Chris,
The funny thing is I sent my pump back to oildyne to have it rebuilt and the spool valve checked for orientation. They charge me $120 or so. That was done around 2002 or so. The strange thing is that the gear worked flawlessly for the first 90 flight hours or so. Free fall test on the ground and in flight were perfect. Locked in 3 seconds or less.

On the test stands with the dump valve open you could easily push up each gear leg by hand. Out of the blue that changed, now on the stands with the dump valve open, you can push the gear leg up but there is building pressure as the gear gets close to the wing. I suspect that I tightened my dipstick to tight causing pressure to build in the resevoir. Could that pressure also be making the spool valve act up?
I thought the spool valve had to be oriented a certain way. Based on some of the posts it sounds like it can work either way.

I installed mine as per the plans. The first dozen or so test retractions, everything worked great. Then the the thing  acted very strange, the pump would run and labor and the gear doors would begin to close before the wheels were in their holes! I was told my spool valve was backwards. I sent the thing in and reinstalled it and everything worked without a hickup until 2 mos ago. I am going to check the dipstick and vent. I don't see how that could be causing the rest of my issues though.

FYI, low side pressure runs around 200 psi during retraction. High side is 0 during extension.

I am absoluteley baffled over how both the high and low sides build pressure during flight.

Thoughts?

Randy.

Christopher Zavatson <Christopher.Zavatson@baesystems.com> wrote:

<<With the dump valve
open, what would cause the building resistance? I
would think that whatever the position of the gear,
the opposite side should be open to the resevoir at
the pump.
>>

 

Randy,

The fluid circuit is really a closed loop.  The only options for returning fluid to the reservoir are either through one of the pressure relief valves or the low resistance back pressure circuit.  Excess fluid needs to return to the tank every time the rod volume displaces fluid volume.  When the rod volume is removed from the system, make up fluid is pulled in from the tank through a one way check valve.  During retraction, the spool valve opens a low resistance path from the low side to the reservoir, eliminating a large pressure build up in the low circuit during retraction. - which also eliminate the possibility of pressure build up due to thermal influence.

One side effect of the correct spool valve orientation and a leak free system is that you cannot manually push the gear up even with the dump valve open unless the spool valve is moved into the retract position.  After opening the valve you still have a closed loop from high to low that is now bypassing the pump.  When you try to push the gear up you get hydraulic lock.  You are trying to add the rod volume to a closed system and the only route back to the reservoir is through a relief valve.  By bumping the pump in the up direction, even with the dump valve open, you can push the spool valve into the retract position which opens up the low resistance path to the reservoir for the low side circuit.  If you are able to push the gear up at all without bumping the pump, I would suspect the spool valve is backwards.  Let us know what observe in the low circuit during retraction.  You can also try the direction-reversal test during retraction.  The response should be instantaneous.

good luck

Chris 

 

 

Chris Zavatson

N91CZ

360std

www.N91CZ.com

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