To return or not to return

[Dorsal]

Looking for comments on the value of a fuel return line on a Rotax 912s. My understanding is that it allows more fuel flow through the line to help prevent vapor lock. Assuming one is burning 3-6 gal/hour how much more flow do you need?
All comments, controversy or suggestions appreciated.

[Slyfox]

I don't have a fuel return. I just put in a new motor, saw all the lines to put in and said, forget it, the other engine never had it. I did have some issues until I balanced the prop. Now the engine runs like a purring kitten. I don't have any firesleeve. I figure the engine area is running real low on temps, unlike a lycoming. This is a water cooled engine and the head temps are around 180 all the time, at least on mine. Firesleeve is an overkill in my opinion. What do I know, I only have about 1500 hrs on my fox. I will NOT put clear hose or bing blue hose in my engine area, it's all auto fuel injection hose. This stuff is real good and last a long time. My oil hoses are 1/2 300psi double braided hose, no firesleeve and doing great. I pull the cowls on a regular bases, about every 10hrs and do an inspection on everything, no problems.

[cap01]

red high temp rtv thinned with mek workes well also to seal up the ends of the fireseal

[akarmy]

For those thinking about a return line on the fuel system. I also did some research as to the fittings & restrictors needed. You can see the details on my site if needed. There's nothing amazing about it, just by the book rotax setup, just wasn't clear to me at first.

http://flying.karmy.com/?p=309

[Slyfox]

I was thinking of taking all my stuff and put it on, so my question, where do you put it into the header tank, could I just tee into one of the fuel lines going into the tank or do you go into the vent line up twards the top of the vent where there is no fuel.

I got all the necessary fitting for the engine side, just need to know where to put it into the header tank.

[akarmy]

Here's how the new header tanks do it. The fuel return is the top right hose fitting with the 90 elbow on it. Basically it just dumps the return into the top of the header. I would think that's very similar to using one of the vent lines.

http://flying.karmy.com/wp-content/uploads/2010/02/20100207-P1010031.jpg

[Slyfox]

looks like if I take a hose and put a tee on it will work.

[Dorsal]

Great! I had just finished convincing myself not to worry about the return line, spent last night re-doing the fuel system to include it. The annoying part was relocating the fuel flow transducer so it is after the return line. I have been going round and round in my head on this issue (a little obsessive sometimes), now it is done and I am sure in some small way safer. I do not have the banjo bolt setup so I created my own manifold, will post a picture later for those who are interested.

[Dorsal]

For those that do have a return line do you use an in-line check valve? It occurs to me that even with the fuel valve shut off the engine side will still see the head pressure of the tanks through the return line.

[hansedj]

Yes we found that same problem, so yes we put a one way check value right before returning to the header tank.

[tucsonchris]

I'm debating this same issue. Here's what I'm confused about.

If my header tank is full--which it would be all the time unless I was minutes from fuel exhaustion--then I have continuous gravity pressure trying to make the fuel flow from the tanks, to the header, and then to the engine. the fuel doesn't care if it travels through the bottom fitting or the top fitting of the header. Having said that, there would be constant back pressure to the fuel return line. But the fuel return line is supposed to dump vapor (and I presume small amounts of fuel, into the header--which it cannot do because it is being met with the fuel coming FROM the header. Now I'm guessing the fuel pump provides extra pressure, but I've seen the little jet inside the banjo bolt through which the fuel and vapor are supposed to travel, and I can't imagine there is much pressure left after that.

Any ideas?

[Dorsal]

The pressure from the pump will be ~4-5 psi, the head pressure is ~1 psi, with the engine running you will get a small amount of flow through the restrictor back to the header tank. My understanding is that this provides a more continuous flow of fuel through the lines when idled down on a long decent on a hot day. This flow is to help prevent the fuel in the lines from vaporising over the hot engine by introducing a steady supply of cooler fuel. Not sure how much this all matters but I believe this is the theory behind the return line.

[Dorsal]

One thing I do on my 912-S that helps a lot is have Firesleeve on all the fuel lines. Firesleeve is an excellent insulator and will probably do as much or more in keeping the fuel from getting hot and vaporizing than the return line will.I agree, I do the same just for that reason, then heat shrink over the ends to keep oil and stuff out of the fire sleeve. I personally don't think the return is necessary but I put it in anyway, couldn't see the harm in it.