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Comment by colechristensen | Hacker News Reader

Ok that's long, one top line thing people tend to miss in these flying explanations is that airfoil shape isn't about some special sauce generating lift. A flat plate generates any amount of lift you want just fine. Airfoil design is about the ratio of lift to drag most importantly and then several more complex effects but NOT just generating lift. (stall speed, performance near and above the speed of sound, laminar/turbulent flow in different situations, what you can fit inside the wing, etc)

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To be more specific,

You can't escape momentum exchange. To generate an upward force, the airplane must exert a downward force on the air molecules.

An airfoil does this more efficiently than a flat plate, essentially using the top shape to create a low pressure area that sucks the air over the top downwards, imparting the downwards momentum, along with deflecting the air downward on the bottom surface. A flat plate pitched upwards "stalls" the air on the top surface, because the air has to travel forward some to fill the gap by the plate moving forward - so this creates a lot of drag as the plate is imparting more forward momentum on the air.

The issue is that to analyze lift using momentum, you have to do statisitcal math on a grid of space around the airfoil, which is super complex. So instead, we use concept of pressure with static and dynamic pressure differences creating lift, because it makes sense to most people learning this, which then all gets rolled up into a plot of lift coefficient vs angle of attack.

And as you dive deeper, you learn more analysis tools. For example, there is also another way to analyze performance of an airfoil more accurately, which is called vorticity. If you subtract the average velocity of the airflow around an airfoil, the vector field becomes a circle. In vector math, the total curl of the vector field is directly correlated to the effective lift an airfoil can produce. This method accounts for any shape of the airfoil.

But under the hood its all momentum.

Nearly everything you wrote here is inspired by reality and mostly incorrect.

Exactly. Airfoil is an optimization. There is a common misconception that planes wouldn't get off the ground if you didn't have airfoil. No, most of the lift (depends on the plane but in the ballpark of 80-90%) comes from the overall shape of the wings. ~20% is from leading edge airfoil deflection dynamics.

And if, say, airfoil was never discovered, we'd probably design the whole wing slightly differently to compensate for it, so the actual difference wouldn't even be 20%.

Airfoil is about as important as winglets, and planes fly without winglets just fine. But nobody points to winglets and says that's the crucial bit that makes the whole thing work.

Two ratios dominate aircraft design. Lift/Drag, Thrust/Weight

To get off the ground Lift > Weight, Thrust > Drag, or to simplify to stay aloft Lift = Weight, Thrust = Drag

Bigger engines weigh more.

To get off the ground you need an engine powerful enough to overcome the drag necessary to generate enough lift.

That is what enabled powered flight especially at the beginning. Wing design with a good enough lift to drag ratio and engine+propeller design that had a good enough thrust to drag ratio to come together for more lift than the aircraft weighed.

I was obsessed with fighter jets in my adolescence. My favorite plane was the F-15 Eagle; its thrust:weight ratio was greater than 1 -- meaning it could take off, point its nose straight up to vertical, and keep accelerating past mach 1. Amazing.

The F-15C (and the F-16), yes.

The F-15E was a different story, which I remember from flamewars at flightsim forums over how slow of a climber it was :)

It is probably obvious, so obvious that no one starts with it? but it took me an absurdly long time to put together that an airplane lifts by moving air down.

Admittedly there is an amazing amount of fluid-dynamic subtly on top of this simple Newtonian problem. But I am surprised that almost no one starts with "An airplane produces lift by moving air down, for steady flight it needs to move exactly as much air mass down as the plane weighs. here are the engineering structures that are used to do this and some mathematical models used to calculate it"

That was what I was taught 30 years ago in university.

To be more precise, we defined or made a shorthand of this downward force W. Originally it stood for weight but we knew it was the downward force that must be counteracted by an upward force called L for lift. Lift by convention was always an upward force.

These are conventions taught and used.

I should also remark the laminar fluid boundary layer only is true for subsonic flight. When you go over Mach one, a permanent shock wave forms in front of the wing. This shock wave disrupts the laminar boundary layer. Lift is achieved by Newtonian breakdown of the air hitting the wings underside. This is why supersonic planes require fly by wire. As computers are constantly issuing commands to ailerons and rudder to prevent stall.

Exactly. Any kid who has stuck a flat hand out of the window of a car at speed knows how airplane wings work. You tilt your hand back and the wind pushes it up. Tilt it forward and the wind pushes it down. Everything else is an optimization.

Was gonna say where is the debate of bernouli vs. AoA/pforce (p-factor), scatter blast shotgun hitting bottom of wing

There is an interactive simulation on the page with a simple plane showing exactly this.

Umm no, at zero degrees AoA as the first diagram on the page shows, a flat plate does not generate lift. But nobody actually questions that a flat shape can generate lift; we all made paper planes as a kid.

But every airfoil has an equilibrium angle of attack (not always stable with velocity) where it generates zero lift. The chordal angle of attack is for convenience because it depends only on airfoil geometry and not ambient velocity, but it isn't a fundamental physical property of the airfoil.

If we treat the angle where zero lift is generated as the base angle for an airfoil, then all airfoils generate lift depending on their angle relative to that, including a flat plane. As the GP says, other properties are the dominant factor in airfoil geometry.

When introducing airfoils I think it is more useful to start from a plane than a traditional airfoil shape; the math and intuitions are much clearer from there.

And with steady level flight symmetrical airfoils are flown at an angle, a cambered airfoil shape being flown at 0 degrees angle of attack vs its chord line would be an unusual coincidence. Wings are mounted at a small angle relative to the direction of thrust and what one would define as a flat line on the fuselage.

Uncambered airfoils also don't generate lift at zero degrees. What constitutes "0" for curved airfoils is convenience. You want lift, you put a flat plate on an angle, anything fancier is for Lift/Drag, Thrust/Weight, etc.

Paper airplanes do not have barndoor wings, though. Most of them have stepped camber through the way they are folded.

It's not finished but I started writing this to clarify: https://entropicthoughts.com/paper-airplane-aerodynamic-stab...

Scroll down to "trim and angle of attack".

(I hope there's nothing embarrassing in there. It's an old, early draft.)

There are about a million places incorrectly "explaining" that airfoils create lift because the top path is longer and this means the air has to go faster. A flat plate would not create lift in that case. The fact that paper airplanes obviously can fly somehow never stops people from repeating this.

HERE WE GO AGAIN...

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