Where does the fuel go?

[DanH]

Originally Posted by BillL

But, is the pressure from the boost pump higher than the mechanical pump? If not then easily it can push fuel back, but if it is higher, then how would it happen then?

Indicated boost pump pressure is typically higher than indicated engine pump pressure, and as you say, if those pressures were real, the engine pump could not force reverse flow back into the feed line. I puzzled over that for a while before realizing the actual pressure in the pump chamber is dependent on diaphragm position; the initial, fully compressed pump spring force is higher than the available force when the spring is extended.

Drat, I bid on a good used Rigol 2 channel scope but missed it. I was not properly motivated. I checked on the PC based ones and did not think the performance was high enough (100MHZ), being digital that is.

Ha! Mine is an ancient BK 15Mhz scope my buddy pulled from the trash bin at Maxwell AFB. I would not have paid money for it, but it has proven to be surprisingly useful. The PC scope software Joel linked looks pretty darn good to me.

[AlexPeterson]

Originally Posted by DanH

Agree, and without the springs and accumulators, there can't be a reverse flow. That's the catch to theories based on pressure oscillation alone...gasoline is, in this context, incompressible. Bulk modulus is around 1.9^5 psi. Delta pressure/E=change in volume. All possible pressures in this case are very low, so the change in downstream volume rebound (necessary for a pulse to move the rotor in reverse) would be some really tiny fraction of a percent. For example, a 10 psi oscillation / 190,000 = 0.00005 volume change

As a practical matter, it's compressible enough to transfer a pressure pulse, but lacking an accumulator, there's no actual fluid movement.

The exception might be a case of entrained air bubbles....a whole bunch of little accumulators suspended in the gasoline.

Dan, I've missed a few days - others have chimed in about the accumulators/springs in the pumps. It was those I was referring to in my post. Perhaps the check valves allow enough reverse flow... Some things may just have to remain a mystery...

Just in case this mystery gets solved, I've had my own flow indication mystery since day one. I have the Floscan mounted on the firewall, between the servo and flow divider. I see no fluctuations at all with boost pump on. However, I see bogus readings for the first cold start of the day for about 2 to 4 minutes. It'll show 5 gph at idle as I start to taxi out, then drop to actual of 1.5 or so in a couple minutes. It has done this every single time over 13+ years.

[bpattonsoa]

Since my -10 is down for painting, I have decided to go to the Red Cube between the injector and the spider. The Flowscan and tube came out today while waiting for paint to dry and a tube went in.

The very interesting persuit of why the increased flow just lost (for now) to the Engineer. Especially one who keeps forgetting to turn off the aux pump and does not trust float fuel senders after one failed in a non conservative direction. ( my wife will never forgive the day the engine in my -6A quit while I was sure there was at least 4 gallons in that tank. The other one read 1/4 full, but we landed right away to get gas about 50 miles from home!)

Two questions for those who have gone down the path of the inline Red Cube without a bracket:

1. What size firesleave goes over the cube?
2. I want use the hose that came with the Lyc and cut it. Which Aeroquip hose/fitting is it?

[BobTurner]

How do the electric fuel pumps work? Is there any way they can be introducing some vorticity (swirling) to the flow, which would cause the red cube's impeller to turn faster?

[rbibb]

Originally Posted by BobTurner

How do the electric fuel pumps work? Is there any way they can be introducing some vorticity (swirling) to the flow, which would cause the red cube's impeller to turn faster?

That is my operating theory and is based on it being the simplest explanation for what is observed.

[Walt]

Originally Posted by AlexPeterson

Just in case this mystery gets solved, I've had my own flow indication mystery since day one. I have the Floscan mounted on the firewall, between the servo and flow divider. I see no fluctuations at all with boost pump on. However, I see bogus readings for the first cold start of the day for about 2 to 4 minutes. It'll show 5 gph at idle as I start to taxi out, then drop to actual of 1.5 or so in a couple minutes. It has done this every single time over 13+ years.

I see exactly the same thing, red cube and AFP injection with the cube in the servo-divider line. Changed the cube a few years ago for another problem and the new one acts the same way. Done all the checks I can think of and finally I've decided this is a phenomenon above my pay grade

[AlexPeterson]

Originally Posted by Walt

I see exactly the same thing, red cube and AFP injection with the cube in the servo-divider line. Changed the cube a few years ago for another problem and the new one acts the same way. Done all the checks I can think of and finally I've decided this is a phenomenon above my pay grade

Walt, that is indeed interesting. I posted some more mystery here...

[Pmerems]

I have a RV-7A with a IO-360 with a Flowscan mounted between the AFP (old style) pump and the engine driven pump. I had a D180 and now a Skyview system.

The Flowscan is mounted in the tunnel in the cockpit. I been flying for several years and haven't used the boost pump during takeoff or landing (please lets not debate this on this post) because of the increased flow rates that occur and the effect on the totoalizer when the pump in on.

Recently I started using the pump during takeoffs and the flow rate is somewhere near 3 gph more then without the pump. But I know I am not burning it or pissing it out.

I started thinking about why the flow rates are higher with the pump on. This post talks about pressure pulses but what about electrical noise/interference?

My fuel pump power wires run in parallel with the Flowscan signal wires since booth the pump and the Flowscan are in close proximity of each other (10" apart). The wires to the Flowscan are unshielded.

Could the electrical noise from the pump motor interfere with the Flowscan signal output and cause the increase in fuel flow? If the Flowscan was mounted under the cowl the wiring wouldn't be in parallel with the fuel pump thus reducing the possible electrical noise/interference.

Any thoughts?

[DanH]

Originally Posted by Pmerems

Could the electrical noise from the pump motor interfere with the Flowscan signal output and cause the increase in fuel flow?

Seems unlikely, given that a lot of us have our Floscan or Red Cube mounted at a location remote from the electric pump, yet have the same indications.

The key is to devise a test to go with a proposed theory. Here a portable o-scope would probably prove or disprove.

[rbibb]

https://drive.google.com/file/d/0Byv...ew?usp=sharing

The image shows what I think is happening. The higher pressure sets up a vortex flow around the impeller so it "sees" a higher velocity flow than the actual flow through the device.

I've never taken one of these apart so the drawing is "notional" only.

To me its probably not worth actually figuring this out as the system works for my purposes. My biggest problem in getting accurate results is forgetting to zero out the total used when I refill the tanks!

Yeah I know - should be on the checklist.