March 2009 UTC Yes it is the vertical component of momentum of this body of air moving with the flow that is increasing And then it does not matter whether you talk about and Eulerian or Lagrangian frame of reference Or whether the air is flowing around the airfoil or the airfoil flying through air at rest far from the airplane This body of air has to be large many chord lengths in diameter say five or more for the effects of the airfoil on the boundary to be small enough to be negligible from a practical point of view a case which not satisfies this is the ground effect where the boundary influences the kinematics and forces 11 21 28