You may be surprised to find out that an airplane can stall at any attitude if the critical angle of attack is exceeded, but it cannot stall at 0 G.
This quote found in the April 2014 issue of Flying magazine, but it seems counterintuitive to me.
Why can't an airplane stall at 0G?
At 0 G the plane does not need to generate any lift, therefore there is no critical angle past which the airfoil cannot generate the lift required.
In addition, a sustained 0 G maneuver is a ballistic arc, which means that in theory your flight path should keep your angle of attack at zero the entire time (in a theoretical, symmetrical airfoiled aicraft). You can't stall because your AoA is constantly 0 degrees and cannot exceed the critical angle.
Look at it backwards.
And that's why you can't stall at zero G. You're always also at zero degrees angle of attack (you can get pedantic, depending how you measure it) when you're at zero G. And stalling is always caused by high angles of attack. And if that doesn't make 100% sense, you should look more into stalls before you dig too deep into how G loading affects stall speed.
EDIT: Many people here seem to be replacing the world "airplane" in the question with the word "wing". 0g flight involves the entire airplane, not just the wing. Sure, you could take a Cessna, strap highly controllable rocket engines to the top of it and drop it from another airplane. Then, use the rocket engines to counteract drag and give downward acceleration equal to the force of gravity. That would give you a stalled wing and 0g.
But that's not really an airplane anymore, and it doesn't really answer the question.
To stall, the wing must exceed the critical angle of attack. That means it must have an angle of attack above zero. That means lift is being created, which means an upward force is acting upon the airplane. That means you are no longer at 0g.
The point isn't that once an airplane is at 0g it becomes unstallable. The point is you have to leave 0g to stall it.
It's also an example of how airspeed is irrelevant to stalling. (Sounds crazy, since we talk about stalling speed!) Stalling isn't about the speed or amount of air flowing over the wings, it's about the angle, path and flow. A wing on the same airplane can be not stalled at 5mph, or stalled at 100mph. It just depends on the angle.
It's probably more accurate to say "you cannot maintain 0 G in a stall". Even a stalled airfoil generates some lift -- the stall just means that the lift coefficient drops below the best the airfoil can do, not that it becomes zero. Since the stalled wing produces lift, and this lift is generally not in a direction where it can be canceled out by thrust, it will pull the aircraft away from the freefall trajectory.
And even that isn't completely true in the theoretical extreme. If we imagine an aircraft with fully variable angle of incidence (never mind the engineering problems in building such a thing), we can imagine rotating the wings up to around 90° angle of attack. Then, by symmetry, they will not produce any lift, yet it will be fully stalled. There will be lots and lots of drag, but sufficiently powerful thrust can offset that and make the net motion of the aircraft follow a zero G / freefall trajectory.
Critical angle of attack or stall angle of attack is by definition the maximum angle at which the stream over the airfoil is still attached. It is also the angle of maximum lift. Beyond this angle, the stream over the airfoil becomes unattached, leading to a sudden decrease in lift.
Now, I think the article is somehow misleading. I think what they mean by stall is the airplane lift not being able to compensate weight. Since the weight in 0G is 0, then obviously it won't "stall" in this definition.
A harder question is whether the airfoil can stall in 0G according to the actual definition. Well, if the airplane is free falling in a straight line with its wings parallel to the ground, the airfoil is stalling, but in reality it will likely enter spiral dive, the airfoil will produce lift (which produces the centripetal force of the spiral) and won't be 0G anymore. An airplane is so that it will naturally adapt to the relative air stream, so it's hard to maintain 0G. You do this following a ballistic trajectory, on which at the top portion of the trajectory you get a short free fall feeling, where the airfoil is not producing lift (zero angle of attack).
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You may be surprised to find out that an airplane can stall at any attitude if the critical angle of attack is exceeded, but it cannot stall at 0 G. This quote found in the April 2014 issue of Flying magazine, but it seems counterintuitive to me. Why can't an airplane stall at 0G?
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Here is a $C_L$ / $AoA$ curve that I took from Wikipedia. The better textbooks say that a stall is that condition in which a further increase in angle of attack will result in a reduction of lift... at the chart. If the airplane can sustain level flight at point $A$, it can sustain level flight at point $B$. Is there a practical way that I can demonstrate sustained flight on the backside of the lift curve without an angle of attack indicator? Or to ask the question another way, Is there a practical way to tell when an airplane has exceeded the critical angle of attack without an AoA meter?
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