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Jaggy_Snake
16th Oct 2012, 05:10
I recently posted the same thread on Airliners.net, so apologies if anyone's reading this twice, but I wanted to get the largest possible amount of opinions.


I'm on an internship (for my final year thesis) at an MRO doing a study of blended winglets, in particular, those installed onto the 767-300ER.

I am interested to hear some views from current Flight Crew on any effects the installation of winglets has on the handling characteristics of the plane.
I have already spoken to a UA test pilot who was recently performing the post-mod test flight on one of their a/c and he suggested that there is little noticeable difference to the way the a/c handles following winglet installation. He did however admit that he wasn't the best person for me to ask as he does not exclusively fly the 767s.

I'm thinking that by adding these devices - they're really quite substantial on the 767-300 - there could be an increase to the dihedral effect of the wing adding some extra roll damping?

As the effective aspect ratio is increased, the Mean Aerodynamic Chord is perhaps shifted towards the tips slightly? Combining that with the presumed aftwards shift of the longitudinal centre of gravity due to the winglet's weight, are there any performance implications upon take-off/landing which are at all noticeable?

As pointed out to me, no further type-rating is required for pilots to fly these modified 767, but I just find it slightly difficult to believe that these devices go completely unnoticed from the controls. I'm certainly not a pilot so any views from current flight crew would be welcome!

Glonass
16th Oct 2012, 11:07
Effects? None!

Telstar
16th Oct 2012, 11:42
I know you asked about the 767 in particular but for some comparison I fly the 737-800. I certainly couldn't notice any difference in handling before and after perhaps apart from taking slightly more time to decelerate but it is a slippy aircraft anyway.

deltahotel
16th Oct 2012, 12:24
Don't think I'm good enough to spot any real handling differences, except that if you do a normal flare you float for ages. They seem to need less flare/a flatter landing attitude.

misd-agin
16th Oct 2012, 14:18
Less drag. Tougher to deaccelerate, more inclined to float in ground effect.

Shallower descent profile. If you have an unmodified FMC(no 'W' next to a/c type) you'll get fast("drag required").

Dg800
16th Oct 2012, 14:33
I believe aircraft with winglets are approved for 2 or 3kts less crosswind limit due to the extra surface area perpendicular to the wind.That seems rather unlikely, seeing as the surface area of even the largest winglet is really minuscule compared to the (lateral) surface area of the fuselage. Even if they did make a difference then that would only affect gust limits, as a steady crosswind will have the same effect on any aircraft, regardless of shape or even size.

Denti
16th Oct 2012, 15:35
Our landing crosswind limit is the same for 737s with winglets and those without (40kts on dry and wet). Take off crosswind limit however is different as the NG with winglets is only demonstrated to 34kts and in my company that is now a fixed limit including any gusts whereas aircraft without limit have a take off crosswind limit of 36kts. So there is a difference of 2kts for take off, none for landing in my outfit.

Jaggy_Snake
17th Oct 2012, 03:32
OK, these replies are useful. I realise the effect will be minimal but I'm just looking for anything which will add depth to my report. Any further comments are always welcome!

framer
17th Oct 2012, 07:25
40kts is a fair old crosswind Denti.
My outfit has a 33kt crosswind limit dry for both takeoff and landing, and 25kts wet. (737-800)
I would be interested to hear if a 40kt limit is common from other -800 pilots.

Denti
17th Oct 2012, 08:55
Actually, it is kinda new to us, we used to have a common take off and landing crosswind limit, but it got changed around a year ago into different ones for landing and take off. Remember though that those limits are including gusts, not steady wind and are applicable to narrow runway operation as well. A 25 kts crosswind limit on a wet runway is fairly low though considering that the autoland crosswind limit is 25kts as well (doesn't matter if wet or dry). Only single engine autoland crosswind limit is lower at 20kts.

Beakor
17th Oct 2012, 09:35
Hi Fergus,

Our fleet of 767 300s has both winglet and non-winglet aircraft and we regularly fly both. We use them for both long-haul and short-haul. The only differences I've noticed on normal revenue flights are the winglet aircraft are more slippery. If you need engine anti-ice in the descent its almost certain you'll also need speedbrake with a winglet jet. The fuel economy on long sectors is also better with winglets, less so on short sectors, probably due to the extra weight of the winglets not being offset by the lower drag on a short flight.

The fine differences in roll stability and cross wind landing behaviour are not noticeable in line flying. They probably exist but you'd need to do a test programme between identical jets apart from the winglets to find out. The biggest differences in practice are between flying a heavy jet (180000Kg) or a light one (100000Kg). Landing, we have the option of F25 or F30 landings (Company prefer F25 unless good reason for F30) which make more difference in handling than winglets or non-winglets. There is no difference in cross wind limits and I haven't noticed any difference in behaviour. Not to say there isn't any but to notice it you'd need to land a winglet jet, then land a non-winglet on the same runway with the same conditions. That obviously never happens in normal line operations.

Sorry that doesn't help your thesis much but best of luck.

Jaggy_Snake
18th Oct 2012, 05:17
Thanks for the informative reply, Breakor. Excuse my ignorance, but what is the relation between requiring anti-ice and needing to apply speed-brake? [Air temp is low, so air is more dense therefore more lift which needs to be ditched by deploying air-brake? :confused:]

This does help my thesis in that I can state with confidence that the winglets don't significantly affect a/c handling. Until now I've been unable to find any reports on this particular subject.

Also, your 2nd paragraph is definitely useful in demonstrating how countless different conditions and parameter mean that no one landing or flight is directly comparable to another.

haughtney1
18th Oct 2012, 07:53
Excuse my ignorance, but what is the relation between requiring anti-ice and needing to apply speed-brake? [Air temp is low, so air is more dense therefore more lift which needs to be ditched by deploying air-brake? ]

Anti ice on the 767 requires bleed air, this means a higher idle setting which means more residual thrust....

Sillert,V.I.
18th Oct 2012, 08:02
Excuse my ignorance, but what is the relation between requiring anti-ice and needing to apply speed-brake?

Most jet airliners use bleed air from the engines for deicing which means you need to carry extra power in icing conditions & the reduced drag of the winglet-fitted airframes means you are more likely to need to deploy a little speedbrake to offset the extra power.

Notso Fantastic
19th Oct 2012, 11:35
I flew B737-700 before and after installation of winglets. I never detected the slightest difference in any handling, and they were quite large- 2 metres, I believe. Some of my colleagues claimed they did detect a difference, but I think it was more in their imagination. I also flew B747s early models without winglets, and the -400 with. Again no difference you can notice. We used to occasionally fly the -400 with a winglet removed- absolutely no difference. It's like filling your car with standard or super petrol- absolutely no detectable difference so take the cheaper one! Though there's always smart ones who claim they can feel the difference!

atldave
31st May 2013, 15:34
Go check out APB (aviation partners Boeing) website, they are the division of boeing that created the winglets and they have a ton of info about them. Im a mechanic working for Delta Airlines installing winglets currently on the UPS fleet. I know UPS also has a winglet line in hong kong currently installing on the other half of their fleet. You should try to get a tour to check out the install, its impressive! Good luck with your theisis

WeekendFlyer
5th Jun 2013, 12:00
Jaggy, some thoughts from a flight test specialist:

the winglets are nearer vertical than horizontal, so their contribution to lateral stability will be tiny. They will shift the lateral position of the centre of pressure outboard slightly, but the shift will be very small - think how much wing area is inboard compard to outboard, and also remember the CL distribution along the wing span favours the inboard part of the wing also, so changes to the tips do very little to the lift distribution as a whole.

Longitudinal CG change will be negligible compared to other factors such as fuel, cargo and passenger distribution in the aircraft.

The winglets main function is to extract some energy fom the tip vortex that would otherwise be lost, and convert it to a small forward thrust force. They also slightly increase the wing efficiency factor, thus reducing induced drag and increasing the L/D ratio. This gives increased range and less fuel burn for a given sector, which saves operators a lot of money, as I am sure you know! This also will affect climb and descent peformance, and potentially takeoff and landing performance, hence the need for a different performance database in the FMS for some aircraft types when winglets are fitted.

As for changes to cross-wind limits, it may be that the flight manual limits are simply that which has been demonstrated by the test team, thus they can gaurantee the aircraft's handling characteristics will remain safe and within certification limits.

Aerodynamically it is hard to see why crosswind limits should be different with/without winglets, unless they are affecting directional stability, which could be possible given their size and orientation. If they make the aircraft slightly more directionally stable, then it would be harder to kick off the drift for a crosswind landing, hence the slight reduction in the crosswind limit.

Hope this helps.

Uplinker
5th Jun 2013, 13:41
Hi WF,

I thought winglets worked by reducing the drag of the wingtip vortex, and this is how they improve efficiency?

Do they really produce thrust from the vortices as well? Can you explain how? I am not having a go, I am genuinely curious.

I would imagine that the increased vertical area of the winglets would affect crosswind performance - they are large vertical surfaces which would generate more sideways force from the crosswind than an aircraft without them?

gtseraf
6th Jun 2013, 06:58
I fly both winglet and non-winglet 300er versions and don't have much to add to what's been said already, except that the winglet machines look really sexy!!:)

WeekendFlyer
6th Jun 2013, 12:58
Uplinker,

as I understand it, tip vortices take a lot of energy from the aircraft because previously still air becomes rapidly spinning air. That increase in Kinetic energy has to come from somewhere, and ultimately it manifests itself as induced drag. Force * velocity = power = energy per second, so the induced drag force * TAS gives you an idea of the rate of energy transfer from the aircraft to the air. This energy transfer creates the vortices, the upwash ahead of the leading edge and outside the wing tips, and the downwash behind the wing.

The vortices are caused by high pressure air beneath the wing forcing its way around the wingtips to try and fill the low pressure area above the wing. This causes airflow towards the wingtips from underneath, and away from the wingtips on the top of the wing. Adding this span-wise velocity component with the velocity of the air coming towards the wing and you get tip vortices.

Winglets exploit the vortices by taking the flow from the top and fuselage-facing parts of the vortex and using it just like a normal wing uses the relative airflow - i.e to generate a lift force. However, for a winglet this force acts mostly horizontally towards the fuselage but also has a forward facing component. This "thrust" component cancels out some of the induced drag force, resulting in lower overall induced drag and thus could be argued to be a form of energy recovery from the vortices. The resulting vortices are therefore smaller and less energetic and the downwash is thus reduced. These changes are small, but enough to lead to significant fuel savings in the long run.


Regarding directional stability - yes, I suspect winglets have sufficient area in the vertical plane to cause a non-trivial sideforce at high sideslip angles. As this will be acting behind the aircraft CG it will cause a small increase in directional stability, which in turn could lead to a slightly lower crosswind limit for takeoff and landing to ensure directional control can be retained.

Hope this clarifies things a bit!

FlightPathOBN
6th Jun 2013, 16:32
Interesting explanation, but unfortunately, the explanation of how vortices are created and the effect of winglets in regards to vortices is incorrect.

Due to the research, it is generally known that the vortices are not wingtip, and rollup is a function of the wing and flap settings.

Winglets have been shown to have no effect on vortex generation.

What winglets do, is influence the laminar flow over the top of the wing. In certain flight configurations, increasing laminar flow as the wing narrows, therefore decreasing drag.

The increased friction due to the winglets, nulls their effect at low coefficient lift states.

Again, to date, there has been no measured effect on vortex generation...

Uplinker
7th Jun 2013, 01:06
Thanks for the replies - I don't know who is right, but I must admit my understanding was similar to WF's - although I had not heard the forward thrust vector theory.

An aircraft model with 'plain' wingtips flying through a smoke cloud clearly shows a rotation of the air at the wing tips. I haven't seen the equivalent thing with a winglet equipped model, so I don't know how the vortices are affected.

I'm not necessarily doubting you, FPOBN, but if winglets just improve laminar flow, then would not the manufacturers fit the little vortex generators one sees on some wings and around the fuselage tail cones of some Boeings, instead of huge great heavy fins attached to the end of the wings, with all the structural issues that implies?

Winglets improve overall fuel consumption by around 4%, allowing aircraft a greater range - the majority of that extra range being in the cruise, when a lower lift coefficient is required. But from what you say, there is no advantage to winglets when a low lift coefficient is used?

Also, you say that the vortices are caused by the wing and flap settings, yet in the cruise obviously the flaps are in - surely this does not imply that the vortices disappear with a clean wing? (and I know they don't - having had to offset our track on more than one occasion to get out of the trailing vortices of another 'heavy' ahead of us when crossing the Atlantic).

I'm confused !

Vc10Tail
7th Jun 2013, 04:33
History of airplane design includes many attempts to significantly reduce INDUCED DRAG.
Unfortunately induced drag is omni present like death and taxes!So all can be achieved is anadesign of winglets to optimize induced drag(vortex drag) by increasing effective aspect ratio and thus reduce induced drag by permitting significant lift to extend al the way to the wing tips.

The initial types of winglets developed were end plates or wing tip fins which airbus has favoured in its designs and still feature on their A380s. Boeing/MDC on the other hand have favoured the more modern blended winglets.

In principle, winglets do the following:-

1) reduce induced drag for a given weight

2)the amount of Induced drag reduction is equivalent to extending the original wing span by half of the winglet height(aproximately).

3)the expected advantages of winglets is less wing bending moments and resultant wing weight increase than with a spanwise increase(albeit of the order of around 2%) .The decision on whether to use winglets in the design process is likely to be dominated by special circumstances. Among these is a non optimum span loading on the original wing,minimum practical skin gauges already used on the outer panel so that the bending moment of the winglet attachment does not require more material, the desire to avoid a span increase for reasons associated with airport gate/ramp facilities limitations or hanger space, and the possibility of flow separation at the winglet root at high lift coiefficient because of flow interference.

A last non- technical factor to incorporate winglets is the desire to look modern ("sexy").


The main benefits of winglets to Aviation are:

Reduced wake vortex for aircraft following a winglet incorporaed aircraft

A more comfortable ride for passengers in turbulence

Less effort from the pilot during the flare to land

Perhaps( please verify this) smoother upset recovery

Improved low speed assymetric flight characteristics(especially within ground effect)

Due to increased directional stability...slight reduction in crosswind handling limitation as mentioned in previous threads (about -3kts)

Slightly improved fuel economy during cruise (about 2.5% savings for a 1500nm stage length)


It would be interesting to review why Boeing cancelled the original 767-400 programe (which incorporated blended winglets) and whether the lack of winglets on 777s is purely due to gate limitations or not cost benefit due to the already supercritical nature of its wing.

Am not sure whether incorporation of winglets. Can result in improved service ceiling as well as improved glide range with both engines flamed out?

Most corporate jets with their quests for pushing the speed/ range envelope have featured blended winglet technolgy and some thing might be learnt from these jet toys of today too. Incidently, the mid air collision between 737(classic??) And Embraer Lineage corporate jet over Brazil a few years back...which saw the loss of the Boeing and succefful landing of the Embraer with a partially torn winglet...might attest to the handling advantaages of incorporating winglets in improving directional stability??(just wrth researching whether their presence played a role at al in the ulimate recovery of that flight..according to the CVR the cockpit crew did well but did not deviate majorly from SOPs.

I hope this general commentary is helpful to your thesis on effects of winglets on handling characteristics of B767. Given the choice to add or not to add winglets..will depend on the residual airframe life and have to be balanced against re-certification obstacles and the resultant cost benefits but in general I expect a god rather than detrimental effect especially for those operators stuck with the old airplane that still has mileage in its airframe when weighed against the capital costs of acquiring a newer airplane with a similar payload range capability!



Improved fuel consumption

FlightPathOBN
7th Jun 2013, 17:43
Winglets are not an optimal solution, they are an average solution. The 4% benefit is a marketing tool, and is not over the entire flight.

Enroute, with the AoA changing, the effective efficiency of the winglets is not for all phases of flight. That is why when looking at a retrofit, the airline must look at the intended use for payback. If the ac is doing short hops, there is no payback.

Again, there is NO reduction of wake vortex associated with the winglets. That early marketing assumption has been negated by NASA, Volpe, and DLR research, as has the assumption that the vortices is generated by the wingtips.

The wake vortices change throughtout the phases of flight with the AoA of the wings and reduction of weight. On Approach, the flap settings have a significant effect on the wake generation. The wake from the same 737-800 with flaps 30, is much different with flaps 40.

Notice the evolution of winglets, sharklets, and xwings?

Wingtip?
https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcRewHYuoq-3beFjoWtytm_6kUtZDeirws7jv-NbgmGhb7yynAM7

http://25.media.tumblr.com/6a83e7a522cc9930f1b76304ff15643f/tumblr_mm911koNsS1sowhpgo1_500.jpg

Vc10Tail
7th Jun 2013, 18:28
With respect, kindly enlighten us as to what the optinmal solution is then...

FlightPathOBN
7th Jun 2013, 18:39
Due to the myriad of variables, there really isnt one.

There was talk of an articulated winglets, but havent heard much more about that.

With the 787 and 350 wing configurations, it appears that winglets are not in the future...

Owain Glyndwr
7th Jun 2013, 21:23
Originally posted by Flightpath OBN
What winglets do, is influence the laminar flow over the top of the wing. In certain flight configurations, increasing laminar flow as the wing narrows, therefore decreasing drag.Could you explain the mechanism by which this is brought about please? [or point me at a source where it is explained]

Vc10Tail
7th Jun 2013, 22:06
If there isn't an optimal solution then I rather think calling it average is just sweeping it off the stage of relevance.That it is an optimized solution for minimising induced drag (perhaps not on super critical wings) is what aerodynamicists speak of, so I suggest you take up that argument with them.I explained quite clearly why it was an optimized solution.I shall rest my argument and beg to differ then.Thank you.

lederhosen
9th Jun 2013, 19:13
We have recently put winglets on our 737 NGs. They seem to struggle in the last couple of thousand of feet of climb to the FMC indicated optimum cruise level. I never noticed this in ten years flying the same aircraft without winglets. Has anyone else noticed anything similar?

Uplinker
9th Jun 2013, 22:44
I have heard too that they have worse climb performance but better fuel consumption.

Dash8driver1312
11th Jun 2013, 08:14
Well done on demonstrating that the outer point of a surface is causing a vortex. There are also vertices coming off the tips of the horizontal and vertical fins, and the winglets. These are just not visible in the images you have posted due to the intensity. (The flaps generating much more down wash than the outer panels at this point)

I have seen aircraft come in trailing vertices from flaps and wing tips.

Spooky 2
11th Jun 2013, 08:49
I believe the payload capability of the 767-300ERW is significantly better than a non winglet aircraft. No changes in certified numbers just perfromance. I know that DAL operates out of KSLC (hot & high) with the 767 non stop to CDG. In the summer there can be some significant payload limitations if a 767 without winglets is used on this sector. Perhaps all the DAL 767-300ER's are now equipped, so this information may not be applicable today.

WeekendFlyer
11th Jun 2013, 09:13
FlightpathOBN,

Can you provide references / links to the research you cite? All of the explanations I have ever read refer o the winglet interacting with the tip vortex in a manner that reduces the strength of the vortex and thus reduces induced drag. Certainly, the toe-angle has an efect on the production of a small amount of thrust force that is not there when the winglet is present.

As for where the vortices come from, the explanation I gave regarding higher pressure above the wing and lower pressure underneath is correct. However, I simplified it by stating that the vortex appears at the wingtip. The reality is that many small vortices form along the length of the trailing edge but they all add up very shortly afterwards and eventually form a large tip vortex. This vortex will form wherever there is an "edge", such as the wingtip and the edge of the flaps (if they are deployed). Clearly the nature of the vortex sheet will change as he wing shape changes due to flap/slat deployment.

Lastly, I am intrigued by the laminar flow argument. It makes sense that lifting the vortex away from the wing tip to the tip of the winglet would keep the vortex flow away from the wing upper surface near the tip, which would, as you say, permit laminar flow to persist over a slightly larger area of the wing. Are you saying that the entire drag reduction from the winglet is due to this effect?

FlightPathOBN
11th Jun 2013, 16:26
Currently, there is quite a bit of research going on relating to the vortices, mostly for reducing spacing on final to increase capacity.

The research is changing the understanding of the mechanics of vortex creation. There are volumes of documentation from the the WakeNet-USA and EU WakeNet (http://www.wakenet.eu/) studies.

Enroute vortex generation is important for fuel savings, but currently, the research is really driven by the military and mid-air refueling operations, and most recently, for unmanned tankers fueling unmanned platforms.

NASA just completed the formation flying with 7 C-17's flying in formation. he attempt was to keep the trailing ac in the uplift part of the rollup of the leading vortex. Over the course of a 5 hour flight, they noticed that the trailing aircraft had to re-position several times to stay in the uplift portion, because the leader vortex was changing as it lightened up and the AoA changed.

For final approach, the more measurements that are being taken, the more the concept of wake generation is changing. One can look up the research and see the images, many of them just a simple wing section extruded in length. This was about all the modelling that could be accomplished, and the concepts regarding wake generation were the result.

There was no impetus to model an actual wing section, change weight and/or angle of attack, and no way to even consider different flap settings. The wing designers and others completely discounted, (and in fact, some still continue to discount) the flap settings.

As a result, winglets were originally marketed as reducing wing vortex generation, thus reducing drag. Airbus did extensive studies with the A380, in an attempt to get the wake sep behind it reduced. They found that the winglet configuration had no effect on vortex generation.

So in regards to winglets, the boundary layer is very sensitive to pressure changes. With the wing taper, as the pressure rises, the boundary layer begins to shift towards the leading edge, leading to turbulent airflow at the trailing. This is the drag that reduces fuel effeciency. It is somewhat easy to envision a wing section in flight, at the beginning, with a higher AoA, that the transitional area would be much greater, than as the ac AoA lessens with reduced weight.

The winglets, at these angles of higher AoA, effectively move the transitional area outwards, increasing the boundary area of laminar flow, the reduction of turbulent flow is less drag, therefore better fuel economy under those conditions. The lower drag economy outweighs the increased friction drag of the winglet itself.

Boeing decided to use the large radius winglets to mitigate boundary separation with the pressure differential at the corner. This lead to a very tall winglet, and the associated friction component.

Airbus decided to use the right angle winglets, and to mitigate this, the leading edge of the winglet is moved into the region of the pressure rise of the wingtip, with the winglets pressure rise region moved behind the wings trailing edge. This places the winglets in a region where the pressure gradient mitigates the wings pressure rise.

more later....

ZimmerFly
12th Jun 2013, 07:30
Airbus decided to use the right angle winglets........until they decided to add the option of "Sharklets" :ooh:

AirAsia becomes first operator of Airbus? Sharklet equipped A320 | Airbus News & Events (http://www.airbus.com/newsevents/news-events-single/detail/airasia-becomes-first-operator-of-airbus-sharklet-equipped-a320/)
(http://www.airbus.com/presscentre/pressreleases/press-release-detail/detail/air-astana-takes-delivery-of-its-first-sharklet-equipped-a320/)

FlightPathOBN
14th Jun 2013, 16:54
The A350 is certainly an interesting winglet. There is also the small flap right at the winglet. I really want to look into that more, as it appears to an attempt at a morph wing condition.

The A350, like the 787, will likely make large core, long lasting vortices, uncoupled due to the landing gear turbulence. So, probably forget about b0, and crow instability. Just avoid them, wherever they may end up!

Back to the turbulence, you will see discontinuities at the flap edges and wingtip, but this is not the rollup load vortex that is the issue at hand.

BARKINGMAD
18th Jun 2013, 11:05
Having flown 737-800s with and without winglets, my personal experience was that for a heavyweight near max landing mass aircraft, my continuous descent approaches needed speedbrake when using non-winglet energy management maths for idle power path control in the winglet version.

Better results were obtained, without destroying valuable energy obtained from the fuel tanks, by factoring my IAS-reduction-to-min-clean distance by increasing it by 50%.

I have never noticed the reported near-ceiling struggle, but we're using 26k engines, which means the -800 does what it says on the tin!

Hope this is of use, though not the right a/c type? :)

FlightPathOBN
19th Jun 2013, 22:31
BM,

Interesting, I dont think I have ever seen a 378 without winglets....

aside from that, why would you use non-winglet calcs on a winglet ac?

Owain Glyndwr
21st Jun 2013, 06:46
Weekend Flyer
Lastly, I am intrigued by the laminar flow argument. It makes sense that lifting the vortex away from the wing tip to the tip of the winglet would keep the vortex flow away from the wing upper surface near the tip, which would, as you say, permit laminar flow to persist over a slightly larger area of the wing. Are you saying that the entire drag reduction from the winglet is due to this effect? Like you I was intrigued by the laminar flow argument so I decided to put some numbers into the equations, using the B737 geometry as an example. Basically, to get the cruise gains Boeing quote one would have to have completely laminar flow over a large chunk of the wing - upper and lower surfaces, from LE to TE extending from the notional 'wingtip' (at the supposed wing/winglet boundary inboard to about 80% span.
I have great difficulty envisaging any flow mechanism whereby a highly turbulent vortex flow springing from the trailing edge of the winglet tip could encourage laminar flow at the LE, let alone the undersurface LE. At 737 cruise conditions the Reynolds Number is about one million per foot, so one would normally expect transition from laminar to turbulent flow about three or four inches back from the LE. Beyond that it is very doubtful that laminar flow would persist aft of any small discontinuity such as the retracted slat/wing interface and I have never, ever, come across a turbulent flow reverting to laminar so that any laminarisation effect has to start from the LE . All of which leads me to believe that this laminar flow argument is a nonstarter.

I reckoned that Boeing ought to know how their wing works so I went to their website where I found : (my bolding)
Winglets affect the part of drag called induced drag. As air is deflected by the lift of the wing, the total lift vector tilts back. The aft component of this lift vector is the induced drag (fig. 1 (http://www.boeing.com/commercial/aeromagazine/aero_17/winglets_fig1.html)). The magnitude of the induced drag is determined by the spanwise distribution of vortices shed downstream of the wing trailing edge (TE), which is related in turn to the spanwise lift distribution. Induced drag can be reduced by increasing the horizontal span or the vertical height of the lifting system (i.e., increasing the length of the TE that sheds the vortices). The winglets increase the spread of the vortices along the TE, creating more lift at the wingtips (figs. 2 (http://www.boeing.com/commercial/aeromagazine/aero_17/winglets_fig2.html) and 3 (http://www.boeing.com/commercial/aeromagazine/aero_17/winglets_fig3.html)). The result is a reduction in induced drag (fig. 4 (http://www.boeing.com/commercial/aeromagazine/aero_17/winglets_fig4.html)). The maximum benefit of the induced drag reduction depends on the spanwise lift distribution on the wing. Theoretically, for a planar wing, induced drag is optimized with an elliptical lift distribution that minimizes the change in vorticity along the span. For the same amount of structural material, nonplanar wingtip devices can achieve a similar induced drag benefit as a planar span increase; however, new Boeing airplane designs focus on minimizing induced drag with wingspan influenced by additional design benefits.
Notice that there is no mention of reduced vortex strength, which is as it should be, because winglets do not change the total vortex strength as FPOBN says.

[Vortex strength is another name for what aerodynamicists call circulation, which in turn is related to lift by the expression:
Lift/unit span equals circulation times airspeed.
If you integrate lift/unit span from winglet tip to winglet tip you will get total lift (equals weight) and total circulation (total vortex strength). With a little manipulation this gives you:
Vortex strength equals aircraft weight divided by airspeed
Span doesn't appear in this expression so adding winglets doesn't change the total vortex strength behind the aircraft]


So the commonly held view that winglets reduce drag by reducing the vortex strength is wrong - they work by changing the vortex distribution to give the same vortex strength.
I now think of it in terms of the old-fashioned expression for induced drag coefficient - Cdi = k*CL^2/(Pi*A). Keeping the vortex strength constant is roughly similar to holding constant CL I reckon, which leaves the winglets working on k/A. They certainly increase span and hence aspect ratio and with careful design one can optimise k (or in the USA optimise Oswald's efficiency factor e)

WeekendFlyer
21st Jun 2013, 13:13
So winglets increase span and thus aspect ratio, and can also improve the wing efficiency if designed carefully, so the CL distribution is optimised along the span. That makes total sense to me, particularly when one applies the induced drag equation!

This raises the quesion: why is there so much discussion ion apparently cedible online sources, stating that winglets reduce the strength of the tip vortices? Clearly they don't, they just distribute the vorticity in a more suitable manner along the span of the wing.

awblain
21st Jun 2013, 16:02
Does the amount of energy being dumped in turbulence off the surface of the wing matter for drag?

An aircraft that weighs 200 tons and has a 50m span, flies at 200 m/s in air with a density of 0.1 Kg/m-cubed needs to deflect the air in its wake at a vertical speed of about 40 m/s to stay up. That requires about 10MW of power. Managing the rotational flow that that descending slab induces compared with the air outside the span is important for reducing that power demand. By comparison, I suspect that having laminar flow over the whole of the wing is not a big deal, unless you're dumping more than 1MW into the turbulent flow. With a 50m span, that would be 20kW per meter of span, which seems to be a lot. It may be true, but I suspect the effects of winglets for sculpting the large-scale vortices behind the aircraft are much more important for drag reduction than changing the boundary layer turbulence properties near the wingtips.

FlightPathOBN
21st Jun 2013, 21:18
Well....Owain, thanks for that detail and for looking into this..

perhaps my understanding of the terminology is at hand.

What I noticed with winglets is a movement of the turbulent air towards the wingtip. I understood this an an increased area of laminar flow, thus less drag.

There are a few things to keep in mind when looking at winglets.

First off, the radical differences in winglet designs, the particular models of aircraft they are installed on, and the outward flap configurations of those aircraft. Noting that there is little performance difference associated with the Airbus winglet vs sharklet, and the general differences between the Boeing and Airbus configurations.

If this was simply a span factor, then wouldnt there be significant differences in the winglets performances (Airbus vs Boeing)?

Secondary, but very important, is the winglet performance differences during phases of flight. There is a marked difference when the aircraft is at the beginning of a long haul (very heavy and the highest AoA) vs the performance when the aircraft is light, with a decreased AoA.
Boeing notes that the benefit from the winglet decreases with decreased AoA, and is negated by the friction drag of the winglet itself.

Finally, I must remind you that I only deal with the measurements from aircraft on final approach. There is significant influence from landing gear, AoA, and especially flap settings/flap configuration, and atmospheric conditions at this phase of flight...

Great conversation :ok:

Owain Glyndwr
22nd Jun 2013, 08:56
Flightpath

Yeah - fascinating but difficult subject!

What I noticed with winglets is a movement of the turbulent air towards the wingtip. One thing that seems common to all the various configurations is that the local lift coefficients are increased in the tip region. This actually means that the vortex strength is increased there (keeping the wing total constant of course). So the argument about reduced drag from reduced tip vortex strength is totally wrong!

First off, the radical differences in winglet designs, the particular models of aircraft they are installed on, and the outward flap configurations of those aircraft. Noting that there is little performance difference associated with the Airbus winglet vs sharklet, and the general differences between the Boeing and Airbus configurations.

If this was simply a span factor, then wouldnt there be significant differences in the winglets performances (Airbus vs Boeing)?Its not just a span factor of course; the detailed design of the wing outboard aerodynamics, the local structural strength all come into it. There is also a considerable difference in winglet performance inside one manufacturer's range of products - from the A300 through to the A350 for example. That may be due to increased understanding of how the things behave, understanding developed over several decades.

Secondary, but very important, is the winglet performance differences during phases of flight. There is a marked difference when the aircraft is at the beginning of a long haul (very heavy and the highest AoA) vs the performance when the aircraft is light, with a decreased AoA. That is certainly true, but the reasons for the differences may be quite subtle. On one aircraft I was associated with we flew back to back performance tests with and without winglets. We found that the benefits were indeed weight dependent, but at least some of that dependency was because the wing aeroelastic twist was greater at the higher weights so that the actual span loading at the tip varied with weight and the winglets were working from a different datum.

Boeing notes that the benefit from the winglet decreases with decreased AoA, and is negated by the friction drag of the winglet itself.True and of course consistent with the benefits being associated with lift dependent drag.

Finally, I must remind you that I only deal with the measurements from aircraft on final approach. There is significant influence from landing gear, AoA, and especially flap settings/flap configuration, and atmospheric conditions at this phase of flight...I was aware of your interest in approach conditions, so permit me a couple of thoughts....

It seems to me to be obvious from the pictures you have posted from time to time that the vortices springing from the outer edge of the flaps must be much stronger that those at the wing tip. I say this because you often see condensation in the 'flap' vortices but nothing at the tip. This would suggest that the pressure drop/vortex velocity/strength is much greater in the flap system.

This in turn is entirely consistent with the expression I quoted earlier - that lift/unit span is vortex strength*airspeed - the lift per unit span being much greater where the flaps are of course.

It also suggests that the danger to following aircraft may be linked more to these flap vortices than to those at the tip. Paradoxically then winglets may reduce wake vortex problems (ever so slightly) because they transfer vorticity from flaps to tip (keeping the total vorticity constant) ie because they increase the tip vortex strength!

HazelNuts39
22nd Jun 2013, 09:26
There is a marked difference when the aircraft is at the beginning of a long haul (very heavy and the highest AoA) vs the performance when the aircraft is light, with a decreased AoA.That doesn't really apply in practice. Since the optimum AoA varies very little with weight, aircraft do one or more step-climbs on long sectors to stay close to optimum AoA.

FlightPathOBN
22nd Jun 2013, 16:26
OG,

The images really were to illustrate the location of the lift rollup, so we are saying the same thing. The flap edge condensation is a good indicator to illustrate that the vortex is not generated at the wingtip.

http://blog.aopa.org/asfblog/wp-content/uploads/2008/11/vortices.jpg

While there may be discontinuities shown at the wingtips or flap edges, it is really the rollup vortex that is the issue.

Flap settings on final have a significant impact on vortex strength. The same 737-800, weighing the same, will make a much different vortex with flaps 30 than 40. Variants with outboard flaps will make a different vortex than ones with inboard (such as 737-400 vs 737-800)

The major finding of the measurements is the issue with b0. Those really nice images of the wake diagram go to hell real quickly when the wheels go down on a large ac. Break that center wing pressure couple, and the vortices act independently, and roll outward quickly. Crow instability gone.
http://origin-ars.els-cdn.com/content/image/1-s2.0-S0376042110000576-gr4.jpg

Given that, consider how effective a half module test is?

In conversations with NASA, DLR, etc...there has never been a model or a test where the aircraft is crabbing sideways in a crosswind. The nice diagrams where the crosswind causes the vortices to drift sideways, and the foundation for closely spaced parallel runways, is completely wrong, shown by actual measurements.

As noted y mnay posters regarding the general assumptions of winglets and vortices, if all of the aerodynamics were understood, why is vortex generation and transport completely mis-understood.


In regards to AoA, I would have to question that, as I frequently notice an attack angle of about 6 degrees at the beginning phase of a long haul enroute flight, dropping to around 3 degrees near end of the enroute phase.

FlightPathOBN
23rd Jun 2013, 19:14
from Boeing on the x wing..

Interesting flow and stresses...

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2013/06/ScreenHunter_69-Jun.-23-11.08-e1372011096253.jpg

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2013/06/ScreenHunter_70-Jun.-23-11.08-e1372011216820.jpg

BARKINGMAD
25th Jun 2013, 19:36
OBN. I had just departed from a company having driven 737 300s, 400s & 500s and joined XL Airways with NGs various.

True to form, the training staff provided NO figures to use for planning descent, and true to form the published descent figures were hidden away in the QRH Perf Inflight pages, which I confess were not on my bedtime reading list!

The "Classic" computations worked fine until the fleet started to acquire new ex-factory and retro-fitted variants, that is where I ended up doing my "test-flying" stint and learning the hard way. Hence my pet topic of energy (mis)management and the lack of easy-to-use rules of thumb being disseminated from the lofty halls of sim/line training.

G-XLAA was a non-winglet 'frame in those halcyon days, and only last week I parked beside a non-winglet fuselage with the -700 label at a European airport, so they're still out there confusing the reggie spotters and maybe the pilots too?

Question for the forum! Do the 'buses of similar weights (sorry, masses!) to that of the 737 variants have similar or significantly different energy calcs? That is assuming clean glide to ILS G/S capture and no heroics involving gear at Vne minus 5 knots, airbrake and other pax frightening measures etc. Just for my further educashun....:)

FlightPathOBN
25th Jun 2013, 20:02
Okay, I think I see your point, especially with the 700 version.

I have experienced the same energy issues with the 37-7 and up...these guys are really slick compared to previous variants.
You can note a significant difference in the wing design, especially the inboard flaps.

and yes, Boeing buried the differences. It became very apparent with RNP-AR procedures, having the design/performance parameters for an "NG", the 37-7 winglet and 37-8 variants had quite a bit of trouble with the many of the tighter turns to final.

A good example was one of the first RNP-AR designs, PSP. The variants could not make that final turn, and eventually, the ASA procedure was abandoned.

Boeing, for the longest time, has tried to keep the 737-NG as a blind NG for access, trying to keep the 'NG' as a CAT C as long as they can...this makes it difficult in many ways, especially the 140kts FAS...

I did not have the same issues with the Airbus aircraft, the data provided has always been much better for design purposes...

Niatpac
19th Aug 2013, 04:51
I fly mainly 757s but the effect is in my opinion very similar,

No noticeable handling differences in normal flight
Gusty X- wind landing more challenging, the wing still flying at 60 knots.
Performance very different particularly clean at low speed 220 knots thrust levers closed less than 1000 fpm rod.
Non of the FMCs appear to be able to compute a VNAV decent out despite have been modified !

Hope that helps