Thrust generated by winglets?
 

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?

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 for a quite old thread that discussed this topic, and also look here for a specific discussion (the Winglets Design link from the other thread.)

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

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.

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!

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.

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!!

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

Mad Scientist:
Thankyou, I hadn't thought about that angle. Interesting link.

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.

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:

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 ... ?

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:

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 ?

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.

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).

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?

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:

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:

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.