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Old 01 December 2005, 17:09   #121
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Originally Posted by DJL

This is incorrect. How do you think the difference in air pressure is created? Remember Pressure = Force x Area and Force = Mass x Acceleration. By moving the wing through the air you're applying a force to the air (this is where drag comes from, as you move around you apply a force to the air to move it out the way and it pushes back) So by forcing the air downwards you make the air force you up. Then we come back to the area that you've moved all the air from above the wing. There's now nothing there - the weight of the air (~1.5kg per m^3 ) is no longer pushing down on the wing (air pressure - which is cause by gravity acceleration the mass of air towards the centre of the earth)
daniel is right. A wing infact generates the majority of its lift from its angle of attack. The lift generated by the bernouilli's principle is not infact the major source of lift (i'm pretty sure of this but I'm prepared for stiff opposition ). Just incase anyone is unsure, bernouilli's principle (as far as wings are concerned) is based on the fact that faster moving air is at a lower pressure, so by pushing a wing profile through the air, you're forcing the fluid (air) to travel faster over the top edge because it has further to travel in the same amount of time than the fluid travelling along the bottom edge.

oooo, I love these discussions
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Old 01 December 2005, 17:19   #122
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ok, what I wrote there is slightly, how shall I put it...wrong.

Increasing the angle of attack just increases the effect of bernoulli's principle, until you reach the stall angle...well, thats what a magic frog told me anyway.
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Old 01 December 2005, 17:29   #123
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gosh the water dyson discussion is going at some pace here


how many of you learned chaps are going to see the rib ship on sunday to see all this in action then?
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Old 01 December 2005, 17:55   #124
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Quote:
Originally Posted by DJL
This is incorrect. How do you think the difference in air pressure is created? Remember Pressure = Force x Area and Force = Mass x Acceleration. By moving the wing through the air you're applying a force to the air (this is where drag comes from, as you move around you apply a force to the air to move it out the way and it pushes back) So by forcing the air downwards you make the air force you up. Then we come back to the area that you've moved all the air from above the wing. There's now nothing there - the weight of the air (~1.5kg per m^3 ) is no longer pushing down on the wing (air pressure - which is cause by gravity acceleration the mass of air towards the centre of the earth)
Slimtim - beg to differ, but Daniel isn't right. No air above the wing? Where does that go then? Lift has nothing to do with the lack of air (and it's weight) and gravity.

You are right that the lift does come from the pressure difference between the upper and lower surface - the air travels further over the top surface - and accelerates to accommodate that, so the pressure drops. The differential in pressure gives the lift.

ANYWAY - you can only go so far with the air and water analogy - 'cos it doesn't really work as water is incompressible, and air is very compressible until it goes supersonic, and then it gets even more complicated.


As far water jets go, as far as I can tell, for the most part we are agreeing on the same things.

Sucking = low pressure, so water travels towards the low pressure. Can we therefore agree that the impeller, which produces a low pressure area, draws water into the intake?

Where I disagree is on the nozzle. If the nozzle is only directional, why is the impeller not just a propeller in a tube? The jetski impellers I've seen are centrifugal - why is that? The only reason I can think of is that it needs to generate higher pressures, to give higher velocities in the outlet. To do that, it must accelerate through a nozzle - so surely it must be shaped with a constriction?

The force comes from Mass X Velocity. Mass is constant through out the jet system (otherwise it would accumulate somewhere) - so the only thing the jet drive can do is accelerate the water to the highest velocity it can - and so an impeller pressurises the water and then the nozzle constricts the water column, letting it return to atmospheric pressure as it accelerates at the nozzle throat.

If it didn't constrict in the nozzle, it would be as efficient as a propeller in a tube.

Yes? No? Burble?
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Old 01 December 2005, 18:27   #125
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My final thought on the matter tonight...

The engine provides the power to pressurise the water with the impeller, the nozzle provides the increase in velocity - and ultimately the change in momentum to produce the required force.

Still not 100% sure about the nozzle...
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Old 01 December 2005, 18:34   #126
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Originally Posted by DGR
As far water jets go, as far as I can tell, for the most part we are agreeing on the same things. [Probably]

Sucking = low pressure, so water travels towards the low pressure. Can we therefore agree that the impeller, which produces a low pressure area, draws water into the intake?
Hmm not convinced, at the waters surface the water pressure is only just above 1 bar absolute (14.5 psi) so its hardly rushing into the jet unit but at speed the hull generates a force of say 80 psi (gauge pressure) or 6.5 bar absolute, now thats more like it.

[QUOTE Yes? No? Burble? [/QUOTE] no but interesting discusion for the long winters night. Makes you wonder about peeps like Bill Hamilton working away on his own in the early days trying to make it work.

Pete
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Old 01 December 2005, 18:38   #127
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The newtonian and bernoulli explainations are both correct - and say the same thing. Both require that the wing to deflect air downwards. Lookup the Kutta condition. A cambered wing acts like a tilted uncambered wing.

The speed differnce over a cambered wing is not caused by the air having to travel faster to travel the greater distance over the top of the wing. The air being deflected downwards by the wing causes the air to circulate around the wing - the direction of the circulation increases the speed of the air above the wing and slows it below. Hence the pressure difference
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Old 01 December 2005, 18:45   #128
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Pete,

I see what you mean, but by 'sucking' I meant that the impeller will reduce the pressure slightly at the impeller face from whatever it is under the hull - i.e. will encourage water go up the intake to the impeller.

I completely agree with you on the the airborne jetboat landing - the pressure under the hull would shove water into the intake replacing the air in there very quickly. I guess that any lag in power would be due to re-pressurising the water flow downstream of the impeller and getting the jet back up to full power?

D...
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Old 01 December 2005, 18:46   #129
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Originally Posted by Pete7
Hmm not convinced, at the waters surface the water pressure is only just above 1 bar absolute (14.5 psi) so its hardly rushing into the jet unit but at speed the hull generates a force of say 80 psi (gauge pressure) or 6.5 bar absolute, now thats more like it.
At atmospheric on a 6" square grid, that could be up to 237Kg pushing the water in, that enough?
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Old 01 December 2005, 18:51   #130
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Pete, I see what you mean, but by 'sucking' I meant that the impeller will reduce the pressure slightly at the impeller face from whatever it is under the hull - i.e. will encourage water go up the intake to the impeller.
the pressure forcing the water into the jet can't be anymore than the air pressure pressing down on the sounding water. Afterall the impellor can't generate a vacuum, the reason the water will flow into the jet is because the surounding water is subjected to an air pressure of 1 bar when the boat is stationary or lots of bars when the hull is on the plane, dianamic stability perhaps generating a large pressure, afterall if Newtons 3rd law applies then the whole weight of the boat is supported on a small running surface hence high pressure.

Quote:
I completely agree with you on the the airborne jetboat landing - the pressure under the hull would shove water into the intake replacing the air in there very quickly. I guess that any lag in power would be due to re-pressurising the water flow downstream of the impeller and getting the jet back up to full power? D...
agreed, guess the folks will find out this weekend if Pressman can get her airborne
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