Return line in rotax

[K5054]

I’m relocating a fuel flow (red cube) sender in my rotax. I think the current set up is too close to the fuel pump on rotax 912ULS and pulses are giving erroneous readings. So I’m lengthening the fuel hose so it has 6 inches of straight hose before and after it (as recommended)

My return line has to be upstream of transducer (so I don’t have to buy two) but I’m wondering how far upstream is allowable.

Not sure if by having a long length of hose between the return tee and the transducer would defeat the purpose of the return line and allow vapour lock as there would be a fair distance of hose to be heated. (maybe 1200mm)

It will all be fire sleeved

[Downunder]

I don't have much straight infront of my red cube but installed about 250mm of straight hard pipe before it and it works ok.

It's also mounted fairly horizontal but with a slight incline.

As you know it should probably return to a tank, but I know a few that just return to a "Collector".

It eliminates the problem of constantly filling one tank...even when turned off.

As long as you're not just circulating hot fuel, I don't see a problem.

[pluessy]

My sender is in the line between the Facet pump and the mechanical pump (which is the longest straight line), with the return line after the mechancial pump. It is important that the return line fitting is after the mechanical pump (I think most engines have it in the 4-way joiner from the pump to the 2 carburetors). I haven't seen any installation where the sender is on the mechanical pump outlet?

[K5054]

My sender is in the line between the Facet pump and the mechanical pump (which is the longest straight line), with the return line after the mechancial pump. It is important that the return line fitting is after the mechanical pump (I think most engines have it in the 4-way joiner from the pump to the 2 carburetors). I haven't seen any installation where the sender is on the mechanical pump outlet?

How do you account for the returned fuel which would be included in the fuel flow?

[pluessy]

at normal fuel pressure (0.3-0.4bar), it is ~3lph and pretty steady (proportional to fuel pressure). I hardly ever look at the fuel flow meter anyway these days, I know I have 2h of flight time in each wing tank, so 1/2 tank is 1h. That leaves me with ~20lt after 4 hours.

I have asked JP Instruments to consider a calibration feature where you could turn the electric pump on (engine off) and the unit would use that fuel flow as default return flow. I don't think they did that

[Franky]

Another technique is to put the fuel flow transducer in line with one carby only and assume that the other carby uses the same amount of fuel, ie balanced. This means you multiply the fuel flow calibration by 2.

This works surprisingly well.

I check the calibration every 500 liters or so and the error is less than 0.5%