# Why is airflow forced to be speed up on the upper surface of an aerofoil? [duplicate]

As the title says: Why is airflow compelled to flow further over the upper surface of an aerofoil? I understand because it has a larger distance to travel, but why can't the airflow travel at the same speed as the airflow underneath the lower surface?

I understand how the Bernoulli effect works but there is no constriction in an aerofoil, why does it speed up with the Bernoulli effect; without the constriction?

I also understand that the there is a wrong theory that airflow over the top of the wing wants to meet the airflow on the lower surface at the end of the aerofoil, at the same time.

• @GregHewgill No that just explained how lift is generated. Although it did mention the concept of the wrong theory, it did not explain why it needs to travel faster Dec 9, 2020 at 1:11
• It does say that air has to have a lower pressure to negotiate the curved, inclined upper surface. Bernoulli (and that answer) tells you that lower pressure equals higher speed and the suction causes air flowing towards it to accelerate. So what is missing? Dec 9, 2020 at 7:12

Airflow isn't compelled to do that by some edict. It just does it. "Why" isn't all that meaningful, compared to a theory or formula with predictive power.

Bernoulli says that, under certain conditions, a fluid's speed and pressure have a certain relationship.

How a fluid flows past a obstruction, and what net forces and torques it exerts on that obstruction, aren't because of Bernoulli.

The closest we can come to a reason for the fluid's behavior is the Navier-Stokes equations. But that's still a model, a simplification.

For an airfoil in conventional steady flight, it's actually more useful to say the reverse: because the airfoil has been carefully chosen to generate lift under those conditions, the pressure above must be less than that below, so Bernoulli predicts that the flow above must be faster than that below. And we can confirm that with CFD simulations or with onboard sensors.

• That is an answer worthy of Philosophy SE. +1! Dec 9, 2020 at 7:14

it is because of the camber provided on the upper surface.the area where flow can move is decreased due to pronounced camber.and according to equation of continuity the velocity will have to be increased to maintain the same airflow