29 March 2009 UTC not so random un indent Perhaps easiest is to imagine an airfoil propagating through a large body of air which is initially at rest Once the airfoil is well inside this body of air by several chord lengths say 5 or more the rate of vertical momentum change of this body of air is equal to the lift Or more precise the reaction force of the wing on the air minus the lift force At the edges of this body of air far from the airfoil nothing is happening Small air parcels are subject to three laws mass conservation the mass of a parcel is constant Its position volume and shape may change Newton s laws the 2nd law states that the rate of change of momentum is equal to the net force on the parcel The 3rd law states that the force by one parcel on its neighbour is equal to minus the force by that neighbour on that particular parcel A force model if incompressible flow is assumed the pressure distribution at the edge of the parcel is such that its volume does not change but its position and shape may change In case of a compressible fluid pressure and volume are related through some thermodynamic equation of state since the parcel mass is constant a volume change implies a density change These three act together In general the influence of the airfoil diminished with the distance from it since the effect of the displaced volume by the moving airfoil is largest in its neighbourhood Hope this gives some clues to pinpoint where the crux of your problem is 21 57