Why does a sycamore seed auto-rotate when falling?

[BruceGuinness]

Homework Statement

Explain ALL of the forces present acting on a falling sycamore seed (maple seed/Helicopter seed) and use this to explain why it auto-rotates in a stable state.
Also explain what other factors may effect flight and autorotation,
Note:[i/]you can factor out many variables by noting that a similar analogue with almost identical flight pattern can be constructed from simply a sheet of paper.

Homework Equations

None Given :/

The Attempt at a Solution

Analysing the forces on the seed, I can see that with a large wing and a small concentrated mass at one end, the seed is likely to tip up. Due to the uneven mass distribution and drag forces. My only other guess would be the moment of inertia in a z axis is trying to align with the axis of rotation. A feat it cannot achieve and hence is constantly rotating to achieve this?

Although I'm not entirely sure.

Help! Thanks :)

 

[Simon Bridge]

If you dropped the seed head-down, with the vane directly upwards, what would happen?
http://www.madsci.org/posts/archives/1999-08/933947038.Ph.r.html
... basic description + resources.
This is a well-discussed phenomena online ... Google is your friend here.