Fuel Pressure loss on Loop upline

[Tim Lewis]

- If you apply takeoff power with the boost pump off (as an experiment), does your fuel pressure drop, or stay up in the 20+ psi range? If pressure drops at high fuel flow, that could point to a fuel flow (volume) related pump performance issue. When mine developed that problem, replacing the engine driven pump solved the problem.

Thinking out loud...
- You can perform the loop with the boost pump on (ensuring adequate fuel pressure) without a problem.
- Thus the fuel system components forward of the fuel pressure sensor location (near the engine drive pump output, I assume) appear to be exonerated.
- In the boost pump off configuration, the engine driven pump appears to generate inadequate pressure.
- It could be the case that maneuver G loads are impacting the engine driven pump's performance for some reason.
- Replacing the pump is fairly inexpensive, and not too difficult. Consider putting that on your list of troubleshooting options. (Recall the lacing cord trick to hold the actuating plunger up during new pump installation).

[SMO]

Here is an update - I bypassed the Aerolab gascolator and went flying. Fuel pressure stayed above 20 psi. Max pull was 3.5 g's.

The Aerolab is designed such that the inlet is at the side of the bowl and the outlet at the top. I had a 90* fitting at the top and thought that maybe that was restricting the flow. So I changed it to a straight fitting and had a more gentle bend with tubing. I greased the orings in the gascolator with Molykote 55, vacuum tested the unit, and reinstalled it. Went flying today and no joy, fuel pressure dropped off to 8 psi.

My next thought is to reverse the flow through the gascolator and see if that makes any difference. My previous unit was an Andair - it had a horizontal inlet and outlet, both at the top, and worked fine. I am wondering if I make the top port the inlet, and the side port the outlet, what difference that would make. I sent an email to the mfg to get their thoughts. The impact I see would be:

• Any debris caught by the filter would end up on the inside of the filter rather than the outside (fyi, my gascolator filter has always been clean).

• The gascolator would hold less water before it ended up going downstream (I have never seen water in my tanks or the gascolator).

• When pulling the filter I wouldn?t have an automatic shutoff (my fuel valve is quite close to the gascolator, I use it to stop the flow of fuel).

Anybody see any other issues?

Some might say just get rid of the gascolator all together. I agree but in Canada the law says I have to have one.

[terrye]

I think from your test of bypassing the Aerolab unit that the problem is somewhere inside it. Looking at the Owner's Manual p7/9, I see the automatic fuel cutoff valve is right beside the inlet. I wonder if this valve is being affected by your 3.5 g pullup and trying to close the inlet port. You might talk to the manufacturer to see if the gascolator can be operated with this valve removed (I know, it's there to prevent fuel draining out when the bottom is opened). I can't think of an easy way to bench test this, so a careful test flight would probably be required.

I don't think it is a good idea to plumb the gascolator to operate in the reverse direction.

[SMO]

Originally Posted by terrye

I wonder if this valve is being affected by your 3.5 g pullup and trying to close the inlet port.

I had the same thought. I took the gascolator apart and looked at this piece in particular but it became obvious that it is not the problem. The cylinder that shuts off the fuel is on top of the rod that is captured by the bottom of the gascolator. It is firmly attached (probably threaded but I am not sure) and it would take a LOT more than a few g to move it - I believe it would have to break first.

I don't think it is a good idea to plumb the gascolator to operate in the reverse direction.

Ok, can you tell me why? BTW, I won't do this unless the mfg says it can be done without issue.

[F1R]

Only a hunch, but I suspect the belly skin mounting could be the culprit as it subjects the entire gascolator to the exhaust pulse .
I am guessing when you had the Andair , that it was NOT mounted to the floor skin.

Either it is the vibration or, it could simply have a lower flow capacity than the Andair. ( set them both up on a bench and time a gravity flow test with the same volume of fuel)

I doubt the pull up is flexing the spar and skin to the point where it distorts the aeoliab and allows air to enter, but it is within the realm of possible.

Perhaps try a foam spacer gasket or else place the entire unit inside of the bottom skin and test fly if you can secure it safely free of the control trunion.

Any chance a flex line in or out of the unit is able to deflect downward with G loading to the point where it restricts flow? Both sides of the gascolator are under suction so the flex lines would not need to droop much to flatten slightly and restrict flow.
Good Luck

[SMO]

Originally Posted by F1R

Either it is the vibration...

I will check the floor for vibration, certainly nothing visible - I suspect my turned down exhausts mitigate that quite a bit.

.... or, it could simply have a lower flow capacity than the Andair.

Not a flow issue. I have burned over 20 gph without the boost pump on, was only burning 12 gph doing a loop today when the fuel pressure dropped.

Any chance a flex line in or out of the unit is able to deflect downward with G loading to the point where it restricts flow?

No flex fuel lines rear of the firewall.

Thanks for the thoughts, one of them will eventually turn out to be the answer. Maybe I just have a weak engine driven fuel pump, and the Aerolab needs a bit more suction than the Andair did.

[DanH]

Originally Posted by SMO

Not a flow issue. I have burned over 20 gph without the boost pump on, was only burning 12 gph doing a loop today when the fuel pressure dropped.

Set gascolator pressure loss aside for a moment, and consider only the effect of flow and G. Pressure loss due to flow increase is a function of line friction. Pressure loss due to G is a function of head, which is elevation change x acceleration. Assume 1 foot of head in level flight, 0.375 ID smooth lines, and 3.5 G. Run the numbers, and you find the pressure drop for 12 GPH at 3.5 G is 2.9 times the pressure drop for 20 GPH is level flight. Head increase has much more effect than flow increase.

Maybe I just have a weak engine driven fuel pump, and the Aerolab needs a bit more suction than the Andair did.

If true, "...needs a bit more suction" would mean the Aerolab unit results in more pressure drop than the Andair. I say "if true", because we don't know it as measured fact. It would be easy to check with a simple experiment.

Heat is also a big factor, and we have reason to believe the Aerolab runs a bit warmer due to its mounting in the belly skin.

Don't blame the engine pump. Remember, when local pressure drops below vapor pressure, bubbles form. Restriction and head reduce local pressure. Temperature increases vapor pressure.

[I-TERA]

Hi Mark,
I also fly an F1 evo, AEIO540 D4A5, two aerolab filter-gascolator installed on the belly with a 90 ? fitting on top, one for each tank.
Only 20 hours flight from the maiden flight.
The fuel pressure normally ranges between 20 and 22 Psi without auxiliary pump. The maximum flow I recorded is 115 l / h, full rich, wide open, 2700Rpm, 1018mb, 22 C ? with 18-20 Psi fuel pressure.
This morning, OAT 20C?,1016, 3500', to check if also my installation is prone to pressure drop, I pulled +4.5 G, 2500 Rpm, 24 Map, 35 l right tank, 65 L left tank, rich, FF 65l/h, controlling the pressure, no variation.
I did it on right and left tank with the same result.
The filters are installed near the exit of the fuel line from the tanks, in the two small bays near the stick, where Mark F. suggests to put the battery.
I do not have any idea about your problem, the hydrostatic head between inlet and outlet of the filter appears to me very small also at 4G and, in all cases, the total head between fuel surface ( in the tank ) and the pump inlet remain the same.
I am obviously very interested in knowing how your tests are proceeding.
If you think I can do something useful I'm at your disposal.
Claudio

[SMO]

Originally Posted by I-TERA

This morning, OAT 20C?,1016, 3500', to check if also my installation is prone to pressure drop, I pulled +4.5 G, 2500 Rpm, 24 Map, 35 l right tank, 65 L left tank, rich, FF 65l/h, controlling the pressure, no variation.

Thank for your response Claudio, good to hear from you. Can you confirm when you did the test quoted above, was your auxiliary pump on or off?

I found 2 new symptoms today I have never before experienced with this airplane:

• The ambient air temp was ~80* F (27* C), the hottest I have flown in so far this year. I was 1,000' AGL and went to change tanks. I do not normally use the boost pump when I do this, but I watch the fuel pressure. I have never seen a drop in fuel pressure in the past and have done this in much hotter temps. Today the fuel pressure started dropping from the 24 psi it was holding, 21, 17, 14, and the RPM started to decay. I switched tanks back and switched the boost pump on. I then climbed to 4,500' AGL and, with the boost pump on, switched tanks. No problem.

• When I returned to the airport I fueled the plane. Next was the hot start. My hot start procedure is full throttle and mixture at idle cutoff. Crank till it starts, pull the throttle back and advance the mixture to half. Done properly RPM never exceeds 1,000 and after 5 seconds the idle smooths out and all is fine. Today it started normally, but then did not want to stay running. I played with the throttle to try to keep it running, but it would not smooth out until I put the boost pump on. Even then it was rougher than normal but ran well enough to taxi to the hangar.

These two events are symptomatic of vapor lock, similar to the g force issue.

I think I will put the Andair gascolator back in and see if I get back to normal operations.

[I-TERA]

Yes, the aux fuel pump was off during the flight.
I do not have your automatic low pressure control ( may I now how it function? )
so the aux pump can't run without intentional pilots action.
My exhaust gas flows near the filters but I see that the flow ( grey smoke tracks) do not impact directly the filter caps . Two Vettermann 3in1. The cowl is not standard to accept a cold air sump.
Claudio