Fuel Pumps

[skippydiesel]

Debate seems to be lagging somewhat, to the point of despiraton ("Peeing while on long flights") so thought I would "shoot the breeze" on fuel pumps;

 

(FYI most of this will be from my experience with Rotax 912ULS & my current aircraft Sonex Legacy/Rotax 912ULS)

 

Boost/Back-Up/Auxiliary

 

As one of the "fixes",  addressing an ongoing problem with low fuel pressure on take-off/climb out, I have recently changed my no name Facet Cube knock off (estimated 3-5 psi, 30 US G/hr) for a genuine Facet Cube  PN 40135, 4-7psi, 32 USG/hr  (yet to be tried). 

 

The Rotax obsessed amongst you will observe, I have exceeded Rotax recommended Boost Pump Max pressure of 5 psi. Reason - RV have advised RV12 owners to retrofit & fitted latest 12's, with this pump. It seem this aircraft was experiencing the same problems as my Sonex/912.

 

In my quest to improve fuel pressure at this critical time in flight ops, I have ensured filters are clean, vents are open, changed fuel return restrictor jet for a smaller one (now changed back to original). The jet fix certainly helped but not enough, so I hope the new pump will solve the problem.

 

Transfer Pump

 

My Sonex is fitted with a Facet  Posi-Flow PN 60303. 7-10 psi, 34 US G/hr. Ssslllooowww!  Despite its claimed flow rate, it takes around 20 minutes to deliver 30L. Have been researching potential upgrades;

 

Facet Posi-Flow PN 60107. 7-10 psi 40 US G/hr - This would be a "drop in" replacement, as I imagine external dimensions are the same as the above (awaiting dimension confirmation from Facet) BUT only improves flow by about .5L/min

Facet Cube PN 40307, 12-15 psi, 50 US G/hr - Significant step up in flow rate BUT two things are of concern ; 1. Dimensionally longer (between fuel IN/OUT sockets)  so not a drop in 2. Given that there will be frictional losses, what sort of  improvement am I likely to experience?

 

Your constructive criticism will be most welcome

 

Sorry about the imperial pressure & flow specifications - that's how Facet express them.

[Thruster88]

The boost pump.

 

Most likely won't achieve the 7 psi due to system loss, the same reason your transfer pump is down slightly.

 

If the pump can overwhelm the carb floats, think cattle trough with the wrong valve for system pressure, and you are comfortable with where that over flowing carb bowl fuel goes then no problem. If the over flow might cause a fire then think about that. Mitigating strategy might be to delay boost pump use before takeoff. 

 

The transfer pump

 

The 90lph you are getting is plenty given you will be transferring fuel while only burning  10-15 lph. Know your aircraft.  Put the transfer procedure in the flight manual😎

[Old Koreelah]

This is topical. This morning I tested my old Facet boost pump. The flow is spluttery, but it’s delivering 45 litres of fuel per hour- twice my maximum burn rate.

[skippydiesel]

Thruster - 

 

Boost Pump

 

The Rotax advice of max 5 psi is specifically to reduce the chance of pump pressure overwhelming the float chamber valve.

Rotax also advise the use of a #35 return flow restrictor, which in my application delivered 7+L/h back to the tank.

The combination of fuel burn at max power (TO & CO) plus the return flow, came close to equalling the max flow from both mechanical & boost pump combined, leading to low pressure at this time.

After determining that the system was working as expected, I looked for a way to improve the pressure - cheapest/easiest was to put in a smaller aperture return line restrictor reducing return flow to 5L/h this produced a small improvement but not enough.

A reference, on Rotax Owner Forum, to  RV's advice to RV 12 owners to install a pump with a higher pressure & flow spec grabbed my attention. On reading the RV advice document I quickly saw the parallel with my situation - hence the installation of the higher capacity pump and the reinstallation of the #35 restrictor jet.

My Sonex is still undergoing a host of major/minor mods, the boost pump change being just one, so yet to see if there is a beneficial change in TO/CO fuel pressure.

 

Transfer Pump

Yes the transfer pump works BUT its so sloooooow! In my limited in flight fuel transfer experience, the longer it takes, the more likely the pilot will be distracted by other tasks/situations leading to venting fuel overboard - not good!.

My dilemma is; the 50 US GPH pump (costly special order from The States) will make a transfer time difference BUT by how much?? given the physical restrictions inherent in piped systems.

Yes my transfer methodology & rates have all been logged in the POH 

 

 

[skippydiesel]

12 hours ago, Old Koreelah said:

This is topical. This morning I tested my old Facet boost pump. The flow is spluttery, but it’s delivering 45 litres of fuel per hour- twice my maximum burn rate.

OK - I now have a low hrs (50) cube boost pump,  surplus to requirement, works fine just a bit low on delivery for mys system,  see above for estimated performance - intersted?

[Old Koreelah]

5 hours ago, skippydiesel said:

OK - I now have a low hrs (50) cube boost pump,  surplus to requirement, works fine just a bit low on delivery for mys system,  see above for estimated performance - intersted?

Thanks Skip, but I’ll stick with what’s working fine. 

[Area-51]

An external Bosch EFI pump off an old W124 mercedes will work great as a transfer pump. Very reliable unit.

[BrendAn]

12 hours ago, skippydiesel said:

Thruster - 

 

Boost Pump

 

The Rotax advice of max 5 psi is specifically to reduce the chance of pump pressure overwhelming the float chamber valve.

Rotax also advise the use of a #35 return flow restrictor, which in my application delivered 7+L/h back to the tank.

The combination of fuel burn at max power (TO & CO) plus the return flow, came close to equalling the max flow from both mechanical & boost pump combined, leading to low pressure at this time.

After determining that the system was working as expected, I looked for a way to improve the pressure - cheapest/easiest was to put in a smaller aperture return line restrictor reducing return flow to 5L/h this produced a small improvement but not enough.

A reference, on Rotax Owner Forum, to  RV's advice to RV 12 owners to install a pump with a higher pressure & flow spec grabbed my attention. On reading the RV advice document I quickly saw the parallel with my situation - hence the installation of the higher capacity pump and the reinstallation of the #35 restrictor jet.

My Sonex is still undergoing a host of major/minor mods, the boost pump change being just one, so yet to see if there is a beneficial change in TO/CO fuel pressure.

 

Transfer Pump

Yes the transfer pump works BUT its so sloooooow! In my limited in flight fuel transfer experience, the longer it takes, the more likely the pilot will be distracted by other tasks/situations leading to venting fuel overboard - not good!.

My dilemma is; the 50 US GPH pump (costly special order from The States) will make a transfer time difference BUT by how much?? given the physical restrictions inherent in piped systems.

Yes my transfer methodology & rates have all been logged in the POH 

 

 

i am not sure i would want to increase pressure. higher flow is what you need. 

[skippydiesel]

20 minutes ago, BrendAn said:

i am not sure i would want to increase pressure. higher flow is what you need. 

Hi BrendAn,

 

In this instance, pressure is used as an indicating of flow. Rotax 9's require a pressure of between 2-7 psi. If flow is reduced (pump unable to maintain sufficient flow), pressure will drop. Its as simple as that.

[BrendAn]

28 minutes ago, skippydiesel said:

Hi BrendAn,

 

In this instance, pressure is used as an indicating of flow. Rotax 9's require a pressure of between 2-7 psi. If flow is reduced (pump unable to maintain sufficient flow), pressure will drop. Its as simple as that.

Ok

[Thruster88]

16 hours ago, Area-51 said:

An external Bosch EFI pump off an old W124 mercedes will work great as a transfer pump. Very reliable unit.

These pumps can produce about 4 bar or 60psi. If the receiving fuel tank became full or there was a blockage in the vent system the tank would most likely burst. Not good.

[Old Koreelah]

Tank vents are a whole other topic; locating them in a low pressure area is important. One European plastic fantastic has a tiny pinhole at rear of wing.

[skippydiesel]

Anyone got any thoughts on the likelihood of a pump, with a higher flow & pressure, delivering a worthwhile improvement, in fuel transfer time, given that the piping (aluminium) will remain as is.

[RossK]

Hi Skip, don't take this as criticism, but you may be looking in the wrong area.

It seems that your fuel delivery system may have a lot of restriction (lines, connectors, pumps etc), which is limiting pressure and flow.

 

As an example, the Sportstar POH actually says the boost pump is on only for start up and emergencies. It's off at all other stages of flight, including take off and landings.

The boost pump is actually an optional extra for the Sportstar, ie - the fuel system will work just fine relying on the Rotax pump alone.

 

I'm not advocating deleting the boost pump, having the boost pump there is more than nice, redundancy in an aircraft critical system is a good thing to have. But I don't think your fuel system should rely on it to give satisfactory performance.

 

[Thruster88]

1 hour ago, skippydiesel said:

Anyone got any thoughts on the likelihood of a pump, with a higher flow & pressure, delivering a worthwhile improvement, in fuel transfer time, given that the piping (aluminium) will remain as is.

Impossible to say without knowing size, length, number of elbows etc. Remember restrictions on the suction side have a much greater effect on pump efficiency than delivery side.

 

I would keep the transfer pump you have and install a timer switch like on your oven, set time based on known flow rate, can still turn off anytime manually.  

 

RossK is correct, the engine pump alone should provide correct flow and pressure, either the pump is faulty or the fuel system is not right.

[skippydiesel]

5 hours ago, RossK said:

Hi Skip, don't take this as criticism, but you may be looking in the wrong area.

It seems that your fuel delivery system may have a lot of restriction (lines, connectors, pumps etc), which is limiting pressure and flow.

 

As an example, the Sportstar POH actually says the boost pump is on only for start up and emergencies. It's off at all other stages of flight, including take off and landings.

The boost pump is actually an optional extra for the Sportstar, ie - the fuel system will work just fine relying on the Rotax pump alone.

 

I'm not advocating deleting the boost pump, having the boost pump there is more than nice, redundancy in an aircraft critical system is a good thing to have. But I don't think your fuel system should rely on it to give satisfactory performance.

 

Hi RossK,

 

"It seems that your fuel delivery system may have a lot of restriction (lines, connectors, pumps etc), which is limiting pressure and flow."

 

Restrictions - for sure;

• Long (pipes) all the way to the wing tanks
• Failure to install a pump bypass as per Rotax installation advice

 

Changes to the fuel system, is to plumb in my (new higher pressure) boost pump & bypass to my 40L header tank. Previously the boost pump would only draw from either wing tank. This change should have two benefits - header tank is pretty much on the same level  (head) as the carbs/ engine & very much closer (frictional losses) to the engine.  The downside to installing a bypass, etc,  is many more joins, additional non return valve & convoluted pipe work. So far only ground check, will be interesting to see what happens on the first TO/CO.

 

"POH actually says the boost pump is on only for start up and emergencies. It's off at all other stages of flight, including take off and landings."

 

Can't agree with any of that. All my training requires;

• Boost On pre start / check pressure / Off / start engine - This is a check pump function & prime of carb float chamber
• Run up bay - Boost On / check combined mechanical/boost pressure / TO &/ CO to safe altitude / Off - This is to ensure continuity of engine function through the most critical phase of powered  flight
• Pre landing - Boost On / Off after after landing - Again this is to ensure continuity of engine function, when in close proximity to very hard ground.

 

"The boost pump is actually an optional extra for the Sportstar,... ""

 

I can accept that a high wing/tank may have good fuel flow (engine continue to deliver power)  even if mechanical pump fails, however a low wing does not have this inherent safety feature.

 

 "...the fuel system will work just fine relying on the Rotax pump alone."

 

Sure - all of use who have forgotten to use the Boost Pump, at one time or the other, know this BUT what happens when the mechanical pump malfunctions????? I suggest it gets a tad quiet,  followed by potential nasty noise a little while later. The Boost Pump is a low cost safety feature that all aircraft (in this class) should feature.

Edited September 22, 2023 by skippydiesel

[skippydiesel]

5 hours ago, Thruster88 said:

Impossible to say without knowing size, length, number of elbows etc. Remember restrictions on the suction side have a much greater effect on pump efficiency than delivery side.

 

I would keep the transfer pump you have and install a timer switch like on your oven, set time based on known flow rate, can still turn off anytime manually.  

 

RossK is correct, the engine pump alone should provide correct flow and pressure, either the pump is faulty or the fuel system is not right.

Hi Thruster,

 

See above answer to RossK for responses to most of your observations.

 

I think RossK was leaning towards the Sportstar concept (no Boost Pump) I cant accept that.

[RossK]

Hi Skip, I'm not advocating no boost pump in your aircraft. I'm just suggesting that the fuel system should be able to work comfortably without it.

 

As to the Sportstar, I agree it's comforting to have the pump on in those critical phases of flight, but when I was last flying with an instructor in the RH seat, his comment was,

"it's unusual, but follow the POH" ie. leave it off. 

[skippydiesel]

Fair enough.

 

"Hi Skip, I'm not advocating no boost pump in your aircraft. I'm just suggesting that the fuel system should be able to work comfortably without it." - it does.

 

"As to the Sportstar, I agree it's comforting to have the pump on in those critical phases of flight, but when I was last flying with an instructor in the RH seat, his comment was,

"it's unusual, but follow the POH" ie. leave it off. " - There are times in life & POH's, when common sense should prevail, no matter instructions to the contrary. My advice use the Boost pump at all critical stages of flight. It costs nothing and may save your life.

 

An unrelated example; I was taught to use carby heat when required. Many years after obtaining my PPL, I was flying with an instructor in Canada - his advice, use carby heat every time you initiate descent power and leave it on until returning to cruise/full power. I thought about it, made sense, cost nothing and adopted the habit and that's what I do now (when in an aircraft so fitted).

[Thruster88]

Skippy, I don't think you are hearing what RossK and I are saying, all aircraft should have normal fuel pressure without the use of the boost pump. All my aircraft certainly do this. The boost pump is for redundancy, it should not be a band aid. Hope this makes sense. 

 

When I got the RV6-A some three years ago I noticed the fuel pressure was dropping from the normal 4-5psi after turning the boost pump off to only 1-2psi. I spoke to the previous owner and he said it had always been that way, so I thought it must just be the way the sensor works(conformation bias). Some months later having looked at the system I realized the fuel pressure had to be indicating correctly. Replaced the engine pump on the Lycoming and it is now always 4-5psi regardless of boost pump use. I dismantled the old pump and could see no fault with it.

Edited September 22, 2023 by Thruster88

[facthunter]

Why not have the backup at critical times with a low wing plane? It doesn't increase the fuel pressure.  Nev

[skippydiesel]

3 hours ago, Thruster88 said:

Skippy, I don't think you are hearing what RossK and I are saying, all aircraft should have normal fuel pressure without the use of the boost pump. All my aircraft certainly do this. The boost pump is for redundancy, it should not be a band aid. Hope this makes sense. 

 

When I got the RV6-A some three years ago I noticed the fuel pressure was dropping from the normal 4-5psi after turning the boost pump off to only 1-2psi. I spoke to the previous owner and he said it had always been that way, so I thought it must just be the way the sensor works(conformation bias). Some months later having looked at the system I realized the fuel pressure had to be indicating correctly. Replaced the engine pump on the Lycoming and it is now always 4-5psi regardless of boost pump use. I dismantled the old pump and could see no fault with it.

 I don't think I disagreed with the idea that the aircraft should be able to fly without a Boost pump - just that mine gave every indication that it might not.

 

I never tried a TO/CO without Boost pump, so I can't say what might have happened, however with mechanical + Boost pump, my fuel pressure dropped below 2 psi (the Rotax min). Engine never lost power/hesitated but my heart did.

 

As mentioned , several fixes were tried, with limited success. Aircraft returned to workshop for a range of improvements (including fuel system) - yet to be tried/flown. 

 

One of the fuel mods, was the installation of a Boost pump fuel bypass (as per Rotax recommendation) - this will remove any tendency for the boost pump itself, to restrict fuel supply (one of the possible in-line restrictions to good flow).

 

When its back in the air, I will try a full power climb, at altitude, without Boost pump , just to see what happens - let you know.

Edited September 22, 2023 by skippydiesel

[RFguy]

Don't forget high altitude vaporization of MOGAS not just hot weather.

So, do consider the need for the boost pump..
AND consider large diameter fuel lines from tank to pump to reduce suction tension (low pressure) which will promote formation of  vapourization bubbles for high vapour pressure fuels (MOGAS). 

Keen readers will be aware I calculated that MOGAS might get bubbles in the fuel lines once over 10,000 feet, depending on summer or winter gasoline, and sure enough at FL125 the other day, (actually climbing through 11400 feet (slowly ha ha ) -  fuel pressure began fluctuating a little periodically (bubbles passing) and slight EGT variations. Switch over to AVGAS tank, no problem..... drop back to 8500, back to MOGAS tank, no problem.....

[Thruster88]

Good info Glen. 

Skippy the concern with your low fuel pressure is the cause. If it is due to some restriction in the supply to engine pump then you will be at increased risk of vapour lock at altitude. In Glens case the PA-28 has large fuel lines and would have been burning only 30 lph, not much difference to the 27lph for a 912 @100% power.

[onetrack]

Don't forget that unions and elbows in fuel lines provide increased flow restriction. 90° drilled elbows are notorious for flow disruptions, swirls and cavitation.