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New 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
New Methinks that ... yer physics Gots Cuth. :-)
New 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
     Here or sports ... - (drook) - (14)
         nice, want -NT - (boxley) - (2)
             Ooh, idea - (drook) - (1)
                 First thing I thought of reading the article. -NT - (hnick)
         Neat. -NT - (Another Scott)
         And should you be surfing sand dunes - (Ashton)
         First thing I thought of, just >>before<< I got to the complaint about limited flight times... - (CRConrad) - (8)
             Brilliant! - (drook) - (6)
                 Yeah, thought of that too. But is it needed; don't they auto-rotate like autogyros/real helicopters? - (CRConrad) - (5)
                     Doesn't that depend on the mass of the turbine? -NT - (drook) - (4)
                         "Turbine" -YM rotor? Mass - YM keeps spinning by inertia? Didn't think so; aerodynamic thing, innit? - (CRConrad) - (3)
                             Metal vs. plastic - (drook) - (2)
                                 Methinks that ... yer physics Gots Cuth. :-) -NT - (Ashton) - (1)
                                     I could have just Googled - (drook)
             You might have pissed-away a few millions of $zlotys - (Ashton)

Major Allison Digby Tatham-Warter was so absurdly British that you can't even read his name without being compelled to brew a cup of tea.
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