Pusher or Sucker ?

Sails.

How do they work ?

Do you believe that they do push you along ?

or

They do suck you off ?

You could cast your vote now.

Suck.

As NASA and Tom Speer (a Boeing aerodynamicist who also designs the wings for the winning US America's Cup team) says, there are basically a bunch of different ways to explain the way sails and wings work. Each of these ways is as correct as the other, essentially. There's also some wrong ways, like the "equal time" concept.

See www.grc.nasa.gov/www/k-12/airplane/right2.html

The "Newtonian lift"/lift from flow turning" theory works best for me. The sail redirects the wind and shoves it out the back. As Isaac Newton said, a force creates an equal and opposite reaction, therefore shoving air out the back shoves the rig forward.

Sure, there is laminar flow, attached flow, boundary layer effects and all that - but they are really about ensuring that the flow moves across the sail efficiently, rather than breaking down into turbulent flow which moves more slowly and less uniformly and therefore you shove less air out the back, and at a lower speed.

Look up Tom's posts on Boat Design Forum. Also look out for Mark Drela, a professor of aerodynamics at MIT and designer of America's Cup wings and world-record human powered aircraft and boats. Those guys really, really know their stuff - and unlike a lot of the stuff around, the theories they speak of also work in real life.

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RAL INN said..
No such thing as Suck just Push.
>>>>.

You'd think macro would know that.

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Chris 249 said..

The "Newtonian lift"/lift from flow turning" theory works best for me

that looks like "pushing" for me
you are saying that air molecules are pressing on the sail or transfer their kintetic energy onto our sail if I understand you correctly...
your vote then is +1 for pushing

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Macroscien said..

decrepit said..

RAL INN said..
No such thing as Suck just Push.
>>>>.

You'd think macro would know that.

with all respect. This is not about me the original question sounds : what do YOU or anybody else think that it is behind our propulsion .
Sort of poll what our SB community thinkand I am completely neutral /unconcerned if that is true or not

I think what RAL INN was getting at, is that the question is invalid, there actually is no such thing as "suck" so you can't ask the question "is it pull or suck".
And my comment was about somebody with science in their name talking about something that doesn't exist.

Maybe a bit pedantic I guess, we know what you mean, is it the lower pressure on the leeward side or the higher pressure on the windward side doing the work?
Well my opinion is it both, it's the difference between the two that causes the lift, so you can vote me as neutral.

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TristanF said..
Not much wind about, clearly

Well there is, but I don't sail in the dark.

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TristanF said..
Not much wind about, clearly

ok that is surprising but still valid answer

you vote neither pulling or pushing is going on

0 push 0 sucking

must be something esle that keep us going over that water and our sail just help us to keep balance straight

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John340 said..
The foil shape of the sail bends the wind around the foil and creates low pressure on the leeward side of the sail and high pressure on the windward side of the sail. This pressure differential pushes and pulls the sail from the high pressure to the low pressure. So the answer is both

Great answer John, that's 2 for both.

The incorrect part of theory, is that "air molecules must move faster over the top to meet molecules at the trailing edge that have gone underneath".
It's the "equal transit" that's wrong, not the difference in pressure!

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RAL INN said..
No such thing as Suck just Push.

You've not seen me attempting to sail this year.

I can assure you there's been a hell of a lot of sucking going on

ok +1 for pushin +1 for pulling registered

BTW.
one may argue that on the picture airplane wing is moving - and pushing air (not vice verse) , air is stationary, wing is moving

in our example wind is moving and sail ( eeeeh we can consider stationery? or moving ? if you like) .

Does it matter or not ?? if so by how much?


and you need to clarify that too:




that is good one




if that means that model air plane on International Space Station doesn't have a lift ( in absence of gravity but still plenty of air around ?)


that is even better:



if that mean that if your wing or sail is indefinitely bid there is no pressure on both sides ?

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Bristol said..

Macroscien said...

. . . You could cast your vote now. . .

I have no opinion. I would like to vote "informal".

Thank you.

Don't be shy. Whatever we vote today is not going to change anything ,
and sail will be pushing you or sucking or even both as usual.

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Macroscien said..

...

if that means that model air plane on International Space Station doesn't have a lift ( in absence of gravity but still plenty of air around ?)

...

Yay, something new :)
I reckon if you threw a model glider in the ISS it would do a back flip and stop.

Just to add to the confusion, on Mars rough is better than smooth when it comes to wings...though there is no "water", so no Martian windsurfers.

... quite aside from the science, it is the feeling of being pulled along by the sail that I love.
So, pulling for me.