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?