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Author Topic: Lift: Bernoulli or Newton or Both  (Read 3737 times)
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Allen Carter
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« Reply #15 on: July 13, 2010, 12:50 PM »

Now Jim, get real.


It's called line.
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« Reply #16 on: July 13, 2010, 12:59 PM »

 Cool Cool Cool

As you approach the top limit of the window with a dual line kite, it's mostly Bernoulli keeping it up there.  As you approach bottom, level with the flier, center window, you get the maximum out of Newton's bit of the equation with comparatively little Bernoulli. 

Don't ya think?

obi
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normofthenorth
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« Reply #17 on: July 13, 2010, 10:44 PM »

No, Obi, I don't!

Here's one: Have you noticed that boat propellers are often shaped with rounded leading edges and swept-back (concave) trailing edges? And have you noticed that modern fan blades are often shaped EXACTLY the OPPOSITE way -- like a boat propeller turning BACKWARDS?

I posted that mystery on a physics website a while ago. An interesting discussion ensued, after everybody agreed (and saw links to photos that proved) that it's true.

The consensus there was that the boat prop shape designed was to maximize the thrust without cavitation -- SORT of like stalling, but more what happens in a denser fluid, where actual voids form -- and the fan shape was to minimize noise. And it was more-or-less a coincidence that they arrived at the exact opposite shapes.
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Norm in Toronto
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« Reply #18 on: July 14, 2010, 03:08 AM »

Huh?  Missing the logic link there...but I am an idiot.

obi
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stapp59
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« Reply #19 on: July 14, 2010, 03:40 AM »

As you approach the top limit of the window with a dual line kite, it's mostly Bernoulli keeping it up there.  As you approach bottom, level with the flier, center window, you get the maximum out of Newton's bit of the equation with comparatively little Bernoulli. 

Don't ya think?

Not sure Obi.  In the center of the window maximum sail area faces the wind for maximum pull and drive.  As the kite moves towards the edges the kites surface area relative to the wind decreases as does pull and drive.  Seems pretty Newtonian ie action/reaction.

If lift 'switched' to Bernoulli at the edges then kites would have a full 180 degree window with strong pull at the edges.  That does not seem to be the case; even with foils.

Most likely a mix of explanation needed and our discussion will not change the actual physics  Cheesy
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DWayne
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« Reply #20 on: July 14, 2010, 05:15 AM »

Steve, you hit the nail on the head. If Bernoulli took over at the top of the window, the kite would pull almost as hard at the top of the window as it does in the center of the window.
Newtonian forces rule here.  Wink

Denny
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mikenchico
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« Reply #21 on: July 14, 2010, 06:40 AM »

Not neccesarily, that would assume the lift generated was equal between both the Bernoulli & Newtonian forces if indeed both have some effect, but the Bernoulli effect is highly dependent on speed through the fluid, Newtonian I would guess is more effected by the speed of the fluid against the sail. Also foils at the top and edges of the windows have slowed down in their movement through the air so even any Bernoulli effect decreases.

I think we are all overlooking the effect of the fairies too.

« Last Edit: July 14, 2010, 06:43 AM by mikenchico » Logged

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zippy8
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« Reply #22 on: July 15, 2010, 06:31 PM »

Question:- if Bernoulli isn't a factor at some point in the wind window then why on earth are the vast majority of duallies shaped like a Rogallo wing, which most assuredly does generate its lift Swiss-style ?

Mike.
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JimB
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« Reply #23 on: July 15, 2010, 09:18 PM »

Bernoulli may contribute some efficiencies in regard to specific designs but Newton is why kites do what they do in the first place overwhelmingly IMO. 
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inewham
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« Reply #24 on: July 16, 2010, 12:37 AM »

They both work together as described in the link I posted and
normofthenorths post.

The reduction in pressure isn't significant enough to generate lift itself, however it contributes to the coanda effect/circulation causing air across the top of the wing to be deflected downward off the trailing edge. That moving air down of the top of the aerofoli is what generates the lift from the top of the wing; bernoulli deflecting the air and the fact that air molecules are moved leads to a bit of newton.

This is seperate from the newton explanation off the front of the kite and you need an aerofoil for the above to work.
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« Reply #25 on: July 16, 2010, 12:51 AM »


This is seperate from the newton explanation off the front of the kite and you need an aerofoil for the above to work.


Just for the sake of a good discussion  Wink

So what's going on with the flexi, it has a aerofoil duilt in the wing that produces downforce?
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Pete
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« Reply #26 on: July 16, 2010, 01:31 AM »

So what's going on with the flexi, it has a aerofoil duilt in the wing that produces downforce?

Yep the profile kicks up at the tail end to stop them flipping over becaus theyre bridled only at the LE (wasn't that a key element of the patent?)

My guess is the net effect, despite that, is that the top of the wing still results in air being deflected down. But that's a guess Tongue

What do I know? I design by feel not by maths  Kiss
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« Reply #27 on: July 16, 2010, 01:50 AM »

Question:- if Bernoulli isn't a factor at some point in the wind window then why on earth are the vast majority of duallies shaped like a Rogallo wing, which most assuredly does generate its lift Swiss-style ?

Mike.
Where is the evidence for the incorrect "most assuredly"  statement, were the Dutch-Swiss theorists to be correct try explaining the extreme downwash (Newton) produced by aircraft, especially rotary wing aircraft, if reduced air pressure above a wing provided sufficient lift (sucking the aircraft upward) you would in effect create an upwash below the aircraft as pressures equalized.
Some real red herrings in the thread, propellors and fans for example, one a minimum slip device designed to propel a vessel through an uncompressible fluid, t'other a static device designed to pump air, (a maximum slip device), totally different requirement hence different blade designs.
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JimB
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« Reply #28 on: July 16, 2010, 03:55 AM »

This is all great but I was serious in my first post in this thread.

It's seems a lot of this theory is being looked at without regard to the lines attached to the kite. It isn't really flying. It's a tethered airfoil, being held in a particular relationship to the wind and being moved about accordingly.

Without the lines, and the proper AOA, there is no "flight". Everything else is secondary to this.

Let's get those pictures of properly bridled tents and Prism kite bags "flying" up here.

I know some of you guys have them someplace.
« Last Edit: July 16, 2010, 04:00 AM by JimB » Logged
zippy8
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« Reply #29 on: July 16, 2010, 12:12 PM »

Where is the evidence for the incorrect "most assuredly"  statement....
If you're going to try to argue that Rogallo wings produce lift as anything other than true aerofoils then I'm afraid I can't help you.

Mike.
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