Quote:
Originally Posted by A320fan
ever since i started my training, i've had a tough time explaining why the maneuvering speed (at least with a Piper Warrior and a C172) is two different speeds. i know it has to do with the critical angle of attack and load factor, but relating the two is what's tough for me. i tried the search function and didn't see anything about this on this fourm, so thought i'd give it a shot. anyone here have any way to easily be able to explain Va?
thanks in advance,
Colin
What do you mean by "two different speeds"? Va decreases with weight but that's a continuous process, so I'm not sure what you are referring to.
But, if you're asking why Va decreases with weight, it's usually a good idea to have a couple of explanations in your toolkit. If you have a student who's mathematically inclined, a too-simple explanation cna make you sound like an idiot. OTOH, if your student is math-phobic, a too-complicated one can leave her with a deer-in-the-headlights stare.
With that in mind, 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.