Fuel flow test

Been reading Firewall Forward and AC 90-89, planning things to do as I finish up. Does anyone (RV builders) dig a pit for the tail, and/or block up their plane to "takeoff " attitude to measure boost pump flow?

I would think that you would get a good equivalent measure if you just added a length of tube or hose to make the pump push fuel up a few feet after the fitting you disconnected from the fuel servo or carb. Just make sure not to run a closed hose back down from the high point to eliminate any siphon assist.

No ditches nearby to put the tail in?

With a cookie cutter plane like an RV, you're probably pretty safe (but not necessarily legal) as long as the fuel flows at all. But the goal of the test is to check the whole system. With some planes, unusual attitudes can do things like unport the fuel pickups, or get the carb higher than the sump (with no fuel pump). Just extending the FWF part of the system upward a bit won't test the system in the rest of the airframe.

The tail down attitude of the 7 yields better (less fuel remaining) performance, as does the 10. It is more that there are no suction leaks with the additional head from the tank to the fuel pump that would be tested. If you air pressure tested the system from wing root to servo, then you can be assured that section is good.

Then test at level and as much nose down as you are comfortable with. Tail down attitude for tail dragger is 12 degrees. Nose down for landing, I was able to get about 8 degrees with a weight on the tail and good blocks under the mains. I don't think a pit is necessary. For an A model sitting the mains on extra blocks, maybe 4" then nose down on the flat should give you an idea of the change. Not as much as you might think.

I pushed the suction screen as low in the tank as it would go (7) and took a picture with a camera internally. If it is sitting on top of the drain, you are good there and should have good readings. Not sure what model you are testing.

Be sure to run the fuel pump at 30 psi and 16 GPM for the test (IO360M1B), or max fuel flow at desired pressure for your configuration. It takes about 150 grams of fuel to fill the system, so the first side to be measured will read higher (fuel remaining) than the second. Yes, I used one gallon for the test, and measured by using a scale. PM me if you have questions. I used a clear section just before the back pressure valve to observe possible bubbles in the flow indicating cavitation somewhere in the system. There should be none.

Since sloshing and slip is not included, you will find for real life usage the fuel remaining test result will yield an uncomfortably low number.

Ron RV8 said:

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And smaller than a thousand words

Thanks for the replies! Beautiful view from your back yard Ron!

I'm just looking for a reasonable way to do this test without excavating at the airport - I'd really rather not put fuel in the tanks until final assembly.

My reasoning for the idea of just adding head pressure for the pump to overcome is thus: If you do a test on the system and measure flow rate as the plane sits on the ground (taildragger 8) and you get well over the minimum rate for the IO-360 - you prove that the pump is adequate for the system as it sits there - pulling fuel through all the joints, turns, valves, and filter. Then if you change the orientation of the plane - the only thing that physically changes is that the boost pump has to raise the fuel another x inches from the tank to the engine pump. In my plane, if you raise the nose to 45 degrees that would add maybe three feet vertically.

I will still need to find min usable fuel for a reasonable landing pitch and go around attitudes - should be able to get those with blocks under the tail for a landing attitude and whatever grade I can find near the hangar for go around pit.

If I need to do the full Monty on these tests, do you have any tips for safely handling the wings after they had gas in them?

Thanks again!

SgtZim said:

If I need to do the full Monty on these tests, do you have any tips for safely handling the wings after they had gas in them?

Thanks again!

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Prior to working in the fuel tank of an operational aircraft I flushed it with CO2, running a hose from the outlet of the CO2 cylinder into the corner of the tank furthest from the filling port. The CO2 flow rate was not high - I just kept the purge going for about an hour. Might have cost me a dollar in CO2. When I was done I could not detect a gas odour in the tank and I have a pretty sensitive nose.

As for "doing the full Monty" in terms of fuel flow testing, I highly recommend it. You will have plenty of things to worry about on that first flight - gas getting to the engine should not be one of them. When I did my flow test I ended up with something well in excess of the required 150% of max engine consumption rate. While the test burned off a half day and took five volunteers to help set up, the peace of mind it provided was invaluable.

For those of us building in Canuckistan, the "full Monty" fuel flow test is a requirement. It was made a requirement for good reason, considering the number of amateur-built airplanes that come to grief in the first few hours of flight as a result of impaired fuel flow. The test costs virtually nothing to perform, thus it is incredibly cheap insurance.

If you have some $$ to spend, a great way to do the test when surrounded by flat land is to bring in a friend with a tilt-n-load tow truck. This device really helps in getting the right nose-up angle.

How to flow test a (low pressure) mechanical pump

I have recently replaced my engine driven mechanical fuel pump (it’s an RV6 with an 0-320) with a new Tempest unit.

Since I had the fuel system partially disassembled, I repeated my pre Phase I fuel flow testing (as outlined in this thread) with satisfactory results.

I have also pressure tested the system against the (closed) needle valve in the carb - engine not running. Boost pump (vintage Facet) makes just over 2 psi. New Tempest makes 4psi (at cranking speed with top plugs removed).

Somewhat disconcertingly however, when I flow test the mechanical pump (at cranking speed into an open jug) I only get 4-6 slugs of flow then air. It’s as if the mechanical pump can’t maintain its prime (without the boost pump running).

I’ve looked extensively for leaks in the system (while pressurized) and found none.

The questions are:
1). Has anybody flow tested a low pressure mechanical pump in an RV and found different/better results?
2). Should I have confidence the mechanical pump will be fine at higher (idle and above) speeds?
3). Anything more to do before a ground run to verify the engine can run at all speeds on just the mechanical pump?

Thanks in advance,
Peter

No joy

Ok, continuing with my “experimentation” I have confirmed I have a problem.

Ground run results: Easy startup using the conventional procedure (boost pump on). 4psi fuel pressure observed. At idle, upon shutting down the boost pump, pressure holds steady at 4psi for a minute or so, then cyclical dips in pressure start and grow to +/- 1 psi as average pressure falls. About a minute later the pressure arrives st zero and stays there. In another minute or so the engine quits (presumably of fuel starvation as the carb runs dry).

I can save it (and restore steady 4-5 psi pressure) at any time the engine is still running by turning on the boost pump.

This is the same behavior that caused me to replace the fuel pump so my problem still exists.

Any advice?

Peter

I suggest you look for air leaks in the fuel system. It sounds like you are sucking air with the mechanical pump.

Carl

Carl Froehlich said:

I suggest you look for air leaks in the fuel system. It sounds like you are sucking air with the mechanical pump.

Carl

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Yes, I’ve looked extensively for air intrusion or fuel leaks, so far without success.
My current suspicion is the fuel selector valve stem … the connections seem sound.

Does anyone have a suggested procedure for testing for air leaks ?

Is is conceivable that flexible fuel lines firewall forward (they are rubber and old) are admitting air but not leaking enough to be noticeable ? Could they be collapsing due to the suction of the mechanical pump ? I am considering replacing them next …

Please keep the ideas coming !
Peter

no leaks

I would think you might want to check the mechanical pump. See if fuel is coming out of the drain between the diagrams, or maybe even going into the oil (I dont know how this could be, but it is simple to pull oil dipstick and smell for raw fuel.)

My reasoning is if the pump is sucking air thru a bad diagram, you would probably see oil or fuel out the drain.

The pump is brand new (thats the first thing I replaced) so I hope it has good diaphragms. That said, the tell-take line is clear, and yours is a good suggestion to check the sump (for smell) so I will do that.

Thanks, and keep the ideas coming. I’m running out on my own …

Peter

You can check your gascolator if you have one. Some older 6's were built with Cessna gascolators and they were designed for gravity feed.

You could pull a small vacuum on your fuel system. Close the fuel valve. Disconnect the fuel line at the carb and connect a "T" and a vacuum pump... https://www.harborfreight.com/multi-use-transfer-pump-63144.html?_br_psugg_q=transfer+pump ... Your fuel system should hold a vacuum to your finger or gauge. The leak should be almost immediate.

gasman said:

You can check your gascolator if you have one. Some older 6's were built with Cessna gascolators and they were designed for gravity feed.

You could pull a small vacuum on your fuel system. Close the fuel valve. Disconnect the fuel line at the carb and connect a "T" and a vacuum pump... https://www.harborfreight.com/multi-use-transfer-pump-63144.html?_br_psugg_q=transfer+pump ... Your fuel system should hold a vacuum to your finger or gauge. The leak should be almost immediate.

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It *is* an early build RV-6, I do have a gascolator, and it *is* likely out of a Cessna. What are the implications of “designed for gravity feed”? This one has provided 220 hrs of satisfactory service till now.

I’ll look into away to create vacuum for the test you suggest. Thanks.

Peter

Cessna used a pull cord to sample the sump. Suction from the engine driven pump can suck air in around the pull shaft. The electric fuel pump places pressure on the seal and holds it closed just like the gravity flow of the Cessna fuel systems.

Most likely, the seal around the pull shaft is subject to fail and cause the air infiltration that you ae experiencing. As you can see, they are not made for low wing aircraft..... and should be replaced.

gasman said:

Cessna used a pull cord to sample the sump. Suction from the engine driven pump can suck air in around the pull shaft. The electric fuel pump places pressure on the seal and holds it closed just like the gravity flow of the Cessna fuel systems.

Most likely, the seal around the pull shaft is subject to fail and cause the air infiltration that you ae experiencing. As you can see, they are not made for low wing aircraft..... and should be replaced.

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Ok. No pull cord or shaft on this one, so that’s not how the air is entering … the search continues.

Issue found!

Just to close the loop, I found my issue. Thanks to all who helped out. The pointer to the harbor freight pump was pivotal.

Using the pump’s ability to apply both positive pressure and vacuum I found a leak at the gascolator. It was unusual in that under pressure there was no leak, but under vacuum a significant air leak into the system appeared.

Disassembling the bowl and resetting the sealing gasket fixed the issue completely.

For posterity, my testing running the engine driven pump (only) resulted in effective self priming and the quick establishment of 4.5 psi (dead headed) and a little over 30gpm sustained into an open container, at starter cranking speeds (with spark plugs removed).

And do be careful reassembling your gascolator (mine is the ACS “Homebuilders Gascolator” from Spruce).

Peter