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
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The geometry is specifically chosen so that as the plane moves forward in steady and level flight, the pressure on the underside of the wing is higher than the pressure on top of the wing, resulting in enough upward lift to balance the weight of a fully-loaded aircraft.
You’re right in assuming angle of attack contributes heavily to lift. That’s how takeoff and landing works. But even in steady level flight the wings still produce lift.
The idea about creating lift by deflecting air downwards remains exactly the same, just for steady level flight, just instead of creating enough lift to make the plane travel upwards it just needs enough lift to balance out with the plane’s weight.
The angle of attack is close to zero but not quite. If you look at how a wing is mounted to the fusulage you will notice that the rear of the wing is much lower then the front of the wing when the fusulage is level. This means that in cruise the fusulage is level and people can walk around the cabin normally but the wings have a significant angle of attack to generate lift.
The thrust and speed play a big part, on take off most jetliners take off with less speed than at cruise altitude. 130 knots vs nearly .7 or .8 mach at 30-40k feet. Flaps help achieve more lift at lower speeds, after reaching certain speeds flaps are raised because there’s enough speed & thrust to avoid stalling. If you simply try to look at this without the speed and thrust components you will be missing vital info related to what you wish to understand.
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.
The wings of a plane are actually angled so when the body of the aircraft is level with the ground, the angle of attack isn’t 0°. Otherwise, how would you get a plane off the runway? Sure they angle the plane up after its lifted off the runway to get up in the air, but how else would they get off the ground in the first place? Also, when the plane is up in the air, it’s moving much faster than it is on the runway, so it doesn’t need as harsh an angle of attack to get the same amount of lift because lift depends on velocity as well as angle of attack.
You’ve taken your fluids right? Have you covered airfoils at all yet? Lift/angle of attack curves are dependent on geometry. While some airfoils don’t produce lift with zero angle of attack, you totally can make ones that do. Check your textbook to see if it has lift curves for different airfoils. I couldn’t tell you why specific geometry does that though, ask your professor, they should be able to tell you and show you the math.
Aircraft mechanic here.
The wings are still pitched up a bit. There’s no magic.
The plane is design to have the wings at an efficient angle in cruise, the wings are slightly pitch up compared to the flight path, and are slightly pitch up compared to the fuselage axis, and the panel flies slightly nose up, this allows the fuselage to also give a bit of lift. It’s a small angle but wings are big, and can carry the plane up even at slow speeds, at cruse speed a small angle will still give a massive lift. Consider that lift=Vsquared in the equation. 400knots give you 4 times the lift you have at 200kts.
Have a look to lift, drag diagram vs angle of attack. That’s a milestone in our theory studies to get a license. It makes clear a lot of stuff.
Get any book titled “principles of flight for pilots”. Awesome read. It’s made for pilots, pilots are not expected to have an university engineering degree, these books are as deep as possible and as simple as possible, a fantastic read.
To keep it simple, if you have enough power to generate enough lift to *gain* altitude, then you can drop the nose a bit and cut back on the power a bit to *maintain* altitude.
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