Wing shapes elliptical or swept tips does it matter?

[Dennis Toth]

Guys,
A little slow week so thought I'd throw something out for general discussion. Is the elliptical plan form lower drag then a wing with swept back tips (i.e. flight streak)?

Best,   DennisT

[Brett Buck]

Guys,
A little slow week so thought I'd throw something out for general discussion. Is the elliptical plan form lower drag then a wing with swept back tips (i.e. flight streak)?

    Hypothetically lower drag, but irrelevant in the larger scheme of things.
 
     Brett

[rustler]

Somewhere along the way I've picked up the impression that eliptycal wings are lower drag in level flight, but are not necessarily the best for aerobatics.
Not sure if I can explain this, but I've also picked up that while an eliptycal wing gives lower straight line drag, it's more because of the elyptical lift distribution this gives from tip to tip, rather than the fact that it's an eliptycal plan view wing. The same effect can also be achieved with a straight plank constant chord wing by e.g. arranging wash-in/wash-out from tip to tip such that an eliptycal lift distribution is produced from tip to tip.
Sorry about the spelling, but hope at least one of them is right.
And at this point I think I'd better quit and leave it to any experts to comment. 

[Scientifiction .]

One thing that bugs me about the peacemaker Type TIPs , is that the ' edge ' / length is longer AFT , so you picture it pivoting on the trailing edge .
Then Bill werwages USA 1 , juno etc have the Leading edge Longest , which some say is the most efficent regarding lift . I.E. Longest Entry , spanwise .

As Tips Vary from SQUARE Vertically as say a Genisis and many typical foam wing types , maybe rounded edges . Some Outswept Aft & Still Square ,
so present a vertical face going outward into the airflow .

But EFFICENT ! ? , at low lift Level , moderate lift - rounds , high lift - squares , and semi stalled / under duress . The Cross Section would have a significant
influance , Particularly with a typical hard edged 1/4 deep 2 in long Adj Leadout cut out on the inner . Some Taped em once leadouts set . notta bad idea .

SO

ELIPTICAL TIPS on a Plank wing, I read give a rectangular spanwise lift graph . i.e. lift per in span constant to near the end , then gone .

where , it said , a Rectangular plank wing had a eliptical Lift per in graph , spanwise , to ZERO at Tips !

This Old Cow , Tho not a TRUE Plank Wing ! As the FLAPS are sorta elliptical !


Was Viceless and accurate , particularly when diheadraled . ( Theres a replica ready to assemble & cover )
20 + knot measured windspeed , some turbulance from large trees . Note the elevator ' balances ' .
a few other tricks , would twang 18 thou & stretch 16 thou solids in those winds .

pity about the funny grauncy noise in the engine, Er Push rod end . Lost Elevator inveerted outoff wingover
with the irvine 40R in it , 10 x 4 3 blade experiment to deal with rough air . pity about the noise in the engine .
Er Duh , I think they say .  

[peabody]

I understand that elliptical wings roll better, which isn't necessarily what we are trying to accomplish in PA.

Have fun!

[Steve Thompson]

Going from memory here...

Another benefit of the elliptical wing, like the Spitfire had, is the wing stalls starting at the wing root and moving towards the tip.  In high angle of attack maneuvering, at the onset of stalling, you would still have aileron control and less tendency to suddenly drop a wingtip/spin.

[Serge_Krauss]

Elliptical lift distributions afford the most efficient wings, when structure is not a consideration. They also have the furthest inboard aerodynamic center (like center of lift), than any but the most tapered of wings (a bit more tapered than the Australian 'Firecracker'). That makes them the best windy-weather shape. Elliptical wings are not necessarily going to produce perfectly elliptical lift distributions though, unless the lift were the same at each point on the wing!. Tip losses then, among other things, affect the lift distribution. SO,...in practice, Hershey-bar-shaped wings are not nearly as bad as one might think, having a lift that trails off toward the tips. Likewise, tapered wings have closer to elliptical distributions than one might initially imagine.

From usual theoretical compromise (Lift equal at every point), all elliptical wings have their aerodynamic center at only 42.4% of their halfspans out from fuselage centers. This makes them less susceptible to gust upsets. For a straight-tapered wing to have such an inboard a.c. point, it's taper ratio (c_t/c_r) must be T = .376. That's pretty sharp. A delta wing, the most extremely tapered wing (C_t = 0), has it's a.c. at 1/3 the half-span.

For the best lift per root bending moment, the wing looks pretty elliptical, but has a bit more span and narrower tips. That means that the root structure can be lighter for the same lift, and the wings are actually more efficient than those with elliptical distributions for that minimum bending moment at the root. They are not the most efficient wings (they're close) overall, but they are the most efficient when allowing for an idealized root structure.

Tip shape can make a significant different, with longest trailing edges the most efficient of any particular configuration. 'sorry that I don't have time to re-read the TR's concerning all of the corresponding effects and efficiencies. Of the elliptical wings, ones with their tips centered at .25-chord were found by NASA to have the "ideal" efficiency, but as the tips are moved back, even past the root trailing edge to a crescent shaped wing, their efficiencies rise up to about 108% of the accepted elliptical "ideal." As noted, such structure would flex a lot. Split tips seem best. Raked back tips do have increased efficiency over rectangular wings.

I do not think that elliptical wings would give up any superiority in maneuvering flight, unless tip stall became a problem. Reports over the years, since the time of Bob Palmer's pseudo-elliptical "Smoothie" seem to give the windy-weather edge to the elliptical wings.

I suppose I should try to answer the question. ^The elliptical wing is more efficient - until the triangular "tip" becomes rounded some with greatly increased tip span. Bets are off, when the wing starts to approximate one of those "elliptical" wings with a straight trailing edge. I have a german report that shows swept-tip superiority. Although the text is an NACA or NASA translation, the diagram symbols are german, and I don't have time to re-translate them. One thing seems certain. The swept-BACK tips are superior to rectangular or swept forward tips with straight, span-wise leading edges.

The pictures show Robert T. Jones' illustration of distortion of an elliptical wing ti get minimum root bending moment and a comparison between a "Firecracker" wing and a straight-tapered wing that would give an a.c. that matches an equivalent elliptical wing.

SK

[Chuck_Smith]

Good points Serge.

I'll add something we often don't consider. I learned when I was doing stability and control analysis of Edwardian-era aircraft (long story, don't ask...) that when the structure becomes very light the "apparent mass" effects of the air you move  as you make control inputs can overcome the conventional stability and control functions and stability derivatives, since these are generally steady-state values.

That's a mouthful but let me try to explain and then perhaps convey why this could have an effect on a CLPA ship.

If you pick up your ship by the nose and twist it along the longitudinal axis, you'll feel a rolling resistance. Part of this is due to the polar moment of inertia (Ixx, Iyy, Izz for all you engineering types) but some of it is due to pushing air out of the way.  The faster you roll it, the more the air matters. If you next do the same thing only rotating along the pitch axis you'll feel the air resistance on the tail - sometimes referred to as the "flyswatter" effect.

The air being moved out of the way becomes part of the "apparent mass" of the aircraft when you do stability and control. Normally, these effects are so small we ignore them, but if you have a very light structure - as in a CLPA ship (or the Wright Flyer ) they can become the dominant term. In fact, without the apparent mass effects, we calculated that the Wright Flyer would have been too unstable to fly, so it's a real effect.

OK, what does this mean for a controline ship? Maybe (probably?) nothing, but one could sort of, kind of, logically get to a line of thought that goes like this - If I'm flying in turbulent air there may be an advantage to the elliptical wing because I can have a smaller tip and the apparent mass effects of the gusts will have less unwanted rolling effect.

From a drag standpoint, a properly tapered wing is almost as efficient as an elliptical wing, so it would seem to me - if I squint and get really esoteric - that the advantage to the elliptical wing in CLPA is that it should fly better in turbulence.

On side note not related to CLPA: Once you get going fast enough an elliptical wing will have ENORMOUS drag.

[Tim Wescott]

Guys,
A little slow week so thought I'd throw something out for general discussion. Is the elliptical plan form lower drag then a wing with swept back tips (i.e. flight streak)?

Best,   DennisT

This seems to be one of the things that we perpetually go through on various boards in this forum.

The answer, for Stunt, is "you're asking the wrong question".  Lowest drag doesn't necessarily equal best all-around performance.

But yes, for any given wing that doesn't have any appreciable dihedral, an elliptical plan form (or better yet, a half-ellipse with a straight trailing edge), gives the lowest induced drag for the span.  But that isn't necessarily the best thing for a stunt plane.

[Gerald Arana]

On our toys, it's simply about looks. 

Right on Ty!  I've always liked round things......oh wait, we're talking about airplanes arn't we!? 

Smoothies and Thunderbirds! Yes!

Jerry

[FLOYD CARTER]

Much of my design effort reflects what I observe at contests.  I notice that some of the very top flyers have wingtips that are just squared off.  So I must conclude that tip shape doesn't matter all that much at our flying speeds. Also, a little drag here and there isn't so bad after all.

Floyd

[phil c]

Going from memory here...

Another benefit of the elliptical wing, like the Spitfire had, is the wing stalls starting at the wing root and moving towards the tip.  In high angle of attack maneuvering, at the onset of stalling, you would still have aileron control and less tendency to suddenly drop a wingtip/spin.

The Spit had an elliptical wing that tapered in thickness from 16% or so to 9% near the tip.  It also had significant washout to prevent tip stall that would mess up maneuvering and landings.  Unfortunately you can't do that with a stunt wing.  The early versions reportedly had heavy aileron control.  I believe that later versions copied the Friese ailerons used on the ME 109 to get better roll rates.

[Scientifiction .]

o.k. for a Library Book , ( The bloke was an Australian ) but not one for MY Library .

And Id thought it was Joe Smith .

Definately Not Wingtips , But Definately Spitfire .

[Scientifiction .]

Pre Mk 21 Wing ,



Mk 21 on , Wing .




Blah Blah Metal Cover Ailerons  ( The Fabric Balloned and Stick Solid @ 300 on Mk 1 & Early Mk II .

Blah Blah Blah Late Wing Six ?? times the TORSIONAL Ridgidity . Blah .

CLIPPED Tips Increased Roll Rate , and reduced Ceiling , Long Tips ( LF ) increased Cieling .

SECRET A.F.C. 146  http://www.spitfireperformance.com/spit12afdu.html





Nearu dah dah dah , ETC .

[Robert Zambelli]

I have two elliptical wing stunt planes as shown.

The Screwball Fury by James Luck is OTS eligible. A very good flyer and excellent for OTS.

The Starlight by Charles Mackey is classic legal and as good as any plane I've ever flown. Extremely stable and very responsive.

Both planes are take-apart and have something in common.
They are extremely sensitive to wing warps.
Both had very minor warps and tended to roll on sharp maneuvers.

Once I got them straight they flew perfectly.

Bob Z.

Bob Z.

[Avaiojet]

The LE cannot be straight. here's a few others. Really nice looking.

[john e. holliday]

Don't leave out Jack Sheek's Spitfire.   A much better flying plane than the pilot until the bell crank post decided to move out of the ply plates.

[Scientifiction .]

This Tells you something , now it stands out , actually .

Shadow from Diheadraled elliptical wing has turned into a straight L E  , Eliptical rear .
One would assume its acting as such aerodynamically , at times , Perhaps hard insides , Hard Outsides if so would conversly be more swept L E & Straighter T E .  ?

[Serge_Krauss]

A couple - well, more - points:

1) While drag can be desirable in stunt, I don't think that induced drag is necessarily desirable, since it saps energy needed in completing maneuvers. That is the drag that is minimized by "elliptical wings."

2) The actual consideration in these performance ideas is elliptical lift distributions. Elliptical lift distributions come, in simplified theory, from elliptical chord distributions (limitations discussed earlier).

3) To have elliptical chord distributions does not limit the shape of either leading or trailing edges (also discussed and cited previously, but illustrated below). These shapes influence efficiency, even extending it past the theoretical best.

4) If memory serves (big 'if' these days),the reason that efficiency discussions always seem to involve drag is that textbook aero theory and derivations are based on this parameter, with the rest deduced from it. It's not from some obsession with low drag. It's "how much lift can you generate with this drag?", which morphs into L/D talk.

5) Since modern stunt models have more than sufficient thrust, the main advantage of elliptical wings is their lower susceptibility to gust upsets, since these act inboard of where they act on other otherwise equivalent conventional wings, thus having less leverage about the roll axis.

It least, that's how I'm seeing it in my dotage.

SK

[Chuck_Smith]

Serge, you are spot on. The Oswald Efficiency Ratio is usually .9 or better on a tapered wing at typical stunt aspect ratios and has nothing to do with maneuverability anyway.

Once the AR is sufficiently high the tapered wing is just fine. Look at modern aircraft optimized for efficiency: They have long tapered wings, not elliptical. Even sailplane designers figured it out by the late 40's and they typically fly between 50 and 100 mph.. A higher aspect ratio tapered wing will give the the same L/D for less weight and less complicated structure. In fact, the calculation for the induced drag of an aircraft includes the AR as variable. So with a tapered wing if you add 10%  to the A/R you have roughly the same induced drag as an elliptical before the add.

Since we're not interested in top speed (where the elliptical wing does well up to about 450 mph) the only possible benefit I can think of is that the elliptical wing should be better in turbulence because, a)properly designed it will have slightly less span and, b)less area at the tip. Assuming everything else is equal. Looks, well that's a different story.

[Trostle]

This thread has evolved to a discussion, not just about elliptical wing tips, but about elliptical wing planforms.  I will toss in another perspective regarding elliptical wings for our Precision Aerobatic CL airplanes.

If there is some real advantage to use an elliptical planform for our PA models, then I would think the top fliers would consistently be using them regardless of the difficulty/complications in the construction.  My own experience goes back to the early-mid 60's when I first started competition using my own design elliptical wing stunt ships with some success.  I then took the numbers from the last elliptical wing and tail design and put them in a design with more conventional straight tapered wing and tail planform with equivalent aspect ratios, wing loading and same power train albeit a semiscale design.  Memory does not serve me the details, other than the straight wing flew better and that design eventually won the Nationals.  The straight wing design flew better in the corners, though it did not have the blinding corners of the modern PA designs.

I do not want to speak for Dave Gierke.  He designed/built that drop dead gorgeous elliptical wing model, NOVI Four, pictured in the recent Spencer post above.  Dave also had his All American and straight wing NOVI models at his disposal.  I do not know if Dave seriously campaigned that NOVI four.  However, he did use his straight wing models to compete at the Nationals.  Surely, if the elliptical wing model would have flown better, it would seem that the elliptical wing model would have been used.

Now, one can argue that Bob Palmer's Thunderbird has had some success over the years by a number of fliers, including a Nationals win by Palmer.  The Thunderbird evolved from his Smoothie/PowWow series with elliptical tips and trailing edges.  There have been an untold number of derivatives based on that Palmer design  The design, though not a true elliptical wing with its straight leading edge, very closely approximates an elliptical wing.  There is no doubt that the Thunderbird is a superb design from late 50's/early 60's - it flew and still flies well.  My experience with All of Palmer's designs is that they have a feel in the turns unlike any other.

Keith

[Avaiojet]

One would think this guy knows what he's talking about.

Contemporary elliptical models with proven performance.
 
A Guru to say the least.

http://discovery-aeromodels.com/en/andrey-yatsenko-shark-ellipse-2-control-line-f2b-model.html

[Serge_Krauss]

Just a reminder...

It's actual elliptical lift distribution that we are really talking about, when we consider performance. As Chuck pointed out, span and aspect ratio make up more of the difference as span increases, making tip losses a smaller part of the total lift. The elliptical wing will approximate that lift distribution, but should not be expected literally to have that distribution. This is for the same reason that tapered and even rectangular wings come closer than expected to producing elliptical lift distributions: all wings have tip losses and other "anomalies" and realities of 3-D air flow and pressure/velocity.

The most significant reason for the inaccuracy of simplified theory then is that the assumption in classical theory of uniform lift over the entire wing is is just too simplistic, even though it gives surprisingly (to me) good estimates. Actually, while assuming this in the basic model of things, including MAC's, all we are really doing in computing the a.c. this way (in primary theory) is finding the center of "mass" of a two-dimensional wing of a particular shape. That doesn't correspond to the actual lift generated at each point on the wing. Even the elliptical lift distribution comes from an ideal gas. So, there are shapes close to elliptical - like tapered or clipped-wing spitfire shapes - that might be better. I would, for example, expect the actual a.c. of a tapered wing to be even closer to the fuselage than the MGC (mean geometric chord) that we've calculated and treated as the MAC. That's one reason I've felt that the average chord, being even further out, might be too much of an approximation (although Howard found something a year or so ago that seemed to make the average chord more viable).

SO,... the ideally shaped wing may be some strange compromise, perhaps structurally impossible to use, and may be only the best wing for certain conditions of altitude, gusts, mass, etc. The literature of tip geometry is very interesting now.

SK.

[Chuck_Smith]

So correct Serge.

One has to lose the picture of 2D flow and start looking at 3D flow.

Even the NACA curves of lift vs. AoA are 2D and there's a correction that needs to be applied depending on the wing's geometry.  Plus, the fuse is in yaw in CLPA and there's a cross-flow vector of the flow field with is playing with the Dutch Roll characteristics and changing the relative AoA from inboard to outboard wing.

It's about flow fields and not streamlines. The classic Bernoulli picture is myth. There's a downwash behind the wing, and a upwash ahead of it.

[Igor Burger]

The plan form or tips shape are important at high lift coefficient, it means not in level flight, but in turns, and since we use flaps, the lift distribution in turns is afected by them much more then by plan form. Plus do not forget prop which makes air flow unequal especially in turns. So I would not expect anything special from elliptical wing ... at least I did not notice anything important.