RudiGreyling

RudiGreyling

Well Known Member
RV12-ators,

I see the Fuel flow sender inline for the main fuel flow, but how much fuel goes back to the tank via the Fuel Return Line, that will cause havoc with your fuel calculations, if it is not deducted?

Anyone in typical 912 installations know?

Rudi
 
Return line

My experience with a 912S is that most planes do not use a fuel return line. It is there for a possible float/Needle valve failure and overflowing of a carb. It hopefully will never have anything in it, at least as far as flow. The line will likely back-fill up to the tank fuel level.

John Bender
 
Which don't

Any of the tube and rag users do not use a return line. We have had 3 - 4 in our light plane club, and they did not even use the collector trays under the carbs. Rans, T-Bird, Skyranger etc. The newer composite planes etc. do.

John Bender
 
It is a fuel return line. I had a Sting 2000 with a Rotax 912s that had a fuel pressure relief valve. This allowed a small amount of fuel to flow back to the tank at all times when the engine is running, especially when you pull the throttle back to idle during decent. I assume this is a safety feature to allow cool fuel to continually circulate through the system. The fuel flow calculations are based on consumption of the engine and the flow meter is adjusted to compensate for the "bypassed" fuel. Once the system is calibrated we always had very accurate fuel flow and comsuption readings.
 
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The requirement for the fuel return line is specified by Rotax.

My understanding is that the carb. floats are very sensitive to excess fuel pressure. Any pressure beyond what they are rated for, and fuel will leak past the needle/seat, drain out of the carb and wash oil out of the air filters, etc.
This doesn't occur in flight with a properly designed/installed fuel system, but it can occur while parked when the fuel system gets heat soaked. The check valve in the engine driven fuel pump prevents any back flow so the expansion of the heated fuel will start to raise the pressure in the portion of the system that is after the engine driven fuel pump, and then the carbs start leaking.
The fuel return line connects to a fitting on top of the engine that has a very small orifice in it. It has a rather low fuel flow rate at normal pressure but provides a pressure bleed off point when the engine is stopped... to prevent the problem described above.

Since the normal operating pressure is relatively constant, the small fuel flow of the return line can be compensated for with the K factor value in the fuel computer
 
I have a fuel return line on the 912 in my RANS S-7. Rans just routed it back to one of those unused fittings on top of the gascolator. I've not had any problems with the fuel system at all (230 hrs.) Steve
 
Hmm

Sounds more like a Mechanical fuel pump/ vapour lock prevention measure (in other words a botch) to me...If indeed it is returning fuel.

I ran two Facet fuel pumps in the wingroots of my old Zodiac for 500 hours with no Mechanical fuel pump and no return...Worked flawlessly.

This was on a Soob (same carbs as the 912) with the carbs on top of the engine.

Frank
 
steve wyman said:
I have a fuel return line on the 912 in my RANS S-7. Rans just routed it back to one of those unused fittings on top of the gascolator. I've not had any problems with the fuel system at all (230 hrs.) Steve
Click to expand...

I could see how this could provide the same result that Rotax wanted but only because it is a high wing airplane with gravity feed to the engine drive fuel pump. Totally different from an RV-12. The RV-12 fuel system design matches what Rotax requires, not what can work.
 
frankh said:
Sounds more like a Mechanical fuel pump/ vapour lock prevention measure (in other words a botch) to me...If indeed it is returning fuel.

I ran two Facet fuel pumps in the wingroots of my old Zodiac for 500 hours with no Mechanical fuel pump and no return...Worked flawlessly.

This was on a Soob (same carbs as the 912) with the carbs on top of the engine.

Frank
Click to expand...


I see far to much of this type of information posted here.
"I did such and such on my plane and it worked just fine, so it should work for yours". This statement would be valid if every other RV were the same.
The only thing here is the same carb. Different engine, different fuel pump, No engine driven pump with a check valve, etc ...
Not relevant.
 
As long as we are talking

I do not have one now to look at, but I think the carb vent tubes exit the carbs just above proper fuel bowl level, and will vent to the outside if the fuel level gets too high. The only return system I have seen is the 'catcher' under the carb that was offered by Rotax. Maybe they will have us remove the factory vent/overflow lines and run them to the return line. I certainly have not seen all installations, but I have seen some. We should know soon ( I hope ).

John bender
 
JBPILOT said:
Maybe they will have us remove the factory vent/overflow lines and run them to the return line.
John bender
Click to expand...

Nope.
As I described previously, the RV-12 return line is for a very small orifice port on the fuel system between the engine driven fuel pump and the carbs.
 
You have seen it I trust

Is there a pressure regulator then ? Are the vent lines still on the carbs ?

John Bender
 
Scott

You are saying there is a small port between pump and carbs. That would seem to indicate constant return flow then, if there is no regulator.

John Bender
 
rvbuilder2002 said:
The requirement for the fuel return line is specified by Rotax.

My understanding is that the carb. floats are very sensitive to excess fuel pressure. Any pressure beyond what they are rated for, and fuel will leak past the needle/seat, drain out of the carb and wash oil out of the air filters, etc.
This doesn't occur in flight with a properly designed/installed fuel system, but it can occur while parked when the fuel system gets heat soaked. The check valve in the engine driven fuel pump prevents any back flow so the expansion of the heated fuel will start to raise the pressure in the portion of the system that is after the engine driven fuel pump, and then the carbs start leaking.
The fuel return line connects to a fitting on top of the engine that has a very small orifice in it. It has a rather low fuel flow rate at normal pressure but provides a pressure bleed off point when the engine is stopped... to prevent the problem described above.

Since the normal operating pressure is relatively constant, the small fuel flow of the return line can be compensated for with the K factor value in the fuel computer
Click to expand...

JBPILOT said:
You are saying there is a small port between pump and carbs. That would seem to indicate constant return flow then, if there is no regulator.

John Bender
Click to expand...

Yes.
As I wrote previously, their is a small port with a very small restrictor orific threaded inside it. It does allow a small amount of constant fuel flow.
 
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I dissagree

rvbuilder2002 said:
I see far to much of this type of information posted here.
"I did such and such on my plane and it worked just fine, so it should work for yours". This statement would be valid if every other RV were the same.
The only thing here is the same carb. Different engine, different fuel pump, No engine driven pump with a check valve, etc ...
Not relevant.
Click to expand...

The carb is the same..the carb does not care what engine is bolted too. My point was that the Rotax insists on using a mechanical fuel pump and then applies fixes to overcome a fundamentally flawed system.

My point was simply that there is a better way to do design the system.

In other words two electric pumps and remove blank off the mechanical fuel pump.

My system is not perfect either but is better than this setup. I have had several Rotax engined airplanes adopt my system with a perfect track record so far.

Frank
 
frankh said:
The carb is the same..the carb does not care what engine is bolted too. My point was that the Rotax insists on using a mechanical fuel pump and then applies fixes to overcome a fundamentally flawed system.

My point was simply that there is a better way to do design the system.

In other words two electric pumps and remove blank off the mechanical fuel pump.

My system is not perfect either but is better than this setup. I have had several Rotax engined airplanes adopt my system with a perfect track record so far.

Frank
Click to expand...

We all know that every design is a compromise.
Apparently Rotax has decided that dealing with the issues of a mech. fuel pump out weigh dealing with the issues and electrical system complexity to make sure there is always power available to multiple electrically driven pumps.
I tend to agree with their decision.
 
Thanks Scott

I skimmed your post too fast. I agree, should work well. I had not seen that used in my experiences. Just spoke to a friend who just purchased a new 912S, and he says it is on his. I'll have to look at it.

Thanks again,

John Bender
 
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