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Comment by Someone1234 | Hacker News Reader

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).

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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...

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