Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Mon, 30 Jun 2003 11:38:47 -0400
Message-ID: <redirect-2449838@logan.com>
Disposition-Notification-To: Brent Regan <Brent@regandesigns.com>
From: Brent Regan <Brent@regandesigns.com>
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Subject: Re:  Legacy sequenced gear doors
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Adam:

Your logic is sound and the proposed system would work, in theory. 
Unfortunately there are three problems (one of which you already 
anticipated).

1) Wet armature spool valves have a low MTBF and are particularly (NPI) 
sensitive to contamination in the oil. It is not really a question of 
if, but rather when it will stick open or closed. This type of valve was 
originally intended for the IV but I was able to convince Lance that the 
manually actuated valve would be a better choice. I am not aware of any 
hydraulic system related gear extension failures on the IV.

2) Spool valves have high internal leakage. Sealing is achieved by a 
close fit (0.0003") between the spool and the sleeve. Sealing is a poor 
term as leakage through the gap is still present and the leaking fluid 
tends to carry contamination into the gap where it is "filtered" and 
causes the spool to stick (see #1 above). The leakage rate for the valve 
selected is 82 cc / min and is typical for this type of valve. Most 
pilots are annoyed if their hydraulic pump cycles every few minutes to 
make up the lost pressure.

3) I would not consider it wise to move an airplane (with an FBO tug) 
while the hydraulic down pressure is zero.

You may want to consider using  pilot operated lock valves and have the 
doors close when the pressure in either line is above a setpoint.

See:
www.kepner.com

Regards
Brent Regan