Rotax 914 Performance Expected or Not?

by JEFF A. in 912 / 914 Technical Questions 8 years ago

Dear Forum members,

I recently acquired a new light sport airplane with a 914UL (no intercooler, fixed pitch prop set to average) and I am trying to understand how it is supposed to perform. See attached screen shots (note the GPH is wrong because when I made the tests the flow sensor was not calibrated).

- At 2000 ft/69F (so 3100 ft density alt) and max continuous power (100% throttle, 87% engine power) I get 35.4 in, 5550 RPM and 114 KIAS/119 KTAS (pretty much expected)

- At 11500 ft/52F (so 14000 ft density alt) and max continuous power (100% throttle, actually I get same result with 115%) I get 24.9 in, 5330 RPM and 90 KIAS (I would have expected 110+ KIAS and the prop, since not at 5500 RPM, is definitely not showing this is too flat of a pitch issue, right?)

I know it was warm. The Rotax manual vaguely says that maximum continuous power is expected till 16000 ft but more precisely shows a table (OM Edition 2 / Rev. 0 page 5-5) calling for 64KW/86HP or 75% power (so not really max continuous power per se which would be 100HP) at 12000 ft and +20C over ISA.

There is no way the 914 is producing 86 HP with 24.9 in, right? Page 5-2 shows that 75% power (86 HP) is achieved with about 32 in and 5250 RPM.

Also it seemed my airbox temperature was well below the 72F (or 88F, I can't figure what the threshold is really) therefore the turbo waste gate was not opened.

Any explanation?

Thanks.

Jeff

Perf3-3100DA-31JUL16.jpg (You do not have access to download this file.)

Perf5-14000DA-31JUL16.jpg (You do not have access to download this file.)

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Re: Rotax 914 Performance Expected or Not?

by Roger Lee » 8 years ago

Roger Lee

126 5.13k 4.56k

Hi Jeff,

You won't get what is in the manual, only at certain times because you're not using an in flight adjustable prop. You'll be relegated to what a fixed pitch can deliver which will leave some performance behind.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Rotax 914 Performance Expected or Not?

by JEFF A. » 8 years ago

JEFF A.

10 10 2

Thanks Roger,
What puzzles me is that if the problem is the prop pitch (I could certainly try a more coarse pitch as it is ground adjustable) why would the prop not overspeed? I was well below 5500 RPM (at WOT). It sounds to me the prop rotating was limited by the power input, exactly like if I had related the throttle, and that seems to be corroborated by the low 25 inch MAP.
Regardless of the prop, if Rotax claims "continuous performance" till 16000 ft that means the engine should output, according to their own chart, about 86 HP. If the prop is spinning in the thin air like crazy (because not correctly pitched) it should reach red line?
I am confused. Is the turbo leaking or something? Why am I topping out at 25 inches?
The prop is a 3-blade carbon fiber DUC Flash (claimed to have some "constant speed" like properties, probably overstated but not a bad prop).
Thanks again,
Regards,
Jeff

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Re: Rotax 914 Performance Expected or Not?

by JEFF A. » 8 years ago

JEFF A.

10 10 2

Re-hi Roger,

I did more researches. But you seem to be a Rotax expert so I would need your feedback too...

My questions were in regards to the engine performance, not the propeller or the airframe, though the poor KIAS at high altitude I was getting is what started me to search for an explanation. As a reminder, I was getting 29.1 inches MAP at 10,000 ft (DA) and 24.9 inches at 14,000 ft (DA).

Basically my turbo engine seems to perform well only around sea-level! It seems to boost by about 10 inches from the ambient air pressure (so I can get 39 in at SL and 25 in at 14,000).

I talked to a Tecnam dealer (who sold a few 914 LSA) and he confirmed what I suspected.

That is, the manifold pressure is expected to stay around 36 inches all the way and beyond 12,000 ft. That also seems to corroborate the statements in all Rotax manuals, that is the engine is supposed to deliver max continuous power till 16,000 ft, which is confirmed by their performance tables and by the statement in the maintenance manual (76-00-00/2.1.2.3): The critical flying altitude for max. continuous power is reached at 4500 m (14,800 ft, 17 in). Because of the low density at this altitude, the actual boost pressure is only approx. 1140 hPa (33.7 in), although the wastegate is completely closed. This means the turbo boosts by 33.7-17=16.7 under those conditions, right? That statement (in Rotax manual) calls out 14,800 ft while earlier statements (like 16,000 ft) or table don't seem to exactly agree but they all concur to confirm that 33 to 36 in should be available all the way to at least 14,000 ft.

So it seems absolutely clear that something is wrong with my engine/turbo configuration, right? I am getting 25 inches instead of 33+ inches at 14,000 ft and proportionally higher MAP at lower altitudes (such as 29.1 inches at 10,000 ft) but still way off spec. Again, that is at Wide Open Throttle (the one at 115% you are supposed to stay only 5 minutes on).

All my sensors are OK because (and they may be a few % off, it does not matter) they all agree at sea level and are all coherent at elevation. For example, 24.9 in is reported at 14,000 ft but because the RPM is so low (when, in contrary, my fixed-pitch prop should be overspeeding) and the KIAS is so low (the GPH in my screen shots, another indication of engine performance, was wrong as not being calibrated but I flew an hour at 14,000 ft and calculated my fuel consumption differently by the gauges and a refill upon arrival and my consumption was very low at 4.8 GPH, a clear indication the engine was working very very lazily, certainly not 100% power, not even close).

Further, during my test at 10,000 ft (DA) (I did not think of doing t at 14,000 ft), I switched the TCU servo off and it did not affect anything at all. Initially I thought the TCU might have opened the waste gate for some reasons (like high airbox temperature) but it is clear it never did that.

So it is clear the turbo in this brand new airplane/new engine is not performing correctly, perhaps never did even when leaving the factory.

My only explanation is that the servo cable is too tight. Since the TCU servo commands opening the waste gate then when the servo is off (I suppose also when the TCU unit is off) the waste gate should be completely closed. Then the TCU can command to open the gate from 0 to 100% depending on conditions. The manual also says: with the wastegate completely open, the engine performance can reach up to approx. 70 kW/94 HP since not all
of the exhaust gases bypass the turbine. I suppose at sea level only though.

If the servo cable is too tight that means the waste gate is always opened a little bit (likely very easy to fix). If the cable was too loose then the TCU would have a hard time reducing boost and I would see the opposite issue (perhaps more than 40 inches at sea level). Another explanation (likely way more costly in term of warranty) could be a leak or a defect somewhere preventing the turbo to ever boost by more than 10 inches.

Would you concur?

Thanks.

Best regards,

Jeff

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Re: Rotax 914 Performance Expected or Not?

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

A couple things:
The cable pulls the wastegate closed, the spring opens the wastegate. Test: with the throttle closed, turn the master ON, let the servo cycle, and turn the master OFF. Take a look at the wastegate: It should be fully closed and the cable tight (spring is stretched).

Check that the airbox rubbers are very tight on the carbs, particularly the left (2-4 cylinders) carb. Visual inspection may look good but you have to grab the airbox and see if you can pull it away from the carbs. This is the most common "loss of boost" issue. It is often caused by someone using the airbox as a handle to move the engine of get themselves up off the ground.

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Re: Rotax 914 Performance Expected or Not?

by JEFF A. » 8 years ago

JEFF A.

10 10 2

Thanks Rob,
So, you are saying the opposite and that the cable might be too loose instead of being too tight and if too loose then the spring acts to open the waste gate. So I will check that.
Yes, when I powered the TCU I could see the servo cycle, I just don't know if the position the waste gate lever rested was really all the way "waste gate closed or not". I supposed the only way to know is to disconnect the cable and see how much the waste gate lever travels in either direction.
I will check that (and the airbox) this week-end.
Thanks.
Best regards
Jeff

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Rotax 914 Performance Expected or Not?

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