RE: converting air to electric retracts.
one other thing. Make sure when you build your screw jack you put a thrust bearing into the design and travel end stops. For thrust bearings i use the ones from model helicopters that attach the main rotors, the ID is 3mm so you'll ned to take the thread off your 4mm screw. If you don't have end stops you'll bend your screw jack if you get any side loads on the gear. If you don't have a thrust bearing you'll wreck the motor in no time at all.