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kavu
7th Jul 2004, 05:02
Hello all

Just wondering if anyone could expand on "Can winglets give extra thrust?"

If you look down on the winglet and have a look at it's cross-section then the lift vector would be pointing out from the thickest part of the chord (vertical) and the drag would be pointing out the rearwards end. (horizontal).

Would the resultant vector from the drag vector then connect to the lift vector........... therefore a slight thrust vector???

In a picture form

A Capital "L"

With the vertical vector being the lift and horizontal vector being the drag. The resultant "" being the thrust vector?

Am I describing this well?

Mad (Flt) Scientist
7th Jul 2004, 05:07
Short answer is YES, thrust can be derived from a well designed winglet. I posted a link to a site that explained this very question quite a while ago; let me try to find it again....it had diagrams that showed what I think you're trying to describe.

ok, found it

look here (http://www.pprune.com/forums/showthread.php?s=&postid=667733#post667733) for a quite old thread that discussed this topic, and also look here (http://142.26.194.131/aerodynamics1/Drag/Page8.html) for a specific discussion (the Winglets Design link from the other thread.)

747FOCAL
7th Jul 2004, 05:48
Your going to have to give me a day cause I just got back from downing a few. Thrust is not "generated" by the winglets. The winglets give the airplane, in most cases, better L/D, such that it climbs better on takeoff and requires less thrust on approach. They also, if designed correctly, improve fuel burn in cruise.

:)

ps. Any person that tells you that that thrust can be "derived" from winglets is not telling you correctly. B

kavu
7th Jul 2004, 09:08
Mad S.

I'll look up the thread.

747FOCAL

I might be mistaken with the correct terminology but the resultant force in the forward direction would be a forward moving velocity - ie forward motion provided by the winglet?

Cheers for the help guys/gals.

Fordhom
7th Jul 2004, 15:35
At the risk of sounding anally retentive (what has four years of undergrad aero eng done to me!).......

The net force generated by a body in flight cannot be forwards - that is, into the flow. However, we commonly resolve the total aerodynamic force on an aerofoil (which a winglet is) into components parallel and normal ('drag' and 'lift') to the chord line of the aerofoil. By appropriate angling of the section it is therefore possible to make some of the 'lift' appear to be 'thrust' (i.e. angled into the flow), although the 'drag' generated will be larger than this component, ensuring the total force generated is not forwards.

Sounds a bit complicated - would be helped by a diagram I guess! The concept is similar in character to the way the lift force generated by the wing of a glider is angled forwards to oppose aerodynamic drag, in addition to the aircraft's weight in unaccelerated (but decending - at least relative to the airmass) flight.

Sorry if I've confused anyone more!

Mad (Flt) Scientist
7th Jul 2004, 17:08
Fordhom

While a body cannot, I agree, generate a net force into "the flow" you have to be careful about "the flow".

The way the winglet can generate "thrust" (i.e. a force forwards along the direction of flight) is by taking advantage of the fact that the flow direction at the wingtip is NOT the flow direction of the gross air mass.

To take a very exaggerated example, if the wing tip vortex were so strong that it was essentially flow moving inboard and spanwise, then an airfoil pointed outboard could generate local airfoil "lift" which was normal to the flow locally, which would end up being forward in gross aircraft terms i.e thrust.

Now obviously the actual angles are not so large as stated there, but it is the case that I can use the angularity of the vortex to generate actual thrust.

If you wish, think in flow momentum terms. If the winglet acts to turn the vortical flow slightly more rearwards (which it does) then there must be an opposite reaction, which is a forward force on the winglet.

The diagrams in one of the links I posted show it better than words can explain.

Eagles Forever
7th Jul 2004, 17:52
How about this for lateral thinking?

Winglets should reduce the drag caused by tip voticies. If drag is reduced then the thrust from the engines will be just a little bit more effective. So you could argue that the winglets produce thrust!!

747FOCAL
7th Jul 2004, 18:07
Mad (Flt) Scientist ,

Where can i get one of these magic winglets? I want to mount one on my car so I do not have to put gas in it anymore. :E

Fordhom
7th Jul 2004, 18:55
Mad Scientist:
Thankyou, I hadn't thought about that angle. Interesting link.

Mad (Flt) Scientist
7th Jul 2004, 19:33
747

Unfortunately, you'll have to install a wing, too. That might take up a few excess parking spaces .... :)

Eagles

It's not just a question of reducing drag caused by the vortices; the energy in the vortices represents excess work done, and so drag. A device which simply destroyed the vortex would arguably reduced the energy in the vortex and hence the drag "caused by the vortex". The nifty trick is to redirect some of that wasted energy in the vortex back into providing something useful. If you were to instrument a winglet appropriately you would really find there was a propulsive force acting. It's not just an elimination of drag force.

747FOCAL
7th Jul 2004, 20:39
Mad (Flt) Scientist,

I personally know Joe Clark, President of Aviation Partners Boeing, I will ask him what he thinks of your claims. I will get back to you. The winglet accomplishes no work on its own without the work of the rest of the aircraft so for you to say the winglet actually produces thrust is either nieve or you just plan do not know what you are talking about. Now the winglet for sure allows the airplane to perform more efficiently, but that is all. By efficiently, I mean climbs better and requires less thrust on approach. The also allow you to reduce throttle in cruise and get the same speed. But if you take the drag reduction equation out, the winglets will not make the airplane faster. :hmm:

john_tullamarine
7th Jul 2004, 20:43
Perhaps the doubters could address the following situations (which appear to me to be similar from one perspective or another) ?

(a) auto-rotating helo rotors

(b) jet engine net thrust analyses (ie, considering the internal flow forces developed on the housings - especially for the intake region)

(c) research which has investigated the way some fish can use body flow generated vortices to provide a serving of additional thrust .... by having lateral oscillations of the tail being at the "correct" frequency to match the passage of vortices shed from the body ...

Not at all trying to be provocative here as my aerodynamicist days are far in the past ... but the issue is of some importance and I wouldn't mind knowing what current thinking is in the backroom trade ....

While I haven't had the pleasure of a beer with Mad (Flt) Scientist, his credentials suggest that we should give him some air to explain his views ... ?

PPRuNeUser0172
7th Jul 2004, 20:45
Mad Flt Sci Fi boy

You clearly have too much time on your hands old chap, someone very succcinctly said it correctly a few post back! Winglets reduce induced drag, by reducing the severity of wing tip vortices and reducing spanwise flow along the wing. This reduction in induced drag will clearly make the engines more effective at any given power setting hence reducing fuel burn etc etc etc.

I think you have to be careful saying that winglets produce thrust, they merely make it more effective.

:ok:

john_tullamarine
7th Jul 2004, 21:01
The question is not so much whether foils can work .. but by what mechanism.

One should note that, in some cases at least, the foil is there either because it looks nice (ie marketing ... a bit like most of the spoilers one sees on the boot lid of many production sedans ...) or the designer couldn't do the sensible thing and stretch the wing.

An interesting topic which, one hopes, will be thrashed to death in this thread ... perhaps some of the site's other practising aerodynamicists might like to wade into the fray ?

Old Smokey
8th Jul 2004, 19:53
There was, and still is, one bizjet manufacturer who stridently purpoted that the winglets produced a forward thrust vector. Wind tunnel analyses that I studied seemed to verify this point, but.....there's no free lunch.

The thrust required to drive the winglet through the air somewhat exceeded the small net forward force generated, the result, a net total reduction in effective thrust available.

On the positive side, the winglets work well most of the time to alleviate drag, thus making the available thrust more effective, but countering this, the B747-400 under certain C.of G. conditions achieves improved Specific Ground Mileage with the winglets removed in about 20% of cases.

Bre901
8th Jul 2004, 23:19
or the designer couldn't do the sensible thing and stretch the wing.
In some specific cases, he just MAY not : standard class and 15m class gliders must have a wingspan less than or equal to 15m, routine checks are performed in competitions.

Another positive effect of the winglet is to reduce the spanwise flow, leading to much nicer low-speed handling and stall characteristics (Discus 1 : stall speed reduced by 5kts).

Thunderball 2
8th Jul 2004, 23:22
I recall that Gulftream was involved some years ago in a hushkit proposal (must have been Spey) that involved a large aft duct shrouding a free turbine connected directly to an outer concentric "bypass" fan. The resultant assembly was expected to meet Stage III noise while reducing SFC and increasing thrust. Don't believe it ever reached the prototype stage.

Maybe this is helpful in understanding the answer to the question that runs through this thread. Would the shroud/turbine/fan assembly itself have generated thrust? No, the engine generates thrust, but the hush-kit assembly captures and organises energy from the engine exhaust that would otherwise be lost, and thereby converts it into thrust which is transmitted to the engine through the husk-kit attachments.

Surely it is the same in principle with a winglet?

747FOCAL
9th Jul 2004, 04:19
Come now people. If you were to mount and infinately lare winglet on an airplane and put it out in a field someplace and wait for a 100 mph wind to come along the winglet is not going push that airplane just because it reduces vorticies at the wingtip. All the winglet would do is lower the wind speed at which point the aircraft would lift off the ground.

:hmm:

ROB-x38
9th Jul 2004, 05:09
More thrust / less drag

Isn't it just a matter of definition?

So taking the blanket statement that a winglet reduces drag - slap one onto a wing in cruise and there is now an excess thrust (or less drag). Has the winglet produced thrust? :confused:

john_tullamarine
9th Jul 2004, 06:10
The foil has a similar effect to increasing span (but with differing effectiveness) .. for the larger widebodies, airport infrastructure dictates the need for foils rather than span increases.

747FOCAL
9th Jul 2004, 06:41
No the winglet did not produce the thrust. It made the airplane more efficiently use the engine thrust is all.

Mad (Flt) Scientist
9th Jul 2004, 16:24
Wow, a popular topic for discussion....who'd have thought aerodynamics could be so interesting....

I think the example that Thunderball 2 gave about the engine noise device is much the same principle. John T's autorotating helo rotors are similar. In both cases the item is extracting energy from the flow to generate a force.

The statement "winglets produce thrust" should not be taken as saying they generate a force without the right conditions. What it means is that at the design condition the winglet extracts energy from the tip vortex - energy which the engines have placed there through overcoming the drag of the rest of the wing - and uses some of that energy to create a small forward force component.

If one were to consider the whole wing, then I agree; the effect of a winglet is to reduce the drag force on the whole wing by some small amount. No questions.

But if I JUST look at the winglet then the local aerodynamic forces acting ON THE WINGLET ALONE are in a slightly forward direction (at the right conditions). You may prefer to call it "negative drag" rather than "thrust", if "thrust" implies a primary source of motive power; I'm using "thrust" to mean ANY forward acting force on the aircraft.

There was, and still is, one bizjet manufacturer who stridently purpoted that the winglets produced a forward thrust vector. Wind tunnel analyses that I studied seemed to verify this point, but.....there's no free lunch. I wonder how far away that data is?

Actually, to extend the analogy, there is such a thing as a "free lunch" - if one is prepared to sift through the garbage discarded by a restaurant, one may in fact assembled a small snack. Winglets are like that; you're using wasted energy from the tip vortex and re-using it. The costs come in things like weight, cost, etc. But aerodynamically they are, indeed, a "free lunch".

Come now people. If you were to mount and infinately lare winglet on an airplane and put it out in a field someplace and wait for a 100 mph wind to come along the winglet is not going push that airplane just because it reduces vorticies at the wingtip. All the winglet would do is lower the wind speed at which point the aircraft would lift off the ground.

Actually, what will also happen is that the plane, if free to roll, would move back more slowly with the winglet because the OVERALL drag would be lower. Clearly some new, extra force is pushing in the opposite direction to the "normal" drag. That force is caused by, and acts at, the winglet.

Of course, the plane would never actually roll forwards, because there isn't enough energy in the air to provide enough force to be extracted by the winglet.....unless....

Put the same plane in the field, free to roll. Install monster winglets and blow high energy air at the winglet FROM THE DIRECTION THE VORTEX WOULD BE. The winglet, acting like a sail, will extract the force it does, as usual. With no forces acting on the rest of the airframe (we only blow at the winglet) which way do you think the plane will roll, forwards or backwards. You all know MY opinion ;)

- this forum needs an "en-garde" smilie ;) -

747FOCAL
9th Jul 2004, 18:20
The winglet acting like a sail is not what we are talking about.

WINGLETS DO NOT PRODUCE "THRUST" IN ANY WAY SHAPE OR FORM. They produce lift and drag reduction is all such that your L/D is better for the given airplane. You can attribute a thrust reduction to the installation of winglets allowing you to fly at a lower thrust to maintain speed without the winlgets, but the winglets are not "thrusting" the airplane to make up for pulling back the throttle.

Keith.Williams.
9th Jul 2004, 18:34
747, your comment that "we are not talking about winglets acting like a sail" is curious in that is a perfectly reasonable description of what they do. We can get an understanding of how winglets produce thrust by looking at how sailing boats manage to sail upwind.

Imagine that we are standing on a beach. We have our backs to the wind and we are looking out to sea. The wind is crossing the beach at 90 degrees and blowing straight out to sea. On the horizon we see a small sailing boat. As we watch, we notice that the boat is sailing from side to side, but gradually getting closer to the beach.

The boat is being propelled by the wind, which is blowing straight out to sea. But the boat is moving upwind towards the beach. By interacting with the wind, the sails of the boat are producing a force, part of which is acting in the opposite direction to the wind. The trick of course is that the boat is not sailing directly into the wind but at an angle to it.

To understand how this is happening we must move along the beach to a point where the boat is moving directly towards us. The bow of the boat is pointing directly towards us, but the wind is no longer coming from directly behind us, but from one side. Because of this angle, the wind acting on the sails produces a force that acts at an angle across the boat, towards the side away from the wind.

Although this force tends to push the boat sideways, the keel of the boat resists this sideways motion, and in so doing deflects part of the force towards the bow of the boat. The overall effect is forward motion of the boat through the water. Because the bow of the boat is pointing slightly upwind, the boat moves slightly upwind.

If we examine the motion of the boat we will find one vector at right angles to the wind and another acting upwind. The surprising thing about all of this is that it is entirely possible for the upwind part of the motion to be far greater than the crosswind part. This means that the boat can sail in a direction that is very close to the wind.

A winglet is in many ways similar to the sails of a boat. By interacting with the air passing over it, a winglet generates a force. The direction of this force is determined by the angle at which the chord line of the winglet is set and the direction of the airflow passing over it. Because of its location out at the wingtip the winglet is in the wingtip vortex. The airflow over an upward pointing winglet is not directly fore and aft, but is angled inboard towards the fuselage. This is the equivalent of the boat sailing at a slight angle to the wind.

Given the correct combination of chord line angle and airflow direction, the force generated by the winglet will act inboard and forward. The inboard component tends to bend the winglet towards the fuselage, while the forward component tends to drive the winglet in the direction of the flight of the aircraft. This forward component of the overall force generated by the winglet is thrust.

In addition to generating this thrust force, the winglet also reduces the overall intensity of the inboard airflow, thereby reducing wingtip vortex strength. This in turn reduces lift-induced drag. This process could also be described as the winglet extracting energy that would have been lost in the wingtip vortex and giving it back to the aircraft by applying a forward thrust force.

The principal problem with winglets is that to be effective they require the correct combination of chord line angle and airflow direction. The airflow direction depends upon aircraft weight, airspeed and configuration, all of which vary in flight. So the fixed angle winglets currently in use can be really effective only within a fairly narrow speed range. This speed range could be increased by using variable angle winglets, but the cost would probably outweigh the benefits.

So why can we not get rid of our engines and simply use winglets. The first problem is that we would need a reliable source of wind to interact with the winglets. Imagine trying to use a sailing boat on a dead-calm day.

The second problem is that the thrust produced by the winglets is far less than the drag produced by the rest of the aircraft. A winglet that could produce 50000 lb of thrust at the start of the take-off run would be, hhhhhmmmmmm, interesting to say the least!

747FOCAL
9th Jul 2004, 19:11
Thanks you said it good. The winglet cannot produce thrust just sitting there by itself. It needs outside input from other sources before it can do anything. So in reality it is generating nothing unless it robs the energy created by something else. :}

Mad (Flt) Scientist
9th Jul 2004, 20:40
I think I used the phrase "extracts energy from the vortex" several times while trying to explain the mechanism.

Thanks Keith, especially for the yachting example which is another good analogy.

Keith.Williams.
9th Jul 2004, 23:09
747,

The statement that winglets produce thrust is not the same as saying that they produce or release their own energy. Winglets, like the sails of a boat extract energy from the airstream and use it to provide thrust. In the case of winglets this energy is of course initially released by the burning of fuel in the engines, but would otherwise be lost in the wingtip vortices. But the action of winglets is not simply a matter of reducing drag.

In an earlier post JT provided another example of this process in supersonic air intakes. It is often said that the convergent-divergent intakes of Concorde produced more than half of the thrust in supersoinc cruise. The engines alone could not produce this much thrust without using a far greater fuel flow.

But if we shut down the engines all of the thrust is lost. The engines need the intakes and the intakes need the engines. Neither alone can produce the extra thrust. But can we really say that the intakes simply reduce drag? No we cannot.

The intakes extract kinetic energy from the airflow and convert it into a form (static pressure) that increases the total thrust produced. But the energy was originally provided by the burning of fuel in the engines.

If winglets simply reduced drag then we could do this in a much more simple manner. Fitting external wing tip fuel tanks for example would restrict wingtip vortices and provide the additional benefit of reduced wing bending moments. Or we could fit simple end plates which would be structurally much more simple and less liable to fall off in flight.

Mad (Flt) Scientist
9th Jul 2004, 23:09
Another example occurred to me:

Almost all fixed wing aircraft have the engine providing thrust along the longitudinal axis, give or take.

Yet somehow all these aircraft manage to overcome gravity by producing a force by extracting energy from the airflow and generating a force - in this case lift.

I don't see anyone claiming the effect of a wing is to reduce the weight of the aircraft, we all accept that lift is a force, despite there being no thrust applied vertically.

So the idea that a winglet - a more-or-less vertically oriented wing, with a bit of toe-out - can generate a similar force, in the forward direction, is not so strange, surely.

Traditionally, forces acting in the forward direction are called "thrust".

palgia
10th Jul 2004, 04:27
Very interesting thread!

I think we have to look at the NET effect that the component has on the aircraft. For example, a wing produces more lift than it weighs, and an engine produces more thrust than drag.
Now the question is, does a winglet OVERALL produce more thrust than drag? (induced drag reductions don't count as thrust) In cruise flight, is the wing dragging the winglet through the air, or is the winglet helping to push the wing (and the aircraft) through the air? How does this change during the different phases of flight?

This is what, IMHO, would determine wherether or not we can say a winglet produces thrust. What do you guys think? I know mad scientist's opinion, and respect it, but I'm wondering if there is any proven scientific data (if there is any such thing) indicating this. (I believe someone must know, because I'd assume that somewhere in the design process of the structure holding the winglet the engineers would need to know whether its gonna have to resist primarily loads pushing the winglet back, or pulling it forward.)

If all a winglet does is reduce drag, I don't think we could considser it to produce thrust although the effect on aircraft performance is the same, and probably greater. In fact, don't you think that there is little doubt that the greatest contribution that a winglet brings is the reduction in drag rather than an addition, if any, of thrust?

As far as the forward component of force created by the winglet, does it overcome the parasite drag created by the winglet itself or does it merely REDUCE the overall drag produced by the winglets by compensating with some amount or forward force?


KeithWilliams,

First of all I thought the sailboat analogy was excellent, I will definately add that one to my bag of tricks :ok:

Not to go off-topic, but regarding the concorde C-D inlet

you wrote:
" It is often said that the convergent-divergent intakes of Concorde produced more than half of the thrust in supersoinc cruise. The engines alone could not produce this much thrust without using a far greater fuel flow. But if we shut down the engines all of the thrust is lost. The engines need the intakes and the intakes need the engines. Neither alone can produce the extra thrust. But can we really say that the intakes simply reduce drag? No we cannot. "

Why not? We definately can't say the intake creates thrust.... rather, I would say they add drag but are the most efficient compromise between what the intake HAS to do and what it can do. From what I remember, the main purpose of the C-D intake is to slow down the air the air to an acceptable intake velocity (about M.5 if I remember correctly). This is a necessity, not a luxury. By slowing down, speeding up, deflecting or otherwise affecting the air in any way you always loose energy. Now in this case, since you HAVE TO slow it down, you might as well do it in a way that increases the static pressure of the air, since you'll have to do that anyway prior to combustion. In other words, its an efficient utilization of what you have (ie. high speed RAM air) to get what you need (slower-speed air with higher density). But don't forget how you got there (at M2.2)... by burning lots of fuel. Only because you NEED that intake doesn't mean it should receive credit for pushing the aircraft.
I would see the intakes more as as an efficient way of performing a nescessary task rather than a thrust augmentation device.


palgia

Keith.Williams.
10th Jul 2004, 10:07
Palgia,

My reason for quoting the CD intake was to illustrate another example of a system whereby energy that would otherwise be lost is recovered and used to increase thrust. Like the winglet, the CD intake does not actually introduce any extra energy into the system, but simply recycles energy that was initially released by the engines.

You are of course correct in saying that the CD intake does not create thrust directly. It actually creates extra drag. But in concert with the engine it increases the total thrust generated. This is different from the winglet, which creates thrust directly.

I am not entirely sure that the use of CD intakes is inevitable, but it certainly increases efficiency. We could for example us some form of inlet lying flush against the side of the aircraft. This would minimise the drag but would not recover the kinetic energy of the air. In effect you would have the required low speed airflow, but at ambient density. The principal problem with this solution is that becuase the intake could not recover the kinetic energy from the airstream, the engine would not benefit from ram effect. The overall result would be a minimisation of the intake drag, but no extra thrust.

Getting back to the winglets, if their only function is to reduce induced drag then the use of tiptanks would have the same effect. If these were area ruled the benefits in high speed cruise would be even greater.

If the objective was simply to restrict the flow of the tip vortex, then a simple end plate aligned directly fore and aft would do the trick quite well. But winglets are not aligned directly fore and aft. Their chord lines are angled slightly outwards. The purpose of this feature is to provide the required angle of attack to generate a forward tilting total reaction, a small part of which is thrust. As MFD stated earlier, by deflecting the vortex flow aft, they create a forward thrust.

montys ex teaboy
11th Jul 2004, 10:02
If anyone out there can get a winglet to produce thrust, then they would become very weathy indeed. In fact they would never have to fly for a living ever again.

Even more money than the person who can get an engine to run on water.

A winglet reduces induced drag. They cannot in any way, shape or form produce thrust.

Keith.Williams.
11th Jul 2004, 14:30
Readers who remain unconvinced about the thrust generating properties of winglets should do an internet search for winglets and see what they find.

The following is an extract of something I found by this means.
The important bit is in the third paragraph. (I'm afraid I haven't got to grips with the process of inserting links yet but you can find the whole item at airspacemag.com).


QUOTE

In 1976, shortly after an energy crisis sent fuel prices skyward, Richard Whitcomb, a NASA aerodynamicist, published a paper that compared a wing with a winglet and the same wing with a simple extension to increase its span. As a basis for comparing both devices, the extension and the winglet were sized so that both put an equal structural load on the wing. Whitcomb showed that winglets reduced drag by about 20 percent and offered double the improvement in the wing's lift-to-drag ratio, compared with the simple wing extension.

The aspect ratio of a wing is the relationship between its span and its chord-the distance from leading edge to trailing edge. A U-2 has a high aspect ratio; an F-104 has a low one. A wing with high aspect ratio will provide longer range at a given cruise speed than a short, stubby wing because the longer wing is less affected, proportionally, by the energy lost to the wingtip vortex. But long wings are prone to flex and have to be strengthened, which adds weight. Winglets provide the effect of increased aspect ratio without extending the wingspan. One rule of thumb says that for an increase in wing-bending force equal to that of a one-foot increase in span, a wing's structure can support a three-foot winglet that provides the same gain as a two-foot span extension.

The airflow around winglets is complicated, and winglets have to be carefully designed and tested for each aircraft. Cant, the angle to which the winglet is bent from the vertical, and toe, the angle at which the winglets' airfoils diverge from the relative wind direction, determine the magnitude and orientation of the lift force generated by the winglet itself. By adjusting these so that the lift force points slightly forward, a designer can produce the equivalent of thrust. A sailboat tacking sharply upwind creates a similar force with its sail while the keel squeezes the boat forward like a pinched watermelon seed.

If winglets are so great, why don't all airplanes have them? Because winglets are a tradeoff: In the highly visible case of the 777, an airplane with exceptionally long range, the wings grew so long that folding wingtips were offered to get into tight airport gates. Dave Akiyama, manager of aerodynamics engineering in Boeing product development, points out that designing winglets can be tricky-they have a tendency to flutter, for example. "We find that it really doesn't matter what kind of wingtip device you use-they're all like span," he says. "The devil is in the details. Span extensions are the easiest and least risky." In the past, winglets were more likely to be retrofitted to an existing wing than to be designed in from the start, but now that is beginning to change. Unlike those tailfins on cars, winglets really work.

Originally published in Air & Space/Smithsonian, Aug/Sep 2001 . All rights reserved.

UNQUOTE

I'm not sure how much Mr Whitcomb or NASA earned from this research, but I suspect that it was not enough to enable them to design an engine that runs on water.

Romeo Tango Alpha
12th Jul 2004, 14:03
Enter the Supercritical Wing... :ok:

Viva La 777, which don't need no steenkin' winglets Stimpy!
http://www.gunboards.com/forums/uploaded/vulch/2004784588_anim_lol.gif

747FOCAL
12th Jul 2004, 14:08
Keith.Williams,

And I suppose just cause I read it on the internet I should assume it is true? :E

Romeo Tango Alpha,

We will see blended winglets on 777s within a few years. Already in the works. They won't be that big though. :)

When it comes down to the winglets producing thrust, they may vector something in a direction that makes it look like thrust, but without the help(WORK) of something else the only pushing they do on the airframe is side to side when the wind is blowing real hard at the gate. :\

Romeo Tango Alpha
12th Jul 2004, 14:26
Well, the 7E7 seems to have blended winglets in the initial design.

Darned thing looks like a shark. Not sure I like it yet!

Kinda like a 777 designed by HR Giger...:bored:

Mad (Flt) Scientist
12th Jul 2004, 15:43
Just because you read it on the internet does not make it untrue, either.

There is plenty of scientifically verifiable data to support the aerodynamic behaviour which has been outlined here. People may choose to believe what they wish, but it will not alter the actual fact of the mechanism at work with a winglet.

You may choose to ignore my opinion, it's no skin off my nose, but I'd suggest you consider carefully something when Keith or John state it. And you could easily look up Whitcomb's research if you were so minded.

747FOCAL
12th Jul 2004, 18:55
Well let's just say your right for sh*ts and giggles.

Does the amount of "thrust" provided from the winglets amount to anything that you can chart vs. what the engines are producing or is it so small it is like hitching an ant up to your car and telling it to pull you to the grocery store?

:confused:

Just got off the phone with somebody that wants to remain nameless, but I can say they market the winglets for Boeing products and also happens to be the owner of the company.....

He says the only way a winglet produces thrust is if they flap and flapping winglets have a very limited fatigue life! :\\

Mad (Flt) Scientist
12th Jul 2004, 20:49
Just did some very rough calcs based some some antique wind tunnel data, so hopefully I won't be shot for using it.

Winglet was found to generate a sideforce (i.e. inboard along the span) of approximately 2% of the total aircraft lift at moderate lift coefficients. The overall drag reduction attributed to the winglet was of the order of 0.2% of aircraft lift under the same conditions.

Some simple trig will reveal that if the winglet "lift" is considered to be angled 1 degree forward of true inboard, the forward acting component would be 2% * tan 1 = 0.03% of the overall lift, or just over 10% of the total winglet effect.

If one were to assume a rather generous 5 degree toe-out effect, then the forward component would be 2% * tan 5 = 0.18% of the overall lift, which would be some 90% of the overall winglet effect.

Now that latter number is clearly too high, and my expectation is that if I were to try to crunch the data to calculate the actual winglet force direction it'd be in the 1-2 degree forward range, which would imply that the forward acting force is worth some 25% of the overall winglet effect.

It would be a lot easier with access to some tip tank type data, but there's none to hand.

So, is it a huge effect? No. Even the overall winglet effect is not huge, after all. But the force effect is a significant contributor to the overall effect of the winglet on the aircraft performance.

....

And if your mysterious friend believes that only flapping aerodynamic surfaces can generate forces, I await with interest his explanation of how a (non flapping) aircraft wing works. :)

john_tullamarine
13th Jul 2004, 02:45
Interestingly, the idea of wingtip flow control devices (excluding endplate devices) did not start with Whitcomb .. it had been a research topic for quite some time. As an undergraduate in the late 60s/early 70s I was exposed to some of the research being done by aerodynamics academics at one particular institution. One such chap tried his best to pressure me into making the subject of my final year thesis the aerodynamics of a rather strange-looking in-cruise device which he had dreamt up over a beer one night ... made that Scissorhand chap's appliance look rather puny ... Indeed, he had an intent, if the research showed promise, to investigate trialling a prototype on an F27.

The Whitcomb solution, though, is a rather elegant (both in the engineering sense as well the marketing ploy used in some areas) outcome of the various research efforts which preceded it.


Actually, what this thread really is saying .. is that we should all do a lot more sailing ... anyone down my neck of the woods is welcome to come along and look at how the sail on a nice little boat works .. see Keith's earlier discussion ... it becomes quite a tightrope exercise when pointing high into wind .. but the fact of the matter remains .. although rig layout and other things exert a material effect, sailing vessels have no great trouble sailing somewhat into wind.

Oktas8
13th Jul 2004, 11:03
JT - I've read a book describing wing tip "sails" or "feathers", much like the outboard primary feathers on an eagle's wings. Is this your Scissorhands device?

I imagine such devices would be easier to rotate to the optimal angle of attack than a winglet, although the reference does not provide quantitative data on their efficiency. FYI it's a book by RH Barnard & DR Philpott.

One wonders what airliners in fifty years will have attached to recover energy from the airflow.

O8

PS - Perhaps this might help clear up some disagreement?

If thrust is defined as a force acting forwards, and if one agrees that there is a small forwards component to a winglet's net aerodynamic force, then there is a thrust force present. Much disagreement here is due to a different definition of thrust I think.

Romeo Tango Alpha
13th Jul 2004, 13:04
I assume (oh dear...) that we are referring to the winglet being canted in (in relative position to the longitudinal axis) towards the fuselage, whereby, because of flow patterns and deflection, it more or less augments the airflow in the area, with the fuselage acting as the opposing augmentation area - kind of like the turkey feather nozzles on afterburning engines. A funnel if you will, or even perhaps a venturi?

If so, yes, there WOULD be a net thrust generated, but how negligable it surely must be! It has to extract energy from the airstream to provide a LIMITED amount of thrust, perhaps at a small increase in FBO because of an increase in parasite / induced drag caused by the winglet.

In reality, I would consider it a "six of one, half a dozen of the other" scenario. The benefits and the negatives surely balance out!

Mad (Flt) Scientist
13th Jul 2004, 13:51
RTA: sort of.

In fact, leave the fuselage out of the equation - the effect of a fuselage on the behaviour of the winglet is very small (consider, for a wing of about 50ft semi span, your winglet might be 3 or 4 ft high - that means the fuselage is some 10-15 times the "winglet span" away from the fuselage, and a good rule of thumb is that to really influence a wing (or winglet in this case) you need any obstruction to be within about one span of the wing; so ground effect is usually important within one span's dimension of the ground, etc.)

The winglet has NO parasitic drag in the aftwards aircraft direction; the forward vectoring of the resultant force takes care of that. And the force isn't negligible; it could be a significant percentage of the overall winglet effect, depending on the flow geometry and the design of the winglet.

mbga9pgf
13th Jul 2004, 14:23
My understanding of this from my aeroeng degree and advanced aerodynamics is that the winglet generates thrust, by extracting energy from the vortex flow at the wingtips. However, this is only so if the winglet has been correctly designed taking an accurate flow pattern into account. Vis a Vis newtons laws are not destroyed; energy is neither created or destroyed. The doubters are also partially correct in my (humble) opinion in that winglets are used to increase Efficiency as opposed to generate thrust, as they also provide a little lift, thus reducing the required incidence and thus marginally reduce induded drag in the cruise. Furthermore, (this is where it gets difficult) the flow triangle of forces taking into account of the extra drag of the winglet actually REDUCES the apparent AOA, thus leading to a further reduction in drag. I believe the savings are less than 1% of operating costs; when translated into total fuel costs for an airline per year however, this saving could run in the tens of millions.

:)
PS, the winglets should also reduce the vorticity on landing and thus reduce any wake turbulence, however, I believe this to be a secondary reason for installing winglets.


By the way, anyone wanting the official line should check out

http://www.fluent.com/solutions/articles/ja133.pdf

which concisely settles any argument.

enicalyth
18th Jul 2004, 07:57
Guys! Any airfoil moving with an angle of attack to the freestream generates lift. Sometimes the lift ain't upward but is forward and is called thrust but it is lift all the same. Take the case of a yacht sailing into wind. You tack, the angle of attack takes effect, the sail bellies out into the wind, the yacht moves forward. Now run out the jib sail. In the narrow gap twixt (slat??) jib and main the air speeds up faster, creating more lift. Even a brick if thrown hard enough flies. Sixteen-ton bricks flew at Binbrook until the 1980s. But you don't need to believe silly old me. Read the texts, go to a wind tunnel, talk to the experts. And to take a leaf out of someone elses book if they don't lift they drag, all things drag, if they don't lift as well (in this case lift means thrust) they don't work.