How do retracts work? Are they spring loaded to the extended position, and pnuematically retracted? I can give lectures on full size A/C landing gear systems, but I'm stupid on R/C ones!

Here's a starter location...
http://www.robart.com/

Well...they can be both really.

You can have the air retracts, which work similarly to the full scale hydraulic retracts. These are generally made by Robart. At least I haven't seen any made by another company yet. In the air retracts a servo generally activates a manual valve to send compressed air to the air actuator...depending of course on which way you want the gear to go. Their is an air tank that's usually pumped up by hand to store the pressurized air.

Other retracts are generally servo actuated. GWS and other companies make these as well as Robart. Very simple in theory - servo moves one way, gear up - servo moves other way, gear down. There is usually a mechanical downlock to keep the gear down and keep the landing stressed from stripping the servo out.

Hope this helps you out!

Originally Posted by Twizter68

How do retracts work?

The majority are pneumatic up and down, with one cylinder being supplied air from a double-action servo-operated valve. Double action in that in one position it pressurizes output line "A" and vents output line "B", and in the other position pressurizes "B" and vents "A". "A" and "B" cylinder ports are connected to achieve the desired resulting direction of travel.

Two manufacturers offer pneumatic extend, spring retract, which makes the plumbing a bit less expensive and also has the 'safety feature' of automatic extension on loss of air pressure.

A third wrinkle is that all the pneumatic setups can be converted to hydraulic, where a second servo is used to drive a hydraulic source cylinder. Whether it's worth the effort is another issue.