the best thing since sliced bread was the invention of spring air retracts. very simple to plumb and you dont have to worry about loosing your air and having a belly landing. i refer them to anyone building a fun fly aircraft. for what it will cost you for the retract servo and the mach. retracts, you can almost buy a set of spring air mains. robart also makes a set of air up, spring down retrracts. i think there the 150 series. i havent tried the robart set, but i have a couple dozen sets of spring airs.

also, i have found that it is easier to make a retract plate instead of retract rails. 1/4 inch cut out of the rib is better than 1/2, and with the plate it spreads out the load of impact better than rails, and you have more area to glue and it doesnt add any more weight.....

just my .02