Doug I disagree that we must spell out what body of air its true for  at least in the introductory section  I believe it is possible to be intentionally vague without prompting the question of but which body of air in most readers minds  Something along the lines of  In accordance with Newtons second law Fma some of the air surrounding the airfoil is accelerated downward  The rate at which downward momentum is imparted to a portion of the air is equal to the lift See Momentum balance in lifting flows for details  Its not a question of length  the issue is whether or not it is a distraction for the reader  and I think a detailed description of control volume analysis at this stage of the article would be a distraction for most readers The more detailed it is the more likely we are to lose our audience Id prefer to keep the intro sections simple Those who want more details can follow the link