All these things are unavoidably part of the same picture
faster airflow on top
less pressure on top
airflow redirected downwards
Result = upward lift
Only the first two are necessarily correct, the third is only correct behind the wing and only accounts for half of the lift, the other half of the airflow is being directed upwards. Even with the first you might come unstuck depending on where you consider the ‘top’ to be.
It is my understanding that the net momentum change of the air is zero, which should keep newton happy because there is no change in momentum of the aircraft…
But does it really matter if the inaccuracies, or ‘simplifications’ yield the correct prediction?
The one thing that really has to happen is the force created by the difference in pressure between the bottom and top of the aircraft has to equal the weight.
The airflow has to be directed downwards to get lift (and the pressure difference between top and bottom is all a part of the same picture).
Obviously this redirection is only local. You don’t end up with a downward flow an hour after the plane has flown by 
But this is not on the topic of how ailerons work, specifically. To say the aileron changes the lift of the wing is a strange way of putting it.
Yes it’s part and parcel that the airflow leaving the wing, is turned downwards but it’s also true that air in front of the wing is turned upwards. Why discard that point?
One thing is for sure the downward turning does not account for the lift mathematicaly.
You could say all the phenomenon are observed in the production of lift.
Stating that ailerons change the lift is a good description IMHO.
The debate would come down to the interpretation of English.
To me, “Ailerons on a wing are designed to give extra lift on that wing to make the plane roll” implies the whole wing, which is obviously nonsense. Also “extra lift” is true only for the down-going aileron. The up-going aileron reduces the lift.
Why discard that point?
Because you must have a net downward deflection on the airflow to generate a net lift.
What causes ground effect? It is the downward airflow, which trails off behind the wing, hitting the ground and getting some back-pressure. Well, that’s my very rough understanding
Supplemental – what creates the increased drag that is made apparent as Adverse Yaw?
The net lift of the – whole – wing with the aileron that’s going down is higher than the net lift of the other wing. It does not matter “where on the wing” the lift is higher. The higher lift of one wing causes the roll. The rest is just nitpicking.
EuroFlyer wrote:
The Bernoulli principle says a profiled wing produces lift – because on the upper side of the wing, the air travels longer and therefore faster, which creates a pressure differential between lower and upper side of the wing, eventually resulting in an upward lift force. We all learned and understood that. That is, if wings are cambered.My example with the hand, though, doesn’t work with Bernoulli. It works on the downward deflection of air which creates a force vector upward. My hand has a flat surface, Bernoulli doesn’t stand a chance, because both sides of the hand are essentially identical and the air travels equally fast on both sides.
No, it doesn’t work like that. Bernoulli’s equations will still work perfectly well with your hand.
Air is a fluid, not a stream of bullets getting deflected downwards ( https://www.av8n.com/how/htm/airfoils.html#sec-fluid ). The airflow over the top of a deflected surface like your hand will still travel faster (note: the “equal transit time” thing that’s bandied about is a load of rubbish). This is true with a barn door ( https://www.av8n.com/how/htm/airfoils.html#sec-visualize-circulation ), of an aerobatic symmetrical wing or indeed anything that produces lift. It’s just that an airfoil surface is an efficient way of doing it, so we use them rather than barn doors. The air ahead of your hand will be affected, too.
Also those who don’t believe cambered airfoils result in air being moved down should try standing under a helicopter (or behind an propeller, in the case of air moving back) some day.
Peter wrote:
Because you must have a net downward deflection on the airflow to generate a net lift.
Except that’s only half correct as the net downward deflection only equals half the net lift ;-) or to put it another way your statement would be equally correct if you transposed the word upward for downward.
I believe if you measured the net movement in the air in say a box say 25m wide around a tb20 it would be close enough to zero.
Trying to explain all this stuff invariably ends up in some circular argument (pun intended).
I think cobalt’s diagram sums it all up best.
Ted wrote:
Except that’s only half correct as the net downward deflection only equals half the net lift ;-) or to put it another way your statement would be equally correct if you transposed the word upward for downward.
Indeed….if lift was all about deflecting air downwards what would be the point of a camber? Peter, the mass of displaced air flux x net downwards acceleration of the air is less than the mass of the aircraft x gravity… and btw…air flowing faster over the top causes lower pressure…ie they are not two different effects… I agree that Cobalt’s post explains it well…
Dave_Phillips wrote:
Supplemental – what creates the increased drag that is made apparent as Adverse Yaw?
When you increase the lift on one wing using an aileron you slow the speed of the air both over the top and the bottom of the wing, the integral of the velocity over the wing is actually lower than the free stream velocity. You can see that clearly in the video bookworm posted
unfortunately the area shown in the video is not quite big enough, (you can’t see the free stream all around the foil)
When you reduce the lift, you do the opposite and the integral of the velocity is closer to the freestream.
If you simply deflected the flow equal amounts the drag rise on each wing would be equal… but of course that’s not what happens.
