The video below shows the sequence of events when the pressure
spike generated during gear extension actually exceeds the low pressure
set-point. The magnitude of the spike is
dependent on several variables. Factors
that tend to make it larger are:
High fluid temperatures (lowers viscosity)
Descending into warmer air (raises high side
system pressure)
Original lower speed .019 cipr gear pump (can’t
absorb returning fluid as fast)
Outback gear (adds downward aero loads on gear)
I lowered my low-side pressure set-point (normally at 800 psi) and went flying.
What you see in the clip is an elevated starting pressure in
the high side circuit. Upon selecting
gear-down, the high-side poppet is opened and pressure is released. The falling gear creates back-pressure in the
circuit and begins to push the spool backwards - allowing the poppet valve to
close off again. When the pressure spike
level passes the pressure switch setting, the pump shuts down. The high side poppet valve is now fully
closed and pressure is locked in both high
and low circuits. The gear is now
only partially extended yet the criteria for the pump to remain off has
been met. This condition is completely
stable. Momentarily pulling the dump
valve allows the gear to continue down.
Permanently eliminating this failure mode simply involves raising the
low-side operating pressure. 800 psi has been shown to be sufficient. The adjustable SUCO pressure switches Lancair supplies, nominally set to 550 psi, are able to be adjusted to 800 psi.
Chris Zavatson
N91CZ
360std
www.N91CZ.net