winglets
 

Greetings for all!

This may be the most asked question or not! but why Boeing 777's do not have have wingtip devices? but 737s winglet program caught like a fire for NGs. I know the latest are designed with ranked but not as popular as NG's original or retrofitted 6 feet winglet. Any ideas?

PS: as the reason behind the wingtip devices is to reduce the induced drag and especially long haul flights, The aeroplane which can virtually connect two cities in the world as they say doe not have any. this itches my head!

Winglets offer the best advantage if the wing design is a little long in the tooth. With modern wings winglets are much less likely to be worth their weight and cost.

Of course a really modern design that has been cocked up - especially in the tip area - could also show some payback.

Boeing 747-400 being a good case!

Or the A320 for instance *cough cough cough? - oh wait, excuse me, those are sharklets, obviously something completely different :D

STBYRUD
 

My point exactly. Since the A320 first flew in Feb 87 I would suspect the aerodynamics goes back to perhaps 1982. Thirty years is a long time ago in the aero business.

Hope your cough gets better soon.

Golly Brian. That is outside my pay grade. I have read that the most modern concepts for wing tip design try very hard to deal with the issues that winglets address regarding flow round the tip. Just look at the 787 for example.

However I do know a bloke who is s hot on such matters so I will try and contact him.

I am relying on old knowledge here, so go easy. Retro fit winglets add a substantial amount of weight when installed on older wings (especially at the wing root), so their cost benefit diminishes. Newer wings have smoother transition zones, reduced tip chord length, and flex more, something winglets addressed in the olden days.

Ah, progress.

Aspect ratio for one I would guess, and modern CAD design. The triple seven wing chord at the tip is quite narrow and is also quite thin. Winglets although have given way to the raked wingtips on more modern designs.

winglets add a substantial amount of weight

.. and can be expected to have an adverse effect on wing fatigue properties due to the additional loads.

How large is their effect on fuel savings? I remember someone saying that the bigger reason is that PAX love winglets and it improves their perceived quality of the fleet. True or BS?

Winglets do several things, sutch as lengthining the wing from an aerodynamic standpoint. Wing flex and rake are more effective as you see boeing evolve from the 777 - 787 and 748 wings. Having said that they are generally a great mod to older designed wings but some argue that it takes a longer leg to really see the profit from them.

To answer the OP's question, the 777 has a raked wing tip (as does the 787 and the 747-8) which does the same job, ostensibly, as a winglet. What it does is change the way the vortices roll off the end of the wing, so the winglet is not necessary.

For the 744: Only one winglet allowed to be removed and IIRC the MEL only refers to a reduction in performance limited weight, something in the region of 9.5T and an enroute climb performance penalty of about 4.5T.:8

777-300 and -200LR have raked wingtips. 777-200 has the standard wingtip.

5-6% fuel savings is the number I've seen (737-800 and 757-200 w/RR's)

Is the 737-800 wing a newer design then the 777's? I'd guess so. So it's not necessarily just a older wing that would benefit from a winglet.

787 wing flexes about 12' from static at 1G. That, along with modern design and materials, has to help with reducing wingtip drag.

It's a 757 wing that somebody put on a copying machine and punched in 70%.

And why, when a winglet is removed, does the airplane not roll uncontrollably if they are so effective?

The 747 wing droops when full of fuel and is substantially higher when low on fuel. Because of the sweepback, the winglets must be out of the airflow direction when either drooped or raised, and only streamlined when at mid fuel. There must be a drag penalty at the beginning of the flight and at the end, so are they really any use?

Or just look good.

I've never understood the high price put on them. Certainly they reduce drag, increase range, but trying to get half a million for them, always seemed a hard sell to me. For the most part they are a bolt on item.

In the really old days they just put a fence about halfway down the wing. Ala Hawker 700 and G-1159. eventually the winglet will go the way of the fence.

The largest gain for the 737 was in 2nd segment climb I believe.

They are certainly not bolt on items. The 767 APB winglets also require the retrofit of massive reinforcement straps inside most of the length of the wing to support the extra weight and forces generated by their addition.

Fuel saving in order of 5-6%. Saving comes from less fuel needed for trip due to drag reduction, and less fuel needed for carrying the additional fuel not now needing to be burnt - and all despite carrying an additional 1-1.5T of winglet around.

Yes, they are serious mods. But they are beautiful. More so than Vortex generators, Strakes, Fences or conformal tanking. Aerodynamics demands of engineers to be artistes, by default, no offense.

Brian Abraham
 

I have contacted 'my' man.

He has explained to me that I have rather oversimplified things in my post about long in the tooth wing designs.

I will not try and paraphrase what he said because he has promised to post something in a few days.

JF

Also produce increased runway performance.

Nice, but they make the plane really 'slick'. The first landings with the winglets are when you really notice the drag reduction. After awhile the slickness becomes the new normal.

Reduction of "effective" span, hence a slight decrease in "Ground Effect"?

Article in the Wall St. Journal today: "Air War: Winglet Verses Sharklet". The wingtip attachment called "blended winglets" is patented by a small Seattle company by the name of Aviation Partners, Inc. They own a 55% share of a joint venture with Boeing to market "blended winglets" to buyers of Boeing jets. 3,500 Boeing jets (100+ airlines) are equipped with winglets.

Aviation Partners had been in discussions with Airbus for 5 years to set up a similar arrangement although Airbus was working the "sharklet" design. Airbus presented the sharklet design to Aviation Partners and they said it infringed on their patent and asked for royalty payments. Instead, Airbus has filed suit in US Federal court to invalidate Aviation Partners' patent which is 18 years old. Airbus claims they are at a competitive disadvantage if they have to pay a royalty fee.

It will be interesting to see how this plays out.

How can AB claim a disadvantage that amounts merely to making less money from another's "Prior Art"? Then again, I think the patent on the "Aileron" lapsed long ago, being an improvement on "wing warping", etc. Yes TD, this will be fun.

And we will both have to pay more for a ticket :ugh:

Darn it.....all along there was me thinking that winglets were to allow for the use of a smaller hangar. Clearly shows what an expensive waste of time my aero-eng degree was all those years ago......

Well I've had extensive experience with and without winglets on 747 all models and 737 200-NG. Although they undoubtedly give lower fuel burns, I have never felt any difference in handling qualities. It wasn't unusual for 747s to fly with only one winglet. Nobody would know unless they looked. Flying 737 300 (without) and NG (with), I noticed no handling difference. I guess I must have been a rather insensitive pilot! I always thought they made Boeings look fantastic and sleek.

Their benefit is cruise only, which is why really shorthaul operators don't invest in them. I've studied the Airbus 'sharklet' design and can only conclude myself that they are having a joke. The Airbus and Boeing designs don't resemble each other in the way that Tristars and DC10s resemble each other. They resemble each other like mirror images do! After that absurd little triangular plate Airbus used to stick on the end of 320-100 wings, we were told they weren't needed anymore, and now they come up with a superficially identical-to-Boeing winglet after all these years. Very peculiar.

I liked the documentary about the Boeing factory and the guy only known as 'the winglet guy'! Works on his own, nobody knew his name, doesn't talk to anybody- just comes in when needed, hammers the winglets into position with a large mallet, and goes away!

They do more than just enhance cruise performance. Boeing Aero number 17 has some further reading if you wish to look.

Doesn't Boeing have any patents on this? Where are the Seattle lawyers; or are the patents expired?

ross M,

Read my post #26 regarding patent ownership. So far, Boeing has been silent on this. The original patent probably has been updated that gives it more life.

Notso Fantastic,

Southwest Airlines in the US tends to be more of a short haul, regional airline, but does have some longer flight segments. They have a fleet of 550 Boeing 737s of which more than 80% are equipped with blended winglets. They are also the most profitable airlines in the US, generally. I am sure they believe every little bit helps and reducing fuel consumption is on top of their cost savings list.

TD

One benefit I have not seen mentioned is the added stability winglets add to a maneuvering a/c in all flight segments. Stability is money in the bank, see: Autoflight.

Boeing Aero 17 -
Gross fuel mileage improvement with winglets was recorded in the range of 4 to 5 percent. Taking into account the weight of the winglet and the related wing structural modifications, the net performance improvement was approximately 4 percent for long-range flights (fig. 6). Low-speed testing showed a significant reduction in takeoff and landing drag and a significant benefit in payload capability for certain operations (fig. 7).


Runway performance improvement, second stage climb improvement, cruise improvement, approach drag improvement.

I believe it also improved climb performance.

Hi, I am John Farley's 'winglet man'. I have direct experience in winglet design at a major airliner OEM and also for light aircraft and sailplanes, amongst other aeronautical adventures.

Before getting into specifics, let's first consider what the winglet is doing, aerodynamically. So, a thought experiment - you're sitting in a chair somehow suspended in space at 5,000' (the height is unimportant, but let's say we're high enough to be out of ground effect, which really kicks in a heights less than half the wingspan of the aircraft). The atmoshere is still. An airliner flies by, before disappearing off into the distance. You can't hear it anymore, but you can feel the air around you moving, having been disturbed by the passage of the aircraft. What's going on?

If you could see the air, you'd be able to see a general downwards motion in the area where the aircraft flew, and a gentle upwards movement to either side of this area. You might notice that the velocity of the air moving downwards is greater than the gentle upwards moving air at either side - and in fact here is our first finding - the net vertical momentum change of the air disturbed by the aircraft must equal the weight of the aircraft. Newton's second law and all that. You will also have noticed, with your air-x-ray specs, that there are two powerful horizontal tornadoes roughly where the wingtips passed by - the 'tip vortices'. These were caused by the airflow at the wingtip rolling up, moving from the higher pressure side on the lower surface around the wingtip, to the lower pressure side on the upper surface. They are an inevitable by-product of generating lift.

Now, if we somehow knew the veolicty of every air molecule disturbed by the aircraft, in both vertical and horizontal directions, we could do some sums. We could calculate the net vertical momentum change - which will equal the weight of the aircraft. The net horizontal momentum change, assuming we've done our sums correctly, and the airliner wasn't sideslipping, should be zero. Let's also work out the kinetic energy of the air in this y-z plane (y being the horizontal direction, and z is upwards), which is 1/2 * m * delta-v^2, summed over all the particles affected. It will work out to be some value. Next, consider what happens if another airliner of the same weight, but double the wingspan, passes by. The net change in vertcial momentum of the air will be the same (same weight), but a greater volume of air behind the wing is affected, because the wingspan is larger, so the vertical velocity change ('downwash') is smaller. Therefore when we calculate the kinetic energy in the y-z plane again, we get a smaller number. Cool!

Any aeroplane that moves along through the air causing less kinetic energy behind it, all other things being equal, must have lower DRAG. Specifically induced drag. That's our second finding - and one that glider pilots have known since the 1930s - that There Is No Substitute For Span. The greater the wingspan, the lower the lift-induced drag. And considering that the induced drag of an A340 cruising along at FL370 is 40% of the total drag, it's an important item to minimise.

We have't mentioned winglets yet, have we? Don't worry, they're coming.

If span is so good, why don't airliners have very large spans, say of 100m? Well, we'd need bigger airport terminals and taxiway spacing for starters, but the other reason is that as the wingspan goes up, so does the wing weight, for a fixed wing area. So we have drag going down with span, and weight going up - at some point there is a 'sweet spot', where things are optimum. A slightly greater span means the induced drag is a little less, but carrying around the additional wing weight isn't worth it in terms of Direct Operating Cost, which is what the airlines care about.

It is common for this 'optimum span' to be larger than what the airports can cope with, and for the A380, which was designed to fit within an 80m x 80m box, the optimum span was actually somewhere between 82 and 84m, although the DOC-span curve was pretty flat between these values. So - how do we 'involve' more air in the generation of lift as our aeroplane flies along? What about bending up the wingtip? We could even call it a winglet!

Let's go back to our chair again. This time an airliner flies by, same wingspan as the first one, same weight, but this one has winglets fitted. The air still gets a net 'push' downwards to equal the weight of the aircraft, but this time the tip vorticies are slightly weaker and in fact, slightly larger. Same overall vertcial air momentum change, but lower kinetic energy. Each winglet has 'diffused' the powerful tip vortex vertically, along the height of the winglet, and in fact the designer of that winglet probably tailored the aerofoil shapes, chord and twist along the winglet to generate a particular level of lift at each station, (the 'lift distribution') in the cruise condition.

So, lift distributions then - every first year aeronautical engineer undergraduate will be taught that for minimum induced drag of a flat wing (no winglets), one must achieve an elliptcial lift distribution. Some elegant maths will be produced with pi and other numbers, good stuff. But, what's the optimum lift distribution for a wing with a winglet? Or a 'wingtip fence', like on the A320? We need to go and read two 1960's NASA reports: (1) A 1962 NASA Tech Report R-139, by Mr. Clarence Cone 'The Theory of Induced Lift and Minimum Induced Drag of Nonplanar Lifting Systems', and (2) A 1968 NASA Contractor Report CR-1218 by Mr. J L Lundry 'A Numerical Solution for the Minimum Induced Drag, and the Corresponding Loading, of Nonplanar Wings'. These give us these optimum distributions and the induced drag reductions we can expect to gain, for any configuration of 'wingtip device'. No magic there, just sound maths and a brilliantly simple way of finding the answer without today's computer programs.

The reports show that the optimum lift distribution for a wing with a winglet, for the same overall lift, has slightly lower lift at the inboard end and significantly more lift at the outboard end, plus some inwards-pointing side load on the winglet itself. Interestingly the winglet doesn't directly add to the lift very much - because it's close to vertical, it's 'lift' has only a very small upwards component - but the 'blocking' effect out the outer wing increases the lift there considerably. The overall effect is larger, less intense wingtip vorticies, for the same overall lift - same vertical momentum change, lower kinetic energy.

Back to the original question regaring why some modern aircraft have winglets, whilst others (B787, B777) don't. I see this as really two questions:

1. Why do some new-design aircraft have winglets, whilst other new-designs do not?
2. Why do some in-service aircraft sprout winglets?

I think the second question is easier to answer, so let's do that first. Company A designs and builds a jet airliner, without winglets. Everybody tries their best during the design and stressing of the wing, but when designing details like stringer and skin thicknesses, some conservatism inevitably creeps in. We can't have a LIMIT:ULTIMATE factor of less than 1.5, can we? So a bit of rounding up goes on here and there, with the result that when the wing is tested, it breaks at say 156% of LIMIT load. Everyone feels relieved and goes for a beer. The airliner goes into production, and a few years later, some bright spark in the project office works out that the extra 6% 'fat' in the wing might be used up by sticking a winglet on, along with some local strengthening at the wingtip (and whereever else is critical for those additional bending loads). Remember that the increase in bending moment at the wingtip is infinite - we had zero BM before, now we have some. There is a surge tank out there, with minimum wall thicknesses, but it will need reinforcing. A few hundred lbs of reinforcement goes in along with the winglet, the LIMIT:ULTIMATE is still just above 150% - on paper - there's no need to break another wing since we can clear this modification 'by analysis', and hey presto we've reduced the induced drag by, say typically, 5%. Minus a bit of extra wetted area for the winglet, and our overall drag standard is still 2% better than before (bigger gains for bigger winglets). Everyone's still happy, and more beer is drunk. Marketing types invent a name for their winglet, to differentiate it and make it 'special', calling them 'sharklets' or 'advanced blended winglets' or whatever. This is my take on the BBJ/737-NG, 757WL, 767WL, A320NEO and others.

New design airliners are slightly different, because the winglet has to 'pay' for itself from day 1, in terms of direct operating cost. This means that the additional bending, shear and torsional loads imposed on the wing by the winglet have to be accurately calculated, as do the aeroelastic effects - the wing bends and twists a little with the winglet flight loads, not to mention flutter margins. And the weight prediction modelling has to be REALLY GOOD. The specific load cases that design certain parts of the wing structure can play a part - for example, if large areas of the wing are designed by the 2.5g manoeuvre case, a winglet that moves the aerodynamic centre at the wingtip further aft (wingtip fence for example) can cause the wing to twist off more at the 2.5g manoeuvre point, washing it out more. This shifts the spanswise centre of lift inboard at this design point, reducing the bending loads; winglet-induced aerodynamic load relief, if you like. This effect may not be the same from project to project, short haul to long haul (short haul aircraft do more cycles, so fatigue and damage tolerance may be the critical design case). The optimum span for the airliner may not be constrained, like it was for the A380, so there may be less gain to be had, if any, for the clean sheet design with optimum wingspan. Of course, when the machine is built and tested, it eventually becomes old, and tricks like question 2 can come into play.

I'll close by talking about winglet effects at different airspeeds. Clearly the winglet has to pay for itself in the cruise, as that is the point airliner wings are optimised for. For a long range aircraft, this is especially important. But at lower speeds, the induced drag forms a greater proportion of the total drag. So a winglet that cuts 5% from induced drag in the cruise (say 2% net total drag reduction, if cruise induced drag is 40% of total drag), will cut say 4% of the aircraft drag at a low speed point, where the induced drag is 80% of the total aircraft drag. So winglets are great for low speed - takeoff lengths, first and second segment climb. A poor winglet design with almost no cruise drag reduction might still be worthwhile, if airport access - second segment climb for example - is limiting.

I hope this helps.

On Glide

On Glide
 

Thank you OG.

Apart from explaining what the OP wanted to know I think this is a good example of why pilots who look for a single sentence that explains some aspect of aircraft design are looking for something that does not exist.

As for those that like to quote their own simple exlanation - nuff said.

JF

Given that the winglet causes a non-trivial change in lift etc. were extra tests performed to validate a asymmetric one-winglet flight?

Are Winglets an MEL item? Also, what sort of problem leads to a one-winglet config. in the first place?

<<Are Winglets an MEL item>> Usually on the Configuration Deviation List (CDL). They might get damaged due to lightning strike, vehicle hitting it on the ground, bird strike etc thus needing to be removed before a replacement in sourced.

On Glide, very much enjoyed your write-up, thanks.

OG
 

Wow, OG phenomenal explaination. Great reading, tks!
:D

On Glide, a beautiful post. Thanks.

TME

WOW. Have you considered a book? Great writing.