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Comment by nvy | Hacker News Reader

>A common misconception about wings is that they need to have the classic airfoil shape to work. In reality, just about any surface can create lift and function as a wing

Left unsaid is why aircraft wings have airfoil-shaped cross sections with cambered (concave-down) shapes: they produce more lift for a given wing loading and angle of attack.

This is why aircraft have flaps, as well. They increase the camber of the wing so that the pilot can fly slower without pitching the nose up, which is important for example when maintaining sight of the runaway on landing approach.

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> You will notice that past a certain angle, the lift starts to decrease, and is replaced by a lot of drag, a force trying to slow down the wing. This is called a stall, and it limits how much lift a wing can create at any given speed. A proper airfoil geometry can generate more lift before stalling, and creates less drag for a given amount of lift, which is why most airplanes use them.

I'm a bit of a novice, so it sounds like this is the same explanation you're giving. What is the article leaving out?

Speed. You'll always stall if you go too slow to get enough lift. Putting the flaps down increases lift and drag and lowers the stall speed.

As for why the article mentions decreased lift whereas flaps down increases lift, that's maybe a bit more complicated. Lift vs. Angle Of Attack is a curve that tops out at about 15°.[0] Flaps often go to steeper angles than that, but I'm not sure if lift actually starts to decrease again in that scenario. Certainly adding a curve to the back of the wing (flaps) isn't quite the same as changing the angle of the entire wing.

[0] https://commons.wikimedia.org/wiki/File:Lift_curve.svg

> You'll always stall if you go too slow to get enough lift.

The amount of lift needed is determined by the load factor. In a turn, or when a glider launches via a winch, the load factor is higher than in level flight.

This means that the stall speed varies with load factor.

On the other side, you can still not be stalled below the straight and level stall speed, if the load factor is low enough. In parabolic flight you can see this, and with a load factor of 0 you cannot stall.

No, flaps increase CL. https://www.researchgate.net/figure/Figure-1-4-Effect-of-fla...

You might be thinking of total lift on a 3D wing, which is something you alluded to re: wing loading. But 3d wings vs 2d airfoils are really different concepts that you seem to be mixing up. Airfoils are advantageous regardless of wing loading (which is an inherently 3d concept) because of better L/D and stall characteristics.

You are correct, I had a brain fart. Not enough sleep the last few days.

Except in dire circumstances, airplanes are not struggling to produce sufficient lift. What matters is to produce it with reasonably low drag, up to all the other constraints that go into making a practical airplane for its intended purpose.

There are some airplanes that can be in a situation of flying, but unable to climb or even maintain altitude, with flaps fully lowered. In such a situation, only carefully raising the flaps will allow it to climb away.

Great explanation! It also decreases the stall speed too; which allows a plane to switch to ‘low speed mode’

It's a really fun set of tradeoffs to be honest. You get more lift at a given speed/AoA, which lets you go slower without stalling. But you get even more drag, which means the engines have to work harder to keep you flying at that slower speed.

Increasing drag is actually useful when landing. too. Trying to land an airplane with very little drag at a given spot is difficult because if you're just a little fast they'll float a long time.

Definitely! Flaps are one of the few “almost free lunch” things in aviation where almost everything else has harsh tradeoffs to consider.

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