Fuel Pressure Numbers--what's right?
in 912 / 914 Technical Questions
12 years ago
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
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
