Fuel Flow Testing

[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

[Jerrytex]

Something that I had thought a while back was a bypass line around the mechanical fuel pump with a one way check valve. That way if the pump quit, gravity and somewhat pressurized fuel (from the gas caps), would be able to make it to the carbs unrestricted by the restrictive mechanical fuel pump. I ended up using the Facet pump instead, figuring that that the electric pump can push enough fuel through if needed.

http://www.siliconeintakes.com/liqui...d--p-9581.html

[Floog]

Nice idea! Just wondering if there's 3 PSI (gravity) to open the check valve?
I, too tested fuel flow before the engine driven pump....

[Jerrytex]

If I remember correctly, I thought I found a couple one way valves with 1/2 psi cracking pressure.

I just checked Summit and they have one-ways with 2 psi cracking pressure. If someone was interested in doing a bypass like this, it would definitely be something to check and make sure that it opened completely with gravity fed fuel. And also to check to see how restrictive the one-way would be.