Fuel flow meter “red cube”

[K5054]

Does anyone have a diagram on how to wire a second “red cube” onto the FC-10 meter for the return line on a rotax. The instructions are too vague

Edited April 23, 2021 by K5054

[skippydiesel]

You dont actually need a second "sender". Perhaps you just want one for the heck of it.

 

Most Rotax 91 range engines have a fixed aperture return line, the dribble of fuel that goes back to the tank can be easily accommodated in your set up menu, under total fuel used.(no need for a return line sender)

[K5054]

Thanks I appreciate your response. I’ve tried that technique before though over a year or two. I tried various k factors to accommodate the return line but the fuel flow varied too much throughout the rev range. At the moment my fuel flow is say 20lph but total with return line is 53lph. This is with the return line orifice that gives the correct fuel pressure. I have also tried putting the transducer on one carb and doubling amount. Again, not accurate. I’m not sure what else to try. At the moment I can get accurate reading by manually shutting return off in flight.

Edited April 23, 2021 by K5054

[cscotthendry]

Another factor might be the location of your sensor. If it's in a high vibration situation like mounted on the engine that might affect the accuracy.

From the numbers you quote it sounds like you're getting 33lph fed back thru the return line. That doesn't seem like what it should be. It should only be a trickle. Do you have an 912iS engine?

[K5054]

I did try a few locations in the engine bay but now it is on the floor below the tank in cool non vibration area with straight hose before and after on cabin side of firewall. It’s a 912ULS. I could try a different return orifice which would bring fuel pressure higher but still in limits. 
If I adjust k factor for cruise rpm, wouldn’t that change with altitude as power drops off making the readings only accurate for where I set it at low altitude? 

[walrus]

K5054, you need to change your plumbing so that the orifice line bleed takeoff is not between the cube and the carburettors. I made that mistake. 

[skippydiesel]

I stand to be corrected; Something is not right about your fuel delivery/return system. The fuel return is more about minimising vapour "lock" than maintain a set pressure. As Scott said your return should be a comparative "trickle". The mechanical pump has the capacity to deliver way more fuel than the 2 carburettors will ever need.

 

My system predates the use of a T fitting, with fixed aperture, having an adjustable/heavy pressure relief system. I had very good results from my fuel computer, using a hall effect turbine sensor, on the main fuel supply line (nothing on the return or boost/alternate supply).

[Franky]

Alternatively, put the “red cube” on one carby only and multiply calibration pulses/litre by 2.

Have used this on a 912ULS with return line for over 5 years.

Typically have less than 1 litre error per 300 litres of flying

Every 500 litres or so drain remaining fuel and check/reset fuel qty.

Franky

[cscotthendry]

My Legend has a weird system. It's not like Rotax specify, but it works.

Essentially, I don't have a return back to the tank. What is there is a return (fed from a restricted tap off the feed to the carbies) that feeds back upstream of the boost pump. The upstream line feeds from the fuel selector valve, essentially connected back to the tank in that fashion. Like I said, it's unconventional, but in 5 years of flying, I've never had a vapor lock and my fuel flow sensor is upstream of all that so I can measure the fuel flow with one sensor and get pretty accurate consumption figures.

To set up my FC-10, I started with their standard K factor, then measured my fuel post flight and used their formulas to adjust it. I get consumption figures that are as accurate as I can measure with a fuel dip stick.

I'm happy enough with the figures I get as I NEVER start a flight with JUST enough fuel for the duration. I always operate such that I should have about 40L left after a trip. I don't like worrying whether one of my fuel pickups will uncover and stop the engine.

[derekliston]

I have a red cube on my Dyson Skyview installation and HAVE NEVER despite a lot of effort been able to make it work!

[K5054]

I think I will sell it and install this

 

https://www.scintex.com.au/products/engine-fuel-consumption-gauge-supply-return?_pos=30&_sid=a33b01d08&_ss=r

[cscotthendry]

55 minutes ago, K5054 said:

I think I will sell it and install this

 

https://www.scintex.com.au/products/engine-fuel-consumption-gauge-supply-return?_pos=30&_sid=a33b01d08&_ss=r

Holy crap!

$1,300 worth?

And what if that doesn't solve your problem?

[skippydiesel]

2 hours ago, K5054 said:

I think I will sell it and install this

 

https://www.scintex.com.au/products/engine-fuel-consumption-gauge-supply-return?_pos=30&_sid=a33b01d08&_ss=r

Get the single sensor + gauge for $900

[Blueadventures]

3 hours ago, K5054 said:

I think I will sell it and install this

 

https://www.scintex.com.au/products/engine-fuel-consumption-gauge-supply-return?_pos=30&_sid=a33b01d08&_ss=r

Maybe ask Derek if there is a convenient time to fly into Warwick and let him look over your installation. cheers.

[K5054]

The k factor can’t go low enough to cater for the return. 

[derekliston]

1 minute ago, Blueadventures said:

Maybe ask Derek if there is a convenient time to fly into Warwick and let him look over your installation. cheers.

Not much point if I can’t get my one to work!!!

[K5054]

Mine is very accurate if I close return line off. It’s the fuel flow when return is open that it cant cope with. I would get a second transducer if I knew how to wire it into my gauge as it says you can (pin 8 but it’s not clear how to.

it would be a lot cheaper to just get another transducer 

Edited April 25, 2021 by K5054

[Blueadventures]

24 minutes ago, derekliston said:

Not much point if I can’t get my one to work!!!

My apology Derek, I miss read your post. Been working on a mates Aztec Zephr today and a bit fatigued:) But seriously no issue with aircraft just engine did not stop last flight, found broken wire and rectified and hooked up the pickup feed for his extra tachometer that he installed. Also been up at the Seaplane gathering at Shute harbour, terrific weekend and great presentations. I have just installed a red cube in a Lightwing so will be interested in how it goes, when his annual and rubber replacement is finished.  Cheers.

[cscotthendry]

6 hours ago, K5054 said:

Mine is very accurate if I close return line off. It’s the fuel flow when return is open that it cant cope with. I would get a second transducer if I knew how to wire it into my gauge as it says you can (pin 8 but it’s not clear how to.

it would be a lot cheaper to just get another transducer 

I just reread the fc10 manual. It seems like adding a second sensor should be pretty easy.

The cube has 3 wires, 2 are for powering the sensor and the other wire is the signal back to the FC10. The second cube gets its power from the same connections that power the first cube ie, their power connections are paralleled. Then the signal wire for the second cube connects to pin 8 of the FC 10.

But now you have two k factors to adjust. One for the supply sensor and one for the return sensor. The K factor is like a “slippage” or fudge factor for the sensor. 
Since you say you get accurate numbers by turning the return off, I'd guess you have the K factor about right for the supply sensor. If you mount a second return sensor near the first one and in a similar orientation, you could use the same K factor for the second sensor as a starting point.

By leaving the number of engines set at 1 in the FC 10 setup, I'm guessing that it will subtract the second sensor's readings (adjusted by its K factor) from the first sensor's readings.

HTH

[K5054]

Actually I had a read again and I think you are right. I have to add another pin to the gauge plug. 
I was wondering if I just pigtail the power from the existing wires and you think I do 👍

I also am guessing that leaving it as single engine makes it subtract 👍

it means I can save a lot as a second cube is only about $250

I will try it out. I’ll let everyone know how it goes. 
thanks for the help 👍👍

[cscotthendry]

5 hours ago, K5054 said:

Actually I had a read again and I think you are right. I have to add another pin to the gauge plug. 
I was wondering if I just pigtail the power from the existing wires and you think I do 👍

I also am guessing that leaving it as single engine makes it subtract 👍

it means I can save a lot as a second cube is only about $250

I will try it out. I’ll let everyone know how it goes. 
thanks for the help 👍👍

Good stuff.

I still think your return quantities bears investigating. From the figures you quoted, it sounds like more fuel is bypassing the carbies than they're consuming and that doesn't sound like it should to me. I'd check that the restricter is there, that it is the correct size, and is in the correct path.

BTW, the local Rotax dealers suggest a carby jet insert as the restricter. From memory, it is an M5 thread and will fit nicely in the end of a brass T that has been threaded with an M5 tap. Rotax sell a cross connector with the built in restricter, but it's very expensive!

Good luck.

[K5054]

I tried a number of different size restrictors in return line starting with the recommended size but that back pressure pushed the fuel pressure too high, so I started increasing size incrementally until it was within limits. This ended up being quite a large bypass hole. Recommended factory return line restriction is 0.35mm

mine is 1.5mm in order to keep fuel pressure  in limits which is over four times the normal. On the up side, there is less chance of getting a blockage 🤔

Edited April 26, 2021 by K5054

[Thruster88]

1 hour ago, K5054 said:

I tried a number of different size restrictors in return line starting with the recommended size but that back pressure pushed the fuel pressure too high, so I started increasing size incrementally until it was within limits. This ended up being quite a large bypass hole. Recommended factory return line restriction is 0.35mm

mine is 1.5mm in order to keep fuel pressure  in limits which is over four times the normal. On the up side, there is less chance of getting a blockage 🤔

The fuel pressure produced by an engine driven diagram pump is done by the internal spring, it cannot be to high if it is the correct pump. Are you sure of the accuracy of your instrumentation? 

[K5054]

The engine is 2018 so the pump is new and correct. The fuel pressure gauge is and old one direct reading though. Can’t really tell how accurate it is. It takes its pressure from Y fitting just before the 1/3 carb

[aro]

3 hours ago, K5054 said:

I tried a number of different size restrictors in return line starting with the recommended size but that back pressure pushed the fuel pressure too high, so I started increasing size incrementally until it was within limits.

The restrictor and return line is not there to adjust fuel pressure.

 

If your fuel pressure is incorrect you should find the real reason why.