Fuel Flow Discussion, Moved from Float Mounting

[Ed.Meyer]

You are right Brooks that the system returns unused fuel. When I first turn it on prior to engine start, I can hear a short burst of bubbling in the tanks as air is purged. I am not sure if the volume pumped stays the same. Fuel pressure varies some with manifold pressure. Higher MAP has higher FP. The same fuel pumps drive the EFII system for an O540 setup so I think the volume being pump is somewhat higher than the engine demand.than our O360.

[Bcone1381]

Having followed this thread and then seeing the safety notice from Bob regarding fuel pumps with his fuel system design I decided to do a test.
...........
My conclusion from this is that there is apparently enough fuel flow capacity feeding from only one tank via 3/8" lines to supply all the fuel the engine needs plus whatever amount the fuel regulator returns to the tank even with the other tank feed open to the vent. I recognize that this is not an exhaustive test but it was enough to boost my confidence in the fuel system as installed.

I think you are right, Ed. You experiment gives good evidence that the first fuel pump in your system is supplied by gravity. Will a change in power change your fuel flow demand? I don't think so, but not sure. I seems to me like your system has a fixed rate of fuel demand out of the tank regardless of the power setting, and system returns unused fuel. Am I right?

[Nev]

I think it’s well established that a single 3/8 line will provide sufficient fuel under gravity feed conditions.

The issue is what happens once a pump is added to the system, and one line then becomes un-ported.
Will the pump suck fuel from the other line, or will it suck air in preference to fuel from the un-ported line, thereby resulting in a stoppage.

[zkelley2]

I think we know, or we all should know that a single 3/8 port and line can feed enough fuel because we've all done the fuel test recommended by the AC and even asked for by most DARs/FAA people.

[Ed.Meyer]

Airplanes that have carburetors have a float bowl that holds enough fuel for a short idle. Ours will not start with the fuel valve off because the pump will not build pressure. Screams loudly in protest. I think the gascolator probably was a factor in supplying a bit of fuel.
I agree that this could be further tested with climb/decent and lower fuel levels. Given what I learned so far, I think that fuel coming from a single port would be enough since each port is a 3/8” line and even with both ports feeding from a single tank, they tee into a single 3/8” line. Doing this test at real low fuel levels I an not willing to do.

[zkelley2]

Your thoughts are what I have experienced Mark. I have started with the fuel selector off on purpose. It does start, and it'll idle for a few minutes, but putting power to it for taxi I can't get 100ft.
If I started the airplane on the runway in position I don't think I could put full power to it before it'd stumble.

[Mark Goldberg]

I have always thought that an engine would start from the OFF position of the fuel valve - maybe. But taxying out and during run up the engine would quit before the plane flies. I have seen this before a few times. But you are probably correct that all planes are different. Mark

[Nev]

This is an interesting experiment Ed. I think that in theory the way you have put the aircraft into a slip, both fuel out-let’s should have un-ported and the engine should have stopped. Given that it didn’t stop, it may be still running fuel from a gascolator or header (I didn’t look at the details of your system), albeit at a reduced pressure. I suspect that the gascolators may be acting as a mini header tank, along with larger diameter fuel lines where used.

It would be interesting to see the same experiment, but instead of an out of balance situation, try an extended steep climb or steep descent. This should unport one outlet, and give the pump an opportunity to suck air in preference to fuel. It may take quite some time before the engine stops because it has to burn all the fuel stored in the lines and gascolator first.

The aircraft that have experienced engine stoppages so far, what we don’t know is how long the fuel stopped feeding before the engine stopped. It may have been several minutes. This effect has implications where the gascolator is plumbed between the fuel selector and the engine. There have been accidents where a pilot has started the engine with fuel selector in the OFF position, and subsequently become airborne, only to have the engine stop at low altitude on climb out. Many fuel systems were designed to prevent this by only having a very limited amount of fuel “stored” in the system (cause the engine to stop before it had a chance to become airborne), but there’s an obvious trade off between that and having sufficient fuel to cater for a un-porting event.

[Ed.Meyer]

Having followed this thread and then seeing the safety notice from Bob regarding fuel pumps with his fuel system design I decided to do a test.

As some of you know, we have a Patrol with the EFII ignition and injection system which includes fuel pumps (main and secondary) to boost fuel pressure up for the injectors. Also, return lines were installed to return the excess fuel from the continuously running pump. All the details of our system installation in the Patrol are in a Beartracks article I wrote a while back.

Not knowing positively whether un-porting of the fuel outlets on one tank would allow air to enter the fuel system and starve the injection system,a concern expressed in this thread; I decided to do a test:

I flew up over my home airport about 4500 AGL with about 9 gallons of fuel in each side. I was using a cruise power setting with about 8GPH fuel flow showing. I turned the fuel valve to the right tank (I usually run both) and then put the right wing low in a pretty hard slip/skid to intentionally un-port the fuel outlets. I expected that fuel pressure would quickly drop and the engine would quit. After about 10 seconds or so, the fuel pressure did drop and the automatic switching to the secondary fuel pump occurred but the pressure stayed low. At this point I knew I had succeeded in un-porting the fuel outlets on that side. I then went back to normal coordinated flight and the fuel pressure slowly recovered in about 10 - 15 seconds. Much to my surprise, the engine never missed a beat through this.

Following this, I switched the fuel selector to both and again did the right wing low slip/skid. I held this for probably about two minutes monitoring the fuel pressure closely. It never wavered in the slightest.

My conclusion from this is that there is apparently enough fuel flow capacity feeding from only one tank via 3/8" lines to supply all the fuel the engine needs plus whatever amount the fuel regulator returns to the tank even with the other tank feed open to the vent. I recognize that this is not an exhaustive test but it was enough to boost my confidence in the fuel system as installed.

[Battson]

Good questions - we tried swapping fuel caps, it appeared to make a difference at first, but then it transpired that it made no difference whatsoever.

We haven't tried swapping fuel caps on the aux tanks yet.

[Bcone1381]

Fuel Cap position....would the pressure differential exist between each cap's vent if each is placed slightly differently at the wing, or the alignment is not exact.......is one cap slightly higher, or slightly more forward slightly angled?

The only explanation I can see at this point is at best theoretical, but might be worth to investigate.

Lets say the L vent pressure is 4 units and the R is a 6 in cruise and causes an imbalance. Then one slows for landing, and extends flaps....it would seem to change the pressure distribution on the wing, and maybe change the distribution at each caps vent, inducing crossflow of fuel thru the selector valve. Would not the crossflow then limit flow to the stem of the valve's "tee" going to the engine?

[Nev]

It’d be interesting to disconnect the fuel lines and compare flow rates L vs R after the selector, before the pump.

[jaredyates]

I wonder if swapping the caps would make any difference?

[Battson]

In our case, we have two Bearhawk 4-place's in the hanger at the moment, and we flew both yesterday. The first machine sucked 60 L (15 gal) out of one tank and didn't touch the other. Mine draws perfectly equally on both tanks, as always.

Same two pilots flew in both machines at once, same day, almost identical setups apart from:
- the plumbing from the tanks to the selector - one has many more fuel drains and lots more AN hardware
- use of aux tanks vs no aux tanks (with gas cap vents)

Literally everything else about the fuel system is the same. Same gascoalator, same pumps, same injection system, same engine, same venting.

It seems to be very fiddly to pin down why the tanks draw at different rates. Perhaps a cross vent would help THAT symptom.

[Nev]

This is probably the most common cause Mark. We used to sometimes balance tank quantities when needed by opening a cross feed valve and deliberately flying out of balance to get fuel across to the other tank. In aircraft without a cross feed valve the fuel can be free to flow through the fuel selector in BOTH to the other tank.