Quote:
Originally Posted by flyguy  
I didn't even bother reading your first response. It was way over my head, but this agrees with what I learned. A heavier plane does not have enough room between the AoA and critical AoA and will stall. A lighter airplane at a lower AoA has a long way to go before reaching critical AoA and that leaves more room to break the plane. |
I like the AoA explanation as well as a fair compromise between the "over the head/lost interest" explanation and the "too simplistic and inaccurate one." My own version of the AoA explanation is (FAQ-in-training):
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This explanation was derived from a number of online and offline sources.
Let's go back to the definition of maneuvering speed. Euphemistically, it's the speed at which an airplane will stall before it breaks due to a gust or abrupt control movement.
Putting it in slightly other terms, it's the speed at which the wings can suddenly go from their existing angle of attack to their critical angle of attack without increasing the load factor (G-force) beyond the aircraft's design. For normal category aircraft, that design maximum is 3.8 G.
Let's fill this out with some numbers. We are flying an airplane that stalls at 15º AoA. At it's normal 120 KT cruise, it's AoA 3º.
What happens if we suddenly change the AoA from 3º to 15º? Because there is (roughly) a one-to-one relationship between increase in AoA and increase in load, we have just increased the ~1-G cruise load on the wings by a factor of 5 G. Too bad we suffered structural damage at 3.8!!
What we're really trying to do to protect ourselves is increase our AoA so that the gap between our AoA and the critical AoA is smaller. How do we do that? We slow down. When we slow down while maintaining level flight, we reduce power and increase pitch, which increases our AoA. So, let's say that flying our hypothetical airplane level at a 90 KTS takes a 5º AoA. Even that small change means that suddenly bridging the AoA gap only involves a 3-G increase, below the 3.8 G damage point.
Why the slower speed for lower weight? Well, in general, a lighter airplane can maintain level flight at a particular airspeed with a lower angle of attack. So the cruise to critical AoA gap is larger at lighter weights. So we need to slow down more to get our cruise AoA where we need it to be to keep the gap manageable.
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