Wolfgang,
The concept I have
trying to describe here and in my hydraulics write-up is nothing new. It is however difficult, if not
impossible, to identify without instrumentation. Most often this phenomenon simply
causes what is perceived as a hesitation during gear extension. Sort of a quick Start-Stop-Start with
the gear continuing all the way down. It becomes quite clear what is
happening if one looks at the system pressures when gear-down is
selected. Analog gauges are best
for catching the interaction.
What you�ll see is a pressure pulse just as the gear starts to
fall. The magnitude of the pulse
is made greater by a few factors:
-
Warm/hot hydraulic
fluid (lower viscosity)
-
Outback gear (adds
gear down force)
-
High-side pressure
well above nominal (descending into significantly warmer air)
-
Original, lower volume
gear pump
When the landing gear is in the
retracted position, it stores a lot of potential energy. This is from the mass of the landing
gear, the pressure on the high side circuit and, with the outback gear, the
aero loads trying to pull the gear down.
It
is useful to look at Figures 9 and 10 in the hydraulics write-up when trying
understand the chain of events.
In Figure 9,
you�ll see state of the pump while the gear is retracted. When beginning the extension cycle,
the pump builds up pressure behind the spool (right side) and pushes it
against the poppet valve on the left as seen in Figure 10. Opening the high side poppet valve
opens a path for fluid from the high side circuit to return back to the
pump. It is not just the pump
moving fluid though. All the
stored energy in the system is released and the falling gear, now also pushing
fluid back to the pump. Given the
fixed volume nature of the gear pump, it can only absorb fluid at a certain
rate. If the gear pushes the
fluid back faster than the pump can absorb it, back pressure builds up in the
high side circuit. With enough
back pressure the spool is pushed back to the right. As this happens, the high side
poppet starts to close off again.
Pressure begins to rise and propagates backwards through the
system. As soon as the low side
pressure rises above the pressure switch set point, the pump is shuts
down.
Most of the time, the shut-down
is momentary. This is because the
pump takes time to spool down and it is still moving fluid as it does so. At the same time, the falling gear is
losing energy. If the pressure
spike was small, the pump side will win, the spool will again open the poppet
all the way and the gear comes down.
If the spike was large, the pump will remain off and you will see equal
pressure in the high and low side, just like in Lorn�s photo. Pressure will be just above the
set-point of the low side pressure switch and the gear will be partially
extended. Mitigation simply
involves raising the low side pressure set-point so that the pressure spike
can no longer affect the switch and pump.
I really don�t expect you to be
convinced. I post these details
for the benefit everyone.
For many years
now, I have been publishing all kinds of reports, studies, diagrams,
maintenance guides and videos of Lancair systems is to promote a better
understanding and safe operation of the LNC2. If I have made errors somewhere,
please point them out. But don�t
just say it is wrong or that you don�t agree. Please provide some legitimate,
logical reasoning, some data or something that makes the point.
To be honest, I found many of
your posts to be a bit worrisome.
They left me with the impression you did not have a full understanding
of the pump operating environment or its operation.
For example, when
discussing spool movement, you claimed the pump could be subjected to 70 g�s.
The actual environment is much
more benign. Vibration
peaks are more on the order of 0.1 G�s.
Below is a link to a vibration study that looks at airframe vibration
in different phases of flight.
The study was done while looking into an engine isolator issue, but is
a useful environmental baseline for anything mounted to the structure.
Another example was a question
regarding the ball and spring in the return line. You had asked what their purpose
was. This would normally a very
legitimate question. My concern
was that you had already designed a circuit to alter pump operation without
being aware of or understanding the function of all the internal parts.
Innovation is a good thing, but
on an aircraft the level of required due diligence is quite high.
Lorn�s pump
definitely had issues. Mounting
the pump behind the baggage bulkhead is not ideal. It discourages good
maintenance practices, just as it did in this case. Being unwilling to remove it from the
aircraft to investigate the internals because it was �so hard to get to�
simply makes it impossible to properly investigate a problem. When I offered to examine the pump,
the response was that these airplanes were built to fly and two weeks of
down-time was too much of a burden.
That was unfortunate. Based on
Lorn�s description of the pumps behavior I strongly suspected the root
cause of its problems would have been immediately obvious upon
examination. I have disassembled
more pumps that I can remember.
All were made to operate normally once configured and adjusted
correctly.
Chris Zavatson
N91CZ
360std
www.N91CZ.net