Winglets - Benefit?
 

What is the average percentage of fuel savings incurred by adding winglets to the 747?

How has the 777 been designed so that no winglets are required?

Grapes,

I've heard figures of 10% being quoted for well designed winglets - whether they actually work is another thing! During my degree, however, the principle has been explained to me and it seems fesable. Only of real benefit to an a/c in the cruise, and thus, better fitted to aircraft with long ranges.

As for the 777, I don't really know why winglets are not used - this is something I have thought about myself. However, it must be appreciated that the 747 wing design is 1960's technology, whilst te 777 is very much later - prehaps thats the key?? Anyone know?

Cheers,

Cuban_8

Currently having winglets installed on the BBJ I fly. Boeing is claiming approx 7% improvement in range. I'm looking forward to making some direct comparisons myself and to see whether this is all hype. Mind you for a mod costing $1.5m thats a lot of fuel to burn before your benefit kicks in!

While I do not know for sure, I suspect the reason they didn't put winglets on the 777 had to do with the fact that it was designed to be efficient in short-haul markets as well as long haul. They increased the span instead, which nets benefits in both short and long haul, as opposed to the long haul benefits of winglets. There has been talk of adding winglets to the 777 for future aircraft designed exclusively for ultra-long haul.

As to the new super-critical wing, the 777 has it, but then, so do the A-330/340, etc., and they do have winglets as well.

I believe 3% is nearer the mark.

If it was as high as 7-10%, would it not then pay for itself to retro-fit all old designs?? (on commercial aircraft at least)

GJB,

Agree totally with your point. The fact is that winglets are an experimental development by the manufacturers. Thus, the likelyhood of obtaining any conclusive information on their performance from them is slim!!

I remember reading an article some months ago (in Airliner World or something) that showed a BA 744 operating with one winglet! From that, I imagine that the effect of the winglet could not be very significant!

Cheers,

Cuban_8

I don't have the numbers handy, but for the MD-11 the lack of even a lower winglet adds a fair amount. Sure, you can operate that way, but you must take into account the increased fuel burn. The numbers are well known and contained in the configuration deviation list (CDL) which is FAA approved in the U.S. by the FSB and FOEB process. The MMEL is also created in this way, and these docs are adopted by most operators around the world.

From memory the MEL permits removal of winglet(s) off the B744 for dispatch with a minimal fuel penalty. And the marketing of the component has always been that it stops spanwise flow and spillage of air from the airfoil to reduce wing tip vortices, but perhaps the biggest driver for thier use is keeping up with the competitor (airbus) and that their use is more for cosmetic rather than truely performance reasons - ie manufacturer seen to be updating a design?



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Its life Jim, but not as we know it!!

Winglets are only useful in the long range cruise. But what about the approach? Do they actually reduce wingtip vortices at all? Interesting to note that the 747-400SR(short range) does not have these winglets fitted- so i'm guessing the cruise length would be the determining factor.

JL,

Winglets (or wingtip sails) do not actually reduce the strength of the trailing vorticies. The vortex strength related to the wing lifting vortex, which in turn is related to the lift generated by the wing. There is nothing that can be done to reduce the trailing vortex strength - bar changing the shape of the wing planform.

Winglets take advantage of the local flow induced at the wingtips by the trailing vortex and use it to both create extra lift and create thrust (believe it or not!). This is how the extra efficiency is achieved. Further, they operate best in steady flow conditions, and thus, are best installed onto aircraft that cruise for long periods of time. For short range aircraft, I can't really see a point in installing them.

Although I have no doubt that they do work to a degree, I'm inclined to agree with tagaroo in that they are probably there for show as much as function - especially when you consider the fact that SOP allows individual sails to be removed. However, I am not an airline pilot, and I may very well be totally wrong!

Cheers

Cuban_8

Cuban_8 - good post. I have been wondering about the benefits of winglets for a while (being a PP with less than 100 hours, I still want to learn everything! :))

I was wondering - how do the winglets improve thrust? For that matter, can anyone explain the physics behind the winglet? What the advantage is, why it works to increase lift (I have an idea) and thrust (lost on that one)?

Thanks! =)

Plap;

At the wing tip the pressure difference between the upper and lower surface MUST be equal to zero, this means that there is a varying pressure distribution along the wing span, hence the formation of the trailing vorticies. Consequently, the flow has a velocity component along the spanwise direction (towards the fuselage on the upper surface) therefore the local flow direction at the wing tip is NOT perpendicular to the mean free stream flow but acutally flows inboard. Winglets, or wingtip sails, attempt to take advantage of the fact that the local flow direction at the wingtip is not perpendicular to the free stream flow. Acting as small aerofoils, there is an effective angle of attack between the local flow direction and the winglet. This leads to a resultant aerodynamic force with a component in the direction of motion - hence the 'thrust' or reduction in drag.

Hope this helps,

Regards

ASI

[This message has been edited by ASI (edited 08 March 2001).]

To: Togaroo

I quote: “but perhaps the biggest driver for their use is keeping up with the competitor (Airbus) and that their use is more for cosmetic rather than truly performance reasons - i.e. manufacturer seen to be updating a design”.

Boeing designed the winglets used on Airbus aircraft under a NASA contract. The design was offered to Airbus at no cost but if Boeing or anyone else wanted to use them they would have to pay a stiff licensing fee to offset the cost of development. That is why only Airbus uses the design. I can only assume that wing tip sails were installed on Boeing aircraft because there would be some operational benefit and not for cosmetic purposes.



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The Cat

Lu

My response was not to generate a Boeing / Airbus slanging match - I was brought up on the seattle metal and prefer the product, my comment was to illustrate why things are sometimes done for marketing rather than pure technical reasons. So if the fences designed by Boeing and used by Airbus were to improve performance greatly then everyone would be using them now. However Airbus is quite clever at exploiting new technology as a part of thier sales pitch, whereas Boeing prefer the tried and tested method.

However the thrust of my post was aimed at the B747-400, essentially a tried and tested product, but with a few enhancements. How to distinguish this product on the jet way as a good marketing strategy (from the 'classics') - add winglets. The reduction in drag 'in cruise' performance another justification for inclusion on the airframe, especially since the MTOW had increased through the use of horizontal stab fuel tanks.

I was not aware of the history of the design of the winglets, but wanted to point out that around the time of their imergence on the A321 (could be wrong that this was the first commercial jet airliner to use them) other manufacturers may have used wing fences or winglets to update the image of the product ie B747-400 and the conglomeration that is the MD-11 (I apologise if I offend anyone about the MD-11)

As my friend Cuban has stated the wing fence or sail has proved its performance in cruise are worth while for certain designs, but as the new wing of the B777 doesnt include them then why werent they included?

Like most things in life an aircraft design is a compromise of many factors - to include or omit certain components are not always dictated by common sense!!

I quite like the look of a winglet - but the BBJ is taking a bit far!!!



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Its life Jim, but not as we know it!!

Speaking to a techie in the hangar at Boeing field he told me that the new 767-400 wing tip is the state of the art as far as drag reduction goes. Forget your winglets especially the 747-400, they are not very beneficial. 3-6% on the BBJ unless its very heavy then it tends to twist the inboard root of the wing negating any benefit. They are working on it though! But next time you see a 767-400 check out the last two or three meters of the wing, it's very different.

Wingtips prevent the large downwash (created by the vortices induced at the wingtips) immediately behind the wings from forming thus keeping a low effective angle of attack.

They might not prove beneficial on the 777 because although they reduce induced drag, they increase parasite drag. It might not be worth it.

I don't understand why boeing would introduce winglets on a very long haul 777. Induced drag decreases with speed and in cruise, the aeroplane is moving at it's quickest. Go figure http://www.pprune.org/ubb/NonCGI/confused.gif

Cuban is correct about how winglet work, the other posts are not. They are designed to be specifically optimized for the cruise configuration. On the MD-11 they reduce fuel consuption 3-6%. Not sure on the 747-400, but I doubt very much they are just marketing.

Although a super-critical wing does not need them as much, they still would help for long haul. They aren't that useful for short haul because you have to choose one flight regime for them to work, and they will actually add drag in other regimes (unless you designed adjustable winglets!).

Consequently, aircraft that are designed to do well in short haul markets have to find other solutions. The 777 used longer wings as well as super-critical airfoil shapes.

Historical footnote is that the MD-11 was originally conceived to have super-critical wings also, but MD chose not to allocate the funding that would have been required. That turned out to be a good thing for the 777, because an MD-11 with super-critical wings would likely burn less fuel for a given sector than a 777 and not be ETOPS restricted. Couple that with the higher payload capability of the MD-11 and we might be in a different world today.

Ceppo,

I'm afraid that there is not much one can do to reduce the strength of trialing vorticies behind an aircraft - and you certianly cannot do it through the use of winglets! Winglets use the vortex pattern shed off the wing to increase airframe efficiency. Further, the downwash angle induced by the aircraft is directly related to the lift produced by the wing - reduce the downwash, reduce the lift produced!

My own thinking with regards to the 777 situation is that due to the increased wing efficiency over that of the 747 or 767, the company has calculated that the net benefit of winglets would be marginal or negetive.

I have been meaning to ask my departments Prof. about this for a while - he's an international authorty on wing design and might be able to sort out our problems!!!

Regards,

Cuban_8

Prof2MDA;

While I agree with the comments made by Cuban, as an aerospace engineering student - 3rd year university -, before making my previous post I referenced current course materials from a once NASA Ames Research Centre associate, so as to describe the physics of winglets.

Without meaning to be funny, I would therefore be grateful if you could explain to me the mistakes made in my previous post.

Regards,
ASI

The current winglets on the 747-400 are more for show than anything else. Call them the golden rivet that allowed for changing the way that Boeing certified performance for the bird. The BBJ actually only gets about 4.5% to 5.5% depending on CG and other conditions. Low speed aero performance is enhanced on most aircraft so MTOW can be greater out of runway limited airports. They also have some noise benefits. The new raked ones are the "tweak of the week" at Boeing. They are no more efficient than a winglet, in fact they do the same thing: increase wing span loading further out from the plane. In rear engined planes at heavy aft CG they can cause problems in stall recovery. If they could design them to last and not fall off the most effiecient wing design known to man would look like the outer part of a turkey wing when inflight. You will find that trim drag has a far greater affect on cruise performance than those winglets.

ASI, sorry, shouldn't have made the blanket statement that only Cuban's posts were correct. I was referring to the posts that implied that winglets would eliminate vorticy.

As to the mention that the BBj "only gets about 4.5% to 5.5%" more efficiency out of winglets, obviously someone has missed the fact that manufacturers will do what they can to tweak 0.5% improvement, let alone those numbers!

As to the 777, I stand by my original reasons for it not having winglets...so far.

Obviously some good brains on this thread, so can I take my previous post one step further.
How will the new 767-400 wing tip reduce drag compared with older designs?
Thanks.

Sorry for the length of this reply...hope you find it interesting...

I have been hitting this subject quite hard over the last while...for the -400 the figures quoted are a 3% increase in cruise efficiency. "Winglets provide a theoretical straightening of the wing vorticies" My understanding is that winglets are a compromise...the same benifits would come from a re-designed wing (777, 767-400) or by an increased span. The 747-400, A340 & 330 all have large wings and are reduced in design philosophy to the size of airports and there gate design...if the wing is too big it will not fit into the available gates which would make it useless...no sense in savings by the wing if it cant fit into an airport. The 747-400 used winglets to save in cruise so they would not need to grossly increase the span. Each wing is increased in span by 6 ft and then has the addition of the 6 ft winglets, though this is generally for cruise the winglets actually increase the a/c's wingspan for takeoff with a full load of fuel. The wing bends or flexes with a full fuel load and will increase the span from 211.5ft to 213ft, a small but effective increase in wing for t/o.

The 777 wing and I am sure the 767 wing are obviously unique. They are very new and have been completely re-designed. Boeing took the 757 and 767 wing and made more improvements on it for the 777 which is the most efficient wing ever designed. The 777 wing is fatter than most conventional designs and was able to take advantage of stronger materials for its build...which I am sure is where the difference comes from now that I think about it...the 777 and generally boeings a/c philosophy seems to be that they will use the more expensive, modern materials that are stronger and would allow for differing designs...the B 777 has a very hight "taper ratio", chord length from the root to the tip. This was and still is not normally used as it requires a very strong wing, and use of very expensive materials to create a stable wing tip...that wont move and flex beyond an optimum range. (all wings will flex and are supposed to but too much would bend and twist the wing, disruppting airflow.)....so without going on for too long, the 777 and 767 are using a different disign which requires more money to build and a higher a/c cost, I am sure that Airbus is using winglets to get similar results at a lower cost for competition reasons. Also just for interest sake the 777 was designed with an optional folding wing. The a/c was designed initially to fit into gates that were used by the DC-10, L1011 and the like...so to fit the a/c into smaller gate sizes or to allow the company some flexibility in its routes and use of gates the 777 was given this folding design like a carrier a/c...that I know of as of yet none have been ordered with it.

I see no point why winglets should be most efficient in cruise; as I remember my aerodynamics lessons winglets are best at reducing induced drag which is increasing with decreasing speeds and increasing angles of attack.

Also, winglets are not only useful for long-range airliners; just look at the canadair regionaljet.
A look at my Airbus 320 operating manual shows that with ONE wingtip fence (small winglet) missing, the fuelflow increases by 1.4%, and the take-off weight as well as the approach climb limiting weight has to be adjusted by 4 %.
Quite a change at the low-speed range, I think

In addition to the above, rumours say that Boeing 777 will be available with winglet retrofits for increased efficiency, just like the Boeing Business Jet.

www.aviationpartners.com has more information about performance benefits with winglets

Max,

Winglets don't _have_ to be more efficient in cruise, but as they are fixed, they are set so that they are creating forward thrust most efficiently under just one flight regime. The manufacturer can choose what flight regime that is. In the flight regime that is not optimized you can actually lose performance.

The RJ obviously has been been optimized for some flight regime, or maybe a bit inbetween (sort of like a fixed pitch prop, it is ok for climb and cruise, but not best for either). Perhaps they figure a small amount in both regimes will net more in that airplane. However, a 777 does either short haul or long haul on any given flight, and you would have to choose which one to optimize. Since the same airframe might alternate from day to day, the other solution is to increase the span with the super-critical wing, as they have done.

Are you sure that the perf penalty for a wing fence missing on the A320 is 4% for the approach climb gradient? Sounds a bit much to me. The missed approach climb gradient at Salzburg is only 3.9%, for contast purposes only!

All IMHO.

Do any of you guys know of any books or websites that illustrate the effects of winglets in different flight regimes?

Thanks for all the indepth replies too folks. My minds soaking it up like a sponge!

Prof2MDA,
I wanted to stress that in my opinion winglets are of most use at low speed, not in high-speed, as some earlier postings indicated. Somebody also stated that Airbus uses them merely for proper looks; if that's the case, I wonder why Boeing's 737's are now available with aviation partner's winglets, they sure are of some use...

Jambo Buana,
by the 4% penalty I was not referring to the climb GRADIENT, but to a calculatory adjustment of the approach climb limiting WEIGHT, which is with G/A thrust, landing flaps and gear still down.

When You mention Salzburg Airport I hope You already had a chance to enjoy the view onto Salzburg Castle...

Cuban,
Yes, the downwash does give the aircraft lift however too much whilst the aircraft is travelling forward at 500+ mph isn't particularly good.
Draw a picture...The effective angle of attack (AoA) increases with downwash meaning that the lift is not perpendicular to the direction of flight but is actually pushing the aircraft upward and backward. (I only wrote that because it says on your profile that you're a student so you'll know what i'm going on about)


Also, winglets reduce the effect wingtip vortices have if nothing else but again this would be mainly at low speeds where the induced drag is at iits greatest.

Not a one of you got it totally right. Putting winglets on the 777 will do nothing for the airplane regardless of what Joe Clark says at aviation partners.

Their are no performance benefits with Raked tips over conventional winglets. LIKE I SAID IT IS JUST THE TWEAK OF THE WEAK AT BOEING.

747FOCAL,

so why did You guys at Seattle sell those 747-400's with winglets ? to give them a certain "new look" ??

Although I like old cars, I prefer advanced models for daily driving..

MAX,

If winglets are optimized for low speed flight they will produce the most benefits at that time. However, if they are set at an angle to gain the most forward lift vector ("thrust") at normal cruise, then at low speeds the angle will likely be such that the winglets have a neutral contribution, possibly worse.

Rather than debate endlessly here, I point you to page 339 of Flightwise, Principles of Aircraft Flight by Chris Carpenter (available at http://www.amazon.co.uk but not on the U.S. site for some reason). Chris is head of Aerodynamics at the RAF College, Cranwell.

It is also discussed in "The Illustrated Guide to Aerodynamics" by H.C. Smith, 1992 Tab Books, ISBN 0-8306-3901-2, pages 286-288.

Also, to those who make the very silly assertions that Boeing adds winglets for marketing purposes, I ask you one question: If that were true, why are they NOT on the B-777? Surely they wouldn't want to give the A-330 the advantage, would they?

The answer is as I have previously stated. Expect to see them on some future "long range only" 777 if there is demand for such a beast.

The 747-400 gains a 2.5% drag reduction on average with winglets.

They get you 3% fuel burn reduction in cruise. Otherwise she is an old girl without much new technology. The 747X will be radically different. I wonder how pilots are gonna feel about 175 kts landings and 200 kts rotate speeds....

Those speeds are just a few knots higher than MD-11 speeds, but I have to wonder why the need for such high speeds? The A-380 lists surprisingly low speeds.

As to how pilots feel about it, as long as the performance numbers show adequate margin for the runway, I have no problem with it.

for some reason us yanks have a hard time getting the same low speed aero performance as airbus but we get em in cruise.

Prof2MDA,

Thanks for a number of good and technically accurate posts. It is nice to see that some people still believe in having a firm technical understanding of a concept before attempting to preach about them!

I would suggest that posts like 747FOCAL's are made with a certian degree of ignorance. Do you have and particular merit on which to state you claim that winglets are useless, or are you merely providing us with your well informed opinion?

Cheers,

Cuban_8

As the B777 is a completely new design, the addition of winglets may have no additional benefit especially with a new wing design? I'm not a designer and havent looked into the design of the B777 wing compared to the B747 so will take a backseat on the technical merits of this discussion however I would like to respond to the marketing side of things.

My comments about marketing were to highlight that although something might not be a great idea technically, it might be incorporated because of other reasons. Perhaps a design is a retrofit on something that has gone through a design freeze or the customer wanted a specific cruise perfomance or penalties would apply and the manufacturer could achieve this through a certain addition in design. Or perhaps other tangible benefits of a component may be sufficient enough to include it in a design even though it may not have a significant technical benefit - ie it makes it look different or adds and aesthetic value? No decision to include a component is done in isolation - everything that goes into or on an aircraft is a compromise.

As this thread has developed it is apparent that the inclusion of winglets for different phases of flight is debatable and very much related to the mission of the aircraft and the benefit - 'percieved' by the manufacturer and the operator.

Perhaps someone from Boeing can comment why winglets weren't included?

scuffle, scuffle - returns to backbench to after putting two pence in.



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Its life Jim, but not as we know it!!

200kt rotation? 175 landing speed....Wow...that'll leave the 747X up a certain creek without a paddle against the A380

It seems for some of you die hard fanatics that 'if it ain't boeing...you're not going?' then you might start having to enjoy cruises from soon onwards

CEPO-You obviously do not understand aircraft performance. Some of the older duffers around here are forgetting that things advance. The new supersonic airplanes they are designing all have rotate speeds in excess of 350 kts. Think of how afraid a worldwar 1 pilot would feel if you took him for a scare ride in an F18... He would think it was crazy. You must think that aircraft are always going to perform the same way and never look beyond that which you have seen for years???? We wouldn't even have airplanes if nobody reached beyond what they had been taught was impossible. Those people that think of themselves as experts close their eyes to all that MAY be possible. That fatb*tch A380 is going to make one hell of a frieghter, but it will never pass safety tests for passenger evac. Okay, here we go, jump down that 6 story safety slide and we can all break our legs together.

As for the rest of you that are reaching for why the 777 does not have winglets. That is one of the most advanced wing designs ever constructed. It was optimized so much that putting winglets on would realize you undetectable amounts of climb performance and very small benefits in cruise. There would be more in looking at drag reduction elsewhere on the bird.