Effect of Retrofitted Winglets on 767 Handling
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
Wake vortices and winglets
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.
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.
Join Date: Mar 2011
Location: engineer at large
Posts: 1,409
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
Flightpath
Yeah - fascinating but difficult subject!
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!
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.
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.
True and of course consistent with the benefits being associated with lift dependent drag.
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!
Yeah - fascinating but difficult subject!
What I noticed with winglets is a movement of the turbulent air towards the wingtip.
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)?
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...
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!
Last edited by Owain Glyndwr; 22nd Jun 2013 at 10:54.
Join Date: Jul 2009
Location: France - mostly
Age: 84
Posts: 1,682
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by FPO
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.
Join Date: Mar 2011
Location: engineer at large
Posts: 1,409
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
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.
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.
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.
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.
Last edited by FlightPathOBN; 22nd Jun 2013 at 15:45.
Join Date: Mar 2007
Location: Another Planet.
Posts: 559
Likes: 0
Received 0 Likes
on
0 Posts
Non-Winglet Calcs.
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....
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....
Join Date: Mar 2011
Location: engineer at large
Posts: 1,409
Likes: 0
Received 0 Likes
on
0 Posts
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...
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...
Join Date: Aug 2013
Location: Gatwick
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
767 / 757 winglets
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
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
