Dynon "RETRN FLOW" parameter...

The Dynon Installation manual for the D180 says to adjust the K-factor for the fuel sensor to make the fuel computer accurate and gives a way to calculate an adjusted K-factor. This seems to work. However, with the Rotax engine the the fuel flow section of the on-screen menus includes a 'RETRN FLOW' parameter. This is set to '0' by default, as there are Rotax engine installations that do not have a fuel return line. The RV-12 does have a return and I am interested in using the RETRN FLOW parameter to get an optimum setting for the K-factor.

I searched this forum and found several threads regarding fuel flow and fuel return but nothing specific regarding the RETRN FLOW parameter. The Dynon manual only states that this function is generally used only by Original Equipment Manufacturers. Hmmm.

I then searched the Dynon Avionics forum and found one post by Dynon Support that states as follows:
"You set the RETRN FLOW in "K" and it subtract this "K" from your flow sender.
If your K factor is 30,000, that means 30,000 pulses per gallon.
If you expect a constant return to the tank of 1.5 GPH, then you would set this to 45,000 (30,000 X 1.5)
This will only work well if you have a fixed return orifice size and a relatively stable fuel pressure."

OK, this explains the RETRN FLOW parameter, and I think it is applicable to the RV-12. The only question is , How much fuel is bypassed back to the tank? I am guessing it is about 2 GPH. The Dynon suggested K-factor for the red cube sensor is 68,000, and with the RETRN FLOW at default '0', I have had to set my K-factor to about 96,000 and still not happy with the result. You can only go to 99,999 with the K.

A better solution may be to set the RETRN FLOW parameter to 136,000 (68,000 X 2) and then further adjustments to the K-factor would follow.

I could be over thinking this. There are over 90 12's happily flying around and no one has brought this up. I guess all those fuel computers are working just fine.

Tony

I have a switch on my electric fuel pump and use it for take off and landing only. I have had to adjust my "K" factor differently from other -12. After 200 hours the Dynon is lazer accurate to show the fuel I have burned. Just set it amd forget it.

Fuel return flow

How much fuel is bypassed back to the tank?

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With the electric pump on and the engine off, all of the fuel is being returned to the tank. Under these conditions, you could set the 'RETURN FLOW' parameter so that the Dynon fuel flow readout says "0". When the engine fuel pump is running, the pressure will go up and so will the return flow. Who knows how much? There is probably some formula (similar to ohms law for electricity) to calculate flow rate with different pressures. It will be interesting to hear about the results of your experiments. It might not matter how the Dynon is set up as long as the end result gives accurate fuel flow. Like Larry said, "The Dynon is laser accurate." Thanks for pointing out this parameter setting that I was not aware of.
Joe

Fuel Flow

I guess I flew too many "unreliable" fuel gauges in my life so I really don't even pay attention to any of them on the 12. I know from the fuel I put in that it burns around 4.7 gph from chock to chock. So if I have 20 gal on board i fly two and one half hours to three hours (15 gal) and refuel. Besides, I have to stop and p** anyway (stop drinking diet cokes? never!). Like Larry says, its been 200 hours and working just fine.

Update...

It may be that the RETRN FLOW parameter is only useful if there is a second flow sensor in the system to measure return flow. The Dynon support post that I researched above sort of implies that is not the case, but I have posted the question to Dynon so as to confirm or not.

Tony

Way to much over thinking!

Some body needs a new project!

Jetguy said:

Some body needs a new project!

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John, you made me laugh , what project, do you think? How about a tailwheel light sport? I do kinda like the Highlander.

Tony

Ok heres one for ya!

Make us some molds for transition fairings for the main gear pans and some for where the main struts go into the fuselage. As well as the where the steps go into the fuselage. Also while your at it make us a fairing for the main gear struts. Hows that sound for a project?

Good ideas...

Jetguy said:

Make us some molds for transition fairings for the main gear pans and some for where the main struts go into the fuselage. As well as the where the steps go into the fuselage. Also while your at it make us a fairing for the main gear struts. Hows that sound for a project?

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But, Oh my, that's fiberglass work .

Tony

Hey, that is exactly MY list of needed efficiency mods! I read recently about the drag of our landing gear legs in a wind tunnel, before and after fairings, dramatic improvement, I would never have guessed they were so draggy. We need to split the list, you work on one part, I will do the others. I love fiberglass work!

Jetguy said:

Make us some molds for transition fairings for the main gear pans and some for where the main struts go into the fuselage. As well as the where the steps go into the fuselage. Also while your at it make us a fairing for the main gear struts. Hows that sound for a project?

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Return Fuel Flow

Tony_T said:

It may be that the RETRN FLOW parameter is only useful if there is a second flow sensor in the system to measure return flow. The Dynon support post that I researched above sort of implies that is not the case, but I have posted the question to Dynon so as to confirm or not.

Tony

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Tony I would be interested in any replies you receive as I am Contemplating either alternative UL 260 or Viking HF-110 Fuel Injected Engine Installations. As both these engines are FADEC with HIGH Fuel preasure & Flow in the Fuel Supply & Return lines to the tank. Im picking they will require 2 Flow flow sensors to calculate accurate Fuel flow Indications on the Sky View Screens. Any feed back from fellow Builders/Flyiers/ Engineers/ whatever would be greatly Appreciated.

Happy Flying John H.

I recall Jan saying something about the fuel return line in the RV-12 not being used at all if I remember correctly. He also strongly disliked the placement of the gascolator and suggested to not use it and to put a drain valve somewhere below the tank and behind the firewall to drain any debris or water before flight.

Dynon response to RETRN FLOW question...

I received a response from Dynon on the question about the return flow function. I'll post the question and the answer for anyone interested. You can also read the Dynon forum here:

My question in the Dynon thread:
I would like to resurrect this thread to ask if the RETRN FLOW parameter is only applicable if there is a second flow sensor in the system to measure return flow?

The response:
Re: Return Flow function
Reply #4 - Today at 4:40pm Quote No, it's to there to simulate a second fuel flow sensor being there on a EMS-D10/D120/D180 that always outputs the same theoretical fuel flow rate (not really what happens in the real world in many conditions, but it can be passable in some configurations). With those products, it is possible to measure the difference between the main fuel flow sensor and a 2nd actual fuel flow sensor for return flow via a Matronics box. SkyView can do it natively.

This indicates that the RETRN FLOW function is there to "simulate" a second fuel flow sensor in the return line.

As to any question of whether the fuel return line is actually used in the RV-12, you will see when you install the fuel lines in the system that the only restriction to flow in the return line is a small orifice in the fitting that attaches to the return plumbing.

My only reason, really, for going through all of this is that my FT-60 K-factor adjustment had to be so high that I may have to adjust it higher than what it can be set at. Using the return flow function would solve that problem.

Tony

Duel fuel flow

Tony_T said:

I received a response from Dynon on the question about the return flow function. I'll post the question and the answer for anyone interested. You can also read the Dynon forum here:

My question in the Dynon thread:
I would like to resurrect this thread to ask if the RETRN FLOW parameter is only applicable if there is a second flow sensor in the system to measure return flow?

The response:
Re: Return Flow function
Reply #4 - Today at 4:40pm Quote No, it's to there to simulate a second fuel flow sensor being there on a EMS-D10/D120/D180 that always outputs the same theoretical fuel flow rate (not really what happens in the real world in many conditions, but it can be passable in some configurations). With those products, it is possible to measure the difference between the main fuel flow sensor and a 2nd actual fuel flow sensor for return flow via a Matronics box. SkyView can do it natively.

This indicates that the RETRN FLOW function is there to "simulate" a second fuel flow sensor in the return line.

As to any question of whether the fuel return line is actually used in the RV-12, you will see when you install the fuel lines in the system that the only restriction to flow in the return line is a small orifice in the fitting that attaches to the return plumbing.

My only reason, really, for going through all of this is that my FT-60 K-factor adjustment had to be so high that I may have to adjust it higher than what it can be set at. Using the return flow function would solve that problem.

Tony

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Hi Tony
So Im trying to get my head around Dynons answer so for the HIGH fuel flows of a Fuel Injected engine( READ HF-110 or UL Power Si260) both large bore fuel lines (Suppy & Fuel Return ) require seperate fuel Flow sensors that Total Fuel flow can be read by a Skyview? I cannot seem to work this one out & as SV cannot read a Injector Pulse rate output for fuel flow I my have to abandon the Dynon for MGL Avonics EFIS ...Pity.!