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Red Cube Fuel Flow K Value

Vantastic12

Member
Patron
Greetings Fellow RV-12'ers.

Just completing Phase 1 testing on N850VH. 14.5 hrs. flying, apx. 20 on the Hobbs. My SkyView touch fuel flow is indicating about 7.5 GPH in cruise, but it's actually only burning less than 5 GPH. Obviously with only one Red Cube, it is only measuring fuel supplied to the engine, not the unused fuel returned to the fuel tank. The Dynon manual says the default K Value is set at 68,000 and to increase it if the GPH is reading too high. Van's says to adjust it up by 10,000.

Question: what is the K Value set at in your RV-12?

Thanks for any responses.
 
I've been fiddling with k-factor on my SLSA.
It came with the upper fuel flow number at 98000 and the lower 68000. I believe one setting is 19 and one 14.

Comparing fuel loaded against fuel burned as well as monitoring the EMS and Moeller gauge I figure my burn is 4.8 gph at 5300 rpm and 4.4gph at 5000
I had the top number set at 87750 and left the lower one at 68000. This brought my fuel flow to read what I was physically burning and showed about 6.2gph for after take off climb and about 2.9 at idle with around 3.8 doing run up at 4000 rpm.

Then of course today at 7500 ft and 5000 rpm on a hot day the cruise burn at 5000 rpm was reading high. I tweaked it up and got it closer to where it had been.

Just my fumble fingered response to your question and ready to learn more about the error of my wYs!
 
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I've been using 90000 for both. Seems to be quite accurate block-block but not necessarily correct at all power settings throughout the flight.
 
I wonder if the mechanism sending the pulses to the fuel flow needs a little time to settle down for a period after say a high power climb to 7500' where I then reduce to a cruise setting of 5000 rpm.

After tweaking a little today I did get a fuel flow indication where I thought it should be as well as shut down fuel very close to where it should be.
 
The single red cube in the RV-12 measures total fuel flow (engine consumption plus returned fuel). While there are two fuel flow adjustment settings possible in the Skyview, only one of them is used for the RV-12 since there is no "return flow" sensor input source. On my RV-12 I believe I have my one K-factor set at 107,000 and I am happy with that accuracy (@ 0.1 gallons difference over a one-hour flight). My other K-factor is set at the default setting of 68,000 although I believe that is not utilized and is of no consequence. I have seen fairly weird fuel flow readings occur during the course of a typical flight but at the end of the flight the overall consumption values are very close. In other words, the instantaneous fuel flow indications can be a bit erratic inflight, but overall the average consumption indication is very accurate.
 
The single red cube in the RV-12 measures total fuel flow (engine consumption plus returned fuel). While there are two fuel flow adjustment settings possible in the Skyview, only one of them is used for the RV-12 since there is no "return flow" sensor input source. On my RV-12 I believe I have my one K-factor set at 107,000 and I am happy with that accuracy (@ 0.1 gallons difference over a one-hour flight). My other K-factor is set at the default setting of 68,000 although I believe that is not utilized and is of no consequence. I have seen fairly weird fuel flow readings occur during the course of a typical flight but at the end of the flight the overall consumption values are very close. In other words, the instantaneous fuel flow indications can be a bit erratic inflight, but overall the average consumption indication is very accurate.

Thanks David I would agree about the somewhat irratic readings in the air and that at the end of the flight things seem to sort themselves out.

I'll try your 'magic number 107000' next time I fly! Thanks!
 
. . . "I have my one K-factor set at 107,000 and I am happy with that accuracy (@ 0.1 gallons difference over a one-hour flight). My other K-factor is set at the default setting of 68,000 although I believe that is not utilized and is of no consequence. I have seen fairly weird fuel flow readings occur during the course of a typical flight but at the end of the flight the overall consumption values are very close. In other words, the instantaneous fuel flow indications can be a bit erratic inflight, but overall the average consumption indication is very accurate."
Concur with that.
My k-factor settings are very close to yours. My 2nd setting is also at 68000. Dynon says it doesn't matter what the 2nd one is set to, because it is ignored by Skyview, since a return counter is not installed.
After a 4-hour flight, my actual consumption is as predicted.

Especially after a nice long cross country flight, just refine the setting as per Dynon Skyview Installation Manual. There is a ratio formula, just follow it and you will be really close. After each long cross country, I re-evaluate and adjust the k-factor as necessary. Only had to change it once in over the last 3 years.
 
Thank you all for your responses.

Much appreciated and very helpful. I must be doing something wrong, I only see an opportunity to input one K Value number, defaulted at 68,000.

I'll do some more Dynon research.
 
New to me with someone else's Van's RV-12 being purchased, and I am trying to sort this out.

My 2012 with Skyview Classic, when I refuel, shows fuel usage much greater than how many gallons it actually needs to fill up the tank. It might say after I have filled up, that I HAD 8 gallons of fuel left, when I obviously had 12 or 13 gallons left, and do I want to match the numbers?

Does an adjustment to the Red Cube number to 107,000 help correct that on fill ups, or does it just correct the indicated GPH consumption rate shown in real time on the GPH gauge?

Or is all my student pilot time spent hardly using any fuel, learning how to taxi straight goofing up the averages? Also been flying somewhat slow in circles, practicing stall work, slow speed work, rectangles, etc, so I am not really keeping the throttle very close to normal cruise values, and fuel flows.
 
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The Dynon Skyview manual shows how to factor displayed fuel used against actual fuel used to calculate a new K factor. That is the only way to get it adjusted accurately. Using someone else?s # is just shooting in the dark.

A lot of low powered operation will skew the accuracy because with an approximately 1.5 you return flow there is only absolute accuracy at what ever power setting it is calibrated for. When calibrated for cruise power it is quite accurate at cruise power but not idle or low power.
 
Thanks, Scott, for the clarification.

I'll leave it alone, for now, doesn't seem to be burning much over about 3.8 to 4 gallons an hour, currently. That will probably change when I start pattern work.
 
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Thanks, Scott, for the clarification.

I'll leave it alone, for now, doesn't seem to be burning much over about 3.8 to 4 gallons an hour, currently. That will probably change when I start pattern work.

Actually, the fuel burn is less doing takeoff / landings than steady-state cruise...
 
I took a different approach to setting my K-factor. I flew a number of flights with the same RPM setting, figured out the fuel burn from the fill up and then inflight I set the K-factor to get the same fuel flow indication. Initially I had a 4.5 GPH flow, but now at 975 hours I see 5 GPH.
 
I recently completed the 5 year rubber replacement, and my indicated fuel flow has suddenly jumped from about 18 litres/hr to 24 l/hr. My k-factor is currently set to 90,000. I don't see why the rubber replacement would have any effect. Don't seem to be using any more fuel than usual and the engine is running OK, so just wondering if anyone else has come across this? Could replacing the carb diaphragms have had any effect?
 
I recently completed the 5 year rubber replacement, and my indicated fuel flow has suddenly jumped from about 18 litres/hr to 24 l/hr. My k-factor is currently set to 90,000. I don't see why the rubber replacement would have any effect. Don't seem to be using any more fuel than usual and the engine is running OK, so just wondering if anyone else has come across this? Could replacing the carb diaphragms have had any effect?

Curious ... Did you also replace the fuel pump as suggested by Rotax? If so, perhaps there is more pressure with the new pump which is causing more fuel to be returned to the fuel tank via the fuel return line?????
 
My fuel pump is four years old as it had to be replaced some time ago and still has a year to run. (Actually, I had intended to install the new pump which I had on hand, but somehow managed to strip one of the inlet threads and turn it into an embarrassing and expensive shelf ornament! Felt like a fool for making such as dumb mistake after all these years).

My replacement hoses were from Aircraft Specialty. They supply a new restrictor fitting for the fuel return line, so i wonder if that might affect the system? Reading past threads it seems some others have their k-factor set to 110,000-120,000, which is well above where my current setting is. i can't see that I'm using 30% more fuel than before, although I need to do a more accurate check. Of course, it may may have nothing to do with the hose replacement, but that's all I can think of at the moment.
 
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Rob, it will be the different restrictor orifice. I had the same issue with apparently increased fuel flow. From memory the TS Flightlines restrictor is a whisker smaller. It seems that over the years Rotax published two different values. It can be drilled out but I’ve never worried about it - hasn’t been a problem. I just adjusted the K factor. I’m not sure it’s spot on but pretty close. I think I landed at 99000 but will check and let you know.

Jack
 
Thanks Jack. That's very interesting. The TS Flightlines/Aircraft Specialty hoses are excellent, but if the restrictor is slightly different to the original it must have some effect. Presumably others have come across this too but I haven't seen it discussed previously. Certainly not a big deal as adjusting to K-factor is easy, but good to have an explanation. Thanks again. Hope you're getting some flying in again after the Covid lockdowns in Vic.
Rob
 
My replacement hoses were from Aircraft Specialty. They supply a new restrictor fitting for the fuel return line, so i wonder if that might affect the system? Reading past threads it seems some others have their k-factor set to 110,000-120,000, which is well above where my current setting is. I can't see that I'm using 30% more fuel than before, although I need to do a more accurate check. Of course, it may may have nothing to do with the hose replacement, but that's all I can think of at the moment.

That explains it ..... I think I know why you are seeing the sudden change in fuel flow. You have a RV-12 that has been flying for a while now so I'm betting you had the original fuel flow restrictor installed.

A while back (can't put my fingers on the paperwork at the moment) Rotax changed the specification for the hole size to be used in the restrictor making it a little larger (one would presume to decrease vapor lock possibilities) allowing more fuel to flow through the fuel lines.

At that time, Aircraft Specialty also increased the hole diameter in the restrictor they supply with their fuel hose kit .... I know, because I received one of the first larger restrictors to retrofit my new AS fuel system kit after the change was made. Since the new restrictor specification allows more fuel to flow back to the fuel tank, you will need to tweak your K factor to get an accurate fuel flow reading.
 
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The original is a brass fitting on the end of the return fuel hose. It attaches to the AN fitting located on the flat portion of the firewall directly behind the engine. The TS/AS restrictor is a separate fitting that goes between that fitting and the fitting on the fuel return hose. It’s hard to install because access is so restricted. Haven’t got a photo but I’m sure someone will.
 
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Where is the restrictor located? Anyone have a picture?

Bob - the first link is to a photo showing how small the hole is in the Aircraft Specialty original fuel restrictor. The pinpoint of light in the center is the hole size ... the later ones have a hole just ever so slightly larger. I never installed the original fuel system as shipped from Van's I went straight to the Aircraft Specialty fuel system (my RV-12 is E-AB) ... However, if memory serves me, the fuel restrictor on the original fuel system has the fuel restrictor inside one of the return hose fittings ... I may maybe wrong about that, but that is how I kinda remember it.

Second link shows a photo of the Aircraft Specialty fuel restrictor installed ... it is the black fitting under the fuel return line on the firewall shelf.
http://4.bp.blogspot.com/-2uBwNNXWnUw/VqmUERt_9oI/AAAAAAAAK7s/e2a-XtXubyo/s1600/DSC01275.JPG

http://4.bp.blogspot.com/-x3aIgSpRTqE/VqmUj1OUrVI/AAAAAAAAK74/ZSQM2XgrNl4/s1600/DSC01280.JPG
 
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Rob, I checked my K factor yesterday and it’s 88000, not 99000 as posted a couple of days ago. I think it was originally 99000 before the hose upgrade. I need a couple of long trips for fine tuning but that hasn’t been possible in recent times.

Jack
 
Thanks Jack. It's a good excuse to go flying, although I don't put too much trust in the fuel flow reading anyway. I'm more interested in how much is left in the tank. Wonder why we apparently got the narrower restrictors when John-G presumably got the modified one quite some time ago.
 
Wonder why we apparently got the narrower restrictors when John-G presumably got the modified one quite some time ago.

I just checked and my email exchange with Aircraft Specialty and it was in June of 2018 .... the original Rotax fuel return orifice specification as stated in the Rotax instillation manual is .014". (a pilot jet 35 or 0.35mm). Apparently Rotax stated somewhere that value was a mistake and it should really be .020" (I just checked the latest instillation manual and the documentation has still not caught up).

So at that time, Aircraft Specialty switched their restrictor size to the new .020" specification (which is the size of the hole in the restrictor I received from Aircraft Specialty to replace the smaller .014" restrictor I originally received). The other option offered was that the smaller orifice could be drilled in the field by the owner.

I have not been following this so don't know if the latest AS fuel line kits for the Rotax 912 have a restrictor size other than the .020" I received.
 
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Thanks John. I ordered my hoses in Feb 2020 but didn't install them until late last year during the annual, so maybe I do have the larger restrictor. It's such a hard place to get spanners into that I'm not tempted to try and find out, so I'll just adjust the k-factor and move on. Good to know about these things though, and shows why the forum is so valuable.
 
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