First of all, Lift is a reaction, not a force in and of itself - you are not sucked up into the air, as implied by EASA questions - if that were the case, where does downwash come from? Indeed, if you relied totally on Bernoulli, a Cessna 172 would have to do 472 knots to fly, so it's not the whole story. There are some who say that Coanda comes into the mix, but this is outwith the scope of exams.
Starting with the flat plate in an airflow, there is a difference in static pressure between the front and the rear of the plate due to the turbulence, from the air speeding up as it goes round the plate and reduces its pressure - this difference is enough to suck the plate backwards, and is similar to what happens in mountains and why you get turbulence in the lee of them. If you could get a situation where there was no change in static pressure from front to back, you would have no form drag, and this is what they measure in wind tunnels.
Now incline the plate at 45 degrees, and you will get some sort of impulse reaction (and downwash) that makes the flat plate want to fly up. Add that to the difference in pressure and we have the first two reasons why anything flies.
But the trick with aerodynamics is not creating "Lift" - we can do that with a plank of wood, as mentioned above. The trick is reducing drag, hence the shaping of aerofoils.
So, on to reason no 3....
Take a square room at sea level, inside which air molecules are bouncing around at random, providing an equal pressure on all surfaces of 2116 lbs per square foot. On a 10-foot square ceiling, that would be enough to support a 737, but we can't magically take away the static pressure on top of its wings, which would make it leap up into the air like startled rabbit. Instead, we can mess with the dynamic pressure by moving forward which reduces the static pressure for us. It is atmospheric pressure from below that pushes the 737 up into the air. It is the same atmospheric pressure that pushes the accelerating airflow over the upper surface against that surface and keeps it there.
Finally (for this discussion), we have wingtip vortices that rotate from the outside to the inside of the wing, providing a additional downwash effect.
The "circular movement of air", BTW, is only an impression due to vector differences, not a real circulation.
The tip vortices make the air trailing behind the tips flow lower than that from the root, which is assisted by the upwash at the leading edge, and which is instrumental in creating induced drag (it is the backwards moment from the difference between the vectors).
Thus, the air doesn’t rotate about the wing, but we get an equivalent effect if you imagine that the airflow over the upper surface is the same as the average speed plus a bit, and that through the lower surface is similar to the average speed minus a bit - rather like the difference between a headwind or a tailwind when it comes to groundspeed.
Last edited by paco; 15th June 2017 at 10:29.