Hi Scratchbuilder46,
I did some searching around RCU and have found a few examples of mechanical gear door retraction systems. As an oh by the way, you can use small micro servos to actuate the gear doors, I used that method as afew other guys have also. This website Oregon Scale Aviation
http://www.oregonscaleaviation.com/ has a sequencer that works either with air/servo systems or vice versa, check out the pics.