Hi Jeff –
Yes, I’m using my Sequenced Gear
System in this plane. The inner gear doors close after the landing gear is
extended for landing. I can say from experience that it provides a huge
reduction in wind noise and cockpit drafts. Naturally I investigated whether my
system could be contributing to the problem. During system development I
changed to a down pressure switch with a set point of 650psi. The stock 550psi
switch didn’t provide enough ‘oomph’ to close the inner gear
doors against the emergency opening springs. I have an older kit with the
narrow diameter door cylinders. Customers with the newer style large-diameter
cylinders are able to use the stock pressure switch.
Theoretically, having a down pressure with
a higher set point should help alleviate this problem since the system would be
less sensitive to pressure buildup on the down side. I believe we can also rule
out pressure buildup due to thermal expansion. My early flights never went
above 5000’ so there wasn’t a big temperature swing. Also, you
would expect to see a gradual rise in pressure due to cold soaking, which
wasn’t the case.
Partway through the build process I also
modified my hydraulic pump by installing the high capacity impeller.
As part of the troubleshooting process I
have removed the inner gear door springs. The gear still extend quickly in both
ground and flight tests as seen in my fourth flight video. http://www.youtube.com/watch?v=fo1qnhBuqS8.
While developing the sequenced gear system I learned that the
pressure switches can be wildly inaccurate, and they behave oddly at the limits
of their adjustment range. As you turn the adjusting screw to increase the
pressure the hysteresis decreases. In other words, the pressure difference
between turning off and turning back on again gets smaller. This can lead to
pump chatter where the gear hit their stops, the pressure rises and the pump
shuts off. Then the pressure drops slightly and the pump kicks in again. This
can happen several times per second and last several seconds. The solution was
to make sure the adjustable pressure switch was set near the middle of its pressure
range.
I don’t have a solution yet but
I’m reading through Chris Zavatson’s excellent write up on the
Lancair hydraulic system http://www.n91cz.com/Hydraulics/Lancair%20Hydraulics.pdf.
I’m beginning to suspect a sticky shuttle valve or some
other internal pump problem, with the pressure switches as a secondary suspect.
On my fifth flight yesterday I had to crack the dump valve to get the doors to
close completely with the gear retracted – and this was with the door
springs removed!
-Adam Molny
7.6hrs Hobbs time and counting
From: Jeffrey W. Skiba
[mailto:jskiba@icosa.net]
Sent: Tuesday, April 17, 2012 9:52
AM
To: lml@lancaironline.net
Subject: RE: [LML] Excessive down
hydraulic pressure
Adam,
Are you running the
Stock Lancair gear system or are you using your gear system ?
Do you have the
first generation pump or the high capacity one ?