Newton vs Bernouilli vs Stokes
Maverick,
The simple arguments usually presented for lift are:
Newton: the wing deflects air down, leading to net descending air behind the wing, and so the air pushes the wing up. (Momentum is conserved by a net huge-scale rotation to keep the whole world static.) This is broadly conservation of momentum.
Bernouilli: Faster flow above the wing leads to a lower pressure above the wing, pushing the wing up. (Yet of course the air behind the wing is still directed downwards.) This is broadly conservation of energy. Compressibility makes it more complex when the speed rises towards that of sound, but that just means the internal energy of the air needs to be taken into account too.
Both descriptions are correct and required.
Viscosity sets how the flow changes as you move from infinity towards the wing, as neither the "Newton" not "Bernouilli" description provides a way to see where the flow streamlines go. To do that, the flow pattern has to solve the Navier-Stokes equation, which will give pressure and velocity fields that agree with, but encompass these other descriptions. "Coanda" describes the viscous connection that shapes the flow to dip behind the wing, so has more of a link to "Newton" in terms of the bulk flow, shaping the streamlines for "Bernouilli".
If you stick to considering the transfers of momentum and energy in the wing-airflow system, then you'll tell no lies. You might not be able to design a wing that way, but you can explain how one works when it's going fast enough or doesn't when it stalls.
Last edited by awblain; 18th Jun 2013 at 12:19.