Why do airplanes usually pitch nose-down in a fully-developed stall?
I've seen this seemingly-simple question discussed on other aviation forums, but there doesn't seem to be a single agreed-upon answer, at least that covers all or most cases.
Here is why I find this question confusing: When we say that airplanes are usually "nose-heavy," what we mean is that the center of lift is aft of the center of gravity. So the lift generated by the wings, in addition to counteracting weight, also produces a pitching moment, rotating the aircraft about its lateral axis, nose-down / tail up. This pitching moment is counteracted by the horizontal stabilizer (or canard wings) to maintain a level attitude.
In a stall, the wings produce significantly less lift. So my intuition would be that the nose-down moment would also be significantly reduced. And indeed, if the wings stalled before the tailplane, I would expect the downforce produced by the tailplane to be dominant, and the nose to pitch even further up. This is obviously contrary to experience.
Here are some of the answers I've seen proposed elsewhere on the internet:
- "The CG is heavy so it falls down."
This doesn't seem like a very good answer. It's not just that the airplane is falling, it's that it's rotating, and all rotation happens about the CG. So the portion of the airplane that is pitching downward is the portion that is forward of the CG, and the portion that is pitching upward is the portion that is aft of the CG. What causes this pitching moment, as opposed to the airplane falling in its previous nose-up stalled attitude?
- "The plane would fall in its previous attitude if it were in a vacuum, but because the engine is heavy, it has more inertia and is less affected by air resistance, so it falls faster."
Maybe? I don't know enough to know whether or not this is right.
- "A stalled wing still produces some lift, and as it stalls, the center of lift moves aft. The greater arm results in a greater forward-pitching moment. So it is the 'nose-heaviness' being exaggerated in the stall."
This also seems plausible, but I don't know. Since stalls and stall recovery are so fundamental to basic flight instruction, I would expect this question to have a straight-forward answer, but maybe it just doesn't? Or maybe there's something I'm missing. Curious to hear people's thoughts.