sanman wrote:
Here are two sites I was reading:

www.metallicpower.com
www.electric-fuel.com

Both these companies are developing zinc-air fuel cells, which have 7
times the energy density of lead-acid batteries.

Could this technology be practical for powering an electric
airplane/helicopter/gyroplane?

There's an electric self-launch glider out there now.

http://www.alisport.com/

Not sure what kind of batteries it uses.