Thanks much for the replies. I am perhaps remiss in not having fully explained the surrounding constraints, which are what I feel are backing me into the hydraulic corner.
The basics of the application are three cylinders each actuating one of three landing gear on a very small aircraft. The current system works but has these uncomfortable (imo) characteristics.
1. It requires a pair of air pumps (for redundancy, motor/pump being considered an unacceptable single failure point) and a tank which must admit to frequent draining of water.
2. Air being compressible the stiffness of the system in resisting gear extension during maneuvering flight requires some kind of alternative uplock which becomes another point
of critical reliability.
A huge advantage to this system is that it is operational and seems to work, i.e. the only leak based failure I'm aware of was mitigated by being noticed in time and a precautionary landing
saved the day. This leak was too large to be overcome by the onboard compressors. I'm unaware of the system having any leak problems subsequent to this initial flight test event.
Reliability in prior art
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Industry practice seems to consider gearboxes (for speed reducing or routing motor drives) and linkages to be reliable (i.e. they needn't enjoy/suffer redundancy. Motor failure and the
absence of electricity however are not acceptable reasons to have to land gear up. It is fairy common to have some mechanism to manually rotate a motor or the motor speed reduction output
to effect emergency extension. Some hydraulic systems (either electric/hydraulic or engine driven pump hydraulic) rely on manual pumps to effect an alternate extension means. Systems which
rely on the pilot moving a lever to mechanically extend and retract the gear have been deployed with no backup.
Back to my problem
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Given the above context my tastes run to simplicity and low weight. Electric actuators put three motors beyond human reach with a material increase in the probability of needing to land with
one wheel partially extended which is worse by far (in this airplane) than landing with all three up.
It has occurred to me that automotive hydraulic clutches (master/slave actuators) are an example of fluid power to use human prime mover over a distance without the routing complication of
mechanical linkages. Current clutch systems have a very fancy throwout bearing that seems to combine the slave cylinder with the bearing, but in the old days there was a
cylinder at the clutch that just operated a pivot bar through the bell housing.
Some boats use a similar approach to steering, with what looks like a hydraulic swash plate piston pump controlling/driving a double acting cylinder at the stern. Some models are powered only by
physical rotation of the steering wheel, others have an additional pressure pump.
My hope is to have a human powered reversible pump that extends or raises the gear. This lets me get rid of the onboard compressors and tank, the fluid (vs air) may obviate the need for up locks,
and since the extend or retract sides of the three pistons are all joined the fact that one cylinder is 3 inch stroke and two of them are 5 inch doesn't even have to have a limit switch
issue as would be present in the electric actuator case. ( Not a big problem that, but has to be part of the failure analysis).
Given all this maybe I should be asking for examples of hand powered cylinders of the 3 or 5 inch variety.
The elegance of using a gear pump that is cranked one way for up and the other for down seems very attractive to me for completely removing any need for valving (no spool valve or similar needed) but
from what I can glean there's always a path internally to vent bearing "blow by" to tank which makes me wonder if I can just crank it backwards to reverse the flow. All the "how to
reverse" info I can find require wrenches...
Thanks for the prior remarks with which I fully agree.
Any further thoughts insights or warnings quite welcome.
Regards,
b
