I may not have used the phrase correctly, but I understand a closed system as being one from which there is no transfer of mass nor energy. I believe that this is a requirement of Bernoulli's theorem. The venturi is a classic closed system, to achieve the effect of a closed system in free air you would have to specify the boundary beyond which the energy/mass exchange is negligible.
If there is a force on the ground, is there also a rise in local air pressure, if so does this imply a rise in pressure under the wing which would also contribute to the lift. Presumably, if the force on the ground is equivalent to the weight of the aircraft there would be no need for a reduction in pressure over the wing c.f. the hovercraft.
Last edited by beardy; 24th Jan 2005 at 10:06.