ORIGINAL: hands without shadows
$18.99 Robot Marketplace
http://www.robotmarketplace.com/products/IL-SAN50.html
There are cheaper suppliers if you want to buy in bulk. Its a fairly common antweight (combat robot class) drive motor. Different gear reductions are available.
For the electronics, just dismantle a standard size servo. Remove the motor and wire in the new gearhead motor. Remove the potentiometer and wire in a new 5k linear potentiometer that is attached to the nut of the retract mechanism. Seal the servo board with heatshrink and some hot glue. Instant proportional retract mechanism that can plug right into your receiver. You can program the endpoints with your transmitter.
Im not sure Im following you on the servo parts use. These are going to need to turn several full 360 degree revolutions to go from UP to DOWN.
Oh wait - thats what the linear pot is for?
If Im understanding you thats a cool suggestion!
That would work well for everything except the gear getting jammed. A current sensor would still be needed I think. Or at least a nice addition.