Fuel Pressure loss on Loop upline

[SMO]

Airplane - F1 Rocket, IO-540 D4A5, recent install of Red Cube between fuel servo and spider. Engine driven fuel pump has ~350 hours.

Pulling 3 g's at the start of a loop I am experiencing a fuel pressure loss from the normal 20+ psi to ~8 psi (engine loses power) after which my boost pump automatically kicks in (power picks back up) and brings the pressure back to normal.

I can fly around all day, pulling 2 g's in 60* banks, and no issues, fuel pressure stays 20+ psi.

Anybody experience the same? Am I correct to assume a new engine driven fuel pump is in my near future?

[DanH]

Consider the changes you made, and start there.

First, it worked fine prior to relocating the red cube, yes?

I assume you're measuring pressure at the engine pump outlet. If so, nothing you plumbed between the throttle body and the divider would make a difference.

I further assume the red cube was previously firewall mounted between the bulkhead fitting and the engine pump. If so, you changed a hose and/or hard line as part of the relocation.

Thus three possibilities; the new lines pick up more heat than the previous install, or one of the fittings has a small air leak, or you installed a right angle fitting and (combined with heat) it is forming vapor bubbles in the flow.

[TS Flightlines]

Mark----did you have previous flights in this configuration and 3g's with no issues?

Tom

[Walt]

Originally Posted by SMO

Pulling 3 g's at the start of a loop I am experiencing a fuel pressure loss from the normal 20+ psi to ~8 psi (engine loses power) after which my boost pump automatically kicks in (power picks back up) and brings the pressure back to normal.

I can fly around all day, pulling 2 g's in 60* banks, and no issues, fuel pressure stays 20+ psi.

20 PSI "normal" is too low (18 minimum), typical pump pressure should be around 24-26, I'd start there.

[SMO]

Originally Posted by DanH

Consider the changes you made, and start there.

First, it worked fine prior to relocating the red cube, yes?

I have had 10 flights since the change where I performed some aerobatics, only the last 3 - 4 have shown these symptoms. BTW, the other change I made at the same time was replacing my Andair gascolator with and Aerolab.

I assume you're measuring pressure at the engine pump outlet.

Correct, just to clarify this is not a measurement issue, there is a corresponding loss of engine power till the boost pump cuts in.

I further assume the red cube was previously firewall mounted between the bulkhead fitting and the engine pump.

Close, it was a Floscan that was replaced, and it was located in the tunnel between the stick well and the firewall. Replacement hose is a straight run.

Thus three possibilities; the new lines pick up more heat than the previous install, or one of the fittings has a small air leak, or you installed a right angle fitting and (combined with heat) it is forming vapor bubbles in the flow.

To add the Red Cube I put a bracket on the front of the fuel servo and mounted the cube there. Not a likely source for additional heat.

Will check the fittings for a leak, although why "G" forces would exacerbate a leak I am not sure, but possible.

No right angle fittings were introduced.

EDIT: This is not fuel flow related, I have confirmed via flight testing.

[SMO]

Originally Posted by Walt

20 PSI "normal" is too low (18 minimum), typical pump pressure should be around 24-26

I should have been more clear, the normal fuel pressure fluctuates between ~22 and ~25. If I look at this on a graph it has a very constant oscillation - is that oscillation normal?

[AlexPeterson]

Originally Posted by SMO

I should have been more clear, the normal fuel pressure fluctuates between ~22 and ~25. If I look at this on a graph it has a very constant oscillation - is that oscillation normal?

Approximately what is the frequency? I've noticed this at about 2400 RPM as monitored by the G3x Touch. I believe it may have to do with the sampling frequency of the engine monitor vs pressure pulses in the mechanical pump. Unverified, but the oscillation goes away when I dial the rpm down from 2400 to about 2350. The period of the osc. I have observed is about 5 seconds, and it will vary between maybe 21 and 25.

[DanH]

Originally Posted by SMO

I have had 10 flights since the change where I performed some aerobatics, only the last 3 - 4 have shown these symptoms. BTW, the other change I made at the same time was replacing my Andair gascolator with and Aerolab.

So check it for an air leak. Seal the inlet and pull suction on the outlet, see if it will hold a vacuum.

Will check the fittings for a leak, although why "G" forces would exacerbate a leak I am not sure, but possible.

If fuel weighs 3x more, it requires 3x the pressure drop for the same lift. More suck means a small air leak becomes a big air leak. Entrained air doesn't pump very well.

Hey, it's the long distance diagnosis game, with the usual lack of information. Maybe right, maybe not, but I usually have a logical reason for a suggestion.

[F1R]

Where did you mount the aerolab relative to the exhaust tips?

Or how many inches aft of the firewall? I am guessing it is somewhere in the center tunnel belly skin.

I am also guessing on take off, below 500ft, you have your boost pump on?

The exhaust pulse is a vibrational shake and bake to anything on the floor skin near the center tunnel. Might just be bubbles manifesting from the increased flow demand when you pull the nose vertical. A 2 G level turn will need less power and fuel flow than a 3 or 3.5 G pull up.

[SMO]

Originally Posted by DanH

So check it for an air leak. Seal the inlet and pull suction on the outlet, see if it will hold a vacuum.

This I will do

..... but I usually have a logical reason for a suggestion.

Of that I have no doubt.