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Please excuse the length of this posting but I want to get all the info in the initial post. Plane is RV12 with Rotax 912ULS, Dynon Flightdek (not Skyview) 120 hours TT. After two partial power losses on takeoff and complete testing of the fuel system,aux pump, and Dynon sender, the original fuel pump was replaced last summer with an off-the-shelf old model from Lockwood. All was well for maybe 30 hours with readings of 3.5-4.2 PSI (in the green) at all times. About 3 hours ago, I noticed the fuel pressure dropping into the mid-yellow range (2.2-2.5) on takeoff, recovering nicely shortly thereafter. It would do this irrespective of fuel on-board or ambient temperature.

Because of my earlier problems, I always take off with the fuel pressure info front and center on the Dynon. I ordered and received from Lockwood a new-style pump. Installed it registered high 5's at low RPM's and high green at cruise. It dipped to low green on takeoff and fluctuated to low green with throttle reduction on return to field.

My last three takeoffs have seen the fuel pressure drop to low 2's, again recovering at about pattern altitude. Again the pressure drops significantly at power reduction, then recovers below 4,000 RPM. Gauge reads 5.2 or better when I exit runway. There has been no takeoff loss of power felt or registered on the data download. Downloads confirm numbers seen on the panel. Fuel flow sometimes shows an increase in concert with pressure drops, sometimes not.

My limited understanding of fluid dynamics tells me that a pressure drop would be expected on initial increase in flow (takeoff) and that reduction of power (approach to landing) would indicate increased pressure. Everybody I talk to says the range of fluctuation should not be this great.

Can someone help me figure this out? The Dynon people have been very up-front about saying that their current sender is not as accurate as they would like it to be but a suitable replacement is not available. The RV12 POH calls for 2.2 PSI fuel pressure before takeoff (RPM not specified) but the aux pump puts out an indicated 2.3-2.6 engine-off by itself. (Return line in RV12 plumbing reduces indicated output, I suppose, from Facet spec.)

For what it's worth, I chased the issue of vapor lock everywhere last summer and this is not the issue. Can't tell if I'm obsessing or snakebit... All suggestions appreciated.

Jim
South Texas
  • Re: Fuel Pressure Numbers--what's right?

    by » 13 years ago


    Hi Jim,

    2.2 to 5.8 is in the manuals. The new style pump is right around 4.5 for most folks. If you run an aux electric pump then that number may be .5 to 1.0 psi higher. Most people do not need to run an aux pump all the time.

    A real fuel pressure issue like you are describing with two different pumps puts a real fuel issue in at a #3 position behind two other issues.
    First this could easily be a grounding issue. They all need to be cleaned and put a wrench on them. Them may be hand tight, but wrench loose.
    The number one suspicion is the sender itself unless it has been proven as good.
    Before I went crazy chasing a possible fuel delivery ghost I would clean and tighten the grounds and replace the fuel pressure sender. You can get them very reasonable from Egauges.com
    The other thing I would do is if you have the fuel pressure sender on the engine then take it off and remote mount it on the firewall. The vibration and heat just kills them. If it has been there on the firewall and hanging upside down then the old fuel gets stale in the line and after a year or so it starts to clog up down inside the sender. I have saved a few like that with some carb cleaning spray in the hole, but I now pull all fuel senders that hang upside down at annual and clean them. I also don't mount any more hanging upside down. Horizontal or upright is far better to help fuel drain away while it sits.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Fuel Pressure Numbers--what's right?

    by » 13 years ago


    Roger, Thank you for the suggestions. I did the ground connection checks last year but will do again. The currently-installed sender is a replacement from Dynon--same style as the one supplied with kit from Vans, which checked out accurate with another FP gauge teed-in... again, last summer. The sender is firewall-mounted, as per ELSA build instructions and the length of the supplied hose may prevent re-positioning it more upright. Will check on this. FYI: Boost pump on RV12 ELSA is wired in to be on 100% of the time.

    I think the fact that the indicated low pressure excursions occur at specific and somewhat repeatable power settings suggests something other than a grounding issue unless vibration at higher RPM's is the culprit. My experience with other Dynon anomalies is that loss of electrical contact initiates off-the-scale numbers, not a mild deviation.

    I think it is possible that other RV12s are experiencing similar numbers but the only time pilots are looking at the fuel pressure page is pre-takeoff (as per POH) and perhaps in-flight when numbers are normal. I will make new inquiries with Dynon to see if they have more user data on that sender.

    Jim

  • Re: Fuel Pressure Numbers--what's right?

    by » 13 years ago


    Hi Jim,

    A sender can go bad at anytime, even when it is fairly new. I would still suspect the sender. The other thing I forgot to bring up is a fuel pressure dampener. I use Dynon and I have one of these in the intake line just before the pump.

    http://www.floscan.com/html/blue/accessorydetail.php?aid=98

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Fuel Pressure Numbers--what's right?

    by » 13 years ago


    Dampener goes in upstream of pump? Who sells these?

  • Re: Fuel Pressure Numbers--what's right?

    by » 13 years ago


    They go in upstream and Floscan sells them. On the website on the link I posted.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


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