What`s the advantage of winglets?

[Warren Jones]

I was wondering about this since I`m restoring a 105" glider with balsa skinned foam core wings. I was thinking of adding winglets but won`t bother if they are draggy or just for looks.

Any opinions please.

Warren

[markschaffin]

You wanted opinions...

Leave them off. It would be very difficult to prove they do anything other than add drag. Difficult enough on a full scale aircraft. Forget about it with a model.

Mark

[vintage1]

I wouldn't be so sure actually.

I think correctly dne they do help. The issue is 'correctly done', of course.. :rolleyes:

[Al M]

I have heard that in one full sized airliner their advantages were offset by the associated weight penalty and they were retained to avoid redesign costs and because they looked cool.

[Sparky Paul]

Winglets are most efficient at a single airspeed.
If the plane isn't at -that- airspeed, the winglet takes some performance.
In essence though, a winglet just adds aspect ratio.
For a model, make the span longer and continue the wing taper.

[Warren Jones]

I figured the same general attitude. At first I thought they acted sort of like tip rudders or something for stability in a bank,....???? wishfull thinking I suppose.

It`s a motorized glider I`m restoring that I picked up at a shop. The owner said it was hanging there for 10-15 years. I imagine a folding prop will be draggy enough rather than adding more drag in terms of "showie" winglets.

I`ve already stripped it. The main wing airfoil looks good and consistent. However I do question the flat plate stabilizer.....hmmmm.

[Warren Jones]

After stripping the wing I see that there was flaps or airbrakes. No manual so can`t be sure. Wonder if I should cut them back in or not. If so then I guess airbrakes are more important?

[ciurpita]

from jon denker's, see how it flies,
"adding a pair of six-foot-tall winglets has no aerodynamic advantage compared to adding six feet of regular, horizontal wing on each side".


http://www.av8n.com/how/htm/airfoils.html#@default336

[ghoti]

Winglets are used to increase lift a tiny bit in competition models when wingspan is already at the limit in length and cannot increase. Other than for that reason they are not as good as a small increase in span. Bill

[Ollie]

If you add winglets, you add the mass at the wing tips and you reduce the nimbleness of the sailplane.

It is very, very hard to design winglets that even improve a small amount of wing lift or even small amount of improved (reduced) drag.

If you are not a world beating aerodynamic scientific type, forget about winglets.

Read an article by Dr. Helmut Quabeck, page 44 of "Quiet Flyer", Nov. 2005.

A final quote by Mark Maughmer

"...the experience this far has shown that it is much easier to design winglets which harm the overall perfomance of a sailplane than it is to design those that produce an overall benefit."

[Brandano]

The best reason I can think for, for adding winglets to a design that didn't originally have them, is to add sacrificial skids for the wingtips so the wing isn't worn down landing on concrete. With properly designed winglets there might be some performance improvement in the measure of reduced induced drag, but you'd need to design them with the help of a wind tunnel and dedicated fluid dynamic computational software. One thing you could do is to make different span removable wing panels to quickly change overall span to suit weather conditions, like some full scale motorized gliders have, and experiment with that

[Julez]

If you want to do something nice to your plane, build a stabilizer with a good symmetrical airfoil on it, perhaps Drela's HT12 or something alike.
It is not more work than building winglets, but the success is guaranteed :)

I read one reason why airliners have winglets, is that they reduce the wingtip vortices during take off and landing. These vortices make the landing strip unusable for a short time, as the following plane could be hit by the turbulences. There is a specific time for each airliner, during which the landing strip must not be used, after this plane has landed or took off.
Therefore, the landing and starting fees of each aircraft also depend on the size and force of these vortices. Reducing them saves money for the airliner company.
Even though they mostly decrease efficiency during normal flight, the extra fuel is still cheaper than the extra airport fees without winglets.

http://en.wikipedia.org/wiki/Wake_turbulence

http://www.onera.fr/cahierdelabo/english/asub9.htm

http://www.nature.com/nature/journal/v443/n7110/images/443385a-i1.0.jpg

[Warren Jones]

Yes, this is what I was hoping to hear. I think I`ll focus on building up an airfoil stabilizer.

Good advice.

Cool pick by the way.

Cheers

Warren

[JetPlaneFlyer]

Julez,
The winglets on most, if not all, 'winglet' equipped airliners are not there to reduce tip vortexes and cut induced drag.. They work by using the energy contained in the tip vortex to generate forward thrust. They actually need strong tip vortexes to work. They were invented by a NASA researcher called Richard Whitcomb. They do actually save fuel in cruise, which is where an airliner is flying most of the time. I’d guess that the winglets on sailplanes work in exactly the same way.

The idea that they reduce wake turbulence separation therefore would appear to be flawed because they don’t, to any large extent, reduce vortexes... In any case the required separation is I believe calculated purely by the aircraft weight, so even if they did make a difference no credit would be taken and no benefit derived.

I’m also not sure what advantage an ‘airfoil’ stab would really make? The optimum stab would be as low drag as possible and to achieve this the best way would be to make it as thin as possible. A sheet stab with suitably profiled leading and trailing edges is probably a good a way to achieve this as any until you start to get to very large size where the stiffness/weight ratio of a thin sheet stab would start to be inadequate. You are more likely to gain an advantage by designing a built up stab that saves weight. Any weight saved at the tail end would save maybe 2-3 times as much in noseweight. I’d be confident that such a weight saving would give more advantage than any fancy airfoil shaped stab could ever hope to achieve.

Steve

[biber]

JPF, the thrust thing you describe is based on just the overall reduction of tip vortices and induced drag.
In the end thats all induced drag is about.

You are certainly right, however, that the WL's are used for performance reasons rather than to reduce wake turbulence experienced by following airplanes.

biber

[JetPlaneFlyer]

JPF, the thrust thing you describe is based on just the overall reduction of tip vortices and induced drag.
In the end that’s all induced drag is about.



Nope... Whitcomb winglets work by using the tip vortex to generate actual forward lift (i.e thrust)... In the process of generating this thrust they do remove some energy from the vortex, which also has benefits, however the forward thrust is the key to their efficiency.

here is an extract from a NASA paper on Whitcomb winglets:

Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices

And from Wikipedia:

The vortex which rotates around from below the wing strikes the cambered surface of the winglet, generating a force that angles inward and slightly forward, analogous to a sailboat sailing close hauled. The winglet converts some of the otherwise wasted energy in the wing tip vortex to an apparent thrust.

You could argue that they reduce drag by offsetting drag with thrust, some would say they are parasitic drag reducing for just this reason, however it’s still important that it’s realised that they do indeed produce ‘thrust’. This lift/thrust production is why the inward 'toe' angle and 'lifting' airfoil are essential for a winglet to operate effectively.

Steve

[biber]

Not quite. ;)

There is no need for a inward toe angle to get that thrust thing, otherwise no Darrieus windmill (http://en.wikipedia.org/wiki/Darrieus_wind_turbine) would ever work.
But they do. :p
Lift is per definition perpendicular to the attacking airflow,
if that airflow is directed inward by a small amount at the winglets' position,
its lift vector is tilt forward by nature.

A winglet shouldn't be considered as some little wing tacked to a bigger one, it's rather the more or less vertical part at the end of a wing.

You can allways have the induced thrust thing even without a vertical part of wing.
The Horten bell shaped lift distribution is one example for that, as it delivers local induced thrust at the tips.

And even Wikipedia is quite distinct by adding the word apparent to the term of thrust.The winglet converts some of the otherwise wasted energy in the wing tip vortex to an apparent thrust.Sic! :rolleyes:

It's almost a philosphical issue, since it's no less about language than it's about mechanics.

biber

[Eric Zanutto]

this is what a winglet does....and im not making this up....I have studied this topic in my classes.

first, you must assume that you yourself can actually make a winglet that works correctly which without CFD or a wind tunnel you can not do. This is not to say that you might get luck but in general it is hard to develop an efficient winglet without these tools and understanding at hand.

what I am about to tell you comes directly from "Aerodynamics Aeronautics and Flight Mechanics" by McCormick

in short, the winglet does have drag and does add drag to the aircraft. but the winglet also produces lift which without the tip vortices would point towards the fuselage. however, the circulation or tip vortices causes an added induced velocity term to the outer side of the winglet. this induced velocity changes the vector of winglet lift and points it more forward in the direction of flight velocity. because the lift vector, which has now been modified due to circulation, is much much greater than the drag vector of the winglet the overall force component of the winglet points forward and towards the fuselage. this effectively produces thrust as some will call it. winglets usually decrease induced drag by about 8%.

another benefit of winglets, if they are properly designed, is that the increase oswalds efficiency and increase effective aspect ratio. however they are definetly best used on heavily loaded wings that fly at one main mission; i.e. one altitude at constant velocity. this is why most transports use winglets.

like I said, they are hard even for a skilled engineer to properly design. I dont think any of us would be able to desing a winglet for our rc aircraft that really gives us any noticible difference in flight performance due to reduced drag. some increased stability, or decreased depending on what you do, may become present. id say for your plane they wont do much


Eric Z

[vyse]

I doubt if Boeing adds winglets to all 737-700, 800, and 900 models just because they look cool. I know of one particular airline, (who happens to be my employer ;) ) that has refit all -700s with winglets. All new aircraft are ordered with them installed, and the refit of all -300 models begins soon. We have seen a substantial savings in fuel costs per aircraft. In a bird that flies in cruise 8 -12 hours a day, the savings are amazing.

I know that modeling winglets might be difficult, and the benefits might not be easy to see, if indeed there are any... But for a 130,000+ pound aircraft, they do make a difference.

[Rickenbacker]

I fly full scale gliders, and we use winglets (supposedly) for the decreased drag, giving better glide ratio. But I've flown the same Discus 2 before and after the installation of small winglets, and the only noticeable effect was a much nicer behaviour when turning near stall speed (which gliders do all the time), the winglets added roll stability at low speeds. Don't ask me how, but they did :).

[nixy]

I fly full scale gliders, and we use winglets (supposedly) for the decreased drag, giving better glide ratio. But I've flown the same Discus 2 before and after the installation of small winglets, and the only noticeable effect was a much nicer behaviour when turning near stall speed (which gliders do all the time), the winglets added roll stability at low speeds. Don't ask me how, but they did :).

Could you expand on 'nice behaviour', please? Sounds very interesting.
Was there more roll for any given aileron stick movement?....more or less rudder needed for any given change in roll? etc.

Cheers

[Lon Enloe]

I just got back from a business trip that included a trip across the tarmac at Chicago/O'Hare on the shuttle between terminals. Having read this discussion, and being up close and personal with the planes, I was taking note of which airlines had put winglets on their airliners, and then I noticed that the winglets (and the ends of all of the other aerodynamic surfaces as well, at least in the case of United's aircraft) had thin metal spikes on them on the downstream side. I would be interested in the specifics of what these are intended to accomplish.

[MarkusN]

I noticed that the winglets (and the ends of all of the other aerodynamic surfaces as well, at least in the case of United's aircraft) had thin metal spikes on them on the downstream side. I would be interested in the specifics of what these are intended to accomplish.
These have no aerodynamic function. They are present also on non-winglet wing tips. They are static dischargers (http://en.wikipedia.org/wiki/Static_discharger). Basically small ligthning rods making sure that static built up in the aircrafts skin is dischaged into the atmosphere before it can interfere with the planes wireless comms.

[groundfx]

I fly full scale gliders, and we use winglets (supposedly) for the decreased drag, giving better glide ratio. But I've flown the same Discus 2 before and after the installation of small winglets, and the only noticeable effect was a much nicer behaviour when turning near stall speed (which gliders do all the time), the winglets added roll stability at low speeds. Don't ask me how, but they did :).
I don't know if this is related, but I built this free-flight rubber powered design (http://www.rubber-power.com/About.htm) because I was interested that it had a flat wing and only used winglets in place of dihedral. The surprising thing to me is, it works very, very well. Can anyone explain why?

[JetPlaneFlyer]

I don't know if this is related, but I built this free-flight rubber powered design (http://www.rubber-power.com/About.htm) because I was interested that it had a flat wing and only used winglets in place of dihedral. The surprising thing to me is, it works very, very well. Can anyone explain why?

This is because you are adding side area above the CG which increases lateral stability.:

Here is an extract from a NASA site:
In a sideslip, there will be a side force caused by the area presented by the fuselage and vertical tail. If the side force acts above the center of gravity, as shown in figure 147 (http://history.nasa.gov/SP-367/f147.htm), there is a roll moment generated that tends to diminish the bank angle. If the side force is below the center of gravity, there is a destabilizing moment set up that will further increase the bank angle.

Steve

[biber]

There are other effects coming into play aswell.
A sideforce and cl induced on a winglet by a sideslip induces lift on the basewing's tips too
and thus a rolling torque.

biber

[mikeyd]

you are all pretty much right there are a lot of benefits in using winglets, They decrease drag, "forward lift" and even in turns they decrease wing tip stalling but only if done properly, on a rc level you won't really notice a difference. However that response on cutting down on airport costs and keeping wingtip vortices from "closing the runway" is BS . I have been flying privatley since I was 13 ( I am now 24) and I have landed in a cessna 152 right behnd a tripple 7 on many occasions at Buffalo international. when landing "next in line" you allways land after the spot the jet touched down. that way you won't experience wing tip turbulance or even worse Jet wash.

[vyse]

I just got back from a business trip that included a trip across the tarmac at Chicago/O'Hare on the shuttle between terminals. Having read this discussion, and being up close and personal with the planes, I was taking note of which airlines had put winglets on their airliners, and then I noticed that the winglets (and the ends of all of the other aerodynamic surfaces as well, at least in the case of United's aircraft) had thin metal spikes on them on the downstream side. I would be interested in the specifics of what these are intended to accomplish.


I believe what you saw were vortex generators. These are installed on everything from small general aviation planes on up to the largest turbojets.

From wikipedia:

"A vortex generator is an aerodynamic surface, consisting of a small vane that creates a vortex. They can be found in many devices, but the term is most often used in aircraft design.

Vortex generators are added to the leading edge of a swept wing in order to maintain steady airflow over the control surfaces at the rear of the wing. They are typically rectangular or triangular, tall enough to protrude above the boundary layer, and run in spanwise lines near the thickest part of the wing. They can be seen on the wings and vertical tails of many airliners. Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow.

A vortex generator creates a tip vortex which draws energetic, rapidly-moving air from outside the slow-moving boundary layer into contact with the aircraft skin. The boundary layer normally thickens as it moves along the aircraft surface, reducing the effectiveness of trailing-edge control surfaces; vortex generators can be used to remedy this problem, among others, by re-energizing the boundary layer. Vortex generators delay flow separation and aerodynamic stalling; they improve the effectiveness of control surfaces (e.g Embraer 170 and Symphony SA-160); and, for swept-wing transonic designs, they alleviate potential shock-stall problems (e.g. Harrier, Blackburn Buccaneer, Gloster Javelin).

Many aircraft carry vane vortex generators from time of manufacture, but there are also after-market suppliers who sell VG kits to improve the STOL performance of some light aircraft."

[Julez]

However that response on cutting down on airport costs and keeping wingtip vortices from "closing the runway" is BS .

You should try to persuade the NASA of your opinion.
Believe it or not, they are still wasting your tax money on B (http://oea.larc.nasa.gov/PAIS/Concept2Reality/wake_vortex.html)S (http://en.wikipedia.org/wiki/Wake_turbulence#Accidents.2Fincidents_due_to_wake_ turbulence).

[skylark3f]

you are all pretty much right there are a lot of benefits in using winglets, They decrease drag, "forward lift" and even in turns they decrease wing tip stalling but only if done properly, on a rc level you won't really notice a difference. However that response on cutting down on airport costs and keeping wingtip vortices from "closing the runway" is BS . I have been flying privatley since I was 13 ( I am now 24) and I have landed in a cessna 152 right behnd a tripple 7 on many occasions at Buffalo international. when landing "next in line" you allways land after the spot the jet touched down. that way you won't experience wing tip turbulance or even worse Jet wash.

As Julez said... Airbus, Boeing and Nasa all spend (A LOT of) money on this "BS". It's not so much a matter of the wake vortices "closing the runway" but a matter of separation standards. These distances are weight based and are only compulsory when flying solely on the blind flying instruments... under VFR it's up the the pilot.

Interestingly, it's these (ICAO sanctioned) distances which are now providing the bottle necks at the world's busiest airports... limiting capacity.

http://www-mip.onera.fr/projets/WakeNet2-Europe/fichiers/publications/publi2004/Elsenaar_Presentation_Wake_Turbulence_London_Nov04 .pdf

Read this presentation for an indication of how seriously the threat of wake vortex encounter is taken in the community. The graphs on pages 6 and 7 are quite interesting... although granted slightly out of date with the introduction of the new category for the A380.

Sky

[MarkusN]

There's no doubt that wake turbulence is a serious issue and consts airport capacity. However, up to now I have not seen evidence that winglets reduce wake turbulence enough to allow significant improvements / use of less safety distance there.

[Ollie]

I haven't see winglets in R/C model sailplane designs that are winners with major contests like F3B or F3J. Are they worth it? Why?

[MarkusN]

In a word: No.

Reasons:

More span is always better than the same length in winglets. In classes that have no spanwise limitation, winglets are no good idea. HLGs (F3K) are the only class where benefits might be possible

Winglets are good at their design-CL (usually high lift). Going away from that, performance decreases and becomes a penalty when far off. F3B, F3J and F3K all have phases during launch with low lift and high speed. Launch heights have extreme impact on results.

Plus, designing (and reliably producing!) a winglet that works well is extremely difficult.


I don't say that depending on flight task a winglet might not have an advantage. It's just that today this effort is better invested elsewhere.

[Ollie]

I like your thinking, Markus!

[Ollie]

Why using winglets in design for any R/C model aircraft world record or country record? Prove it, if you can.