Use of K factor and RETRN setting - fuel flow.

[BigJohn]

Took my first real cross-country this past week and checked my Dynon fuel used numbers against actual usage for both legs.

First leg:
Dynon: 7.6
Actual: 6.1

Second leg:
Dynon: 8.1
Actual: 6.5

OK, so this is telling me the Dynon is measuring more fuel flow than is actually occuring. Acording to the Dynon manual, I am to divide the Dynon number by the actual fuel used, which I did, and in both cases this comes out to a factor of about 1.246. Instructions then say to multiply your present "K Factor" by this number, and increase the "K Factor" to the new amount. Here's the rub. My K Factor was already set to 96,500, the number I got from Tony T. and used as my original setting. After multiplication the new factor comes out to 120,239. Oops! The maximum allowed by the Dynon is 99,999.

Not quite sure where to go with this. Will Dynon increase the size of this field in a subsequent release of the software? Should I try to use the "Return" value that is currently set to zero?

John Peck
RV-12, Rotax 912ULS
D180
FT-60

7/3/11 - Changed title of thread to enable better searching.

 

[BigJohn]

Need detailed information on the RETRN parameter.

Since the first post in this thread I have done more research. It appears as though there must be a way to use the RETRN parameter to account for the fuel returned to the tank in the Rotax 912 installation. However, I am lacking a really good coherent explanation as to how the RETRN parameter works. There are some thread entries here and in the Van's forum that offer some information, but that is not specific enough for me to know how to use the RETRN parameter in conjunction with the K Factor to get an accurate fuel flow indictaion from the fuel system in the RV-12. In the RV-12 design, the fuel is pumped by an electric boost pump through the FT-60 sensor and then into the engine driven pump. After that a portion of the fuel is returned to the tank. There is no sensor in the retuurn line.

I am hoping to get a response from Dynon support on this. Mike? Anybody?

 

[trevpond]

Re: Need more digits in K Factor field for FT-60.

Hi,

You need another FT60 red cube for the return fuel. The efis will subtract one from another and hey presto, you will have the amount of fuel used.

Trev

 

[BigJohn]

Re: Need more digits in K Factor field for FT-60.

Trev,

I don't think it's quite that easy. I don't believe the D-180 has an input for a second transducer. I don't need perfection, just something reasonably close and I think (hope) there must be a way to achieve that using some combination of the K Factor and the RETRN parameter.

John

 

[dynonsupport]

Re: Need more digits in K Factor field for FT-60.

Return Flow assumes a constant flow back to the engine, expressed in terms of a hypothetical sensor that's in the system that's ALWAYS and constantly outputting the K-factor that's indicated in that setting. So when you put a number in the return flow, it's subtracting a second imaginary sensor's worth of fuel out of the main fuel reading, and the second imaginary sensor is always at the same K-factor. So it can work if there was constant fuel flow in the return flow line, which as we understand it isn't the case in Rotax installations. So the results probably won't be perfect, but it can get many installations closer to truth than having just the single sensor input. Ideally, you want a second sensor, which the D180 doesn't have the inputs to accommodate by itself, but it can be done with the Matronics fuel flow computer box.

 

[BigJohn]

Re: Need more digits in K Factor field for FT-60.

Dynon Support - Thanks for the reply and the explanation. Armed with that information, I went to the airport today and did the following:

1. Set the K factor back to 68000, the recommnended value for the FT-60.
2. Went into the RETRN field and while running the electric fuel pump, with engine off, noted the fuel flow indication.
3. Began incrementing the high end of the RETRN field. As I did this the indicated fuel flow went down. When the RETRN value reached 130000 (one hundred thirty thousand) the fuel flow indication was down to zero.
4. Ran the engine. I calculated that at 4000 RPM I should see a fuel flow of about 3.5 GPH. While holding 4000 RPM, I adjusted the K factor to achieve an indication of 3.5 GPH. This value turned out to be 79000.
5. I flew the airplane. Indicated fuel fows were very close to what I believe they should be at various power settings.

I realize this method may not yield perfect results, but I think I am pretty close - certainly closer than before with the K factor at 99999. On my next cross-country flight I will compare calculated fuel burn to actual, and tweak the K factor further as specified in the manual.

 

 

[BigJohn]

Update and new mysterious high flow rate glitch

Over the past month I made several cross-country flights, and tweaked the K factor as specified in the manual, while leaving the RETRN parameter set as above. Got it to the point where my fuel used (by Dynon) and fuel added (at the pump) were within .1 gallons. Pretty darn good, and was starting to feel confident in the Dynon readings.

Then - on my most recent trip, on climbout from Muskegon, MI, I got a high fuel flow alarm and a reading of over 10 GPH, up from the usual full throttle climb flow of 6.5 GPH. I leveled at cruise and the high reading continued. I was clearly not really burning that much gas, as the engine continued to run fine and my actual consumption was normal for the leg. When I pulled the fuse for the electric boost pump, the flow returned to normal. This is on an RV-12 where the pumps are connected in series. This happened again on the next leg. Sudden onset, electric pump off back to normal, electric pump on back to high rate.

Anybody else seen this behavior? :-?

 

[longhorn]

I've been having the same problem recently with the fuel flow.  I've adjusted the "k" factor up and down with not much change in the outcome on Dynon fuel used vs. actual.  I'm going to incorporate your solution above and see how it comes out.  thanks for the troubleshooting.

 

[dynonsupport]

longhorn - note that the previous poster was using a specific solution in order to simulate return flow to a fuel tank. If you have a single fuel flow sensor and no return fuel flow line, you simply need to use the methodology in the installation manual to adjust the K-factor.