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