness there at all. It appears to be much thinner than the 9% the wing is usually said to have.

[vintage1]

I am glad you said that. I have just built an elliptical wing for a sport model, and the section is VERY thin at the tip.

I can confiorm all the unpleasant buiilding problems with the planform :) Worst was getting the ribs shaped up.

I cose it because it looks nice :)

Too much wind to test fly, but I'll check that tipstall later in the week hopefully and show you the shattered balsa :)

[tim hooper]

Vintage1,

It may not be that bad!

I've been flying my elliptically-winged Scorpion (44", 23oz) for a few weeks now, and it shows no tendancy to tipstall. I think this due to the washout I built into the wing by using a twisted build board.

tim

[Sparky Paul]

That board is about the only way -to- put washout in an elliptical!
Makes too much sense! :)

[Sail 'n Soar]

They look pretty, but why else would you go elliptical? Full size designs gave them up years ago as extra cost without any offsetting benefit.

[Sparky Paul]

It isn't always about dragging the last little bit of "efficiency" out of a shape... Sometimes they just look good up there in the sky!

[vintage1]

Originally posted by Sail 'n Soar
They look pretty, but why else would you go elliptical? Full size designs gave them up years ago as extra cost without any offsetting benefit.

Sparky has the right of it. AND despite there not being any washout, it doesn't seem to tipstall !I put that down to a VERY thin wing section at the tip. Probably 5% or less.

Plan looks like yours Tim!

[JRuggiero]

When I put the question of thin wingtips on older aircraft designs, like the B-17, I got a response on the reduction of weight at the tip for better roll response. I didn't find that satisfactory, but I didn't know why There were some other responses I didn't understand and quickly forgot. There was a response, "Those old designers knew what they were doing." Which I interpreted as as sort of a backhanded...

Now it shows up in this discussion, so I'll ask the question again: Doesn't a thin wingtip encourage tip stalling?

Jim

[Ollie]

It depends on the airfoils in question. With airfoils of the same family and mean line camber you can find examples where thinning at the tip results in tip stall tendency, no tip stall tendency and tip stall margin. This is something you don't want to generalize about.