lml@lancaironline.net Mailing List Archive

Adam,

Now that is very interesting. Observing pressure in the low circuit while charging the high side looks very much like the 320/360 system sees when the spool is reversed. Excess return flow is forced to exit the loop via the low side pressure relief valve. Now being that you have a Legacy, the system should be replenishing fluid from the reservoir during retraction. Most like Figure 10 in the following document. (Cylinders extending)

http://www.n91cz.com/Hydraulics/Lancair%20Hydraulics.pdf

Your pump is behaving as though the internal return line to the gear pump is blocked thus forcing the fluid past the relief valve. Concerning is that the pressurized low circuit isn't readily bleeding down. In fact, it starts following the high pressure in the lower plot. This could be the result of spool return springs.

Noteworthy is that the first retraction looks normal.

Can you pull the manufacturing date and part number off the pump?

Chris Zavatson

N91CZ

360std

www.N91CZ.net

From: Adam Molny <adam@validationpartners.com>
To: lml@lancaironline.net
Sent: Monday, April 16, 2012 5:14 AM
Subject: [LML] Excessive down hydraulic pressure

On my fourth test flight I tested the normal and emergency gear extension/retraction and everything worked fine. My first landing attempt resulted in a go-around due to gusty winds. When I extended the gear again on downwind nothing happened. After a few seconds of head-scratching I momentarily cracked the dump valve. That fixed the problem and the landing was uneventful (if a bit firm). http://www.youtube.com/watch?v=fo1qnhBuqS8

The attached JPG file shows the up and down hydraulic pressure readings throughout the flight. The x-axis is time, with the flight beginning at 20:32Z and ending at 21:12Z. You can see the gear retraction tests beginning around 20:43 and 3500' altitude. Notice that after the last test the down pressure remains around 500psi for about 90 seconds even though the gear is retracted. The up pressure fluctuates a bit then the down pressure drops to near zero.

The bottom graph is a closer view of the landing sequence. The go-around occurs at 21:05Z. When I retracted the gear the up pressure went to 950psi as expected, but the down pressure fluctuated around 500psi. This kept the down pressure switch open, preventing gear extension.

Does anyone have an idea as to what's causing the pressure buildup on the down side?

Thanks,

Adam Molny

N181AM 6.1hrs hobbs time

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