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?

[SWeidemann]

I just finished a two day Rotax 912 Series service seminar (Leading Edge Airfoils in Lyons, WI) where it was pointed out that a auxiliary fuel pump is recommended in any installation using the 912.

In my Model 5 Tri Gear, I do not believe I have a pump on board other than the mechanical engine driven one. It works fine, but now I'm wondering if I should put one in? Should I be doing fuel flow testing or just keep flying what I have? (It does have a lot of time on it with no known incidents of any kind.)

Another point, is: Should I have a metal header tank instead of the plastic one ? My plastic one works however is a little "weepy" around the pipe joints.

Skot
N24V

[kmach]

I have a facet electric fuel pump after the header, before the engine pump.
I don't use it all the time in flight , I test it before takeoff and know its there if needed.
I like the redundancy of having one.

[HansLab]

Me too (I have a facet electric fuel pump after the header, before the engine pump. I don't use it all the time in flight , I test it before takeoff and know its there if needed. I like the redundancy of having one.).
Low cost (in space, technique and money) - high revenues.

[herman pahls]

I would like to revive this thread that has been dormant for 3 years.

Two years ago I purchased a flying model 4 Kitfox/ 912 uls and have felt guilty that I have not done a gravity fuel flow test.
I somewhat pacified my guilt and installed and inline Facet fuel pump between the header tank(behind the seat) and the fuel shut off valve under the panel.

Today was the test day and I am very disappointed with the results.
In post #20 it was stated that Rotax recommends an additional back up pump on all 912 installations and from what I learned today(at least with my installation) I could not agree more.
My fuel installation is Kitfox manual all the way.
If I did not already have a back up fuel pump I would ground the airplane.

I believe that Jiott is correct when he suggests that many have not tested their fuel system and assume that it will be OK.

I agree that the gravity test should be done at the lines going to the carbs and not at the inlet to the fuel pump.
What good is fuel at the Rotax mechanical pump inlet if it will not adequately flow through it and get to the carbs?
My testing was tail up in the position where you would do weight and balance.
I removed the fuel line going to the Tee that feeds both carbs and I measured only 3.75 gallons per hour.
I burn 5.5 gallons per hour at cruise and 7-8 on take off at sea level.
I lowered the tail to the ground and the flow completely stopped.
I had been concerned with how small the hole is in a "red cube" fuel flow transducer which is between the Rotax pump and the Tee.
I was assured the red cube flows plenty for engines that have twice the flow of a 100hp 912.
I turned on the Facet fuel pump and did not bother to measure since it was night and day how much more flow there was compared with gravity alone.

I then removed the inlet fuel line to the Rotax pump and got 12 gallons/hour with gravity alone and the fuselage in flying attitude.
This flow was through the turned off Facet pump, the shut off valve, the firewall fittings and the Napa #3095 filter which is just before the Rotax pump.

I assume Rotax recommends a back up pump because the stock pump appears to be a major restriction when the engine is not running.
I have yet to test tail low or tail high.
My results show that I would be better off with gravity alone (without the Rotax pump restricting flow) in level flight if the pump failed.

There is also the issue of what happens to the fuel flow when electric and mechanical pumps fail.

I recently had a the Rotax pump start to fail.
My warning light is set to come on when pressure is 3 psi or less.
I had to complete the flight on the Facet to keep more than 3 psi.

Rotax supplied me at a reduced price with the new improved pump that has a vent line.
Even this new pump will occasionally drop below 3 psi at full throttle take offs.
This pump usually puts out 5-6 psi in cruise, so it bothers me when it dips below 3 on take offs.
I found posts on Matronics where others have had the same issues with the Rotax pumps.

Your thoughts and opinions please.

herman