REFLECTOR: Low Hydraulic Pressure or Pump Check Valve Leaking?

Wed Aug 10 07:40:58 CDT 2011

I am new to this list so I hope my comments below do not hash over things already said on this forum.  If they do please use your delete keys.  I have owned and operated Freeflight Composites, LLC for ten years now helping folks build and maintain their canard aircraft, and have bee an approved Cozy service center and recently was approved as a Velocity service center as well.   I hope to be able to contribute once in a while, and I know I will be able to learn from folks on this list just about all the time.  I have been lurking for a few weeks and the knowledge and civility of the folks who  participate is impressive.  Thanks for letting me be a part of it

One thing to remember about a hydraulic cylinder is that the rod displaces oil thus there is more oil on the piston end of the cylinder than there is on the rod side.  So, a hydraulic cylinder will not retract due to an internal leak inside the cylinder  even if the pistons seals are bad.  There is not enough room for the extra oil on the piston side to transfer to the rod side because of the displacement of the rod itself. An external leak in say a fitting or valve in the pump control will allow the cylinder to retract.  With the gear up there will be less oil in the reservoir than when the gear is down. All that being said either the oil level was low to start with or there is an external leak, or there was air in the system that took a few cycles to work through.  The question John asked about whether the pump continued to run or just the light stayed on was a good one.  In flight you should be able to tell if the pump is cycling by watching. the ammeter, they usually  jump or flicker when the pump turns on. I believe that on a Velocity the cylinders must extend to let the gear go down so an internal leak in the cylinder or elsewhere could cause the pump to cycle too often or even run continously. It is likely normal for the pump to cycle every few minutes but much more often than that indicates a problem with either an external leak or an internal bypass that bleeds the pressure in the system off causing the pump to cycle on and off as the pressure switch tries to maintain system pressure.  Hope this helps at least a little.

Burrall Sanders
www.freeflightcomposites.com

  ----- Original Message ----- 
  From: john at velocityaircraft.com 
  To: bobj at computer.org ; Velocity Aircraft Owners and Builderslist 
  Sent: Wednesday, August 10, 2011 5:22 AM
  Subject: Re: REFLECTOR: Low Hydraulic Pressure or Pump Check Valve Leaking?

  Bob,

  The reservoir has more fluid up than down because you have spacers on all the cylinders acting as stops.  

  As for gear not operating when you change the switch position, this can be caused either in flight going through a major temperature climate change or sitting on the ground in the sun for a long period of time.  Both pressure switches "think" they are triggered.  If you are in flight and put the selector down and nothing happens first push the reset switch on the panel, this forces the pump to run in the down position.  If you are taking off and the gear does not come up then select gear down, push reset button, then reselect gear up and everything should be fine.  You dont want to hit the reset button with the gear in the up position because it is wired to run the pump in the down position.  This is very bad for the relays in the gold box.

  As for fluid volumes in reservoir, have you changed a cylinder out lately?  It could be working air out of the system.  If not then you have a leak somewhere.  Could be internal however if you are loosing fluid this probably not the case.

  Is the pump shutting off when the gear comes up but you just have a red gear unsafe light?  A lot of times the cause if the Nose Gear Door Micro Switch missing.  I would also inspect and open and shut your dump again to make sure there is not junk stuck to the ball on the inside, I have had this happen before and the gear will start to fall in flight, this will happen with internal leaks as well if you high pressure switch is going bad.

  Best,

  John 

  On Wed, 10 Aug 2011 00:37:32 -0400
  "Bob Jackson" <bobj at computer.org> wrote:
  > I'm stuck in Vernal UT after flying from Orlando, 
  >heading for Oregon.  After
  > take-off from Vernal on the third and last leg to 
  >Portland, I got an unsafe
  > gear up light.  After returning and investigating (no 
  >problem getting the
  > gear back down), I noticed the hydraulic reservoir was 
  >down more than a
  > third of the way from the top (we normally keep our 
  >fluid level (with the
  > gear down) about ½" from the top of the reservoir.  I 
  >refilled the reservoir
  > and tried again.  This time I noticed the pump (light) 
  >was cycling even on
  > the ground, and got the same 'Unsafe UP' light 
  >indication after take-off.
  > 
  > 
  > 
  > Does anyone have an ideas or suggestions?  I assume that 
  >the pump can't
  > create enough UP pressure because the up-side pressure 
  >relief valve won't
  > let it.
  > 
  > 
  > 
  > Attached is a PDF copy of the Mercruiser Outdrive 
  >Maintenance manual - pg 20
  > has a good diagram of the internal design of (our) gear 
  >pump, including
  > shuttle valve, pressure relief valves, etc.  Can anyone 
  >explain:
  > 
  > 1. why the pump's tank level is different for the Gear 
  >UP and Gear DOWN
  > directions (isn't there the same amount of fluid on both 
  >sides of the
  > pistons?)
  > 2. if you've ever seen a situation where the both the UP 
  >and the DOWN
  > sides get pressurized so that neither pressure switch 
  >will any longer allow
  > you to change the gear's position (different problem 
  >from mine in Vernal,
  > but just asking since I heard of it once)?
  > 3. What could cause my hydraulic reservoir to go down by 
  >1/3 during
  > just the first two legs from Orlando, when it's never 
  >gone down rapidly like
  > that before.  We don't seem to have any big leaks in the 
  >overall hydraulic
  > plumbing that I've been able to find yet.
  > 4. Is there any way for the hydraulic pump or reservoir 
  >to 'vent'
  > fluid, other than from bad hydraulic connections that 
  >are external to it?
  > 
  > 
  > 
  > Thanks for the help,
  > 
  > Bob Jackson
  > 
  > N2XF
  > 

------------------------------------------------------------------------------

  _______________________________________________
  To change your email address, visit http://www.tvbf.org/mailman/listinfo/reflector

  Visit the gallery!  www.tvbf.org/gallery
  user:pw = tvbf:jamaicangoose
  Check new archives: www.tvbf.org/pipermail
  Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.tvbf.org/mailman/private/reflector/attachments/20110810/1b0fc606/attachment.htm>