Re: What makes a sycamore seed spin as it falls to the ground?

The technical term for this phenomena is autoration. There's a nice web page on it at http://www.copters.com/pilot/autorotation.html. You can locate numerous photos of "helicopter seeds" with a search on Google's Images locator. and a nice write-up at http://waynesword.palomar.edu/plfeb99.htm

To quote from http://waynesword.palomar.edu/plfeb99.htm#helicopters

"Helicopters (also called Whirlybirds) include seeds or one-seeded fruits (samaras) with a rigid or membranous wing at one end. The wing typically has a slight pitch (like a propeller or fan blade), causing the seed to spin as it falls. Depending on the wind velocity and distance above the ground, helicopter seeds can be carried considerable distances away from the parent plant. The spinning action is similar to auto-rotation in helicopters, when a helicopter "slowly" descends after a power loss."

What's happening is this: the weight of the seed itself is enough to orient the whole structure (seed and "helicopter"). This is important, because if the orientation changes -- for example, if the whole thing turns updside down -- then the whole airflow around the structure will change and it will not autorotate anymore.

So, how does it autorotate? Well, take a close look at the shape of the "helicopter" part of the structure. You'll see that it is shaped such that any air flow upwards along its length will be re-directed outward. You'll also see that the curvature is something of a dog-ear, it doesn't curve straight down, but rather down and to the side a bit. It's this redirection of air "to the side a bit" that causes the rotation.

Other than collecting seeds and dropping them yourself (which is probably most instructive) you can try some of these projects to get a better feel for what's happening

http://bcbrown.simplenet.com/secondary/1st6weeks/labs/heli.html

http://muttley.ucdavis.edu/Curriculums/Nature/ seed_helicopter_howto.html

http://www.eecs.umich.edu/~coalitn/sciedoutreach/funexperiments/agesubject/lessons/other/Maple_Seed.html (this one also has some good diagrams)

http://www.iit.edu/ ~smile/ bi96m4.html