Fuel flow testing with Facet back up pump....looking for numbers

[Jerrytex]

I installed a Facet back pump up on my KF 5 build. I just did the fuel flow test and it was pitiful. No where near the requirements for 15 degrees nose up. In my situation, it Barely trickled out with the tail down on the wheel, and got right at 10 GPH level right at the mechanical fuel pump with about 5 gallons in each tank. Of course if I fire up the Facet pump, it comes out like a garden hose. I do know that the Facet pump does restrict the flow, So I am thinking that the the 150% rule for gravity feed is out the window if one uses a back up pump..... Just curious to see if this is same results others are getting with the installed Facet pump?

Other than bypassing the back up pump, I don't think there is a way get the numbers required.....?

[Dave S]

Hi Jerry,

Since nobody has any input yet - I'll take a stab at it.

The bigger picture is that the kitfox fuel system, which in my opinion is about as bulletproof as you can get, functions as a hybrid rather than a pure gravity feed system. The header tank is certainly gravity fed and that has considerable head pressure on it due to the vertical distance to the wing tanks in all attitudes other than upside down with negative Gs.

A few things - 1) The highwing standard airworthiness aircraft like the cessna and piper short wings which are pure gravity feed have the fuel delivery point to the carb below the engine providing considerable head pressure. 2) With the 912xx powered kitfox, tailwheel on the ground, and particularily if you have the big rock hopper tires, there is very little head pressure to the fuel pump inlet from the wing tanks - while the header tank still has plenty of head pressure on it. 3) Continental powered kitfoxes have the carb on the bottom back of the engine so gravity feed alone is just fine. 4) Low wing aircraft depend entirely on the fuel pump but that is not our concern.

The other point to consider is the fact that the facet fuel pump is not the only source of hydraulic friction to the fuel flow. There is a considerable length of hose/tubing between the wing tanks and fuel pump - that by itself provides a resistance to the flow. Try filling 5 feet of 5/16" hose with water and see how much it takes to blow it out with lung power if the hose is level. Then there is the gascolator or filter and the fuel valve.

Bypassing the facet will remove that variable from the equation; however, it remains to be seen how much of an increase in flow you would see unless you try that with the nose up. Might be interesting to know.

What I think, is that adequate fuel flow with the plane in level flight attitude rules out any obstruction or kinked hose - and that is what you are trying to determine. I am not convinced that a blanket standard applied to a hybrid fuel system is completely applicable. Think of it this way - take a low wing airplane with a mechanical fuel pump and do a gravity test????? The fuel system testing really needs to take into account what the design of the fuel system is - pure gravity, hybrid (like the kitfox) or relying entirely on the fuel pump.

FWIW - I know full well that in part of my flight testing, the fuel pump inlet was quite a ways above the level of the fuel in the wing tanks - both with and without the facet running the engine was running just fine.

[napierm]

I installed a Facet back pump up on my KF 5 build. I just did the fuel flow test and it was pitiful. No where near the requirements for 15 degrees nose up. In my situation, it Barely trickled out with the tail down on the wheel, and got right at 10 GPH level right at the mechanical fuel pump with about 5 gallons in each tank. Of course if I fire up the Facet pump, it comes out like a garden hose. I do know that the Facet pump does restrict the flow, So I am thinking that the the 150% rule for gravity feed is out the window if one uses a back up pump..... Just curious to see if this is same results others are getting with the installed Facet pump?

Other than bypassing the back up pump, I don't think there is a way get the numbers required.....?

In my planes I have a one-way check valve in parallel with the pump. I put it so the ball housing is parallel to the floor.

https://www.aircraftspruce.com/catal...checkvalve.php

[Jerrytex]

Thanks Dave. Its kind of disheartening when you don't get the results one would expect, but in retrospect, I think your right about the hybrid system. I think that having the back up pump is worth it. You just have to realize that your testing is going to be a little different, mainly just making sure there are no obstructions.

I like the idea of a one way valve. I might incorporate that as well.

Thanks

[Dave S]

The parallel check valve isn't talked about a lot, however, the DAR who inspected our plane (his day job was doing airworthiness on large aircraft modifications) also suggested that to me. I don't know what the failure modes of the Facet are, however, it if were to get corked up the bypass would save a person's bacon. In my recent engine change, I installed the Rotax fuel bleed line which the newer 912s are set up for and got a check valve for that while getting a second one for just what you are suggesting to go next to the facet pump. Testing the breakout pressure of the check valve indicated it takes 1/2 PSI to open it.

I have been following your progress and and I think you have good reason to be happy with your build Looking good!

[Dusty]

I run a bypass check valve on my facet and mechanical pump,
At one stage I had a facet clone (which appeared identical). It failed ( was delivering 1.5 psi instead of 3.5) and when turned off became a total engine stopping blockage. I had been flying like this for a while, the full extent of the problem only became apparent during fault finding when I clamped the bypass off believing this had failed. Out of habit I also run the backup pump if I will be below 1000 ft agl.