Fuel Flow Testing

[jiott]

I just finished my gravity fuel flow testing on my SS7. It was per the EAA recommended procedure: gravity only, engine off, at various angles, flow measured from the fuel line disconnected right at the Rotax 912s carburetors. Results as follows:

• Level flight attitude-5.8 gph total combined flow at both carbs with 1/4 tank fuel in both wing tanks. EAA (FAA) recommends 125% of maximum T/O fuel consumption for systems with a fuel pump (Rotax has a mechanical fuel pump and T/O fuel consumtion is 7.1 gph). My flow rate is 82% of 7.1 gph so it does not meet the standard. I do not have any auxiliary electric pump.
• 10 degrees nose down attitude-3.1 gph flow with 1/4 tank fuel in both wing tanks. EAA (FAA) recommends at least 25% of T/O consumption. My flow rate is 44% so it easily exceeds the standard.
• I could not do the 25 degrees nose up attitude because at that angle I got zero flow with only 1/4 tank fuel. The wing pickup port was uncovered and the header tank is too low to provide any head. I am guessing I would need at least 1/3 or more fuel in the tanks to get any flow in this extreme nose up angle. I ran out of fuel to pour in so will maybe do this test later on.

What do you guys think about my results so far? I am somewhat concerned about missing the recommendations for level flight, but feel that with the engine fuel pump turning I will have plenty of flow, plus the effect of ram pressure in the tank cap vents. How many of you even bother with this test?

My fuel system is built totally by the build manual with no deviations other than the addition of a Dynon fuel flow sensor (inline with straight fittings in and out). This sensor is generously sized for this flow rate.

Jim

[Av8r_Sed]

Is it possible to temporarily bypass the fuel flow transducer and repeat your level flight test? Also, you have to cast a suspicious eye at any valves in the system. These are often restriction points.

[jiott]

I only have one main fuel shutoff valve in the system.

Jim

[jtpitkin06]

My kit was supplied with a brass step down adapter coming out of the tank. Then fuel had to flow through a ninety degree brass elbow with a hose barb. Brass 90 degree fittings are notorious for restricting fuel flow.
It is better to use a larger elbow to change direction at the tank, then step down with a straight barb fitting to attach your hose. I suspect that is where your flow resistance is located.

The best set up would be to use AN or Airquip full flow ips-to-hose fittings.

John

[SkyPirate]

don't forget your fuel caps get pressure in flight , if you could somehow charge your tank to equal the static pressure in flight,..an old ASI to gauge the pressure at cruise?it cant be much pressure but it might increase the flow a bit

[DesertFox4]

Jim- you should see a better flow rate than that. You might try it at the firewall fitting and see if it is still slow there . You can always start working your way back to the fuel tanks checking flow as you go. I know it's a pain but this system has to flow correctly for safe flight.
Your air return line must be routed properly and not pinched anywhere as it goes from header tank back up to the right wing tank.
I'll assume you sloshed your fuel tanks as prescribed and checked your fuel strainers for blockages. Check your fuel caps to make sure they are letting air in as fuel flows out. Blow through the ram air tubes and make sure they are open and functioning. Just a couple things I could think of quickly.
Photos of your system may help with suggestions. Good luck.

P.S. fuel filters should be considered also. Did you check them for contamination or blockage?

[jiott]

I was hoping to get some actual test flow rates that you guys got when doing this test. All my fittings and routings are kit supplied and per the book. Is it common to have to upsize the kit fittings as John suggested? The manual doesn't even say to do this test. I have a suspision that many builders just build by the book, then go fly.

Jim

[Dorsal]

I know I built mine according to the manual, checked that I had decent flow but did not measure it and flew it. I may measure the flow and let you know.

[Dick B in KY]

Took me a while to find my run sheet, but here are the flows on my SS7 (taildragger attitude) at the firewall where the mechanical fuel pump hose connects (this is where it counts).

Right side wing tank - 1/2 gallon in 75 seconds
4 gallons in 600 seconds (10 min)

Left side wing tank - 1/2 gallon in 90 seconds
4 gallons in 720 seconds (12 min)

More than enough flow for a 912ULS that burns 7 gallons per hour at full power. Hope this helps.

Dick B

[HighWing]

Jim,
I think Dick B has hit on something. I went back and re-read your first post with your measuring point - "right at the Rotax 912s carburetors." I had always measured at the hose right at the inlet of the mechanical fuel pump. I think the first thing I would do is repeat the tests at the point before the pump. At least in my installation, the lines from the pump to the carburetors are quarter inch ID and would add some additional friction not to mention internal resistance in the pump itself.

My testing was done in the tail down postion - the most adverse gravity feed positoin - and finding it adequate, that was it. So, I guess it does indicate, as you suggest, that some of us are not as thorough. But then, I guess, the proof is in the pudding. Fuel flow has not been a common issue with the Kitfox factory designed fuel system. In my mind, I have relied on the test to indicate whether something I have done has compromised the design. However, adding the suspenders, I did install the Facet fuel pump immediately down stream of the header tank with the on switch within thumb distance with my hand on the throttle. I did have one instance where I was glad I had it, but it was from a maintenance issue - my bad.