Strangedays,
As with most things in aircraft, the conventional approach was arrived at after considerable experimentation "in the market", Landing Gear included. I too like the simplicity and good stability of inward retraction, putting the wheels into the wing root area where you are likely to have the volume to do it. Solid, you know how its alignment works, doors are easy to arrange, etc.
Air struts as gear actuators? In a plane this light, why not just crank it down?
While I am not familiar with the specifics of aircraft landing gear actuation nor with the actuators and CO2 cannisters you have available, most hydraulics are rated for operating at 100 to 200 bar, while pnuematics usually run 5 to 10 bar, and Russian radial engine starter systems run about 60 bar. So, your pnuematics would generally have to be heavier...
The reasons that we do not like pnuematics at pressures that hydraulics run to has to do with failure mode management and practicality.
There is little energy stored in high pressure liquid (it does not compress much). If you develop a crack in a hydralic actuator, it leaks. Do the same in a pneumatic actuator, and it can burst rather explosively.
High pressure systems need to be kept extremely clean to maintain those high pressures, or both the pumps and the seals wear and leak. Hydraulics allow this with closed systems and filters.
Now it is true that many larger aircraft have a last ditch emergency gear extension system driven by pressurized gas. There is a substantial N2 bottle connected to the gear down side of the actuators, connected seperately from the regular hydraulics, released by manual operation, and it has sufficient pressure and volume of N2 available to drive the whole system down and locked despite substantial leaks. It is not intended to be reversed, and it is not intended for regular use. I have it on good authority from airline pilots that the usual bump-bump-bump of gear locking down is mild compared to the pnuematic emergency system... Design for the gear hitting the stops pretty hard .
Anyway, your pnuematic actuator sizes for reliable operation are probably pretty small for a bird this size, so they could be a good trade. Remember that you have to have enough gas left in your cannister each time to take the gear through a whole cycle, or it will stop in the hard part... I suppose that you could have a backup smaller cylinder too, but even two of the little CO2 bottles for driving keg beer or pony bottles for diving will weigh nearly as much as an electric-hydraulic pump that is commonly available for airplanes. Oh, you were doing this for cost. Hey a CO2 bottle every flight would be peanuts compared to the fuel bill on your bird and lots less expensive than the electro-hydraulic pack from an aircraft source.
Billski
:wail: 
