Microlighters

Hi,

I have a Aquilla trike with a Rotax 912 motor. As Instruments I had the Stratomaster Extreme X MKII fitted.

Presently I am unable to get accurate fuel flow readings.
The fuel flow sender fitted is the unit supplied by MGL Avionics. The low range jet has been inserted. The Installation is that typical of the 912 motor. The path of the fuel is: tank > Mercedes Filter > mechanical fuel pump (Pierburg) > T piece for return flow to tank > fuel flow sender > T piece to individual carbs. Plastic fuel lines have been used.
With this installation, the indicated fuel consumption is about 19 liters/hr with the motor idling, and a bit above that with cruise power. After having studied the fuel flow sender instruction manual, I assume that I have a pulsating problem.
The only way that I can get a realistic fuel flow reading is if I install an impractically long fuel line, and block the return line to the fuel tank. Installing a fuel filter behind the fuel pump also did not solve the problem. Of interest is the fact that the fuel flow readings are affected if the return line to the tank is blocked, even though the T piece where the return line branches off, is situated before ie. upstream of the fuel flow sender.

In the instruction manual the installation of a pulse damper or accumulator is recommended with a pulsating pump. Does anybody know how I could build a pulse damper, or accumulator, or where I could source one.
Would using rubber hose, instead of plastic hose help solve the problem?
Would increasing the diameter of the fuel hose from 8mm to 12 mm help solve the problem?

I e-mailed Rainier of MGL Avionics, and he suggested that I fit an additional fuel filter downstream of the fuel pump. Having two fuel filters downstream however does not sound practical to me.

Any other suggestions will be highly appreciated.

Regards,
Arnulf

I have installed the ff sensor in the following seq on a 582; fuel tank > fuel filter > FF SENDER > pump (the fuel line from the sender to pump is +- 40cm rubber hose)

Been using it for 5 months now and found it very accurate at 12 l/h on average.

Hi siasd,

Thanks for the answer.
The installation you have will not work on the Rotax 912, as there is a return flow from the fuel pump to the tank. So if the fuel flow sender is installed before the branch to the return line, it will register the pump flow, but not the flow to the carbs. The question is if it is safe to block the return flow.

Regards,
Arnulf

I bought a 2nd hand 912 motor. When I stripped it out of the old plane I did not see any fuel return connection. I also looked at a 912S installation yesterday and this is how he has it as well. From tanks to t-piece, single outlet to the fuel filter, install Fuel Flow meter, between filter and mechanical fuel pump.

Thanks for the info Morph.

Regards,
Arnulf

Hi Arnulf
Yes you do need the return fuel line. The purpose of the return is to vent any excess pressure produced by whatever pump or pumps you have in the system back to the tank, thereby preventing the excess pressure from overcoming the float valves in the carbs.
Do you have the required jet in the return line that restricts the return flow? If so is it the right size? The one on my system is about the size of a main jet, just a bit of drilled brass pushed into the line snugly. The best way to check is to either copy one on a system in use or else measure the fuel system pressure if you have access to a guage, should be about 4-5psi if memory serves me right. The Rotax maintenance manuals available to download on the net have all the right gen.
Hope this sorts it out.
Rick

Hi Ricky,

thank you for your answer.

At the moment I have blocked the return flow to the tank, and the fuel flow sensor gives a realistic reading.

I have in the mean time done some research on the fuel flow return line, and its purpose, and fully agree with you. I will also try and find a solution so that my system works with a return line.

As you correctly state, the return line serves two purposes:
1.)To purge the system of hot fuel, to prevent vapour locks. This is more critical if the fuel line runs under a cowling in a fixed wing aircraft, than in a trike, where the fuel line is open. I have been told that it is a legal requirement to fit a return line in a fixed wing aircraft, however I have not verified this info. Perhaps the builders of fixed wing aircraft could give more reliable info on this aspect. Rotax cover themselves by stating a limit of 36°C for the fuel temp. A bit unrealistic I would say in summer. Must I put my fuel in a fridge?

2.) to prevent excess pressure in the fuel system. Rotax gives a limitation of 0.4 bar fuel pressure to the carbs. The standard mechanical fuel pump delivers a pressure of 0.3 bar. So I think that I am safe in this respect. I am considering to fit an auxilliary electrical fuel pump, then I definately will fit a return line.

I had a flow restrictor in the return line. But possibly the jet was to big. The jet size was 2.5 mm. I will experiment with smaller jets.

Regards,
Arnulf