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