Wolfgang,
Textbook examples are often oversimplified and fall short when applied in the real world.
". . . when you start to push your car you have to apply a lot of pressure
. . . but less pressure to keep it rolling."
Your car example leaves out critical external forces acting on the system.
This 'car' has a massive compressed spring sitting behind it and the car pointed downhill. When you let it go, you don't have to push it, it pulls you - and with a big jolt.
During extension, the pump is not having to apply any pressure or do any work to get the gear down. Note that the gauge went back down to zero
after the spike.
If you observe the retraction sequence (I know you need a plane first) you'll see a completely different picture. To get the gear up, the pump has to do work. Pressure will come up to about 500 psi and stay there while the gear is in transit.
The risk with the extension pressure spike is that it can push the spool backward and close off the poppet valve. When this happens everything stops. The
system cannot relieve pressure on its own because the poppet valve has closed and the pump has stopped running. You will see equal pressures in both circuits with the gear partially extended.
In order to observe this first hand, I suggest getting a hold of a 320/360, installing some pressure gauges and doing a couple hundred landings in different temperature conditions. You can then raise and lower the low-side pressure set-point and observe the relation ship between low side pressure setting and near-miss hesitations and the full lock-ups. You'll
also see how these are affected by OAT and initial high side pressure.
The hesitations are sort of a quick Start-Stop-Start with the gear continuing all the way down. During a full lock-up you'll hear the pump start up for just a fraction of a second. You'll hear the gear drop from the wells and then all goes silent. A quick glance at the pressure gauges will show ~600 psi (stock set-point) in both high and low circuits.
Chris Zavatson
N91CZ
360std
www.N91CZ.net
On Friday, May 16, 2014 7:42 AM, Wolfgang <Wolfgang@MiCom.net> wrote:
Of course there's a pressure spike. Standard in all
hydraulic systems.
That's what happens when you start pumping fluid
faster than the inertia of the gear can follow.
Think about it
. . . when you start to push your car you have to
apply a lot of pressure
. . . but less pressure to keep it
rolling.
So the pressure spike can open up the switch
. . . fine
. . . then the pressure gets relieved as the gear
starts to move
. . . and the switch closes again
. . . and the motor runs again
. . . until the real end of stroke makes the
pressure go high and stay hi holding the pressure switch open.
This is a non-problem. . . . let it go
already.
Wolfgang
----- Original Message -----
Sent: Thursday, May 15, 2014 7:33
AM
Subject: Hydraulic Pressure Spike
Video
Using my iPhone I took a video of my pressure gauges while extending my
gear on a flight today. It is starting to warm up out
here. We're not cooking yet, but it is enough to start observing
the hydraulic pressure spike which was the subject of a previous LML
thread.
It was a bit bumpy on downwind, but the gauges are still clearly visible
in the video.
The high pressure circuit was just a hair under 1,500 psi when gear-down
was selected. One can hear the pump fire up and release the gear,
The high side pressure begins to drop and then the spike immediately hits the
low side. It reached 450 psi in this clip. If the spike reaches the low
side pressure switch setting, the pump solenoid will open and the pump will
shut down. I have my low side pressure switch set to 800 psi which has
proven to be high enough to avoid any gear hesitations and/or extension
failures.
Chris Zavatson
N91CZ
360std
www.N91CZ.net