Rotax 914 Publised Torque Curve

by Ken Ryan in 912 / 914 Technical Questions 9 years ago

I don't understand the published torque curve for the Rotax 914.

[Click for Link to Graph]

The graph shows two separate torque curves. One is called "take off" and the other "propeller curve." They are significantly different for the same rpm settings.

What do these differing curves represent?
Why is it that only the 914 shows torque this way, and not the 912 engines?

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Re: Rotax 914 Publised Torque Curve

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

Conrad can probably give a better explanation but here is what I remember being explained:

The 914 has two phases so they need two curves: "Take off Performance" means full throttle 115% throttle position and fine pitch on the prop (max 5800 rpm). The wastegate is fully closed and the enrichener solenoid is activated to give an extra rich mixture. Also called "military power" or "balls to the wall"

Continuous power is 100% throttle position (max non-military power)
A bit of the German translation is lost here: Full throttle in the context of "continuous power" is not full throttle as far as the throttle arm hitting the stop, it is full throttle to the 100% position as determined during a TCU calibration. see drawing:

The propeller curve is different as it is determined on a dyno with the load of the eddy-current brake always increasing to the max amount the engine will accept to simulate the demands of a propeller. This is different that the industry standard power curve and I think the reason for this curve is so you know what power you can get at different prop pitch settings.

throttleposition.jpg (You do not have access to download this file.)

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Re: Rotax 914 Publised Torque Curve

by Ken Ryan » 9 years ago

Ken Ryan

28 360 928

Appreciate the effort Rob. it helps. But please, anyone else who understands this please chime in. I'm still struggling a bit. Is the enrichener solenoid specific to the 914?

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Re: Rotax 914 Publised Torque Curve

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

Yes, the solenoid is only for the 914.

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Re: Rotax 914 Publised Torque Curve

by Ken Ryan » 9 years ago

Ken Ryan

28 360 928

Now that I've had a couple of days to think about this, it is making more sense to me. One thing that was throwing me was why the propeller curve would go all the way up to 5800 (because it seemed that rpm would only be accomplished in take off mode). Finally it dawned on me that by flattening the pitch on an adjustable propeller those high rpms could be achieved without engaging 115% throttle.

So that leaves me wondering, okay why did Rotax only feel it necessary to publish this sort of a torque curve for the 914 and not for the 912?

The thing that originally sparked this question was the following statement, found in a paper published by rotax-owner.com

"Fixed pitch propellers (on the 914) are not recommended due to the engines non-linear power curve."

[Reference Link]

And yet, there are many happy campers using the 914 with a fixed pitch propeller. The 914 with a Catto fixed pitch prop is a very popular (and one of the best performing) combination on Highlanders and SuperSTOLs. So what gives? 914 + fixed pitch propeller? Yes or no?

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Re: Rotax 914 Publised Torque Curve

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

"So that leaves me wondering, okay why did Rotax only feel it necessary to publish this sort of a torque curve for the 914 and not for the 912?"
I am not completely sure of your question? because the 914 engine has two phases (struggling to find the good term for that): 100% continuous and 115% take off.
The 912 does not have this two-curve necessity as it is not turbocharged and has no "military power" throttle setting.
A constant speed prop is ideal for the 914 as you can utilize the best rpm and throttle position to get the best power, for both take-off and cruise.
A fixed pitch prop is always a compromise but on a STOL aircraft like the highlander you have so much drag a high cruise speed is not important so you can pitch for max climb with a long prop and be pretty darn happy.
Just don't pitch the 914 prop too heavy/coarse. Can you imagine the pressure/load on the pistons at only 4800rpm and 40inHG?? It was not designed for that. Fast moving pistons wear less as they have less transition/side load time and more rpm's give more power.

Thank you said by: Ken Ryan

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