Aerodynamics is so complicated compared to other disciplines

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henryk

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https://www.youtube.com/watch?v=QKCK4lJLQHU

Dough McLean, retired Boeing Technical Fellow on fire debunking a lot of even elaborate theories of aerodynamics. It's worth 48 minutes of your life. Be forewarned it is full geek at full throttle.


Enjoy.
=iff you have in minde the kinetic energy of the air particles,
aerodynamic fenomena are very simple!

=NO atractive forces between particles,only ellastic scattering...

-Vaverage=circa 500 m/s.\1800 km/h !\
 

Kingfisher

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https://www.youtube.com/watch?v=QKCK4lJLQHU

Dough McLean, retired Boeing Technical Fellow on fire debunking a lot of even elaborate theories of aerodynamics. It's worth 48 minutes of your life. Be forewarned it is full geek at full throttle.

Enjoy.
I just got off a 787 and wondered why it only has moderately upturned wing tips. According to McLean, local measures to take out the vorticity at the tip does not affect the overall flow field and are thus useless to reduce induced drag. Does that mean winglets were just a fashion and actually have no effect? Surely, gains in fuel efficiency are measurable and these data would prove otherwise?!

Also interesting his look at wingtip mounted propellers, which he says have not the expected effect either. I had hoped for a tilt rotor plane with short wings they would have the effect of an increased span, as I thought a winglet does.

It makes sense that Bernoulli's equation as the sole explanation for lift by assuming the air travels faster over the top of a cambered airfoil to meet the air flowing over the flat bottom must be wrong, since it totally does not explain how a symmetrical airfoil, or upside down flying airfoil, is generating lift. But obviously a cambered airfoil produces more lift for a given angle of attack than an uncambered, doesn't it? Is that explained by Bernoulli, the difference between the two?

Seems a fair bit of unknowns and myths still out there, given the time we've been flying. I just had this idea:
-Draw a triangle between the angled-up chord line and the horizontal line under the airfoil
-Multiply the area of the triangle by the span. This will give the volume of air that is displaced if the airfoil travels one cord length.
-if the airfoil is cambered, add the area between the straight chord line and the cambered line to the triangle to account for the extra volume. If the airfoil is flying upside down, subtract the area instead.
-assume a certain velocity and divide it by the chord length to get chord length/time.
-assume the volume is displaced down by the height of the triangle in the time it takes to travel one chord length to calculate the acceleration of the air volume.
-Calculate lift force F = volume x density x acceleration

Does this explain lift sufficiently?
 

Aviator168

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Does that mean winglets were just a fashion and actually have no effect? Surely, gains in fuel efficiency are measurable and these data would prove otherwise?!
No. He didn't say winglets produce no efficiency gain in other area (thrust) either.

Like henry said. The whole thing is about momentum of fluid molecules behind the "bump". Look at the propulsive wing. You will see the same air stream as a wing soon after you turn the motor on even it is not moving and not in a moving air stream.

 

mcrae0104

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Seems a fair bit of unknowns and myths still out there, given the time we've been flying. I just had this idea:
-Draw a triangle between the angled-up chord line and the horizontal line under the airfoil
-Multiply the area of the triangle by the span. This will give the volume of air that is displaced if the airfoil travels one cord length.
-if the airfoil is cambered, add the area between the straight chord line and the cambered line to the triangle to account for the extra volume. If the airfoil is flying upside down, subtract the area instead.
-assume a certain velocity and divide it by the chord length to get chord length/time.
-assume the volume is displaced down by the height of the triangle in the time it takes to travel one chord length to calculate the acceleration of the air volume.
-Calculate lift force F = volume x density x acceleration

Does this explain lift sufficiently?
No, the flow field affected is much larger than just the little triangle below the wing. What you're describing is basically like this:

image.jpg
(see http://www.av8n.com/how/htm/airfoils.html)
 

Topaz

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I just got off a 787 and wondered why it only has moderately upturned wing tips. According to McLean, local measures to take out the vorticity at the tip does not affect the overall flow field and are thus useless to reduce induced drag. Does that mean winglets were just a fashion and actually have no effect? Surely, gains in fuel efficiency are measurable and these data would prove otherwise?!...
You can think of winglets as simply a continuation of the wing, even though they're turned upwards. It's best if people stop interpreting winglets as "reducing the tip vortex" and start thinking of them as increasing the effective span of the wings. Span is a strong driver of wing efficiency, which is why sailplanes have long wings. Winglets are a way of increasing the effective span of a wing without increasing the physical width of the wing - usually because you have some kind of span limitation.

For airliners, it's usually the width of the available gates at airport terminals. For sailplanes, it's the span limitation of the racing class rules - 15 meters, 18 meters, etc. Use of winglets allows the airplane to behave as if it has a greater span than the actual width of the wings.

The 787's wing tips are the way they are because they actually were able to make wings of a span suitable for the needed design mission of the airplane, and still fit inside the standard airport terminal gate. If the airplane had needed even more span than it has, you can bet the wings would've had winglets instead.
 

BJC

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Summary statement from McLean's book:

In general, in the choice between winglets, horizontal span extensions, and other tip-device configurations, there is no clear-cut favorite for all applications. In terms of the basic physics, the benefits they offer tend to be comparable. Which choice is favored for a particular application depends on the details of the baseline airplane design, both aerodynamic and structural, and on the mission objective. And the differences between the choices are usually not large.
As mentioned above by Topaz, many new designs / updated designs are constrained by existing airport gate spacing.


BJC
 

Jay Kempf

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You can think of winglets as simply a continuation of the wing, even though they're turned upwards. It's best if people stop interpreting winglets as "reducing the tip vortex" and start thinking of them as increasing the effective span of the wings. Span is a strong driver of wing efficiency, which is why sailplanes have long wings. Winglets are a way of increasing the effective span of a wing without increasing the physical width of the wing - usually because you have some kind of span limitation.

For airliners, it's usually the width of the available gates at airport terminals. For sailplanes, it's the span limitation of the racing class rules - 15 meters, 18 meters, etc. Use of winglets allows the airplane to behave as if it has a greater span than the actual width of the wings.

The 787's wing tips are the way they are because they actually were able to make wings of a span suitable for the needed design mission of the airplane, and still fit inside the standard airport terminal gate. If the airplane had needed even more span than it has, you can bet the wings would've had winglets instead.
Correct. Look at the flow field around most untreated wingtips and the vortex pulls inboard significantly the farther inboard the vortex is pulled the bigger the loss due to the shape of the tip that causes the effect. Span loading and AR really drive efficiency of a planform. So anything that gives you longer effective span gives you larger effective AR and lower span loading. What winglets do is keep the wing that isn't winglet working all the way to the winglet reducing losses. At least that is the way it looks to me when I see bad and good flow fields around the wingtip. Having the wing tip chord drop off to a tiny number with sweep and polyhedral also works.
 

Dana

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It makes sense that Bernoulli's equation as the sole explanation for lift by assuming the air travels faster over the top of a cambered airfoil to meet the air flowing over the flat bottom must be wrong, since it totally does not explain how a symmetrical airfoil, or upside down flying airfoil, is generating lift. But obviously a cambered airfoil produces more lift for a given angle of attack than an uncambered, doesn't it? Is that explained by Bernoulli, the difference between the two?
Bernoulli's equation doesn't "explain" lift; rather, it's useful in describing the pressure distribution around a curved surface immersed in a fluid flow. It's not the whole story.

A symmetrical airfoil produces lift only at a nonzero AOA. At that point it is no longer symmetrical with respect to the relative wind.

Dana
 

Glider

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I doubt that, in a highly swept wing, a winglet does anything significant to increase the effective span of the wing because the spillover of air occurs far behind the flow field of the majority of the wing span. That, and a highly swept wing would seem to have span wise flow (momentum) (top and bottom) that (at least partially) interferes with the action the winglet is intended to counteract.

I do imagine that the turning effect of the winglet can redirect some of the momentum of the spillover air, providing some forward thrust at the each wingtip, even for a highly swept wing. This would seem to require the spill-over to begin ahead of the wing.

I am not an aerodynamicist, but that doesn't stop me from running off at the keyboard.
 

autoreply

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You can think of winglets as simply a continuation of the wing, even though they're turned upwards. It's best if people stop interpreting winglets as "reducing the tip vortex" and start thinking of them as increasing the effective span of the wings. Span is a strong driver of wing efficiency, which is why sailplanes have long wings. Winglets are a way of increasing the effective span of a wing without increasing the physical width of the wing - usually because you have some kind of span limitation.

For airliners, it's usually the width of the available gates at airport terminals. For sailplanes, it's the span limitation of the racing class rules - 15 meters, 18 meters, etc. Use of winglets allows the airplane to behave as if it has a greater span than the actual width of the wings.

The 787's wing tips are the way they are because they actually were able to make wings of a span suitable for the needed design mission of the airplane, and still fit inside the standard airport terminal gate. If the airplane had needed even more span than it has, you can bet the wings would've had winglets instead.
Whether span is constrained or not, winglets provide a benefit. It's a common misconception, but for a given induced drag (effective span so to speak) a wing with winglets will provide both a lower wing bending moment (lower structural wing mass) and a lower wetted wing area (reducing profile drag).

Plenty of evidence too, every single modern aircraft with an AR>35 has winglets (save one). Span constraints don't apply there.

Whether it's discrete "perpendicular" winglets, or gradually increasing dihedral doesn't matter as much. The up- and backwards swept tips on the Dreamliner, A350 etc all work the same way. Continously curved tips are just a lot harder to analyse, which is why they're later on the stage.
 

skier

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Whether span is constrained or not, winglets provide a benefit. It's a common misconception, but for a given induced drag (effective span so to speak) a wing with winglets will provide both a lower wing bending moment (lower structural wing mass) and a lower wetted wing area (reducing profile drag).
I'm not sure I understand what you're saying here. Can you elaborate?
 

Jay Kempf

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Whether span is constrained or not, winglets provide a benefit. It's a common misconception, but for a given induced drag (effective span so to speak) a wing with winglets will provide both a lower wing bending moment (lower structural wing mass) and a lower wetted wing area (reducing profile drag).

Plenty of evidence too, every single modern aircraft with an AR>35 has winglets (save one). Span constraints don't apply there.

Whether it's discrete "perpendicular" winglets, or gradually increasing dihedral doesn't matter as much. The up- and backwards swept tips on the Dreamliner, A350 etc all work the same way. Continously curved tips are just a lot harder to analyse, which is why they're later on the stage.
I've heard the reduced bending moment before. This seems on it's surface very counterintuitive unless the camber of the winglets are reversed from the main wing providing a reverse moment at the tip. If that was the case that would be like cutting off the last portion of each wing and gluing it on the other's wing tip.
 

autoreply

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I'm not sure I understand what you're saying here. Can you elaborate?
If we taken an existing, planar wing with a given induced drag, spar weight and wetted area and we add two options that both have the same amount of induced drag and the same amount of maximum lift (stall):
1) a 1 ft tall winglet
2) a 2 ft long span extension


Then the winglet will have less than a third the surface area of the span extension of (2), in practise yielding about half the extra drag that the span extension caused.

Further, the winglet has disproportionally more lift inboard and nothing beyond it's original tip. If we assume structural wing weight scales with total bending moment (it often doesn't) the winglet will yield in less extra structural mass than a tip extension if both reduce induced drag by the same amount.
 

Kingfisher

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If we taken an existing, planar wing with a given induced drag, spar weight and wetted area and we add two options that both have the same amount of induced drag and the same amount of maximum lift (stall):
1) a 1 ft tall winglet
2) a 2 ft long span extension


Then the winglet will have less than a third the surface area of the span extension of (2), in practise yielding about half the extra drag that the span extension caused.

Further, the winglet has disproportionally more lift inboard and nothing beyond it's original tip. If we assume structural wing weight scales with total bending moment (it often doesn't) the winglet will yield in less extra structural mass than a tip extension if both reduce induced drag by the same amount.
I find it hard to understand that a smaller vertical surface has the same or greater positive effect as a larger horizontal addition. If you take it to more extreme, shouldn't it mean that a carrier aircraft with wingtips folded up should perform the same or better than if the tips were folded flat?
 

plncraze

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The winglet changes how the low pressure is distributed along the upper surface of the wing. Is it like an end plate?
 

Topaz

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Whether span is constrained or not, winglets provide a benefit. It's a common misconception, but for a given induced drag (effective span so to speak) a wing with winglets will provide both a lower wing bending moment (lower structural wing mass) and a lower wetted wing area (reducing profile drag).

Plenty of evidence too, every single modern aircraft with an AR>35 has winglets (save one). Span constraints don't apply there.

Whether it's discrete "perpendicular" winglets, or gradually increasing dihedral doesn't matter as much. The up- and backwards swept tips on the Dreamliner, A350 etc all work the same way. Continously curved tips are just a lot harder to analyse, which is why they're later on the stage.
Given that the span limitation and not having to redesign the wing is the stated reason by Boeing, Airbus, and third-party suppliers for adding them, the textbooks say the same, and the engineers from Boeing I know say the same, I think I'll stick with their explanation, thank you.

Or are you saying they're all wrong?
 
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