Thrust generated by winglets?

[747FOCAL]

No the winglet did not produce the thrust. It made the airplane more efficiently use the engine thrust is all.

[Mad (Flt) Scientist]

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]

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.]

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]

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]

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.]

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]

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]

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

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.]

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]

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.]

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]

Enter the Supercritical Wing...

Viva La 777, which don't need no steenkin' winglets Stimpy!

[747FOCAL]

Keith.Williams,

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

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]

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...

[Mad (Flt) Scientist]

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]

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?



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]

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]

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]

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.