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Comment by belter | Hacker News Reader

Its complex...The example normally given is, the wing is shaped a little flat in the under side and curved on the top. So that would explain the flow as you mentioned. However when an airplane flies upside down, its not sucked into the ground ;-)

It seems nobody really knows:

"No One Can Explain Why Planes Stay in the Air"

https://www.scientificamerican.com/article/no-one-can-explai...

Edit: Added brief from article above:

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- On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs.

- There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.

- Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.

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View on HN

It’s a common misunderstanding that the underside of a wing is flat and the top part curves. A paper airplane with thin flat wings still gets lift though there are several issues trying to scale this up. Similarly many aircraft will happily fly upside down.

Wings need to support the weight of your aircraft while being light this means they need to be reasonably thick especially using the obvious choice of storing fuel inside them. The first obvious choice is a teardrop shape which gets lift from being angled up similarly to the way a flat wing does.

Real wings don’t quite use a teardrop shape, but if you look at the front most part of a wing you see it curves both down and up. https://en.wikipedia.org/wiki/Angle_of_attack#/media/File:Ai...

Kelly Johnson caused a stir in the engineering community when he came up with the F104 Starfighter, with it's thin and almost flat wings.

Yea, start throwing around enough thrust and you can fly just about anything. The F-15 got to the point where one wing was optional: https://theaviationist.com/2014/09/15/f-15-lands-with-one-wi...

There is more to that F-15 incident than just incredible thrust; the F-15's wide fuselage provides significant body lift.

https://en.wikipedia.org/wiki/Lifting_body#Body_lift

They can also fly sideways. Body lift is why it can land, thrust is why it could fly.

If an aircraft flies level upside down it will lose altitude towards the ground (as opposed to right side up wherein given adequate thrust it should keep its current altitude).

In order to stay at a fixed altitude upside down you have to bring the nose of the aircraft up several degrees (increasing based on air speed).

If wings only generated lift in one direction (i.e. towards the curved side), then even flying with your nose up would pull you down if you are inverted. What people here are missing is that curved wings in level flight generate lift, but any shape of wing can generate lift with a positive angle of attack. Just stick your hand out the window while driving on the highway and tilt it slightly, you'll see.

> "Just stick your hand out the window while driving on the highway and tilt it slightly, you'll see."

this is really all the intuition most people need to understand flight, even if it leads to an incomplete understanding. it's easy to feel the air pushing on the bottom of your hand when you tilt it up (or top, when tilted down). what's not obvious is that there is also lift created on the top side at the same time, but that can subsequently be learned in high school physics (or fluid dynamics in college, which is where it really stuck for me).

Every aircraft has the wing set at an incident angle relative to the axis of the fuselage. Usually to generate enough deflection force for level (relative to the fuselage) flight at cruising speed.

Upside down flight requires you to basically inverse this deflection, but it isn't because of Bernoulli lift.

Wings can and generally do have zero degree angle of attack lift.

The 747 wing is at a 2° incidence angle relative to the body, which allows the body to be level with the direction of travel at cruising altitude/speed. An Airbus A320 has an incidence angle of about 5° at the body, twisting to -0.5° at the tip (many aircraft have such complex wings, but the aggregate is an important incidence angle). Every Cessna has a significant incidence angle.

The overwhelming majority of aircraft have an incidence angle relative to the body for the reason stated. So rather by "typically", could you name a single aircraft that doesn't have such an incidence angle? An SR-71?

As to "0 degrees angle of attack lift", such lift is close to negligible. Maybe you mean the body of the aircraft is zero degrees, but then we loop back to the core point again.

Wings, at least on small civil aircraft, generally DO have a positive angle of incidence where angle of incidence is defined as the relative angle between the chord line of the wing and the longitudinal axis of the fuselage.

I think it's easier to think of inverted flight as normal flight for a negative AoA. If the airfoil is symmetric -- as almost all aerobatic aircraft's are -- then it's functionally identical and inverted flight becomes a coordinate system "trick".

My favourite two "explanations" of flight are 1) dP/dt for air is greater down than up; and 2) Kelvin's circulation theorem, but alas that one is not very pub-friendly...

When I first saw the Bernoulli's principle demo of the floating disk at the science museum as a kid it made me mad. And when I actually learned about it in high school physics I still didn't like it. Reading that article now is very satisfying :).

In seriousness though, there is a big difference between "bottom up" causality-focused theories and these derived principles based on complicated notions of steady states. Even when the student is too junior not to have any choice but use the latter, I think the difference needs more emphasis.

Also the 3rd law model of flight is so much easier to understand they should teach it first.

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