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Author Topic: Lift: Bernoulli or Newton or Both  (Read 3953 times)
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chilese
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« on: July 10, 2010, 04:03 PM »

Last night Kent and I got into a mild argument (reference Monty Python for how effective we were) on what makes kites fly.

I am of the opinion it is mostly vector forces (Newtonian).

Kent is of the opinion it is the lower pressure on the back of the kite sucking the kite away from the pilot. (Bernoulli)

I yielded to the point that there was a little bit of the Bernoulli effect.
Kent yielded that maybe 3% was Newtonian.

Well, then I went to Wikipedia and the experts get into debates on the subject.

http://en.wikipedia.org/wiki/Lift_(force)

Now I'm convinced it's almost all Newtonian or:
1 You couldn't launch a kite in zero wind (velocity on the back side = 0)
2 Kites wouldn't be able to hold a stall
3 Revs shouldn't be able to change direction

Maybe those inflatable water foils have some Bernoulli stuff going on, but still not much.

Someone argue with me or help me convince Kent he is merely a naysayer.  Roll Eyes
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johnfarl
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« Reply #1 on: July 10, 2010, 04:11 PM »

I agree with the Newton thing.  But the Bernouli adds a force vector parallel to the ground in line with the lines that is a Newton thing.

That is why in low wind you step back to launch and then keep the kite moving to get a force vector from the Bernouli.

John

PS I think
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Allen Carter
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« Reply #2 on: July 10, 2010, 06:07 PM »

Typical NASA, long winded, but interesting:

http://www.grc.nasa.gov/WWW/K-12/airplane/kitefly.html

http://www.grc.nasa.gov/WWW/K-12/airplane/kitefor.html

Even a kite modeler...

http://www.grc.nasa.gov/WWW/K-12/airplane/kiteprog.html

« Last Edit: July 10, 2010, 06:09 PM by Allen Carter » Logged

Allen, AKA kitehead
inewham
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« Reply #3 on: July 11, 2010, 03:03 AM »

I've played around with modelling software and the pressure difference is so negligible that it would never suck any sort of wing upward on its own. As you say, intuitively you can guess its newton; half the time a kite is stalled or about to stall. However both arguments leave out circulation, which kind of brings both together:

http://www.avweb.com/news/airman/183261-1.html

So both, kind of...

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stapp59
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« Reply #4 on: July 11, 2010, 06:47 AM »

I liked your article Ian, thanks for posting!

Intuitively for me, Newton forces best explain kite aerodynamics as wind speed and wind quality seem most important to overall flight performance.

Sport kites are all about three basic performance areas:

- The overall shape of the kite and sail determines how the sail catches the wind (pull) and how it slips off the trailing edge (forward speed). Think about bridling a sturdy piece of cardboard or an open trash bag, maybe even a certain make/model kite bag.  Cheesy

- The standoffs, channel, LE shape, and bridling control how the kite carves through the air during 2D flight.  Think straight lines, curves, and corners.

- The shape, balance, and bridling also determines how the kite spins and rotates during slack line maneuvers in 3D 'flight'.  Think of a controlled falling leaf.  Wink

The forward speed of the kite is certainly secondary and most affected by wind speed, angle of attack, and drag.

Venting experiments show me vent size is most related to reduced pull though vent shape and location have a large impact on overall handling and performance.

Interesting topic. I have a lot more questions than answers...
« Last Edit: July 11, 2010, 07:02 AM by stapp59 » Logged
mikenchico
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« Reply #5 on: July 11, 2010, 09:34 AM »

I haven't studied it either nor do I have the resources, physical or mental. But I have given it some thought during these discussions. I've also come to the conclusion that it is predominately Newtonian forces on most kites. There are exceptions most notably with foils. In the controllable kite field I would point out the Flexi Foil where the pull definitely increases as the speed increases even though the surface area presented to the wind decreases, so there has to be a Bernoulli effect taking place IMO.

Some SLK foils must also exhibit some Bernoulli lift, if not the rib design would not have much effect on those kites performance. But again many don't seem to rely on Bernoulli lift, cheap pocket foils which fly at a very low angle for example. But the Hagaman Para-foils flew at a very high angle and presented a foil shape to the wind rather then a flat surface, those must've been receiving much of their lift from the Bernoulli effect.

Some other soaring kites such as Delta's and Genki's may receive some lift from the Bernoulli effect. Although with any flat surface kite it isn't really from air traveling over the opposite surfaces at differing speeds causing a pressure differential, but that the air on the upper surface is being disturbed into eddy's thus decreasing pressure, does that then take it out of the Bernoulli theory?

I don't believe that at any time except under perfect laboratory conditions could the lift on a surface presented to moving stream of liquid be explained completely by either Bernoulli or Newtonian physics, especially when that surface is constantly presented at varying angles to that flow. And kites reside in a very imperfect environment.

To me it's more a debate on the percentage of lift recieved from the two effects.

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"Living is easy with eyes closed, misunderstanding all you see" John W Lennon

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« Reply #6 on: July 11, 2010, 11:44 AM »

The correct answer is absolutely "both", but not in the way you suggest. There's no conflict between the theories and equations of Newton and the theories and equations of Bernoulli.

For every action there IS an equal and opposite reaction. So if a kite is being pushed upwards (or sideways, etc.) with a force of X, you can be completely sure that the air is being pushed in the opposite direction with the same force. Newton was right, and his theories are right.

The Newtonian explanations also have the advantage that they are very simple, and "intuitive" to those of us who "get" physics at all. But they don't tell you HOW a specific kite of a specific shape, makes the direction of air flow change to make the "push".

Bernoulli added a bunch of (much more complicated) math and physics to help explain the "HOW". And it's generally conceded to be completely accurate and correct, too. HOWEVER, Bernoulli's explanations suffer from two problems: (1) The math and physics are very complex, and (2) the common simplification -- which I call "High School Bernoulli", like saying that all lift is due the "sucking" from the pressure drop from the higher air velocity around the "outside" of an asymmetrical curve -- is JUST PLAIN WRONG!! (That "sucking" is a relatively minor effect, the curves don't have to be asymmetical, and the two air streams don't actually meet at the same point!)

In addition to the links above (which I haven't followed today), there are a couple of reasonably simple web-sites with names like "How airplances REALLY fly" that elucidate the issues and explain the folly of "High School Bernoulli". (There's also an excellent web-site, maybe from the Netherlands, explaining how sailboat sails produce "lift", and how they don't.)

But there's no conflict between Bernoulli and Newton. All correct descriptions of reality are correct, and consistent with each other! Even the High-School Bernoulli effect of the low-pressure area on the outside of a sail or wing does exist, though its force is inadequate by itself to explain lift. But the way it works also deflects the airflow over the top of an airplain wing, so it exits the wing heading DOWNwards, which creates (the identical amount of) Newtonian force/counterforce. The fancy math all adds up. It's not as if Newton explains X% of the lift and Bernoulli explains the other 100-X%, because they EACH explain 100%, just from different perspectives.

Another name that belongs here is Coanda. The Coanda effect elaborates how and why a fluid (like air) can be induced to follow the outside of a curve in the first place. When the airflow follows that curve (and the corresponding curve on the INside or windward or "near" of the sail or wing), you get lift (Newtonian AND Bernoullian). When the flow is interrupted or induced to "leave" the outside surface -- either because the angle of attack is too square, or because something "knocked the wind out of the sails" -- the lift stops. Coanda helps explain when that flow will stay attached, to create the lift, and when it won't.
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Norm in Toronto
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« Reply #7 on: July 11, 2010, 02:34 PM »

Screw the Newton and Bernoulli concepts, kite flight is magical as far as I'm concerned.

Ron
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« Reply #8 on: July 11, 2010, 02:37 PM »

Another take on the Newton and Bernoulli concepts is that one blows and the other sucks. Cheesy

Ron

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inewham
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« Reply #9 on: July 11, 2010, 03:43 PM »

...the way it works also deflects the airflow over the top of an airplain wing, so it exits the wing heading DOWNwards, which creates (the identical amount of) Newtonian force/counterforce.
...
Another name that belongs here is Coanda.

Thats whats explained in the link i posted above - its got some nice animations showing how the air above the wing is deflected downward.
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Gamelord
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« Reply #10 on: July 12, 2010, 02:43 PM »

John, I wouldn't call it an argument...it was just a very serious discussion - wouldn't have been near as serious if the wind had been better. Smiley

Besides, I am right so it doesn't matter. LOL  Cheesy Cheesy
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711jrp
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« Reply #11 on: July 12, 2010, 02:47 PM »

If that one is messing with your head, here's one for you..

How about the flexi stackers, thier foil shape is upside down like a race car wing,

took me a while to work out.
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Pigs might fly, but only if you bridle them right!!

Pete
stapp59
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« Reply #12 on: July 12, 2010, 04:59 PM »

If that one is messing with your head, here's one for you..

How about the flexi stackers, thier foil shape is upside down like a race car wing, took me a while to work out.

Have not seen these.  I assume the reverse airfoil help reduce pull as the wind and forward speed picks up...
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lylenc
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« Reply #13 on: July 12, 2010, 06:36 PM »

Both explain lift. Why the chicken crossed the road was already taken ...

to get a lift to the kite field.
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Craig     Walla Walla, WA     Just One More!
JimB
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« Reply #14 on: July 12, 2010, 09:32 PM »

String makes kites fly.

No string=No fly.

Simple.
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