REFLECTOR: Gear dump valve
Ron Brown
romott at adelphia.net
Sun Mar 5 10:22:38 CST 2006
I believe there is a lot more pressure across the opening valve just as you crack the valve. After all, 1500 psi or so is used to raise the gear. The pressure switch cuts the pump off, but there is still 1500 psi there. The pump will run again when this pressure drops a few pounds. Now, if you slowly open the valve until the gear starts to drop, you are bleeding 1300-1500 psi fluid across the ball and seat.
Mine worked fine during the building process and then the rapid cycling of the pump after the 2nd landing. Perhaps there was a piece of trash that damaged the valve. Tightening the stem packing did not remedy the problem. I ordered and installed a new valve - problem fixed for 215 hours of flight.
Ronnie
----- Original Message -----
From: Al Gietzen
To: 'Velocity Aircraft Owners and Builders list'
Sent: Friday, March 03, 2006 6:46 PM
Subject: REFLECTOR: Gear dump valve
I also wore my valve out during construction. It started leaking internally
not long after first flight. What I would do is raise the gear with the
pump, then lower it slowly to certain positions so I could work on the gear.
This resulted in the valve leaking through which led to the pump running
about every few seconds when on the ground. I could open and close the
valve and it would improve the situation but only temporarily. So, be
careful opening/cracking the valve with pressure on the system.
Ronnie
I did the same cycling of the valve without affecting the valve. The pressure at the valve necessary to 'hold' the gear at an intermediate position is relatively low; I don't remember what I measured, but IIRC, less than 100 psig; which should not damage the valve.
One factor that could affect the valve is the very high pressure left in the system at the end of the gear down stroke. As verified with a pressure gauge, the pressure spikes up to over 1,000 psig (to the internal bypass pressure of the pump). This results from the fraction of a second that the pump continues after the pressure switch breaks the circuit.
Leaving this pressure in the system for long periods after lowering the gear certainly stresses the ball seat in the valve, and, of course, the seals in the cylinders. I concluded that it is good practice to momentarily crack open the dump valve after the gear is down and locked, and the master is off, to relieve the pressure. It is otherwise likely to eventually find its own path to reduce pressure.
Of course I have no direct evidence that this is necessary or helpful, but it makes sense to me.
Al
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