Fuel Return

Re: Fuel Return

by Ian Watt » 9 years ago

Ian Watt

7 18 0

Hi Phil
I am just finishing a Sling 2 with the Rotax 912is Sport (low wing with no header tank).
I have been running the engine this week and it has not skipped a beat - very smooth.

You do not need a header tank and in fact with a low wing aircraft there would be all sorts of plumbing and pumping issues with a header tank. With a high wing it is much easier.

However, you do need to make sure you don't get any air into the system, or if you do, the system is designed to purge it very quickly for an engine restart.

Make sure your feed and return lines are at least the minimum size specified to ensure high volume circulation. I used AN-6 lines.
Also, I would ensure you have the bypass orifice as specified in the manual. This is positioned just after the fuel pump and allows any air to bypass to the return line before entering the engine. It also means the pressure can be released from the rail.

We were doing some testing this week (engine not running) and had the bypass orifice blocked off. We pumped some fuel which all worked fine then switched to an empty tank. Air was quickly drawn into the system. We turned the pumps off. As the bypass return orifice was blocked some residual pressure stayed in the engine (20psi from memory). When we went to pump fuel again, with either one or both pumps on, th fuel didn't get to the engine as it couldn't overcome the pressure in the engine.

When we opened the bypass orifice, the head pressure was relieved in about 3 seconds and the engine had fuel again when turned to the tank with fuel.

So, I think if you design your system to reduce the possibility of air going into the system, or rapidly purge it if it gets into the system it should be fine.

Ian

Re: Fuel Return

by Phill Hooker » 9 years ago

Phill Hooker

7 17 0

Thanks Ian, I have done over 120 hours now in the two machines(sportscruiser) for an aircraft manufacturer and no problems encountered. I imagine air bubbles generated after the pressure regulator as depicted in the skytech ch750 just go back to the tank via the return line and vented out. Do other high wing aircraft with the Is engine have header tanks or is this an unnecessary gimick?

Re: Fuel Return

by Joe Harrington » 9 years ago

Joe Harrington

1 26 0

Guys,
I am just finishing up my 912iS Zenith CH750 so I can give you some insight on the need for a header tank. The reason that the header tank is desirable in the CH750 is that the wing tanks are large and flat-bottomed. Also the outlet port is out the back of the tank rather than the bottom so even in level or coordinated flight, it is entirely possible to start sucking air with 8 or 10 litres of fuel in the tank. Uncoordinated flight could even be worse, so the header tank allows one to maximize the fuel in each tank rather than flying around with 15 or more litres of unusable fuel. So aircraft that have tanks that have more positive outlet drains and have dual port fuel selectors should not need header tanks. I hope that makes sense.
Cheers,
Joe

Re: Fuel Return

by Phill Hooker » 9 years ago

Phill Hooker

7 17 0

Thanks for that Joe, answers my question perfectly. I notice in that video there is no by-pass for the fuel filter which is a requirement by the Rotax 912is installation manual. Any reason why not needed for the 750?

Re: Fuel Return

by Iannis Pardon » 9 years ago

Iannis Pardon

0 2 0

Indeed, the bypass seems to be after the fine filter in the skytec design. In the rotax manual, the bypass is before the fine filter. I notice that there are no checkvalve in the video too.