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Fuel flow and percent of power

hevansrv7a

Well Known Member
Does this make sense to you (RV 12 carb only)?

Aviation Consumer estimated BSFC for this carb engine at 0.44. Since my C-150 was .45 this works for me. Less is better, btw.

So 100 HP is maximum fuel flow of 7.333 gph if .44 is correct.

Since our mixture is more or less constant the gph should be an easy analog for power. Is the gph actually constant? Or is the Bing adjusting it? Even in cruise?

I would rather use the Dynon calculation but haven't figured out how to show it on the 20% screen.

I would like to learn about this.
 
The Bing 64 carb has a diaphragm that controls Venturi slide and tapered main jet needle. The diaphragm raises and lowers the slide based on altitude and that is why there is no mixture control. Actually on the RV-12, because the FWF stays sufficiently warm, there is also no need for carb heat. Icing is never a problem. The 912ULS, for all practical purposes, has mechanical FADEC engine control with only a power lever for the pilot to manipulate. Pretty clever...
 
Aviation Consumer estimated BSFC for this carb engine at 0.44. Since my C-150 was .45 this works for me. Less is better, btw.

So 100 HP is maximum fuel flow of 7.333 gph if .44 is correct.

Since our mixture is more or less constant the gph should be an easy analog for power. Is the gph actually constant? Or is the Bing adjusting it? Even in cruise?

To HevansRV7A's original question - BSFC varies by the operating condition (RPM, Ambient pressure) of the engine. As a result, you cannot say that a fuel flow of 50% of max is 50% of max horsepower.

Aviation Consumer was probably comparing BSFC at a "normal" cruise condition (75% power possibly) unless they specified otherwise.

That said - the only parameters you have to describe the operating condition of the engine are fuel flow and RPM. You just have to remember that the HP generated is not linear with respect to the fuel flow.

If you need to get the HP at any operating point you will need engine data that shows the HP at various combinations of ambient pressure and RPM.

Aviation consumer's numbers look a little high. the POH I have for the RV12 lists the ULS fulke consumption at 5500 rpm as 6.3 GPH, not 7.3. That matches with the REMOS GX withthe 912ULS I'm flying (while I build the 12) - I use 6.3 GPH for a climb condition for flight planing.
 
From the research document

This should shed some light....
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Thanks for that!

Much of the paper was above my non-engineer's head. But I think at least two things stood out.

1. The chart provided by Rotax suggests that maximum gph is higher than fast cruise as found in the POH. Figure 6. See the 75% dot vs. TakeOff or 29" x >5800 RPM.

2. The Litres per Hour vs RPM (assume full load?) is not perfectly linear but it's close. The two lines are a little different, too. They suggest that the best SFC is between 4500 and 5000 RPM but that would also be at full load. Therefore, it's not necessarily true for those RPM's in a partial power cruise. I am referring to the final graph in the paper. The data that's given cannot be used to compute an actual SFC, just a relative one. the relative flatness of the lines suggest that SFC does not vary much and certainly not as much as it would with pilot-controlled mixture.
 

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Dynon won’t do it for Rotax

Does this make sense to you (RV 12 carb only)?

Aviation Consumer estimated BSFC for this carb engine at 0.44. Since my C-150 was .45 this works for me. Less is better, btw.

So 100 HP is maximum fuel flow of 7.333 gph if .44 is correct.

Since our mixture is more or less constant the gph should be an easy analog for power. Is the gph actually constant? Or is the Bing adjusting it? Even in cruise?

I would rather use the Dynon calculation but haven't figured out how to show it on the 20% screen.

I would like to learn about this.

Dynon support responded that it only works for Lycoming and Continental engines, not Rotax. I have more info on BSFC which I will share later.
 
We were running a 912 on our test stand back in 2007 with a wideband and found the carb delivered an AFR of right around 13 to 1 all the way from 3750 rpm and up. It was remarkably good.

Be aware that power is affected not only by RPM, MAP, AFR, IAT and ignition timing but also by humidity and barometric pressure acting on the exhaust flow. Any device not taking these parameters into account will not display an accurate % of power. Barometric pressure has a not inconsequential effect on power.
 
We were running a 912 on our test stand back in 2007 with a wideband and found the carb delivered an AFR of right around 13 to 1 all the way from 3750 rpm and up. It was remarkably good.

Be aware that power is affected not only by RPM, MAP, AFR, IAT and ignition timing but also by humidity and barometric pressure acting on the exhaust flow. Any device not taking these parameters into account will not display an accurate % of power. Barometric pressure has a not inconsequential effect on power.

That's really interesting but isn't at a bit on the rich side for economy? Is there a way to guess at BSFC from 13:1? Was the AFR higher or lower below that RPM?

Thanks.
 
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