Effect of Retrofitted Winglets on 767 Handling

[FlightPathOBN]

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]

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]

The effects of winglets on transport jet aircraft

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 hangar 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]

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?

[Vc10Tail]

FightPathOBN

With respect, kindly enlighten us as to what the optinmal solution is then...

[FlightPathOBN]

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]

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]

FlightPathOBN

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]

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]

I have heard too that they have worse climb performance but better fuel consumption.

[Dash8driver1312]

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]

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]

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]

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 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]

Airbus decided to use the right angle winglets

........until they decided to add the option of "Sharklets"

AirAsia becomes first operator of Airbus? Sharklet equipped A320 | Airbus News & Events

[FlightPathOBN]

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]

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]

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]

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). 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 and 3). The result is a reduction in induced drag (fig. 4). 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]

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.