Robert Moore wrote in message .8...
I predict incidentally that compressed air servo systems will
replace hydraulics in many aircraft. Safety, economy and ease
of service will promote that.

Just for your information, the YAK-52s being imported into this
country are pneumatically operated, the brakes, flaps, landing
gear, and even the engine starter. The starter is most unusual
in that the compressed air is injected into the top of the cylinder
heads just past TDC via an air distributor similiar to the spark
system.

Zoche aero diesels also use compressed air starters. They are lighter
and more reliable (can't short out)

http://www.zoche.de/
"Reliable starting at low temperatures - patented pneumatic start
system provides instant manifold pressure. Cold start and acceleration
to 2,500 rpm within a second has been demonstrated. Start air
reservoir is refilled by a manifold air driven free piston pump. If
necessary this pump can be operated on any 2 bar (28 psi) air supply."


Bob Moore

Focker when they designed the F27 Friendship (as a DC3 replacement)
purposefully avoided Hydraulics. Hydraulics leak oil (air leaks
aren't to bad) and need special technicians. Hydraulic fluid is
expensive, corrosive and is flamable. Engine bleed air can supply a
pneumatic system.

I think under-carriage opperation or even flap positioning isn't too
difficult but I suspect that the highly precise and dynamic
requirements of flight surface positioning and servo assistance is now
within the capability of pneumatic servo positioning systems.