how do planes stay in the air once they reach cruising altitude?

Question

I think I understand how airplane wings create lift when taking off, but I can’t wrap my head around how they stay in the air once they reach cruising altitude.

My understanding of how wings work during takeoff is: by changing the wing’s angle of attack, air molecules under the wing are deflected down, in turn creating lift on the wing. And air molecules that go over the wing are not met with that resistance from the wing, so they move faster than the molecules underneath, creating a pressure difference which also helps generate lift.

But when the airplane reaches cruising altitude, the wings seem pretty much horizontal, so it seems the angle of attack is not nearly as large as it was during takeoff. So at that point how do the wings generate enough lift to stay in the air?

Thank you

Edit: I’m studying mechanical engineering and have taken a fluid mechanics class so I have a little bit of background knowledge, but am still having a hard time understanding this

In: 3

Answer 1

Couple of things:

> My understanding of how wings work during takeoff is: by changing the wing’s angle of attack, air molecules under the wing are deflected down, in turn creating lift on the wing. And air molecules that go over the wing are not met with that resistance from the wing, so they move faster than the molecules underneath, creating a pressure difference which also helps generate lift.

Both these factors (change in momentum, and upper and lower surface pressure differences) are consequences of the same effect. It’s not that changing momentum produces some lift and pressure differential produces some lift. **Both phenomena are present whenever lift is generated**. If you were to measure the air momentum change, it would equal the lift force. If you were to measure the pressure distribution and integrate it on the wing surface, it would also equal the lift force.

With that mind, note that **cambered wings generate lift at zero angle of attack**. That means that they both deflect air downwards, and have a pressure differential on the upper and lower surfaces. Ergo a plane can fly steady and level at zero angle of attack. In practice though, wings are usually set on the wing at a slight angle of a few degrees.