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Scooby_simon View Drop Down
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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 05 at 9:47pm
Originally posted by Stefan Lloyd

Originally posted by Scooby_simon

We don't use the "force" of the air to move us along; we use the pressure difference as a result of the speed differences across the front and the back of the sail(s)

And the pressure difference results in a force. Force = pressure x area. No force, no movement (Newton's 2nd law).

 



So Stefan,

Sailing upwind your sail area pushes you "up wind".  Nope.

It is the pressure difference between the wind that goes around the front of the mast and sail(s) and the back of the mast and sail(s) that moves you up wind - i.e. you are "sucked" upwind as the wind goes slower around the front(windward) vs the back.

Down wind, yes F = PressurexArea does have more of an impact, but most high performance boats are still "sucked" along downwind as the AW is forward of abeam.
 

Edited by Scooby_simon
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 7:25am

Originally posted by Scooby_simon

Sailing upwind your sail area pushes you "up wind".  Nope.

You seem to think I said something different to what I actually said. As you say, on anything but a run, the pressure difference arises because of the aerofoil effect of the sail. I never said otherwise.

But if you think sail area doesn't come into it, try sailing upwind with a pocket-handkerchief and see how far you get. Why do you think yachts put up smaller sails upwind when it gets breezier?

Force = pressure * area is just basic physics. Not opinion and not a subject for debate. Actually it is the definition of what pressure means. One pascal = 1N per square metre.



Edited by Stefan Lloyd
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Post Options Post Options   Quote Jon Emmett Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 8:53am
Hmmm. I am going to sail in Weymouth this weekend good old cold North breeze, maybe if I am lucky with snow. Next week I sail in Brazil in around 30 degrees. Funny the wind speed will be about the same. It will be interesting to see if I feel noticeably less overpowered!!!
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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 11:24am
Originally posted by Stefan Lloyd

Originally posted by Scooby_simon

Sailing upwind your sail area pushes you "up wind".  Nope.

You seem to think I said something different to what I actually said. As you say, on anything but a run, the pressure difference arises because of the aerofoil effect of the sail. I never said otherwise.

But if you think sail area doesn't come into it, try sailing upwind with a pocket-handkerchief and see how far you get. Why do you think yachts put up smaller sails upwind when it gets breezier?

Force = pressure * area is just basic physics. Not opinion and not a subject for debate. Actually it is the definition of what pressure means. One pascal = 1N per square metre.



Yes sail area is VERY important because this controls the pressure difference between the front and back of the sail. 

This difference is a result of the difference in the speed of the air across the front and the back of the sail. 
So yes the density of the air is small factor(but as someone above explained with a worked example is was a small one for just simple "push")   We then come to the point where we need to decide what the wind extra "push" in the wind can provide as "thrust" via the sails by creating greater low presure area behind the sail(s).

So I agree that colder air is heavier - obvious.
BUT how much effect does this really make ?
how much more thrust(low pressure) do you get by running  slightly denser air around the sails.

I don't know how to calculate this and IMO it's going to be almost not worth worrying about.  Yes cold air is going to blow you over slightly harder when you are standing still (or sitting on the start line waiting to go).  But once sailing along it will provide a litte more thrust (a very little).  But not much.

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....



 
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 12:54pm

Originally posted by Scooby_simon

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....

Well yes, and that would be me, as it happens. Mathematical modelling of flows over aircraft wings was precisely the area I specialised in during my first degree. I have already done the calculations you suggest earlier in this thread. To summarise, you will get more lift from colder air but I doubt the difference is noticeable in practice.



Edited by Stefan Lloyd
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Post Options Post Options   Quote yellowhammer Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 12:56pm

Aircraft cruise high (low dencity) and cold ... gives higher speed at design Mach No. for wing and reduced drag. Not sure it reads across to a 0.03 Mk No (20 kts) at sea level.

Dencer air = more molecules in contact with sail = more drag, another factor to consider.

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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 05 at 1:29pm
Originally posted by Stefan Lloyd

Originally posted by Scooby_simon

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....

Well yes, and that would be me, as it happens. Mathematical modelling of flows over aircraft wings was precisely the area I specialised in during my first degree. I have already done the calculations you suggest earlier in this thread. To summarise, you will get more lift from colder air but I doubt the difference is noticeable in practice.

 

 So we were agreeing with each other

Excellent.

 

What I was trying to say is that the few degrees of them and thus density of the air was not really worrying about.

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Post Options Post Options   Quote Entyplod Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 05 at 4:08pm

Redback asks what difference moisture makes the the density of air. The answer is actually the oppopsite to what one would think - moist air has a lower density than dry air. Odd, but true - and here is the boring mathematical bit to prove it (remember Gas Equation from school):

Density = Pressure/RxTemperature

R= Gas constant, which for water vapour is 1.6 times dry air. Hence density of moist air less than that of dry air.

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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 05 at 4:10pm
Originally posted by Entyplod

Redback asks what difference moisture makes the the density of air. The answer is actually the oppopsite to what one would think - moist air has a lower density than dry air. Odd, but true - and here is the boring mathematical bit to prove it (remember Gas Equation from school):

Density = Pressure/RxTemperature

R= Gas constant, which for water vapour is 1.6 times dry air. Hence density of moist air less than that of dry air.



I knew that bit.  Just did not want to complicate it any more for now.....


 
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 27 Nov 05 at 11:58pm
I'm impressed by you guys.
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