|
|
Chris, I am interested in your comment about the “Old VEP pressure switches”. The 360 kit I am building comes with those pressure switches. What do you recommend we should use in place of them? Thanks again. Rob Stevens Perth, Western Australia. From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Chris Zavatson
Sent: Tuesday, 29 January 2013 11:56 PM
To: lml@lancaironline.net
Subject: [LML] Re: Thermal influence on Hydraulic Pressure Yep, the failure modes are completely unrelated to each other. There is a second failure-to-retract mode. In fact, it is the only one I experienced in my plane. It was with the old VEP pressure switches. They would stick open on occassion. Then the problem is not too much pressure, it is not having any. Failure to retract can be a bit more serious if in IMC or taking off at high density altitude. The failure to extend can be fixed by simply raising the operating pressure of the low side circuit. I have looked at pumps spanning the 95 to 2011 manufacturing time frame. The only one that would lock up on the ground had spool return springs. Since the 70g rattle-your-spool controversy appeared, I have also been measuring the spool resistance in every pump I get my hands on. -haven't found an outliers yet, but I will keep looking. While waiting for Wolfgang's answer, some 200/300 series pilots are becoming concerned that they can't get the gear down when needed. First, Lorn's secondary problem is that he didn't look for the "gear down and locked" lights before landing. While the gear down pressure switch saw enough pressure to not allow the pump to start, the failure to recognize that the gear was not down contributed to the belly in landing. To allow people to be more comfortable about the hydro-electric system they should be reminded of this 1. The failure to RETRACT the gear because of high pressure on both sides is a problem because of the safe design. Opening the dump valve doesn't resolve the problem because the system is in a static state with the gear down and locked. There are no physical forces able to change the system condition - no G-maneuvers, no slips or skids, nada. 2. The failure to EXTEND the gear because of high pressure on both sides is different. Opening the dump valve (allowing down side fluid to flow to the upside) will allow the gear to "emergency" extend because of gravity on the mains and the nose gear gas spring. OK, maybe the nose gear might not extend all the way at 120 KIAS (max gear extension speed), but the pressure lock has been broken and the pump will start if the switch is down and the pump breaker is closed. Close the dump valve to complete the process. As a matter of fact, assuming everything is normal and the gear down switch is selected but nothing happens, it may merely require momentarily cracking the dump valve to relieve the hydraulic constipation. Failing to retract is a nuisance. Failing to extend is, uh, more serious, but resolvable. Scott Krueger AKA Grayhawk I seem to recall you dismissed the idea of a three-way valve previously. <<That's a sound and positive fix. - - However it requires changing the dump valve, running a return line and pump modification (replace the reservoir). ................................... <<..........Automatic action by my module can prevent these problems for both up and down gear operation. Could you please explain how your module will prevent the gear extension lock-up failures. Recall Lorn's photo after his extension failure and gear up landing. Both pressure gauges showed around 500 psi. The system is hydraulically locked with one switch open and one closed. This has been covered at great length before. Some take the position that "real pilots" don't need anything but the dump valve. I have proposed two answers, 1) replace the dump valve that only opens a port between the HI and LO sides with a dump valve that dumps back to the pump reservoir. 2) install a small electric module I make across the pressure switches that runs the pump to relieve the over pressure in the "wrong" line automatically. ----- Original Message ----- Sent: Saturday, January 26, 2013 2:07 AM Subject: Re: [LML] Re: Thermal influence on Hydraulic Pressure This has happened to me even at the beginning of a flight, if the temps in the hangar have gone from cold on a previous day, to warm on flight day. I guess cold temps shrink the fluid and suck more from the reservoir into the lines, and when it warms, the pressure builds on both sides of the system, so that the "up" and "down" pressure switches are both open and the gear won't move until I pop the bleed valve. Nasty when the gear won't retract just after blasting off into a low overcast. On Jan 25, 2013, at 7:23 AM, randy snarr wrote: . I dont like having to open the dump valve but it is necessary once in a while especially when it is cold. The pressures on both sides of the system climb due to the engine heat and the gear sometimes will not move without dumping the pressure.
--
For archives and unsub http://mail.lancaironline.net:81/lists/lml/List.html
|
|