These are popular with jet flyers for gear doors:
http://www3.towerhobbies.com/cgi-bin...?&I=LXES27&P=0

You will have to firmly hold gear doors in the up & down position, not just let
them fall because the slipstream & propwash will have them flapping.
Drive them with a servo or air ram & don't forget they have to open/close
at the correct time.

Lots of guys use a gear door sequencer to achieve that:
http://www3.towerhobbies.com/cgi-bin...?&I=LXCJE1&P=7

John.