Most aerodynamic wingtips?

[stjobs]

I'm working on a micro DLG (4 oz, 36" WS). I want to streamline the design as much as possible to extract the most performance from simple, cheap building materials and techniques. What is the most efficient (lowest drag) wingtip design? Should the LE curve at the tips, should the TE angle forwards, or what? Are winglets helpful?

[Ollie]

Study the Apogee best type design!
http://www.charlesriverrc.org/articles/apogeehlg/markdrela_apogeehlg.htm
Winglets are not helpful.
http://aero.stanford.edu/WingCalc.html

[stjobs]

In the link, Mark Drela says the curved wing shape is basically for looks... I wonder if I should just go with a swept-back tip like on warm/hotliners?

[Ollie]

By making the lift distribution ellipical is for lowest drag. This is a combination for AR, swept, twist, airfoil change along span, planform and range of attack angle that makes the lift distribution ellipical for lowest drag. Don't trying a fancy tip. The complex combination is enough.

[Andy W]

Let me make sure I'm clear on this, as I have a wing I'm making that has squared off tips right now (comes with heavy plastic end caps). You're saying just a semi-eliptical tip shape would be just as good as (or better than) anything?
..a

[Berdie]

Ollie's right about the elliptical wings being the best for drag. I know on real planes tapered wings (trapezoidal) are quite a good approximation for elliptical ones, easier to build as well!

[BMatthews]

For broader chord wingtips the raked back tip is better than most.

The Apogee wing planform is actuallly a good one in general principles and I'm not sure if Mark's reference is to the curving in general or the elliptical taper. And if it IS the taper then is the taper bad due to the short tip chord and the Reynolds number situation or for some other reason?

Generally as I understand it a shorter tip chord is good because a shorter tip chord reduces the size and strength of the tip votices that causes a lot of drag. But like most things it's probably a trade off between smaller votices and reduced efficiency due to scaling (ie: Reynolds numbers)

[biber]

Just for thought. Look at manned gliding aircraft, as Berdie sayd there the ellipse is sufficiently approximated by two or three trapezoids (per half wing). Some of them use more than three trapezoids but that won't give much aerodynamical advantage since even the dual tapered wings (ASW24, ASW27, LS8, etc.) have lift distributions that differ far less than 5 percent from the ellipse. The manufacturers would build ellipsoids if that would give a reasonable gain in performance with acceptable effort.

As Berdie suggested above, the advantage of some trapezoids over the perfect ellipse is that it's far more easy not to ruin the wingsection/airfoil during the manufacturing process or repair(!). Modern airfoils should be built accurately. If slightly changed they may respond with increased drag by some more than only 5 percent.

So, if you don't know a wingtip design that definately is better than a simply cut off (multi-tapered) wing it's better to stick to the conservative designs and get a reliable performance.

[globemaster3c17]

Winglets are not helpful.

How's that? That statement goes against my aerodynamics class, not to mention the aviation industry. Winglets reduce wingtip vorticies, which are the cause of a lot of drag.

[HELModels]

How's that? That statement goes against my aerodynamics class, not to mention the aviation industry. Winglets reduce wingtip vorticies, which are the cause of a lot of drag.

Different Ballgame, Different Ballpark

[ChuckA]

Properly desinged and adjusted winglets do work on models. Ref. May, 1980 Model Aviation. I flew a winglet model in 2-meter for 20 years until my eyes made me give up 2-meter sailplanes.

[biber]

I would not say Winglets aren't good at all. But it is to easy to ruin a models performance with them. And it's rather hard to design winglets that definately improve overall performance without any drawbacks in e.g. low lift high speed situations. Btw, the well known Prof. Maughmer is reported to have said similar. It is much easier to design a very good wing if you don't do it with a winglet.

biber

[BMatthews]

How's that? That statement goes against my aerodynamics class, not to mention the aviation industry. Winglets reduce wingtip vorticies, which are the cause of a lot of drag.

Winglets are good over a relatively small range of operation. This is why some sailplanes used them early on and then switched to the Schumann planform. The Schumann or generally raked back or curved leading edge style, is not as good as winglets within the narrow range of operation but overall from very slow to very fast the raked back style offers more advantage to aircraft that must be efficient over the wider range. Airlliners still use winglets because they spend most of their time flying in that narrow cruise range and the winglets offer more fuel savings within that range. Outside of that range they have been shown to produce MORE drag than a simple raked back style.

[Salto]

Bruce,

What exactly is this Schumann planform? Do you know of any links to good info on fullsize sailplane design?

Graham.

[BMatthews]

It was the original shape of the crescent style wing as developed by Schuemann (I got the spelling wrong the first time)

Heres a link to a site that has an article by the man himself. As you'll see it's the origin of the faceted style of crescent wing we now take for granted.

http://www.scrollsander.com/Soaring-Articles.htm

[kristianb]

My personal opinion, no scientific fact to back it up:
To add wash out on the wing gives a wing were the wing tips dont add any lift around the wing tip at speed, so it´s then only a question of minimizing aerodynamical drag at the wing tip (thin profile and streamlined rounded tip). Then there is no pressure difference between upper and lower side at wing tip and very little vortex around the wing tip adding very little drag in this area.
At lower speed and higher angle of attack wing tip and the full span of the wing is useful for creating lift at landing and take off.
vbr KristianB

[Ollie]

The best tips are the right and left tips, which are more distant between left and right tips. The span is much better than the shape of tips.

[SoCalGliderFlyr]

Tips 0, Span 1

[acvar]

My personal opinion, no scientific fact to back it up:
To add wash out on the wing gives a wing were the wing tips dont add any lift around the wing tip at speed, so it´s then only a question of minimizing aerodynamical drag at the wing tip (thin profile and streamlined rounded tip). Then there is no pressure difference between upper and lower side at wing tip and very little vortex around the wing tip adding very little drag in this area.
At lower speed and higher angle of attack wing tip and the full span of the wing is useful for creating lift at landing and take off.
vbr KristianB

You have unwittingly just put a winglet on your wing. There really is no difference between a flat wing tip and a winglet (some books clasify them as equal). They both perform the same functions. They both create parasitic drag since they have drag and do not produce lift. If you designe them just right for your flight envelope they reduce overall parasitic drag by reducing tip voricies. I doubt most if any in these forums are doing all the math/wind tunnel testing required to get them right. In short adding wingtips/winglets is probably hurting the performance of your planes.

[kristianb]

Hello Acvar,
I fully agree when You compare a wash out wing tips with winglets, they normally add drag. Winglets are however much more critical to design. Maths are over estimated in aerodynamic forums, what have produced 99% of all sucessful aerodynamic designs are wind tunnel trial and error. For a tight flight envelope= an airliners normal speed, winglets can give some fuel savings comparable to a larger wing span (which gives other disadvantages).
A wing with wash out have one big advantage over a wing with winglets, it definitely contributes to the production of lift at lower speed= at start and landing!
vbr KristianB

[SoCalGliderFlyr]

I swear I saw an AgCat on TV last night that had winglets.

[kristianb]

Oh AgCat, that crate must be 98% parasitic drag! I doubt that even a pair of 100 billion dollar development cost winglets could make any noticeable difference! (I saw a windmill in Germany with wing lets, but those guys are very scientific!)
vbr KristianB

[Aeroengineer1]

For all those that are debating about the value of winglets for model aircraft, first one must consider the size of the aircraft being considered. Model aircraft can range in size from 30'+ wingspans down to inches. Even for these micro sized models, winglets can help to improve performance. To show that I am not just blowing smoke I have included a link to a research paper done by one of the professors at Arizona State University. The major points of the paper include that when adding a winglet, the design must be carefully considered. A poor design will produce no positive effects, and indeed, may produce negative effects. Also addressed is that similar performance can be achieved by increasing the wingspan (ie making the model more closely resemble the ideal infinite span). I think that one must also realize that winglets do not really increase performance, better said, they allow for the recovery of losses. Hence they are a help for the wing to reach its maximum theoretical efficiency.

http://frc.asu.edu/publications/reas_paper-2003.pdf

[Ollie]

This is a combination for AR, swept, twist, airfoil change along span, planform and range of attack angle that makes the lift distribution ellipical for lowest drag.

The AR can bad or good. The sweep can bad or good. The twist can bad or good. The airfoil can along span can bad or good. The range of attack angle can bad or good. There are other conditions too. It is only the best combination of all items where the design is for minimum drag.

It is true that an aircraft can fly because there are many, many conflicts of design items that can make a plane fly.

[Gregory66]

NASA Whitcomb winglets are very aerodynamic. as well as NASA tip sails.

Regards,

Gregory

[Ollie]

Add the list of design items, bad or good Whitcomb winglets. It is only the best combination of all items where the design is for minimum drag.

[Ollie]

http://www.nesail.com/articles.php
"Wing Planforms and Tip Shapes"
By Cameron Ninham
"In the recent past a lot of research has been done on wing planforms and tip shapes to reduce induced drag and increase overall performance of a wing. In particular, a lot of attention has been devoted to crescent or “crescent-moon” shaped planforms.


"Various studies and research were done by respected aerodynamicists such as C.P. Van Dam, R.T. Jones, S.C. Smith and I.M. Kroo. Most of these looked into theoretical analysis and methods of calculating induced drag and lift of different wing planforms. Almost all of these studies concluded that a crescent shaped wing created less induced drag than the classical elliptical planform. This raises the question: “What is the optimum planform?” Unfortunately the answer to this question is not so simple. There are several factors that need to be taken into consideration, and in the following paragraphs we will look into these a little more. Having a wing planform that produce less induced drag or that has greater performance is great, but what about handling? If a planform has inherently bad characteristics (read: it handles like a dog) it is no fun to fly, or you have to be on your toes 100% of the time to keep the wing flying at an optimum condition. So let’s look at these factors. There are two planforms that have emerged as a good compromise in handling characteristics and performance. This first is the well known Schuemann planform, and the second is the crescent planform.


"The typical Schuemann planform consists of three linear tapered sections, with the first section break occurring at approximately 50% of the semi-span, and the second at approximately at 75% of the semi-span. The trailing edge (TE) forms a straight line from root to tip and has no sweep. The leading edge (LE) tapers backwards. Schuemann did some tuft studies and discovered that under certain conditions a separation channel can form along the entire TE of the wing. When the TE has some forward sweep it causes an increase in the spanwise flow component, flowing inwards towards the root. This increased flow aggravates the problem and increases the chance of the separation channel forming. The separation channel reduces lift and also increases drag. Schuemann discovered that if a planform is used in which the TE forms a straight line, with no sweep (such as is the case with the Schuemann planform), it drastically reduces (and possibly eliminates) the chance of a separation channel forming along the TE, and thus increases performance. Also, with improved flow conditions at the tip better handling characteristics can be achieved (e.g., stall behavior).



"The crescent planform was studied in detail by various people such as R.T. Jones, C.P. van Dam, S.C. Smith and I.M. Kroo. The crescent planforms studied here comprise of two main variations. The first consists of a crescent (or “crescent-moon”) planform in which both the LE and TE sweep backwards in an elliptical fashion. The second variation consists of a planform in which the LE has a crescent or elliptical curvature, but the TE forms straight line. In various theoretical studies it was shown that the computed induced drag (total drag) was less for both cases of the crescent planform than compared to the elliptical case. Van Dam went on to test the crescent planform in wind tunnel experiments at NASA Langley. It was concluded that the crescent planform exhibited better handling characteristics that the elliptical case. Over most of the operating range of angle of attack (alpha) the crescent planform attained higher values for CL. Of interest was stall characteristics during pre-stall, stall, and post-stall conditions. Stall behavior for the crescent wing was benign and gentle, whereas the elliptical case showed a much more abrupt and unsteady behavior. Pitching moments for the crescent wing was higher, particularly at stall conditions. However, the greater pitching tendency of the crescent wing during stall and would cause the nose of a sailplane to drop down and result in a faster recovery than the elliptical wing. The crescent wing maintained attached flow, at the tip, over a greater range of angle of attack, especially during pre-stall, stall, and post-stall conditions. This also indicates better handling characteristics during stall conditions and reduces tip stalling tendency.


"Studying the above sections, and from personal experience, I have come to the following conclusion. I believe the best planform, one that will give an optimum or good compromise between performance (lift and drag) and handling characteristics, is a crescent planform. Two cases in particular come to mind: The first consists of a planform with a crescent or elliptical LE and has a straight TE over most of part of the span, with either the outer part of the wing, or tip panel, consisting of a crescent shape which curves backwards. The second planform is a crescent wing in which the LE is also crescent or elliptical, but has a straight TE. Of course, due to building and construction difficulties, both cases can be approximated by using several tapered sections - a good example of this is the NSP Bird of Prey."


Note: Cameron Ninham is a graduate student in aeronautical engineering at the University of Illinois. He has been involved in the wind tunnel project, working with Dr. Selig.

[Sparky Paul]

Competition tends to sort out the good from the rest.
In areas where you might expect to see winglets, TD gliders, the Shueman planform as described is the overwhelming choice.

[JRuggiero]

"...a planform with a crescent or elliptical LE and has a straight TE over most of part of the span, with either the outer part of the wing, or tip panel, consisting of a crescent shape which curves backwards." (Ollie)

Several Multiplex ARFs, notably the Easy Star, use this wing planform.

Jim R

[ElectroLawndart]

Let me see if I hear this correctly.

Winglets and Tip Sails can boost proformance of a airfoil only in a certain part of the flight envelope?

Efficiency starts to suffer if you step outside of this flight regiem or design or repair them wrong to the point that them could actually become a detriment.

So that for me to take advantage of them I need to fly my aircraft smooothly at a specific speed.

If this is true, it sounds like that there are a million ways to get it wrong but only a few ways to get it right.

Dart

[Sparky Paul]

That's precisely it..
Those specialized tips have a limited performance range where they are 'better".. the range where they aren't is larger.

[biber]

In general WLs help at high lift situation and don't at low lift, but still add drag then. At high lift the decreased induced drag overcompensates the parasite drag. There basically exists for any WL design a certain airspeed, where its win and loss equals out. Going faster the WLs spoil the performance, going slower the WL improve performance. The hard point is now to design the WLs in such a manner that this certain speed is as high as needed (and as low as possible to get the biggest positive effect out of it at lowspeed). In designing WLs it surely is very hard to hit the little spot where the WLs will be more an advantage than a drawback. On the other hand it is so very easy to design WLs acting like little ancors that I would be carefull with that stuff.

Biber