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.

[jiott]

I agree, the hose to the fuel pump seems way more reasonable as a measuring point. The fuel pump may have a lot of resistance thru it when not turning. I will recheck flow at inlet to fuel pump.

I would still like to hear some more numbers and how others did the testing.

Jim

[t j]

My Rotax engine installation manual lists a maximum fuel pump height above the fuel tank. I have a nose tank that serves as the header tank in my kitfox. The fuel pump is about level with the bottom of the header tank.

The gascolator is the low point in the system and is well within the maximum height below the fuel pump.

So, I tested the flow at the outlet of the gascolator, tail low, one gallon remaining in the nose tank (header) and zero fuel in the wing tanks. flow rate 14.3 gal/hr. Zero unusable.

I then tested the rate from wing tanks to header tank to be sure the wing tank flow to header tank would keep up with the engine fuel burn should I forget to switch on a wing tank and run the header dry. Left tank 11.8 gal/hr. .75 gal unusable. Right tank 11.25 gal/hr, .75 gal unusable.

I have since found that I can actually drain all the fuel in the wing tanks by slipping but I think the definition of unusable is in level flight so left the plackard on the wing tanks. I did subtract that unusable fuel weight from my empty weight though. 9 pounds:)

[SkyPirate]

just my 2 cents worth,..just thinking if its a gravity test, due to concern of normal operation in flight if the fuel pump fails or is turned off,..wouldn't testing the system from the furthest point from the fuel tank be best,..ie carb inlet, to include flow thru the pump? or a by pass of the pump if needed?

[t j]

Don't know about the 912 but rotax two strokes won't run very good without a fuel pump in a kitfox.

[jiott]

Based on some of the comments here I did some additional fuel flow tests. In flight level attitude and 1/4 tank of fuel in each wing I measured the gravity flow right at the inlet to the mechanical fuel pump. It was 15.0 gph. I removed the inline fuel filter (Kitfox supplied) and did it again and got 16.8 gph. These values are over 200% of Rotax 912s max T/O consumption. I put it back together with a new inline fuel filter for good measure.

These values sound real good to me; apparently there is quite a lot of resistance going thru a non-operating mechanical pump plus the additional smaller lines to the carbs (flow in this configuration was 5.80 gph as previously reported).

What this tells me is that with a complete fuel pump failure (broken shaft, etc.) I will not get full T/O power, but all other flight conditions of cruise and descent should be fine.

What do you think?

Jim

[Dorsal]

Sounds good to me

[Dave F]

I am surprised that your final inspection / Inspector does not ask for a fuel flow test or at least the report/results.

Also testing with tail on ground is a good start but
YOu should test at hose end at Carbs.
in level flight
again with tail on ground

and again at max climb angle
-I think that you will find that you will need to elevate your mains a fair bit to achive Max climb angle - try to at least 15/20 degrees.

If you have fuel tank selector then try in every configuration

[Dorsal]

Dave, I am curious as to what one is testing for? clearly the system should be tested for any restrictions which arguably could be done at any attitude that give some head pressure. If the test is to insure full power in the event of a pump failure then that is a different story. I tested to insure that my engine can produce full power with the pump operating from the header tank alone (negative head pressure) and, in the event of a pump failure, I will have enough flow to maintain flight and land (to the sound of a low pressure alarm :) ). As an aside my DAR did not ask any questions about any of this other than my statement that I built to the manual.

[Dave F]

You are testing for fuel flow

YOu will need 150% of the maximun fuel required at WOT for your engine on gravity feed

125% I think on pump driven


But if your pump fails you should knjow what flow you get with a failed pump with gravity alone.

[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

[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/liquid-air-adapter/aluminum-one-way-check-valve-8mm-barbed--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.