I'm trying to compare the performance numbers of the 915 and 916 options in the latest 3.2 version of the POH, but I'm confused. The numbers are given as % power, which is confusing since those aren't going to be the same powers for the two engines. But more importantly, the 916 tables don't make sense.
There's a column in the speed and range tables labeled "95%" with a footnote saying "max continuous power". But max continuous power for the 916 is 137hp and max power is 160hp, so max continuous is 85%. Furthermore, if we compare the cruise speeds tables at, say, 75% power, they are the same (actually, the 916 is one knot slower). That doesn't make sense, because 75% power for the 915 is 0.75*139.5 or 105hp. For the 916, 75% power should be 0.75*160 or 120hp. It can't be the case that the 916-equipped plane goes a knot slower with 15 extra horsepower.
It's also very strange that the climb numbers, which supposedly are stated for Vy at max continuous power, are 800fpm for the 915 and 940fpm for the 916. That's a 17% increase, but the difference in max continuous power is only 2hp or 2%. Maybe the MT prop is that much more efficient but, if so, why are the cruise speeds at max continuous power the same?
There must be something incorrect in how power the figures are reported. #slingsupport care to comment?
POH 916 performance figures don't make sense
POH 916 performance figures don't make sense
Blog of various projects: https://blog.familjenjonsson.org/blog/
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TSiDreamer
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Re: POH 916 performance figures don't make sense
Hi Lutorm,
I looked at the power figures for the two engines using the Rotax Operators Manuals. Although I get slightly different numbers, I agree with what you have to say. Using the Rotax numbers, the power figures at ISA standard temperature and pressure (15°C & 1013mB): 915iA C24 vs 916iA C24
5 Minute Max Power @ 5800rpm | 100% Throttle = 104 kW(139.4 hp) vs 100% Throttle = 117kW(156.8 hp) both at 100% power
Max Cont Power @ 5500rpm | 100% Throttle = 99.17 kW(132.9 hp) = 95% power vs 98.7% Throttle = 101 kW(135.4 hp) = 86% power.
To me, the most remarkable aspect of the 916 is its fuel efficiency. At max continuous power, the 916 produces 2.5 hp more than the 915 but it consumes 11.8% less fuel to do so. That works out to 36 minutes greater endurance with 50gal of fuel and likely over 100nm additional range!
As for the POH inconsistencies, we may need to wait for one of our fellow builders to give us some real numbers gleaned from their flight testing.
Mike
I looked at the power figures for the two engines using the Rotax Operators Manuals. Although I get slightly different numbers, I agree with what you have to say. Using the Rotax numbers, the power figures at ISA standard temperature and pressure (15°C & 1013mB): 915iA C24 vs 916iA C24
5 Minute Max Power @ 5800rpm | 100% Throttle = 104 kW(139.4 hp) vs 100% Throttle = 117kW(156.8 hp) both at 100% power
Max Cont Power @ 5500rpm | 100% Throttle = 99.17 kW(132.9 hp) = 95% power vs 98.7% Throttle = 101 kW(135.4 hp) = 86% power.
To me, the most remarkable aspect of the 916 is its fuel efficiency. At max continuous power, the 916 produces 2.5 hp more than the 915 but it consumes 11.8% less fuel to do so. That works out to 36 minutes greater endurance with 50gal of fuel and likely over 100nm additional range!
As for the POH inconsistencies, we may need to wait for one of our fellow builders to give us some real numbers gleaned from their flight testing.
Mike
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TSiDreamer
- Posts: 5
- Joined: Tue May 21, 2024 9:33 am
- Location: Northern Colorado
Re: POH 916 performance figures don't make sense
Hi Lutorm,
I looked at the power figures for the two engines using the Rotax Operators Manuals. Although I get slightly different numbers, I agree with what you have to say. Using the Rotax numbers, the power figures at ISA standard temperature and pressure (15°C & 1013mB): 915iA C24 vs 916iA C24
5 Minute Max Power @ 5800rpm | 100% Throttle = 104 kW(139.4 hp) vs 100% Throttle = 117kW(156.8 hp) both at 100% power
Max Cont Power @ 5500rpm | 100% Throttle = 99.17 kW(132.9 hp) = 95% power vs 98.7% Throttle = 101 kW(135.4 hp) = 86% power.
To me, the most remarkable aspect of the 916 is its fuel efficiency. At max continuous power, the 916 produces 2.5 hp more than the 915 but it consumes 11.8% less fuel to do so. That works out to 36 minutes greater endurance with 50gal of fuel and likely over 100nm additional range!
As for the POH inconsistencies, we may need to wait for one of our fellow builders to give us some real numbers gleaned from their flight testing.
Mike
Edit 11/4: I created a table comparing fuel Flow for the 915is vs. the 916iS at various power setting in an attempt to make some sense of the flight manual performance figures.
I looked at the power figures for the two engines using the Rotax Operators Manuals. Although I get slightly different numbers, I agree with what you have to say. Using the Rotax numbers, the power figures at ISA standard temperature and pressure (15°C & 1013mB): 915iA C24 vs 916iA C24
5 Minute Max Power @ 5800rpm | 100% Throttle = 104 kW(139.4 hp) vs 100% Throttle = 117kW(156.8 hp) both at 100% power
Max Cont Power @ 5500rpm | 100% Throttle = 99.17 kW(132.9 hp) = 95% power vs 98.7% Throttle = 101 kW(135.4 hp) = 86% power.
To me, the most remarkable aspect of the 916 is its fuel efficiency. At max continuous power, the 916 produces 2.5 hp more than the 915 but it consumes 11.8% less fuel to do so. That works out to 36 minutes greater endurance with 50gal of fuel and likely over 100nm additional range!
As for the POH inconsistencies, we may need to wait for one of our fellow builders to give us some real numbers gleaned from their flight testing.
Mike
Edit 11/4: I created a table comparing fuel Flow for the 915is vs. the 916iS at various power setting in an attempt to make some sense of the flight manual performance figures.
Re: POH 916 performance figures don't make sense
I'm seeing better numbers that your table with my TSi with 916. What could be off in your numbers is that 100% power is actually 5500 RPM. Takeoff power at 5800 RPM reads 105-108% (max 5 minutes).
I routinely fly cruise at 85% power, which gets me +/- 8.0 gph fuel burn (7.8-8.1, depending upon altitude, pressure, winds, etc.). This usually is about 130-135 KIAS. At 5500', I'll see TAS around 140-142; at 6500' 143-145; at 7500' 147-150; at 8500' 148-155.
For comparison, at 90% power, I'm getting 9.0-9.2 gph, and only seeing TAS 2-4 knots more, so not worth the increased burn.
At 80% power, I'm seeing 7.1-7.5 gph, 3-5 knots less TAS.
To get the above numbers at 85% power, I have to reduce throttle to 82-83%, watch the GPH drop, then slowly notch it back up to 85%. if I slowly from 100% to 85%, it seems to stick at 8.5-8.7 gph, for some reason.
I routinely fly cruise at 85% power, which gets me +/- 8.0 gph fuel burn (7.8-8.1, depending upon altitude, pressure, winds, etc.). This usually is about 130-135 KIAS. At 5500', I'll see TAS around 140-142; at 6500' 143-145; at 7500' 147-150; at 8500' 148-155.
For comparison, at 90% power, I'm getting 9.0-9.2 gph, and only seeing TAS 2-4 knots more, so not worth the increased burn.
At 80% power, I'm seeing 7.1-7.5 gph, 3-5 knots less TAS.
To get the above numbers at 85% power, I have to reduce throttle to 82-83%, watch the GPH drop, then slowly notch it back up to 85%. if I slowly from 100% to 85%, it seems to stick at 8.5-8.7 gph, for some reason.
Re: POH 916 performance figures don't make sense
Ok so that's interesting. So you say the 100% is "fraction of max continuous", which would make sense, I think that's a fairly common way of stating things. In the tables, though, it definitely does not define max continuous as 100%.
Blog of various projects: https://blog.familjenjonsson.org/blog/
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TSiDreamer
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Re: POH 916 performance figures don't make sense
The data in the table is pulled straight from the respective engine Operators Manual published by Rotax. All of the data used the entry for standard temperature and pressure. The rated power is also from the same source, however, the actual power from the fuel flow table is slightly greater. In the notes, I mention that all percent power figures are based on the Takeoff or Maximum power at 5800 rpm for that engine, not the Maximum Continuous Power at 5500rpm. Max Eco Power is the point where there is an obvious inflection in the brake specific fuel consumption at 5500rpm as the throttle % is decreased from 100%. Also I selected an altitude of 15,000’ for purposes of comparison, as I figured the majority of the cross country flights would be at 15,000’ +/- 3,000’
If you are seeing lower fuel flows than what is in the table, then you are almost certainly producing less power than the datapoints in the table. The power output is incredibly sensitive to small changes in throttle position. A 2% difference between your throttle position and the reference throttle position in the table could easily be a difference of 10-20HP, thereby explaining the lower fuel flows. Likewise, a 50-100rpm difference from the exact value in the table will also skew your observed fuel flows. Lastly, a slight calibration error in the fuel flow display software can also produce erroneous numbers, though I suspect most pilots engaged in this thread have carefully calibrated their fuel flow display.
I hope this helps! I’m still incredibly impressed by the remarkable efficiency Rotax was able to deliver with the 916 vs. the 915 engine. I wish Rotax would provide a detailed explanation of how they achieved this efficiency and also how they were able to increase the TBO to 2000 hours. With regards to the TBO extension, I recall seeing somewhere that Rotax changed the crankshaft bearing sizes to cope with the extra BMEP needed to generate the extra horsepower. I believe this is why the 915 may not receive the longer TBO in the future. I may be wrong on this and I ask that anyone that knows more about the inner details behind this engine to jump in and correct me if I’m wrong.
Mike
If you are seeing lower fuel flows than what is in the table, then you are almost certainly producing less power than the datapoints in the table. The power output is incredibly sensitive to small changes in throttle position. A 2% difference between your throttle position and the reference throttle position in the table could easily be a difference of 10-20HP, thereby explaining the lower fuel flows. Likewise, a 50-100rpm difference from the exact value in the table will also skew your observed fuel flows. Lastly, a slight calibration error in the fuel flow display software can also produce erroneous numbers, though I suspect most pilots engaged in this thread have carefully calibrated their fuel flow display.
I hope this helps! I’m still incredibly impressed by the remarkable efficiency Rotax was able to deliver with the 916 vs. the 915 engine. I wish Rotax would provide a detailed explanation of how they achieved this efficiency and also how they were able to increase the TBO to 2000 hours. With regards to the TBO extension, I recall seeing somewhere that Rotax changed the crankshaft bearing sizes to cope with the extra BMEP needed to generate the extra horsepower. I believe this is why the 915 may not receive the longer TBO in the future. I may be wrong on this and I ask that anyone that knows more about the inner details behind this engine to jump in and correct me if I’m wrong.
Mike