Fuel Flow Test

[Birdseyeview]

I did my first fuel flow test today on my series 6 tricycle geared Kitfox. I measured fuel flow at the end of the inlet hose to the fuel pump on my 912ULS and I have no other fuel pumps (ie: electric backup) and there is a new Rotax fuel filter in the line. I put 6 gallons of fuel in each wing tank to start the test. I repeated the test at various aircraft attitude positions from 0-18 degree nose up (at 5 degree increments). 18 degrees puts the tail skid on the floor. My bird is not the STI version, just the normal config. Since this is my first flow test with unflushed tanks and lines I may repeat it with a new filter to see if the results improve. The EAA provided fuel flow instruction sheet states I should have a minimum flow of 125% of the takeoff consumption, which is 1.25 x 7.1 gph = 8.9 gph. My test results so far indicate that there is plenty of margin until I get up to just under 15 degrees nose up attitude. Does anybody out there have a number for the normal takeoff attitude (nose up degree)?

[Dave S]

Birdseye,

While this does not include all possible pitch attitudes during operation, I was lucky enough to have an excellent photographer snap a number of detailed inflight photos of our plane in flight during testing near the airport, including one which was taken at 90 degrees off the flight path.

Using my protractor across the photo shows a 7 degree nose up pitch with a normal, no flaps departure climb. Use of flaps would reduce that somewhat.

FWIW, I have never been able to bury the tailskid in the tar either taking off or landing.

There is going to be a reduction in gravity flow as the nose is raised. One point with the Rotax is since the carbs and pump are on top of the engine, not underneath the engine, if a person stands off to the side of the plane it is pretty easy to visualize how close the fuel level in the tanks will approach the level of the mechanical pump inlet and it is easy to understand why a rotax needs an engine fuel pump even on a high wing.

I believe the results you are observing are normal. A person also has to consider that in flight there is some pressurization of the tanks.

During in flight testing, There was never any fuel supply problems with full power and pitch angles to 30 degrees (I did not test beyond 30 degrees). Since it is entirely possible to pull the nose up beyond 30 degrees, a person will realize that the inlet to the fuel pump can likely be positioned above the fuel level in the wing tanks under certain unusual flight conditions - again, the Rotax needs an engine fuel pump so its not only about gravity flow.

[PapuaPilot]

I pulled up one of my flights from the G3X database. What I found is after takeoff while doing a Vx climb at 60 mph the pitch was between 13 to 16 degrees. When I went to my normal climb profile of 70-75 mph the pitch was 7-8 degrees. I have a 125 HP engine installed and it was using 9.5 to 11 GPH at the time.

Remember that the pitch is going to vary depending on what HP engine is installed. The real caution would be with higher HP engines that use a lot more fuel and can pull a much higher AOA in a Vx climb.

[Birdseyeview]

Thanks for the input guys - this is very helpful info.