[sjtrny]

> but so far as I can tell nobody, at least in the hobby size quadcopter world, has done so

It's not the motors that are the problem. They can spin freely any direction. It's programming the ESCs to be reversible.

Anyway it's already been done a hundred times over:

- Fixed pitch props but reversible ESCs https://www.youtube.com/watch?v=UoReJULASkw

- Variable pitch prop styles exist such as HobbyKings Reaper https://www.youtube.com/watch?v=mAFxkHAiimY and Curtis Youngblood's StingRay http://curtisyoungblood.com/V2/products/quadcopters/stingray....

Autorotation on quadcopters is possible with the variable pitch configuration.

[bigiain]

>> but so far as I can tell nobody, at least in the hobby size quadcopter world, has done so

> It's not the motors that are the problem. They can spin freely any direction. It's programming the ESCs to be reversible.

True - poor wording on my part. And even the ESC programming is a "solved problem" - all the brushless RC cars run ESCs which understand how to run brushless motors in both directions.

So actually reversing the thrust is, as you say, already happening. What I've not seen (yet) is a controller board with software designed to use that ability to stabilise and safely land a quad with one motor out.