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About the pressure building in flight, I have a
seperate light that illuminates anytime the pump is running. Sometimes it
cycles on during flight to repressurize. Do you know if your pumps are
doing that?
Steve Rosenzweig
LNC360
----- Original Message -----
Sent: Thursday, November 23, 2006 6:27
AM
Subject: [LML] Re: Electrical /Hydraulic
system questionslancair235/320
Bob,
You describe the exact same thing as I am
experiencing.
I have pressure gauges on the high and low side. In my case,
both the high an low sides sometimes creep up in pressure during flight. Just
today I noticed everying normal after takeoff 1300 high and 0 on the low.
After 20 mins or so the high was 1700 or so and the low was 400. Gear came
down normally. If the low side reaches 600 psi the gear will not come down
without cycling the dump valve. I theorize that the down side pressure switch
shuts off the pump because of this pressure.
Ironically, The first time
this happened was when I flew to the last labor day fly in at Redmond. It has
happened several times since. I also noticed one day when I pulled the
airplane out of the cool hangar and parked it out in the warm sun for an hour.
When I got in the low side was very high and the high side was not far behind
it. It alarmed me enough that I opened the dump valve and equalized the 2
sides. Don't remember what the pressures were. They equalized at something
above 0. Maybe around 6-700 psi. It seems like increased heat increases
pressure in the entire system. I am going to check the dipstic(not the pilot)
and make sure it is not tight. I believe mine is.
To answer your
question about what the down side pressure is during retraction, it is running
around 200 psi. The upside is 0 when the gear is being lowered.
Thanks
for the input...
Randy,.
"Bob B."
<rbelshe@comcast.net> wrote:
:
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,
It would seem that there is no one answer for this
problem.
On
my 235, with the plane on jacks;
I
can open the dump valve and easily push any of the gear
up. Spool valve is backward?
When
I do the direction reversal test the gear reverses instantly.
Spool valve is OK?
When
I put the gear up and then open the dump valve, all 3 drop within 2-3
seconds.
Years ago, I experienced a few instances of the gear not coming
down when the gear switch was put down. This situation was always
relieved by briefly opening the dump valve. It stopped happening, so
I did not change anything.
Am I
just lucky, or what?
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