Originally Posted by
Uplinker
I have always been confused about this. At flight school we were told that 2/3rds of a wing's lift came from the top surface 'sucking' the wing upwards - owing to the air speeding up over the top lowering its pressure relative to the air underneath the wing.
But when looking at a propellor in flight school and commenting that since it has a similar profile to a wing, does the propellor mostly 'suck' the aircraft along?, I was told oh no, a propellor works by pushing air behind it. And it manifestly does push air behind it.
And more recently I have seen it asserted that an aircraft flies by its wings pushing down the same weight of air as the aircraft weighs, and certainly a helicopter's blades do push air down.
So, which is it, or do wing and propellor do both?
This is two different scopes of the explanation. They don't contradict, but it can be confusing since they use some of the same terms.
The first one is a fine-grained detailed view of the action around the wing, where we can see the individual area contributions above and below. How much sucks from above vs. how much pushes from below? Here it is broken down by individual non-overlapping contributions. And as we've seen from the diagrams of all the little arrows or manometer tubes, the answer is that more sucks from above.
But there is also the larger-scale view where we're not concerned with the detailed view above. And in this view, the wind/blade simply
moves air downward/backward, and this movement we tend to call pushing, and it accounts for 100% of the force. (When it comes to props and rotors this is called the actuator disk model, where we're not concerned with individual blades and all the complicated things that happen between and around them... it's just a magical disk that imparts momentum, and for a certain class of problems, this abstraction is still good enough to work with.)