"Turbine" -YM rotor? Mass - YM keeps spinning by inertia? Didn't think so; aerodynamic thing, innit?

Post #418,448 by CRConrad 6/6/17 8:32:30 PM 6/6/17 8:33:13 PM Reply	"Turbine" -YM rotor? Mass - YM keeps spinning by inertia? Didn't think so; aerodynamic thing, innit? Vague recollections from pop-sci books or articles in my teens, which were in another century, so no guarantees for anything. But OK, so add an emergency battery. Or a parachute, if that's lighter. (OK, more liable to wind drift -- but OTOH, a big signal flag to find it by.) -- Christian R. Conrad Same old username (as above), but now on iki.fi (Yeah, yeah, it redirects to the same old GMail... But just in case I ever want to change.) Edited by CRConrad June 6, 2017, 08:33:13 PM EDT "Turbine" -YM rotor? Mass - YM keeps spinning by inertia? Didn't think so; aerodynamic thing, innit? Vague recollections from pop-sci books or articles in my teens, which were in another century, so no guarantees for anything. But OK, so adda an emergency battery. Or a parachute, if that's lighter. (OK, more liable to wind drift -- but OTOH, a big signal flag to find it by.) -- Christian R. Conrad Same old username (as above), but now on iki.fi (Yeah, yeah, it redirects to the same old GMail... But just in case I ever want to change.) Collapse All History
Post #418,450 by drook 6/6/17 8:59:40 PM 6/6/17 8:59:40 PM Reply	Metal vs. plastic On a full-size chopper the blades are probably composite, but are very long compared to the diameter of the (usually turbine) motor. The motor is metal, but spinning far faster than the rotor. Which has more inertia? Dunno. But on most drones it's plastic blades connected to an electric motor. The blades probably have very little inertia compared to the mass of the drone. And electric motors don't "coast". To auto-rotate you need something that can build inertia on the way down to be used to soften the landing. Unless I really misunderstand how auto-rotation works. ... So I just looked it up and the motor is disengaged during auto-rotation, so it's purely the stored energy in the rotor that's used. So definitely not going to work with the super-light blades on drones this size. -- Drew
Post #418,458 by Ashton 6/7/17 2:24:21 PM 6/7/17 2:24:21 PM Reply	Methinks that ... yer physics Gots Cuth. :-)
Post #418,459 by drook 6/7/17 2:34:43 PM 6/7/17 2:34:43 PM Reply	I could have just Googled http://boards.straightdope.com/sdmb/showthread.php?t=728502 The rotors of a typical quad-rotor drone have very low inertia, and fixed pitch. They'll come to a stop almost immediately if the power fails. Oh yeah, fixed pitch. I completely forgot about that. And in more detail: The reason quadcopters work so well using such cheap parts is the same reason they autorotate only slightly better than a brick. The fixed pitch rotors are far cheaper and mechanically simpler than the mechanism a conventional helicopter uses. However, in order to make a fixed patch aircraft hover so well, you need to be able to respond to changes in roll/pitch/yaw extremely rapidly. So you need extremely low inertia rotor blades, relative to the inertia of the quadrocopter body and the torque from the motors. This low inertia means if you lose power, they will slow to a speed at which they provide negligible thrust very rapidly, and the quadcopter crashes like a brick. This is also why large quadcopters don't use longer blades, but instead become octocopters and larger. -- Drew

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