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Comment by jobigoud | Hacker News Reader

Air moves faster on the upper side, creating a pressure differential.

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.

------------------------------------------------------------

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...

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.

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.

Why does the air move faster on the upper side of the wing?

It's not because there's a magic force that requires air particles parted be the leading edge to rejoin thier partner at the trailing edge.

The air particles on the upper surface reach the trailing edge much sooner than the ones under the wing.

Because the pressure on the top is lower :) (this is half-serious: the whole problem with these explanations is that cause and effect for all of these variables is not straightforward: you can see from the navier-stokes equations they are all dependent on each other).

Kind of. Actually the real ‘cause’ in my understanding is 1) the curved geometry of the suction (upper) side of the aerofoil and 2) the fact that the flow remains attached to it. Everything else - you can actually approximate the curved surface to a circle and apply equations of circular motion to a parcel of air to satisfy yourself with why the flow is accelerating. And Newton’s 3rd law explains how lift is generated on the wing. In my view there’s no need to use Navier-Stokes to explain how an aerofoil works, if you simplify the geometry to make a special case.

Most of the lift comes from the suction side.

Actually, if you really want to test an explanation, try to apply the same reasoning to explain how a sailing boat can sail upwind (or at least up to about 45 degrees off).

The devil is in that last detail. "Flow stays attached" is a description of the properties of the flow, not an explanation for what causes attached flow or why attached flow matters. It's semicircular reasoning to say that the plane gets lift because the flow stays attached... Attached flow and lift are correlated, but they may be two phenomena caused by the same underlying property.

Does not move faster either. Otherwise, a flat wing would not work, and they do.

Gravity or force creates the pressure differential. Wing pushes on air below it. (Why birds fly.) Additionally, for moving wing, edges create vortices that create local pressure differentials. (Why helicopters and planes and birds work better than floating pieces of paper.)

Wings work very similarly to performance ship hulls in this regard.

For anyone who’s ever tried building a robotic bird, there is a lot more intricacy to how birds fly than just “pushing air”. A better article might have been, ‘we still don’t understand how certain species of bird fly so efficiently’

Surely it does move faster, because it's lower pressure/you're putting less resistance on it?

If I have a wing shaped like ∖, air going in -> direction, which is what you need to generate lift with a flat wing, then the air on the bottom is running into the wing and slowing down, while the air on the top is being pulled into the region the wing swept clear of particles and speeding up.

It does move faster. This can be readily observed in wind tunnel tests, and is a source of many issues once you get into transonic flight when the airflow can reach supersonic speeds while the plane in subsonic. Flat wings must be inclined to cause the air on the top side to move faster. The vortices cause air to move at different rates.

My understanding is that the air moving over the top of the wing is compressed against the air above it in the atmosphere, like a venturi. This may be extremely simplified but it's what we were taught in flight school.

My Newton's-laws-only bullshit:

bottom air:

"bounces" down, simple enough. Force on wing up and back.

top air:

bounces up off front of wing (because it's not infinitely thin), but then is unimpeded by wing. It get's slightly more compressed at the very front, but then as the wing goes down this big gap is left. The air isn't going to bounce on the air above significantly because air compressed and this is laminar flow to boot: Viscosity > internia-ness.

The about-to-be-vacuum means the bottom air pushes the wing up more easily, usually to the point where there is no more vacuum, just low pressure. But if you go really fast (or are a hydrofoil?) then there might be an actual vacuum.

The vacuum "initially" just accelerates the air vertically, but once things get going since the airfoil "carves out a triangle", the air might speed up horizontally too. There is air behind it (front re aircraft heading) pushing on it but not air in front which is getting "untraffic jammed" away.

There we go, I think this accounts for everything in the article without any Bernoulli. Screw Bernoulli.

You might get something out of this 2013 talk by former Boeing engineer Doug McLean on misconceptions about lift. [0]

[0] https://youtu.be/QKCK4lJLQHU?t=834 (watch for 5 minutes to get some idea of his main points, or 35 minutes to watch in full. The link will skip the introduction.)

I thought it was mostly because of the slight upward angle of the wing which creates air compression under the wing and suction above the wing.

If you try to move a flat object through water, it creates pressure at the front and suction at the back. If you tilt it diagonally (and move it right to left), you get pressure in the bottom right and suction in the top right.

This is true of a symmetrical aerofoil (e.g. most helicopters) but not for an asymmetrical aerofoil (most fixed wing aircraft). It is true that a slightly positive angle of attack generates more lift than none (because the pressure/lower side starts making a contribution)

Correct. Still incomplete. Angle of attack causes a vortex at the trailing edge which has nothing to do with raw air speed and everything to do with fluid dynamics (which involves speed but is much more complex)

Short version is that you created a hole (lower pressure area) in air which it now tries to fill. Air and gasses have finite limited velocity known as speed of sound, which is why you get these pressure differentials while the wing is moving. With a flat wing, they're rather small and low pressure vortex is located behind the wing. In an angled wing, some of it is located below the wing and the air trying to fill the low pressure area exerts a lift force on the wing. (It's unlike a balloon. Bernoulli has very limited impact, unlike essentially wind.)

I’m not sure I fully agree. Do you not get this trailing edge vortex with an asymmetrical aerofoil at 0 angle of attack? (Just less strongly because less pressure difference between suction and pressure sides)

You do. And you get small vortices on wing surfaces too.

Isn't the intent of a smooth aerofoil design to prevent the formation of vortices on the trailing edge? They're inevitable at the wingtip, but in controlled flight most wings are trying to produce laminar flow, right?

In my understanding, if you increase angle of attack sufficiently to generate vortices on the upper surface, then you aren't efficiently transferring downward momentum to the air your wing is shedding, and you lose lift, which causes aerodynamic stall. Am I missing something?

I'm confused. I was in a plane that flew upside-down, and it didn't fall down. Am also a pilot.

A common and wrong explanation. The pressure differential is created by the fact that the air is being pushed into the lower side of the wing.

It's much simpler than that anyway. The wing forces the air downward, so the plane must be forced up.

Overall you are correct that the plane receives an upward force due to the air it interacts with, and that the air receives an equal amount of force downward. In level flight the vertical force components must equal zero (or the plane falls/rises).

But equally, if the plane is forced up, the air must be forced down. Cause and effect are not obvious from a force diagram.

Another common and wrong explanation. The air is pushed down by induced rotation. An inclined wing is one way to do it, but is not necessary.

I don't know what you are trying to say.

Sorry. An inclined wing just means the wing is at an angle relative to the airflow. An induced rotation means that the wing causes the airstream to turn, so the airflow around the wing has a circular, rotating component.

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