Postby Alan King » September 6th, 2013, 11:09 am
the simple answer is that you can convert an air cylinder to run on a fluid, the choice of fluid is important, but as said any leak will in fact be worse due to oil leaking in fuselage with possible severe side effects such as CG issues ( small amount of fluid can cause exciting problems particularly when flowing in the tail area or a bit of fluid in a receiver can of course cause complete loss of control an easier solution may be to use an onboard air compressor such as a lightweight conversion of an off the shelf car pump, this works if you have the capacity to carry the weight.
the next best solution after air is electric and in fact some swear by it, i personally prefer air units but plumbing is crucial, i would go so far as to say buy better quality air fittings and adapt these for use.
I too have looked at hydraulic systems for retracts and in fact run cylinders on glycerine which acts as a lubricant and oil in one, the issue is the plumbing and oil tank besides an effective pump. the other really good alternative is air up spring down which is quite easy to do even to air up and down systems, a spring in the right place can get the gear down if the puff fails, of course off the shelf air up spring down systems do exist.
so the perils of hydraulic are leaks, failures, oil loss, air in oil and pumps the merits are more accurate and scale operation but this can be achieved through other means so the simple answer is keep it simple and check more often, just as you would if flying a full size aircraft or look at spring down system.
