At lunch I looked over your excellent report on the 320/360 and Legacy hydraulic systems. We have the pre-2005 Legacy hydraulic pump we’d modified for 28 VDC and changed
to the high capacity pump. I’d never heard anything from Lancair about a change in the internals as you described it – symmetric spool, spool centering springs, 150 psi backpressure circuit removed, etc.
Of course we have the three way dump valve to handle the hydraulic cylinder rod retraction/extension on dump valve activation. But, it seems everything thing else must
be 320/360 type internal on the spool and backpressure circuit.
I’m not sure what changes we should make, if any, to have a system that isn’t prone to these pressure lock ups with temperature changes… Suggestions? I think William
Ford’s Legacy kit number is close to ours so he likely has a similar setup…
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My involvement with the Legacy pumps started when they began showing up in 320/360s. Lancair wanted to only carry one
configuration for both planes. This is when I learned of the return springs having been added to the Legacy pump. Apparently no one really thought through what would happen during thermal cycling. Personnel turn-over at both Parker and Lancair left everyone
scratching their heads as to why this change was made. All agreed it had the potential for lock up. They was little motivation, however, to do anything about it. The Legacy had a 'get out of jail free' card with the three way dump valve . The 320/360 was
a little more problematic. The most I could get from Lancair was an offer to include a note with each 320/360 shipment stating something to the effect "this pump may lock up your hydraulic system on a warm day".
While Parker was still in the aviation business, they were supplying parts free of charge to revert Legacy pumps back to the original 320/360 configuration while retaining
the higher volume gear pump.
The Legacy fleet unfortunately has a mixture of pump variants. Early unit were likely 320/360 pumps. Then came the higher volume gear pump and a symmetric spool. After
that, the return springs (which also removed the return line back pressure spring). If an early model pump ever went into Parker for servicing, the internal configuration may have been 'upgraded'.
In my opinion, the best configuration for the Legacy is:
1. high volume gear pump - larger fluid volume in the system.
2. symmetric spool - system has cylinders in both extension and retraction orientation
3. no spool return springs - they add no benefit, only lock up potential
4. add/retain the 150psi back pressure circuit in the reservoir return line - belt and suspenders to ensure the spool cannot get stuck at a travel endpoint. Note that
the return line from the dump valve (third line in the center) bypasses the 150 psi circuit. Only fluid coming from within the pump is subject to the 150 psi back pressure.
The Legacy may also benefit from a higher low side pressure setting, especially for those with older, low volume gear pumps. The higher volume gear pump may be able to
outrun the pressure surge that is generated at the initiation of gear extension. This is where pressure gauges are useful. You'll see the pressure spike on the low side upon selecting gear down. If the spike hits the low side pressure setting the pump will
shut down. 800 psi works well for the 320/360. The higher volume gear pump lowers the risk, but I can't put a number to it without some data.