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Comment by TylerE | Hacker News Reader

Stall speed is a misnomer.

Stalls occur due to exceeding the critical angle of attack and can happen at ANY speed.

it's a misnomer but airspeed is life. we're not talking about people stalling during air combat or aerobatics, we're talking about doing it in the pattern. simply keeping your speed up and knowing the speed you will stall at based on bank angle should be enough.

I think that was his point. To calculate if you are going to stall in a final, you need to know your weight and flap setting to see if you will exceed the critical angle of attack when doing the turn, exactly because the speed isn’t constant.

No, you just need an angle of attack sensor. These are increasingly available for small GA planes.

stall AoA depends on flaps though?

Yes, and some sensors do take flaps position into account (or you compromise and have it display the AoA with a typical approach setting).

You need an AoA sensor to see if you are currently stalling or are just about to stall. To predict if you will stall at a specific point in your approach before you’re there, you need to know your weight, too.

Whoever is downvoting this: Stop. It's the key point here. Planes don't need to know their weight to produce a stall warning; AoA is a great metric, and GA planes not having an indicator or warning based on it is astonishing.

I thought the point was predicting a stall by knowing the approach speed will be so low that you will stall, not detecting a stall just before it happens

You can't control the weather, if there's a micro burst during landing you are in trouble.

GA planes have a stall warning horn (based on AoA). They just generally don't have a AoA indicator (though that might be a good idea indeed), relying on indicated airspeed instead, which (for given airplane mass) has a one-to-one [1] mapping to AoA in unaccelerated flight. That's why the concept of stall speed exists.

That is the case since, in unaccelerated flight, we need weight == lift, so

   W := m g == L := 1/2 rho v^2 c_L S

with m = mass, g = earth gravitational acceleration, 1/2 rho v^2 = fluid dynamic pressure which is measured by the pitot tube and displayed as indicated airspeed (well, a function of it), c_L = the coefficient of lift, and S = wing area.

Now, weight is constant (for given airplane mass, in unaccelerated flight), and so is the wing area. The coefficient of lift depends on the AoA, and dynamic pressure has a monotonic one-to-one relationship to IAS. Thus you have the relationship between IAS and AoA.

[1] Unless you get to "the back of the power curve" (the coefficient of lift increases with AoA, then decreases again, until it drops off in a stall). Let's not go there.

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