Gear Extension Failure video

Submitted by Anonymous on

Forums: 

From: Chris Zavatson <chris_zavatson [at] yahoo.com>
Sender: <marv [at] lancaironline.net>
Subject: Gear Extension Failure video
Date: Sat, 17 May 2014 11:41:39 -0400
To: <lml [at] lancaironline.net>

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:

  1. High fluid temperatures (lowers viscosity)
  2. Descending into warmer air (raises high side
    system pressure)
  3. Original lower speed .019 cipr gear pump (can’t
    absorb returning fluid as fast)
  4. 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

Gear extension failure video

Submitted by Anonymous on

From: Tom <n20087 [at] yahoo.com>
Sender: <marv [at] lancaironline.net>
Subject: Gear extension failure video
Date: Wed, 21 May 2014 08:54:06 -0400
To: <lml [at] lancaironline.net>
Message Header


Undecoded Message
Just to add to what Chris has presented, I experienced the same issue that started this thread in the early days of proving out my gear during phase 1 testing.  I had two instances of the gear locking up in the air.  Funny enough it never happened while the aircraft was on jacks until I fiddled with the pressure switches to prove the point.  I did have a few things confounding the  problem such as a reversed spool in the pump and one pressure switch with hysteresis all over the map.  In any case, the clincher was instrumenting the system with pressure gauges on both sides of the circuit as described by Chris.  Things behaved exactly as demonstrated in the video.   I knew my low side pressure setting was off and the root cause but I had not gone into the detailed analysis as presented by Chris to figure out what was going on inside the pump.  Having spent some time around high powered hydraulic servo valves and associated systems in my professional career I cannot find fault with Chris's conclusions and am very grateful for his effort and insight


Btw.. I did upgrade my pump to the higher flow rate which really speeds up the gear cycle.  I also installed a quality three way dump valve which was not entirely necessary but it gives me piece of my mind that I can relieve pressure under every circumstance but the biggest benefit was from removing the crappy unit that was part of the kit.  

In at least 400 gear cycles since dialing in the system I have had only one gear retract failure which was due to heat soaking in the florida sun which was easily resolved by a quick twist and reset of the dump valve

Tom

Lnc2 
315 hrs