flightmedic72
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787 Winglets?

Wed Dec 16, 2009 9:32 pm

I noticed yesterday during the first flight of the 787 that the wings lack the winglets common to nearly all next generation aircraft to improve efficency and performence. Is this because the new wing design eliminates the need or will they be added later?
 
Acey
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RE: 787 Winglets?

Wed Dec 16, 2009 9:38 pm



Quoting Flightmedic72 (Thread starter):
Is this because the new wing design eliminates the need

Correct. As is the case with the raked wingtips on the 777-200LR, 777-300ER, and 767-400ER. Having said that, on renderings of the 787-3 that I have seen, there are winglets.
If a man hasn't discovered something that he will die for, he isn't fit to live. -- Martin Luther King, Jr.
 
BMI727
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RE: 787 Winglets?

Wed Dec 16, 2009 9:46 pm

Boeing has been somewhat ambiguous as far as winglets vs. raked wingtips. The 767-400 and 777 along with the P-8 have raked wintips while the 737, 757, and 767 have gone the winglet routes. I suspect that at least part of it has to do with gate space. After all, gate spacing isn't really an issue for the P-8. The 787-8 and -9 (and most likely the -10 as well) will have the raked wingtips while the -3 will have winglets, assuming that it ever gets built.
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Stitch
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RE: 787 Winglets?

Thu Dec 17, 2009 4:29 am

The 787-8 and 787-9 used raked wingtips, while the 787-3 is expected to use winglets to keep the span shorter to fit in 767-sized gates at Japanese airports.
 
tdscanuck
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RE: 787 Winglets?

Thu Dec 17, 2009 4:43 am



Quoting BMI727 (Reply 2):
Boeing has been somewhat ambiguous as far as winglets vs. raked wingtips. The 767-400 and 777 along with the P-8 have raked wintips while the 737, 757, and 767 have gone the winglet routes.

It's not really ambiguous, it just depends which requirements path you had to follow.

Span restriction > winglet

No span restriction >
Retrofit? > winglet
No retrofit > raked wingtip

Tom.
 
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HAWK21M
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RE: 787 Winglets?

Thu Dec 17, 2009 9:59 am

Partially related....The Wings looked very highly tapered upwards at the tip.Is this normal or was minimum fuel being carried.
regds
MEL.
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Stitch
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RE: 787 Winglets?

Thu Dec 17, 2009 2:53 pm



Quoting HAWK21M (Reply 5):
Partially related....The Wings looked very highly tapered upwards at the tip.Is this normal or was minimum fuel being carried.

The Seattle Times reported TOW was 195t, which is 25t less than the original spec MTOW. She was intended to fly for over five hours, so I expect her tanks were reasonably full.
 
BMI727
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RE: 787 Winglets?

Thu Dec 17, 2009 11:16 pm



Quoting Tdscanuck (Reply 4):
Span restriction > winglet

So would there be the possibility that the BBJ2 would be offered with raked wingtips at some point?

Quoting Tdscanuck (Reply 4):
Retrofit? > winglet
No retrofit > raked wingtip

Just out of curiosity, why can't raked wingtips be retrofitted? Is it because they require more structural mods than the winglets?
Why do Aerospace Engineering students have to turn things in on time?
 
tdscanuck
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RE: 787 Winglets?

Fri Dec 18, 2009 1:59 am



Quoting HAWK21M (Reply 5):
The Wings looked very highly tapered upwards at the tip.Is this normal or was minimum fuel being carried.

That's normal...if you look at the empty 787's in the factory when you're on the tour, or headed for paint before they're fueled, they've got a pronounced upward deflection at the tips. It's probably more pronounced with certain fuel loads, but it's in there right from the wing jigs.

Quoting Stitch (Reply 6):
She was intended to fly for over five hours, so I expect her tanks were reasonably full.

Probably full mains, but max endurance on a 787 has got to be 18 hours, so not very close to full fuel load.

Quoting BMI727 (Reply 7):
So would there be the possibility that the BBJ2 would be offered with raked wingtips at some point?

In theory, I guess so, although I've never seen it offered up. I've never found hard proof, but I've been told many times that the original driver to put winglets on the 737 at all was to make it look better for the biz jet customers.

Quoting BMI727 (Reply 7):
Just out of curiosity, why can't raked wingtips be retrofitted? Is it because they require more structural mods than the winglets?

You got it. A winglet shifts the lift distribution of the wing outboard, which cases higher stress in the wing, but the winglet itself contributes very little lift since it's mostly vertical. The raked tip shifts the lift outboard in a similar fashion, but also increases wing area and generates lift in its own right. As a result, the stress change is bigger and you need more excess margin to retrofit a raked wingtip than an equivalent winglet.

Tom.
 
nomadd22
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RE: 787 Winglets?

Fri Dec 18, 2009 2:42 am

Wouldn't a raked wingtip also generate something of a twisting force to the end of the wing, which would seem to require a wing desinged to take that tp in the first place?
I got the idea that the efficiency of raked wingtips compared to winglets got more favorable the higher the airspeed, so longer range planes, whch spent a higher percentage of time at cruise, would be more likely to have raked tips where shorter range would do better with winglets.
Anon
 
tdscanuck
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RE: 787 Winglets?

Fri Dec 18, 2009 2:59 am



Quoting Nomadd22 (Reply 9):
Wouldn't a raked wingtip also generate something of a twisting force to the end of the wing, which would seem to require a wing desinged to take that tp in the first place?

That sounds right to me but, of all the possible forces, a wing is best suited to withstand twisting because it's a closed torque box. I'd be kind of surprised if there were wings out there that are critical in torsion.

Quoting Nomadd22 (Reply 9):
I got the idea that the efficiency of raked wingtips compared to winglets got more favorable the higher the airspeed, so longer range planes, whch spent a higher percentage of time at cruise, would be more likely to have raked tips where shorter range would do better with winglets.

I can't really picture the aerodynamics behind that, but it certainly could be true. It's definitely true that a properly design raked tip will give you better performance than a properly designed winglet on the same wing, so I suspect it's simply a default to raked tips unless something (span restriction, retrofit, etc.) forces you another way.

Tom.
 
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jetmech
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RE: 787 Winglets?

Fri Dec 18, 2009 4:46 am



Quoting Tdscanuck (Reply 4):
winglet



Quoting Tdscanuck (Reply 4):
raked wingtip

One of the intuitive explanations given for the function of winglets is that they form a physical barrier at the wing tip, which reduces the ability of the air under the bottom of the wing - which tends to flow towards the tip - from interacting with the air over the top of the wing, which tends to flow toward the root. Reducing the interaction of these two streams near the wing tip then reduces the magnitude of the wing tip vortice. However, this explanation is harder to apply to raked wingtips.

Thus, I suppose a better explanation that would apply to both cases is that one wants to reduce the difference between the span wise components of flow at the wing tip. This would require that the pressure difference between the top and bottom surfaces at the wing tip are minimised, which also has the effect of reduce the lift produce at the tip.

Is this a good explanation for the basic desired function of winglets and raked wingtips? In lieu of any other penalties one may need to pay, would the ideal case be one where the lift distribution tapers to zero at the wingtip?

Low aspect ratio wings and one without tip devices produce strong vortices at the tip, which implies that the pressure difference is high. Does this mean that the lift coefficient is non zero at the tip with these configurations? Is a non zero wingtip lift coefficient observable and / or producible in reality?

Regards, JetMech
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tdscanuck
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RE: 787 Winglets?

Fri Dec 18, 2009 5:51 am



Quoting JetMech (Reply 11):
Is this a good explanation for the basic desired function of winglets and raked wingtips?

Not particularly, unfortunately. This is one of those cases where the intuitive explanation is nice, but not all that physically accurate. Both winglets and raked tips work the same way; they decrease vortex-induced downwash across the wing, which lowers induced drag. A winglet is just a raked tip that's been bent up.

Quoting JetMech (Reply 11):
In lieu of any other penalties one may need to pay, would the ideal case be one where the lift distribution tapers to zero at the wingtip?

The lift always drops to zero at the tip, because there's nothing to maintain pressure differential. The ideal (aerodynamic) case for non-transonic lift distribution is an elliptical distribution, but for a variety of reasons in real transonic aircraft you want closer to a triangular distribution. Either way, it will go to zero at the tip.

Quoting JetMech (Reply 11):
Low aspect ratio wings and one without tip devices produce strong vortices at the tip, which implies that the pressure difference is high.

Not exactly...you can build two wings with the same pressure difference and the same lift (i.e. same area) but the high aspect ratio one will have lower induced drag and weaker vortices. The major contributor is that, in a high aspect ratio wing, most of the flow is far from the tip, so most of it isn't influenced much by the ability to "sneak" around the end of the wing. In a low aspect ratio wing, much more of the wing is close to the tip, so it feels the effect more.

Quoting JetMech (Reply 11):
Does this mean that the lift coefficient is non zero at the tip with these configurations?

No. Even with a winglet or raked tip, the lift still has to drop to zero at the tip.

Quoting JetMech (Reply 11):
Is a non zero wingtip lift coefficient observable and / or producible in reality?

The only way I think you might be able to do that is some kind of powered lift (suction or blowing in the right places) but I'm not sure anybody's ever done that explicitely as a tip treatment.

Tom.
 
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HAWK21M
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RE: 787 Winglets?

Fri Dec 18, 2009 8:13 am



Quoting Stitch (Reply 3):
The 787-8 and 787-9 used raked wingtips, while the 787-3 is expected to use winglets to keep the span shorter to fit in 767-sized gates at Japanese airports.

What would be the approx comparative performance of a Raked v/s blended wingtip on the type.
regds
MEL.
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MSNDC9
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RE: 787 Winglets?

Fri Dec 18, 2009 4:41 pm



Quoting Stitch (Reply 6):
The Seattle Times reported TOW was 195t, which is 25t less than the original spec MTOW. She was intended to fly for over five hours, so I expect her tanks were reasonably full.

The Seattle times reported the TOW was 430,000 lbs, which is 72,500 lbs less than its current design max takeoff weight of 502,500lbs, so yes, it probably had a substantial fuel load.

Welcome to the US. This is a US aircraft, and yes we still use pounds.
 
nomadd22
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RE: 787 Winglets?

Fri Dec 18, 2009 5:52 pm



Quoting Tdscanuck (Reply 10):
Quoting Nomadd22 (Reply 9):
I got the idea that the efficiency of raked wingtips compared to winglets got more favorable the higher the airspeed, so longer range planes, whch spent a higher percentage of time at cruise, would be more likely to have raked tips where shorter range would do better with winglets.

I can't really picture the aerodynamics behind that, but it certainly could be true. It's definitely true that a properly design raked tip will give you better performance than a properly designed winglet on the same wing, so I suspect it's simply a default to raked tips unless something (span restriction, retrofit, etc.) forces you another way.

I was picturing Rutan's explanation of the winglet when he came up with the Vari-eze. He explained how it decreased drag and increased lift by preventing the turbulence on upper and lower tip surfaces from the low and high pressure areas meeting. I just get the feel that that area of turbulence would move further back as you got closer to mach speeds, so the aft rake would help more than a vertical winglet at those speeds.
Anon
 
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DocLightning
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RE: 787 Winglets?

Fri Dec 18, 2009 6:53 pm



Quoting Acey (Reply 1):

Correct. As is the case with the raked wingtips on the 777-200LR, 777-300ER, and 767-400ER. Having said that, on renderings of the 787-3 that I have seen, there are winglets.

Raked wingtips offer the benefit of a winglet twice as high as the wingtip is long. The disadvantage is that you can make winglets as tall as you want, but the wingtips increase the span.

What I've never understood is why no Airbus has raked wingtips.
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tdscanuck
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RE: 787 Winglets?

Sat Dec 19, 2009 5:01 am



Quoting DocLightning (Reply 16):
What I've never understood is why no Airbus has raked wingtips.

The A350XWB appears to have them, although the rake is much more gradual than the sharp kink seen on the 777 and 767-400ER. The A380 is span restricted, so I don't think it was an option for them. The A330/340 appear to share the same aerodynamic heritage as the 747-400 tips and, as previously discussed, probably aren't good candidates for a retrofit raked tip.

Tom.
 
GST
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RE: 787 Winglets?

Sat Dec 19, 2009 4:17 pm



Quoting Tdscanuck (Reply 10):
Quoting Nomadd22 (Reply 9):
Wouldn't a raked wingtip also generate something of a twisting force to the end of the wing, which would seem to require a wing desinged to take that tp in the first place?

That sounds right to me but, of all the possible forces, a wing is best suited to withstand twisting because it's a closed torque box. I'd be kind of surprised if there were wings out there that are critical in torsion.

As lift decreases towards the tip, similarly your applied bending moment also decreases, so that it is zero at the tip. If you put a wingtip divide onto the wing, you generate a massive point bending moment at your wing tip, that tries to twist the wingtip upwards and towards the fuselage.

If you assume that your typical wing spar is the shape of a square bracket, like this [, the vertical part takes all of the shear loads, and the horizontal parts take all of the bending moment loads (in reality each takes a little of the other, but this is a good approximation). Your shear force will still go to zero towards the tip, so the vertical part of the beam can become very small, but by putting a large point moment on your wingtip,l your horizontal booms must remain thick (read: heavy) right to the tip, you you need to be sure that your wingtip device will give you enough performance to pay for itself.
 
dynamicsguy
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RE: 787 Winglets?

Sat Dec 19, 2009 9:54 pm



Quoting GST (Reply 18):
but by putting a large point moment on your wingtip,l your horizontal booms must remain thick (read: heavy) right to the tip,

The spar doesn't act on it's own. It is the skin which resists most of that moment. The job of the spar flange is mostly to transfer loads between the skins and spar web.
 
Western727
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RE: 787 Winglets?

Sun Dec 20, 2009 3:02 pm



Quoting MSNDC9 (Reply 14):
The Seattle times reported the TOW was 430,000 lbs, which is 72,500 lbs less than its current design max takeoff weight of 502,500lbs, so yes, it probably had a substantial fuel load.

On the very first flight of an experimental, uncertified and unproven aircraft? Sounds irresponsible to me.

Quoting Tdscanuck (Reply 8):
Probably full mains, but max endurance on a 787 has got to be 18 hours, so not very close to full fuel load.

This sounds more logical. I'm confident in assuming much of the bulk of the payload was made up of the testing equipment, and possibly some ballast water (if the aircraft at the time had the testing ballast tanks fitted around the inside of the fuselage, that is)...and not fuel.
Jack @ AUS
 
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Stitch
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RE: 787 Winglets?

Sun Dec 20, 2009 4:17 pm



Quoting Western727 (Reply 20):
On the very first flight of an experimental, uncertified and unproven aircraft? Sounds irresponsible to me.

Some of the critical initial tests (like flutter) probably need to be performed with a decent fuel load in the tanks to accurately reflect how they will respond when loaded for a long-distance customer revenue flight.

Since she only flew three of her scheduled five hours, I wonder how close to MLW she was when she touched down at BFI?
 
tdscanuck
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RE: 787 Winglets?

Sun Dec 20, 2009 6:28 pm



Quoting Western727 (Reply 20):
On the very first flight of an experimental, uncertified and unproven aircraft? Sounds irresponsible to me.

Irresponsible how? "Experimental" is a big of red herring...in this context, it just means it's not certified under the transport category FAR's (yet). The plane is certified (just as an Experimental). "Unproven" is also a little complicated; although you should never underestimate the complexity of systems integration, all of the systems and structures on the plane have already been validated individual, and most of them validated in concert via things like the gauntlet testing.

It's obviously not zero risk, but given the gap between the known limits and the first flight limits (no big load factors, no high speeds, no radical maneuvers) I don't really see how a slightly lower weight would really provide much risk reduction.

Quoting Stitch (Reply 21):
Since she only flew three of her scheduled five hours, I wonder how close to MLW she was when she touched down at BFI?

Flight test aircraft can land above MLW (well, any aircraft can, but the paperwork is easier when you're on an Experimental ticket). I have no idea if they actually did or not but, provided you control sink rate, it's not a big deal.

Tom.
 
Western727
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RE: 787 Winglets?

Sun Dec 20, 2009 7:19 pm



Quoting Tdscanuck (Reply 22):
Irresponsible how? "Experimental" is a big of red herring...in this context, it just means it's not certified under the transport category FAR's (yet). The plane is certified (just as an Experimental). "Unproven" is also a little complicated; although you should never underestimate the complexity of systems integration, all of the systems and structures on the plane have already been validated individual, and most of them validated in concert via things like the gauntlet testing.

Irresponsible in that I'd assume one would want to minimize risk on the very first flight by not maxing out on fuel and instead putting on only, say, half its fuel capacity - and that is only a broad, uneducated guess for I am not qualified to give precise numbers. I mean, why push the envelope that early if there are no intentions to fly a long first flight? Likewise, one would not have wanted to take the 787 out on its very first flight at MTOW.

On the second (or third) flight - sure, fill 'er up, if the flight test team feels the data from the first flight (flutter characteristics, for example, as stated in another post above) justify doing so at that point.

Quoting Tdscanuck (Reply 22):
I don't really see how a slightly lower weight would really provide much risk reduction.

What I meant was only not to have the bulk portion of whatever the total payload weight might be - in fuel as in a max-fuel-capacity set-up. My apologies for not being clear on that.
Jack @ AUS
 
roseflyer
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RE: 787 Winglets?

Sun Dec 20, 2009 8:44 pm



Quoting BMI727 (Reply 2):
I suspect that at least part of it has to do with gate space. After all, gate spacing isn't really an issue for the P-8.

The P-8A does not have winglets because they did not handle icing conditions very well in analysis. The P-8A has to be able to operate in a much different environment than a commercial 737. It has to be able to operate at extremely low altitudes (as low as 50ft) for hours at a time in icing conditions. The raked wingtip handles those conditions better. While fuel efficiency is important, the P-8A is designed more about performance and capabilities since it has a different mission.
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jetmech
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RE: 787 Winglets?

Mon Dec 21, 2009 1:44 am



Quoting Tdscanuck (Reply 12):
Both winglets and raked tips work the same way; they decrease vortex-induced downwash across the wing, which lowers induced drag.

Fair enough. But what could be an easy to grasp yet robust explanation into the physics of reducing the vortex strength at the wing tip?

Quoting Tdscanuck (Reply 12):
elliptical distribution

IIRC, an elliptical lift distribution reduces the overall vortex strength produced by a wing, as vortexes are actually produced off the entire trailing edge of the wing. However, I'm thinking the tip is the most important area as it may have the greatest difference in span wise components of flow.

Quoting Tdscanuck (Reply 12):
drop to zero at the tip.

I see. So I suppose wingtip devices try to modify the flow field in the area immediately inboard of the wing tip to realise an advantageous benefit? If this is true, what exactly is the flowfield modification one is trying to achieve?

I suspect lift is mainly a pressure driven force, with a pressure differential between the lower and upper surfaces being essential. This pressure differential also produces span-wise components of flow, which I suspect are heavily responsible for producing vortexes.

Do winglets and raked wingtips provide a greater span over which one can "taper" off the lift distribution and thus reduce the span-wise components of flow and hence vortex strength?

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
tdscanuck
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RE: 787 Winglets?

Mon Dec 21, 2009 2:30 am



Quoting Western727 (Reply 23):
What I meant was only not to have the bulk portion of whatever the total payload weight might be - in fuel as in a max-fuel-capacity set-up. My apologies for not being clear on that.

Ah, gotcha. Yeah, that makes sense...I don't see why you'd pack more fuel than you really needed, unless it was part of some flutter condition for the wings. Ballast blocks are easy to come by.

Quoting JetMech (Reply 25):
But what could be an easy to grasp yet robust explanation into the physics of reducing the vortex strength at the wing tip?

You did a pretty good one:

Quoting JetMech (Reply 25):
Do winglets and raked wingtips provide a greater span over which one can "taper" off the lift distribution and thus reduce the span-wise components of flow and hence vortex strength?

All wingtip devices are designed to make the wing behave like it's got more span; this gets you closer to the ideal case of an infinite (or 2D) airfoil, which has zero induced drag.

Quoting JetMech (Reply 25):
IIRC, an elliptical lift distribution reduces the overall vortex strength produced by a wing, as vortexes are actually produced off the entire trailing edge of the wing.

Correct. There are some really awful wingtip devices out there that are based on the totally erroneous belief that the vortex comes off the tip alone. The whole trailing edge is shedding a big vortex sheet...this quickly rolls up into the "wingtip" vortex within a span length or two, but the source of the vorticity is the whole wing, not just the tip.

Quoting JetMech (Reply 25):
However, I'm thinking the tip is the most important area as it may have the greatest difference in span wise components of flow.

The tangential speed from a vortex drops off rapidly with distance. Since the area near the tips are closest to the vortex, they feel its effects most.

Quoting JetMech (Reply 25):
So I suppose wingtip devices try to modify the flow field in the area immediately inboard of the wing tip to realise an advantageous benefit? If this is true, what exactly is the flowfield modification one is trying to achieve?

They actually modify the flowfield on the whole wing (since we're subsonic, changes anywhere propagate everywhere). The flowfield modification they're trying to achieve is to reduce spanwise flow.

Quoting JetMech (Reply 25):
I suspect lift is mainly a pressure driven force, with a pressure differential between the lower and upper surfaces being essential. This pressure differential also produces span-wise components of flow, which I suspect are heavily responsible for producing vortexes.

Life is entirely a pressure driven force. Reynolds numbers for transonic flight are so high that you can mostly ignore viscosity for lift purposes, which just leaves you with pressure. The span-wise component of flow is entirely responsible for the vortex sheet coming off the back edge, which is what produces the vortices. If you could have no span-wise flow, you'd have no trailing vortex and no induced drag.

Tom.
 
BMI727
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RE: 787 Winglets?

Mon Dec 21, 2009 6:17 am



Quoting Tdscanuck (Reply 26):
Life is entirely a pressure driven force.

So is lift.  Wink

Quoting JetMech (Reply 25):
But what could be an easy to grasp yet robust explanation into the physics of reducing the vortex strength at the wing tip?

A wing works entirely by creating a pressure differential with higher pressure below and lower pressure above. As in any pressure differential, the fluid will try to equalize the pressure. It tries to do this by flowing around the wingtip from bottom to top, creating a vortex in the clockwise direction when looking from the front.

This vortex lessens the lift on an area of the wingtip. A winglet or wing fence keeps the point where the high and low pressure air meets away from the wing, restoring lift onto the tip. In essence, the effect is similar to increasing the span or I suppose more importantly the aspect ratio. Increasing the aspect ratio will of course lower the induced drag of the wing.

I know less about how they reduce the strength of the vortex, but I suspect is has to do with the fact that the winglet, or raked wingtip, creates far less lifting force than the wing. Less lift force means less pressure differential, which means a weaker vortex.
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tdscanuck
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RE: 787 Winglets?

Mon Dec 21, 2009 4:49 pm



Quoting BMI727 (Reply 27):
I know less about how they reduce the strength of the vortex, but I suspect is has to do with the fact that the winglet, or raked wingtip, creates far less lifting force than the wing.

The strength of the vortex is basically a measure of how hard the work is working. Since the wingtip device lowers induced drag, the wing doesn't have to work as hard and the vortex strength goes down.

This is somewhat circular (no pun intended) since it's the vortex that causes the induced drag in the first place, but most aerodynamics are circular. It's not "this-causes-that", it's"this alters the flow so that we get all these other effects."

Tom.
 
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jetmech
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RE: 787 Winglets?

Tue Dec 22, 2009 3:22 am



Quoting BMI727 (Reply 27):
A winglet or wing fence keeps the point where the high and low pressure air meets away from the wing, restoring lift onto the tip.

Fair enough. The down wash produced by the vortex system tilts the lift vector backward causing induced drag. However, I suspect that this phenomenon occurs off the wing behind the trailing edge.

Quoting BMI727 (Reply 27):
Less lift force means less pressure differential, which means a weaker vortex.

Sure. But I think the root cause of the vortices is the span wise components of flow caused by the pressure differential.

Quoting Tdscanuck (Reply 28):
Since the wingtip device lowers induced drag, the wing doesn't have to work as hard and the vortex strength goes down.

Interesting! So part of the improvement provided by wing tip devices is due to the global effect it has on the main plane. I suppose this is similar to the roll-on benefits one gets when they remove structural weight from the airframe. One of those lovely win-win situations!

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
tdscanuck
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RE: 787 Winglets?

Tue Dec 22, 2009 6:07 am



Quoting JetMech (Reply 29):
But I think the root cause of the vortices is the span wise components of flow caused by the pressure differential.

Very much so. If you have a true 2D airfoil (equivalent to an infinite wing or a wind tunnel with a full-span wing) you get zero shed vortices precisely because you don't have any span-wise flow.

Quoting JetMech (Reply 29):
So part of the improvement provided by wing tip devices is due to the global effect it has on the main plane.

Almost all of it, in fact. Getting this point wrong is behind most of the bad explanations for how wingtip devices work...the wingtip device alters flow on the entire wing (on the entire airplane, actually) and it's the global impact of that that gives you the total effect. This is why tiny wingtip devices don't do much. If you're altering flow over large areas, you need large devices.

Tom.
 
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Faro
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RE: 787 Winglets?

Tue Dec 22, 2009 10:43 am



Quoting Tdscanuck (Reply 26):
Quoting JetMech (Reply 25):
So I suppose wingtip devices try to modify the flow field in the area immediately inboard of the wing tip to realise an advantageous benefit? If this is true, what exactly is the flowfield modification one is trying to achieve?

They actually modify the flowfield on the whole wing (since we're subsonic, changes anywhere propagate everywhere). The flowfield modification they're trying to achieve is to reduce spanwise flow.

I imagine this is especially relevant to swept wings. If your wing is straight, spanwise flow will be minimal and caused mainly by the lateral airflow component off the fuselage impinging on the wing root. Is this so? Theoretically, if you have a straight wing with relatively high aspect ratio, you will have next-to-nil spanwise flow?

Faro
The chalice not my son
 
tdscanuck
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Joined: Wed Jan 11, 2006 7:25 am

RE: 787 Winglets?

Wed Dec 23, 2009 1:41 am

Quoting Faro (Reply 31):
If your wing is straight, spanwise flow will be minimal and caused mainly by the lateral airflow component off the fuselage impinging on the wing root. Is this so?

Assuming that the wing is roughly at the fuselage midpoint, the spanwise flow due to fuselage displacement should be pretty minimal (since it needs to close back up to fill in at the back). Even if the wing is straight, you've got high pressure underneath that wants to get on top. It can't do it easily over the front (airspeed) or back (sharp trailing edge) but it can easily go around the ends.

Quoting Faro (Reply 31):
Theoretically, if you have a straight wing with relatively high aspect ratio, you will have next-to-nil spanwise flow?

Basically, yes. That's why induced drag drops off with high aspect ratio...more of the wing is far from the tips and isn't impacted by spanwise flow. At the limit of infinite aspect ratio, spanwise flow and induced drag go to zero.

Tom.

[Edited 2009-12-22 17:42:14 by tdscanuck]

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