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Old 01-20-2021, 09:40 PM
PaulvS's Avatar
PaulvS PaulvS is offline
Join Date: Aug 2019
Location: Western Australia
Posts: 558
Default Rotax fuel return

I realise you have a Lycoming, but it may be of interest to look at Rotax 912 documents and parts for their recommended fuel system design for that engine. There is a standard restrictor part, I think it may be .040, that goes into a tee after the fuel pump and returns a portion of the fuel back to a tank.
Paul vS (yes I'm also a Van)
Building RV-6A #22320 O-320 FP. Wings and tail complete, working on fuselage
Flying my Aeroprakt A-22 STOL and the aero club's RV-9A while I build
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Old 01-21-2021, 10:05 AM
Richard RG Richard RG is offline
Join Date: Jan 2021
Location: British Columbia, Canada
Posts: 21

Thanks for the feed back Paul. Once I open up the fuel system and see what is or is not installed, will then have a better idea how best to proceed. There is a Rotax dealer in my home Province, so may be able to source a restriction orifice from them. I'd be ok with recirculating fuel if I knew exactly the flow rate and remained the same rate regardless of power setting.
Also, having an isolation valve on the recirculation line could be useful. Being able to isolate the return line and operable in flight could be useful if I ever decide to burn mogas again. Mogas is appealing due to the fact it burns cleaner than Avgas and always a good thing to reduce operating costs if doesn't compromise safety.
As others have done, Avgas in one tank for takeoff, climb to altitude and landing. Switch to mogas at altitude. In order to do this, I would have to isolate the recirculation line. If there are any indications of vapor lock during flight and turning boost pump on doesn't remedy the problem, could have option number 2, opening the valve on recirculation line if its within reach...
Think John Denver. All good things to think about. Again, thanks for the feedback.
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Old 01-22-2021, 07:58 AM
Snowflake's Avatar
Snowflake Snowflake is offline
Join Date: Oct 2009
Location: Sidney, BC, Canada
Posts: 4,114

Congratulations on the new (to you) RV-3B. I've seen the plane many times since the builder finished it.

The builder was an MD-RA inspector if I recall, so would have had a lot of exposure to many different types of amateur-built aircraft over the years. So it's very strange that he would have chosen to do something so fundamentally wrong as to plumb an unrestricted return line into only *one* tank.

The only way I could see how this would make any sense is to have a plan to always fly on the tank with the return line, until it was empty. Then switch to the other tank, and fly along while using half the gas in the engine and the other half gets pumped into the empty tank? No, that can't be it either.

I know you are thinking that there must be a reason for this return line being installed how it is... But know that very few RV's have a return line at all on a carbed installation. I'd venture to say that yours is the *only* RV in Western Canada with one.

I would strongly recommend removing the return line and capping the ports. Or at the very least, put a valve in the line, and close it. If you ever have a problem with vapour lock on a hot day, open the valve, switch to that tank, and get started, but close the valve once things are running again.
Rob Prior
1996 RV-6 "Tweety" C-FRBP (formerly N196RV)
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Old 01-22-2021, 08:21 AM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,873

Originally Posted by Richard RG View Post
Thank you for your reply and suggestion, re drilling out an orifice to 0.040 into JB weld applied to the interior of a prepared AN fitting of the fuel recirculation line. Though I've used JB weld, or Permatex metal epoxy in quite a few applications, think that it not wise to use this approach in an aircraft fuel system.
The thought of potential particulate breaking free of the JB weld is not appealing, due to fuel incompatibility or erosion, best case scenario being scoring of piston rings and cylinder walls. Worst case scenario, being blockage of gascolator or plugged jets?
I appreciate the feedback and am waiting (almost 2 months now) for ACS to deliver a new fuel valve. Hoping I might find a restriction orifice in the fuel return line to the left tank when I open up the fuel system. Thanks for taking the time to offer your suggestions.
Yes, I agree there are risks, which is why I suggested that solder is a safer choice. Just giving options, as many like to make their own decisions. Also, I mentioned that an .040 orifice worked for MY application, which is 25 PSI. You will need to calculate an orifice size based upon your PSI and desired flow rate.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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Old 01-22-2021, 08:28 AM
rv8ch's Avatar
rv8ch rv8ch is offline
Join Date: Feb 2005
Location: LSGY
Posts: 3,670
Default snubber or pulsation dampener

Might also be able to find a snubber of the right size. They are also sometimes called "pulsation dampeners".
Mickey Coggins
"Hello, world!"
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Old 01-22-2021, 09:55 AM
Richard RG Richard RG is offline
Join Date: Jan 2021
Location: British Columbia, Canada
Posts: 21

Hello Rob,
The builders name was Hal and yes, he was an MD-RA an AME and I think had a few other credentials as well... very interesting and nice man.
I'm thinking he installed the return line for a reason. Its obvious in the build that weight and simplicity were at the forefront of his mind when he built the RV3, so must have added weight and complexity for a reason?
Since discovering the return line, I now do as you suggest when managing the fuel, though not running the left tank empty before switching. Have only run the left tank down for 30 minutes before switching and was obvious I switched too soon.
It's lucky I was doing relatively short local flights and watching the fuel situation very closely. It seems I had been pumping fuel out the left tank vent, all the while scratching my head trying to figure out the odd fuel levels and higher than expected fuel consumption. It appears I (we) hadn't done as good a pre-purchase inspection as I should have.
I am currently sourcing an isolation valve to install in the return line and will be checking for a restriction orifice.
The plane is based in Sechelt, PM me if you'd like to stop by, we have good coffee in the club house. Thanks for the reply

Last edited by Richard RG : 01-25-2021 at 10:38 AM.
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Old 01-22-2021, 11:14 AM
mburch's Avatar
mburch mburch is offline
Join Date: Feb 2005
Location: Oregon
Posts: 1,256

If you have any pictures of the return line plumbing, I'm sure others would be as interested as I am in seeing them. Something very unusual going on with this aircraft.
Matt Burch
RV-7 (last 90%)

Any opinions expressed in this message are my own and not those of my employer.
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Old 01-22-2021, 11:51 AM
Richard RG Richard RG is offline
Join Date: Jan 2021
Location: British Columbia, Canada
Posts: 21

I'm still waiting for the replacement fuel valve. Once it arrives, I'll remove the cowling and look much closer again at the fuel system. If I can figure out how to, will post some photos. Cheers
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Old 02-02-2021, 09:58 AM
Richard RG Richard RG is offline
Join Date: Jan 2021
Location: British Columbia, Canada
Posts: 21
Default Fuel return line

I've attached links to a couple of photos of the fuel return line. Not the best photos. The return line is the lower fuel line without fire sleeve and can be seen returning the firewall bulkhead fitting. This line returned fuel only to the left tank. I did find a restriction orifice in the return line and with my moderately accurate measuring method appears to be a .046" hole to restrict fuel flow back to the left tank.
Using the fuel boost pump to empty the remaining fuel in the tanks, pumped for 30 seconds into my wife's favorite measuring cup, through the fuel return line. Thirty seconds of pumping yielded 200 ml. I extrapolated the 200 ml to be 24 litres, or 6.35 US gallons per hour being returned to the left tank. I expect that during normal operations the recycle rate would be higher as there was very little fuel left in the tank when this rudimentary test was done. Greater fuel head pressure to the pump would likely increase the pump throughput.
I suspect that the fuel return line was installed to provide cool mogas fuel to the FWF fuel system and reduce the chance of vapor lock.
I've opted to remove the recycle line and at the same time fire sleeved fuel lines FWF. For the time being I want to keep fuel management simple and use only avgas. Possibly in future I may experiment with the fuel return line and using mogas, but would opt for a smaller diameter orifice than was originally installed. I think the system has merit, but would require accurate and trustworthy fuel gauges.
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