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  #51  
Old 12-31-2017, 10:03 AM
rv7charlie rv7charlie is offline
 
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Quote:
Originally Posted by maniago View Post
Ive been wondering about this AFP note. I get that with EFI youre really running a risk of not being about to over come the bubble because youre relying on the electric pump to loop the fuel, but why would it be an issue with the RSA style? The engine pump is designed to overcome dry injector lines, and pull fuel thru some head height (or how else do we dry start a new motor?), so the AFP pump is immaterial - the engine pump will pull fuel thru it and "prime it". No?
I put ~190 hrs on a Swift that had Bendix injection (Lyc IO-320, then IO-360 engines) that had an STC'd aux tank. Standard practice on cross countries was to TO-main, switch to aux, run the aux dry, and switch back to main. You always knew within a few minutes when the aux would run dry, so you just monitored the fuel flow/pressure gauge & switched at the 1st drop in pressure & hit the boost pump. Yes, occasionally you could get distracted and the engine would cough/sputter before switching. The engine driven pump has no problem recovering; it'll just recover faster if the boost pump pushes fuel up there.

If the AFP pump is a gerotor or roller vane style pump (and it's almost certainly one or the other), the [AFP] statement makes no sense as written. Gerotor and roller-vane pumps are positive displacement pumps; they will self-prime at any level of lift you're likely to see in the planes we fly. Self-priming by definition means that there will be a bubble of air on the output side of the pump after the pump self primes; it means that the pump is pulling air through itself to get the gas up there. That air has to go somewhere; it doesn't just disappear. So....when the boost pump self-primes, there will always be a (temporary) bubble of air on the output side of the boost pump.

If AFP used a turbine style pump in that assembly, they made a grievous error in pump selection.

EDIT: Just realized what they might be talking about. Positive displacement pumps must have a regulator, to avoid damaging downstream stuff or stalling the pump. The regulator on that pump (and the other brands that are configured similarly) bypasses excess fuel *back to the intake of the pump*. I can see how the pump could fail to pump (gas), once there's air in the system around the pump. It's a lot easier to move air than fuel, so the pump *system* can vapor lock *itself*, if air ever gets in the line.

Something to think about, while flying over that forest, if you think there's ever any risk of unporting your fuel pickup while operating on the boost pump....

Would not be an issue, if the regulator bypass went back to the tank, as is done with the auto style injection systems that are gaining popularity on this forum.

Last edited by rv7charlie : 12-31-2017 at 11:16 AM.
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  #52  
Old 12-31-2017, 01:32 PM
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airguy airguy is offline
 
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Quote:
Originally Posted by rv7charlie View Post
Would not be an issue, if the regulator bypass went back to the tank, as is done with the auto style injection systems that are gaining popularity on this forum.
Also applies to heat buildup while operating at low fuel flows, since the pumps use the fuel flow itself to cool the pump motors. This was the driving factor for me to route the return lines all the way back to the tank.
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Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
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  #53  
Old 12-31-2017, 02:03 PM
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maniago maniago is offline
 
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Quote:
Originally Posted by rv7charlie View Post
I put ~190 hrs on a Swift that had Bendix injection (Lyc IO-320, then IO-360 engines) that had an STC'd aux tank. Standard practice on cross countries was to TO-main, switch to aux, run the aux dry, and switch back to main. You always knew within a few minutes when the aux would run dry, so you just monitored the fuel flow/pressure gauge & switched at the 1st drop in pressure & hit the boost pump. Yes, occasionally you could get distracted and the engine would cough/sputter before switching. The engine driven pump has no problem recovering; it'll just recover faster if the boost pump pushes fuel up there.

If the AFP pump is a gerotor or roller vane style pump (and it's almost certainly one or the other), the [AFP] statement makes no sense as written. Gerotor and roller-vane pumps are positive displacement pumps; they will self-prime at any level of lift you're likely to see in the planes we fly. Self-priming by definition means that there will be a bubble of air on the output side of the pump after the pump self primes; it means that the pump is pulling air through itself to get the gas up there. That air has to go somewhere; it doesn't just disappear. So....when the boost pump self-primes, there will always be a (temporary) bubble of air on the output side of the boost pump.

If AFP used a turbine style pump in that assembly, they made a grievous error in pump selection.

EDIT: Just realized what they might be talking about. Positive displacement pumps must have a regulator, to avoid damaging downstream stuff or stalling the pump. The regulator on that pump (and the other brands that are configured similarly) bypasses excess fuel *back to the intake of the pump*. I can see how the pump could fail to pump (gas), once there's air in the system around the pump. It's a lot easier to move air than fuel, so the pump *system* can vapor lock *itself*, if air ever gets in the line.

Something to think about, while flying over that forest, if you think there's ever any risk of unporting your fuel pickup while operating on the boost pump....

Would not be an issue, if the regulator bypass went back to the tank, as is done with the auto style injection systems that are gaining popularity on this forum.
So this is all good stuff. The operative question then is if the AFP has this problem, but your Swift pump (and presumably any stock RSA system e-pump) does not:
1)how do those pumps differ from the AFP
2)what make and model are those pumps
3)why would anyone choose to use anything but those pumps?

I havent decided on a boost pump, so this is an important issue for me, and until this thread, I hadnt even considered it an issue. Thanks for any help.

------
Edit: Ok I get it. Weldon pumps are Piper stockers, 8120G or 8150s. I now get that the AFP is meant for EFI - guess I've been ignoring that, despite it being obvious. I now also get why the Andair pumps are the obvious go-to pumps for the RSA style inj for EAB. I'll leave this post here; maybe it'll help someone else in the future.
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Mustang II (FoldingWing) "Firewall Fwd stage.Yea!" IO-360B1E,RSA,C2YR-BF/F7666-2,Superior sump,James; 2xHXr,MiniX,EIS,480,327,240,SL30,(maybe)Echo; SteamAlt,AS,VSI
Don't be a hater; I'm a cousin with thin wings!
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Last edited by maniago : 12-31-2017 at 02:27 PM.
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  #54  
Old 12-31-2017, 02:37 PM
rv7charlie rv7charlie is offline
 
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Look carefully at the wording of the AFP warning:
"WARNING

Airflow Performance and Bendix/Precision fuel injection systems are non-returning systems. In the event that a tank is run dry in flight, an air lock will be formed on the out let of the pumps. It is possible that the auxiliary pump will not pick up fuel, as the auxiliary pump cannot create enough air pressure to over come the flow divider opening pressure, thus displacing the air and resume pumping fuel. It is not recommended to run a fuel tank dry in flight without adequate testing and proper documentation of the procedure for this operation."

To me, this reads as if they're talking about the stock engine driven pump, in addition to the electric aux pump. The statement goes on to say that the AFP aux pump can't make enough pressure to drive the air past the flow divider opening pressure. My understanding (I could be wrong) is that the flow divider operating pressure is at most a few psi (one of the reasons 'hot starts' are a pain; fuel boils in the injector lines and likely, in the feed to the flow divider, as well). The electric boost pump should be able to generate at least 30 psi; more than enough to open the flow divider. The only thing that makes sense to me is that the bolus of air is looping around the boost pump and its regulator circuit.

The most common 'old school' FI boost pump I'm familiar with is the Weldon.
https://www.weldonpumps.com/weldon-fuel-pumps

Very pricey and fairly heavy/big.

Look, the AFP (and other) experimental pumps obviously work quite well; I just have a hard time understanding the way they worded their warning.

to airguy:
On the issue of fuel heating, I asked Don at AFP that exact question several years ago via PM and his response was that they've tested at elevated temps and 'Even tests where 90 degree fuel was run in the pump continuously at idle flow was OK for 20 minutes or more'.

I've never seen any issue until this thread and seeing that warning, which got me thinking about a bolus of air 'looping' in the pump/regulator.
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  #55  
Old 12-31-2017, 03:29 PM
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maniago maniago is offline
 
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Quote:
Originally Posted by rv7charlie View Post
Look carefully at the wording of the AFP warning:
"WARNING
Yup I read it, and I get it. AFP is a better "RSA" inj. But the same pump warning is not present on the Weldon site nor the Andair site. Did they forget to tell us that? Are we being mislead? Or is AFP being over cautious with their product? Or worse? Dunno. Not enough information.

But I think since I have a Bendix, I'll stick with what Piper did. I'm gonna hazard that the designers knew pilots would run a tank dry, on purpose or not. And I'll hazard that they chose the weldon because it can push past dry lock. Or we'd have had a lot of planes in the dirt in the 60s, 70s, 80s etc.

I think its safe to say however that dry tanking an inj motor is not optimal, because its a lot of air to push thru the small inj lines; and restart when it comes, may not be as heart reviving (and perhaps not with enough instantaneous full power) as it is with a carb motor.
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Mustang II (FoldingWing) "Firewall Fwd stage.Yea!" IO-360B1E,RSA,C2YR-BF/F7666-2,Superior sump,James; 2xHXr,MiniX,EIS,480,327,240,SL30,(maybe)Echo; SteamAlt,AS,VSI
Don't be a hater; I'm a cousin with thin wings!
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Dec2017
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  #56  
Old 12-31-2017, 04:59 PM
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I think the intent of the air bolus warning was that the AFP cannot pump air to enough pressure to push the air through the flow divider and pick up fuel if the output is already pressurized. If the AFP pump can catch its prime with fuel, it can certainly push the air out the flow divider and through the injectors, I know because I tested that.

On the heat loop issue - Don tested for 100LL, but the issue is aggravated when running mogas with higher vapor pressure, especially with 91E10 autofuel winter blend. It was easy enough to install return tank lines, I took the safe path.
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Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
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  #57  
Old 01-04-2018, 03:21 PM
Doug Eves Doug Eves is offline
 
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Have to admit reading only the first few posts on this thread before getting bored. Run out of fuel on purpose? Ridiculous.
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  #58  
Old 01-04-2018, 03:29 PM
rv7charlie rv7charlie is offline
 
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Greg,

To be sure we're on the same page, are you talking about a 'typical' a/c injection system, and saying that you modified the AFP *boost* pump to return the bypassed fuel to the tank instead of back to the tank input?

Or are you talking about automotive style high pressure electronic injection (SDS, etc where there is no engine driven pump) where the norm is to return bypassed fuel to the tank?

Charlie
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  #59  
Old 01-04-2018, 04:07 PM
moll780 moll780 is offline
 
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+1 .....

Quote:
Originally Posted by rightrudder View Post
I'm a proponent of always having an hour's worth of reserve fuel!! Just simpler, with less that could potentially go wrong that way. My typical longer stints are about 3 hours, which leaves closer to 1.5 hours reserve.
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  #60  
Old 01-04-2018, 04:31 PM
YellowJacket RV9 YellowJacket RV9 is offline
 
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Did it today so I could calibrate a new fuel dipstick. Circling 4000' above 3 long runways, plenty of remaining fuel in other (right) tank. The fuel flow and pressure alarms went off at the same time, before any change in operation. Plenty of time to hit boost pump and switch tanks without so much as a hiccup. Total non-event, and reinforced the muscle-memory of what to do should it happen inadvertently. I am very fuel-conservative so I don't ever plan on being anywhere close to the limits, but the NTSB reports are full of people who never planned on being there. I also spent some time practicing crosswind landings in windy, gusty conditions. I did it today because I had plenty of outs, and never had to land, and did plenty of go-arounds. So that I will be better prepared if I do someday HAVE to make it happen. A little risk now to make me better prepared for a worse situation later.

I'm sure not everybody would want to do it, and that's fine, but for me it was preferable to draining fuel on the ground and the risks that go with that.

Chris
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Last edited by YellowJacket RV9 : 01-04-2018 at 04:33 PM.
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