Zenith Aircraft Builders and Flyers
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912is return line
Has anyone had experience with installing a 912is in a CH 650. There is an excellent video by Sky Tech and Rotax for installing the 912is in a 750. The 750 uses a header tank which would be overly complicated in a low wing 650, as gravity can’t be used to keep a header tank full. The video makes a big deal about gaseous bubble being returned to the engine via the supply line.
I’ve gotten as far as using a duplex fuel switch to switch both supply and return line from wing tank to wing tank but what needs to be done to insure gasses will not be returned to the engine.
Thanks
Replies to This Discussion
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Permalink Reply by Matt Pope on
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As long as the return lines enter the fuel tanks above and away from where the fuel drains from the tank to the engine, I don't think you need to worry about it because the gases in the returning fuel will rise to the top of the tank, so in theory you'll always be drawing bubble-free fuel from the bottom of the tanks... at least that's my understanding. In the 750's header tank, the fuel outlet and return ports were very close together and the tank volume is small, so it was more of a risk.
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Permalink Reply by Guy Magnuson on
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Hi Matt,
I appreciate the response. I saw some videos from people installing return lines in RV7s. They tended to run the return line two or three bays outboard of the supply line or all the way to the outboard end. The RV7 tanks are not welded tanks like a 650 leading edge tank so the tanks were open.
I only want to do it once. I am surprised the return line location has never been tested by Zenith. Aeromomentum and Rotax fuel injected engines require return lines and are used in 650s, and most likely ULPower fuel injected engines also used returns lines. Only Viking says you don’t need one?
Per the Rotax / Sky Tech video the consequences of bubbles in the supply line is pretty much a certain emergency landing. Sky Tech indicated they couldn’t figure out how to do it. I think that may have been a case of market demand for bush planes over low wing aircraft so just not worth the R&D like they did for the 750. All the rest of the install they did for a 750 to keep rotax components in spec works for a 650.
I get it, hot air rises and so should hot fuels but I would feel much more comfortable about some design and testing rather than guessing it will probably work ok. Or at least the experience of someone who has done it. The fuel pumps are sucking out fuel at 43 psi. I would have a tendency to put in some sort of baffle between return inlet and supply outlet to insure bubbles go the right way. Then again I was in three emergency landings in my first 60 hours. One was caused by a fuel system failure, an often overlooked but very important design parameter.
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With the return line plumbed to the far side of the tank from the supply line (or on the same side but, say, 12" away) then there would be plenty of space between them for warm returning fuel to mix with cool fuel in the tank, and any gas bubbles would quickly condense back into liquid or rise above the outlet level, so I don't think a baffle would necessarily help, but to ease your mind it might be worth talking to a local EAA technical advisor (join EAA if you're not a member) or a Designated Airworthiness Representative (DAR) about best practices. They will have seen many different setups and you'll be able to trust their advice more than advice from some random guy on the internet :)
I know the mental anguish of designing your own fuel system... I have spent dozens of hours on mine!
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Permalink Reply by Eric Genzlinger on
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Viking has the fuel pump and regulator inside the header tank, so the fuel return is also in the header tank. This is why auto’s don’t have vapor lock, no hot fuel returning to the tank. Check out the 601/650 Viking install videos on YouTube. He explains the EFI fuel system real well. It should apply to all EFI engines.
I am NOT a Viking customer, I have a Jabiru with a carb. But I am interested in EFI systems.
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Reviving this thread, how does the Sling 2 handle the return line?
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The Viking system has the fuel pumps and regulator in the header tank, therefore the return is in the header tank already.
Look at the Viking installs for 601/650 on you tube. He uses a micro header tank mounted in the same position the gascolater goes but under the fuselage.
Heck you could use the micro Viking header tank which already has the fuel pumps, regulator and return inside. Just run one fuel line from the header to the engine.
I am not a Viking customer, I have a Jabiru but I was curios about how to instal EFI on a 601/650 low wing.
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Do you mind posting it?
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The installation video is informative. The header tank is mounted under the fuselage so the wing tanks gravity feed into it. The header tanks have the automotive fuel pumps (43psi) and regulators inside. This is a typical auto design except autos only have one pump and regulator in the tank. I would think this setup could be used on any fuel injected engine like a Rotax or UL. The external fuel pumps are heavier and use about 6 amps each while these auto fuel pumps only use 2 amps each. Viking sells these header tanks on their website with all the components.
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Also, no fuel selector valve. If you want to shut off the fuel you shut turn the pumps off.
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