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#1
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vortex ring state at any point during an auto??
It it possible to get into vortex ring state (or settling with power)
during an autorotation? A friend at my radio control helicopter field claimed that you shouldn't do a vertical autorotation, because you might enter vortex ring state if you do. This doesn't seem right to me based on my limited knowledge of the aerodynamics of helicopters. Clearly, during an autorotation the main blades are developing lift, just like a glider's wings are generating lift when it descends at a constant velocity. And so, presumably there is a vortex at the tips of the blades. But in an auto, it seems like you would be descending out of this rotor tip vortex; the wind is driving the blades, rather than the blades driving the wind. Furthermore, the inner counter-rotating vortex that develops during vortex ring state would seem not to be possible during an auto, because there is no down-wash over the intermediate part of the blades. The air is going up through the rotor disk the whole way out from blade roots to tips. (This is all just seat-of-pants intuition; I hope someone with aerodynamic knowledge can say if my intuition is right, and what the aerodynamics of an auto are, and why in that regime settling with power can't happen.) The one place I can (just barely) imagine it might be possible is during the brief moment at the bottom of an auto when you crank up the collective to exchange rotational inertia for lift to cushion your landing. At that point it seems like you are adding energy to the rotor head other than that which is coming from the descent through the air. My supposition is that you can turn the blades using the engine, or you can turn the blades using the stored rotational inertia in the blades, and in either case you might be able to induce vortex ring state. Is this true? Thanks a million for any thoughts or comments, Greg |
#2
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"Greg Johnson" wrote in message m... It it possible to get into vortex ring state (or settling with power) during an autorotation? No. A friend at my radio control helicopter field claimed that you shouldn't do a vertical autorotation, because you might enter vortex ring state if you do. He's talking ********. To reach "that" stage, the air must be travelling DOWNWARDS through the rotors. In autorotaion, the air is travelling UPWARDS and turning the blade. (Well the AIRFLOW is upwards due to the downwards trajectory of the heli) This doesn't seem right to me based on my limited knowledge of the aerodynamics of helicopters. You're right, its not right. Clearly, during an autorotation the main blades are developing lift, just like a glider's wings are generating lift when it descends at a constant velocity. Exactamundo (almost:-) And so, presumably there is a vortex at the tips of the blades. But in an auto, it seems like you would be descending out of this rotor tip vortex; the wind is driving the blades, rather than the blades driving the wind. Correct. Furthermore, the inner counter-rotating vortex that develops during vortex ring state would seem not to be possible during an auto, because there is no down-wash over the intermediate part of the blades. Or over any part of the whole machine. The air is going up through the rotor disk the whole way out from blade roots to tips. Indeed it is, and one day I'll read the whole bleeding post before I start typing :-))) save myself a lot of work if I did:-) (This is all just seat-of-pants intuition; I hope someone with aerodynamic knowledge can say if my intuition is right, and what the aerodynamics of an auto are, and why in that regime settling with power can't happen.) The seat of your pants is working well from where it's sitting:-) The one place I can (just barely) imagine it might be possible is during the brief moment at the bottom of an auto when you crank up the collective to exchange rotational inertia for lift to cushion your landing. Not going to happen there either. You need to have POWER into the blades (and lot of it) plus a descent rate exceeding 300fpm. At that point it seems like you are adding energy to the rotor head other than that which is coming from the descent through the air. Actually, you're USING the energy already stored, so you're losing energy not adding to it. My supposition is that you can turn the blades using the engine, or you can turn the blades using the stored rotational inertia in the blades, and in either case you might be able to induce vortex ring state. Is this true? No. More than one criteria must be met to induce VRS, and one is a LOT of power going into the blades (not being used as it is when pitch is pulled, but DRIVING the blades when pitch is pulled) and the other is that rapid descent rate (300fpm typically). If both aren't present, you're not going to find yourself in the ****. -- Beav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk |
#3
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You still up and about Beav?
-- Alan Remove the dots to reply http://heliweb.users.btopenworld.com/ "Beav" wrote in message ... "Greg Johnson" wrote in message m... It it possible to get into vortex ring state (or settling with power) during an autorotation? No. A friend at my radio control helicopter field claimed that you shouldn't do a vertical autorotation, because you might enter vortex ring state if you do. He's talking ********. To reach "that" stage, the air must be travelling DOWNWARDS through the rotors. In autorotaion, the air is travelling UPWARDS and turning the blade. (Well the AIRFLOW is upwards due to the downwards trajectory of the heli) This doesn't seem right to me based on my limited knowledge of the aerodynamics of helicopters. You're right, its not right. Clearly, during an autorotation the main blades are developing lift, just like a glider's wings are generating lift when it descends at a constant velocity. Exactamundo (almost:-) And so, presumably there is a vortex at the tips of the blades. But in an auto, it seems like you would be descending out of this rotor tip vortex; the wind is driving the blades, rather than the blades driving the wind. Correct. Furthermore, the inner counter-rotating vortex that develops during vortex ring state would seem not to be possible during an auto, because there is no down-wash over the intermediate part of the blades. Or over any part of the whole machine. The air is going up through the rotor disk the whole way out from blade roots to tips. Indeed it is, and one day I'll read the whole bleeding post before I start typing :-))) save myself a lot of work if I did:-) (This is all just seat-of-pants intuition; I hope someone with aerodynamic knowledge can say if my intuition is right, and what the aerodynamics of an auto are, and why in that regime settling with power can't happen.) The seat of your pants is working well from where it's sitting:-) The one place I can (just barely) imagine it might be possible is during the brief moment at the bottom of an auto when you crank up the collective to exchange rotational inertia for lift to cushion your landing. Not going to happen there either. You need to have POWER into the blades (and lot of it) plus a descent rate exceeding 300fpm. At that point it seems like you are adding energy to the rotor head other than that which is coming from the descent through the air. Actually, you're USING the energy already stored, so you're losing energy not adding to it. My supposition is that you can turn the blades using the engine, or you can turn the blades using the stored rotational inertia in the blades, and in either case you might be able to induce vortex ring state. Is this true? No. More than one criteria must be met to induce VRS, and one is a LOT of power going into the blades (not being used as it is when pitch is pulled, but DRIVING the blades when pitch is pulled) and the other is that rapid descent rate (300fpm typically). If both aren't present, you're not going to find yourself in the ****. -- Beav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk |
#4
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"Alan" wrote in message ... You still up and about Beav? I was, and then I went to bed, but I got up again this morning and went out. I got back though and here I am, sitting here with a cuppa in my hand thinking about getting out of this chair and transfering to another where can have a drag. Good thnking that.... Gone-) -- Beav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk |
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