Rotax 914 Static/Cruise RPM

by JEFF A. in 912 / 914 Technical Questions 7 years ago

Folks,
A while ago I asked about good values for static RPM (914UL with ground adjustable prop). Recently I saw a post from Roger Lee (see end of this email) advising that 4700 RPM is a good static number and 5600 RPM is a good cruise number.
I have two questions:
1. a 4700 ground static could go lower when climbing at Vx, like 4650 or 4600, is that a problem?
2. Is 5600 good for cruise, I thought the max continuous power for long periods of time was supposed to be 5500?
Thanks again,
Best regards,
Jeff

Right now you're more under pitched and if you are over 5800 rpm at WOT then you may not even have a good climb prop. You can go too far either way with pitch and eventually it is a losing proposition. Strive to get 5600-5650 rpm in wide open throttle (WOT) in level flight at your average altitude. Start with a 4700 static on the ground rpm to get you in the ballpark. Then go fly and see what the WOT rpm is in level flight. Then make your final adjustment to get your 5600-5650 rpm.

Re: Rotax 914 Static/Cruise RPM

by Roger Lee » 7 years ago

Roger Lee

126 5.14k 4.58k

Hi Jeff,

The 914 is better served with an in flight adjustable prop and you'll lose out on some of its power and good flight characteristics. I know you can't use that so a balance should be struck between climb & cruise which also affects fuel use. During my prop research project I found different props would have different static rpm on the ground to achieve the same WOT setting in the air. Some props I could use around 4800 - 4900 and some had to be down around 4650-4750. So don't get hung up on static rpm. It just gets you in the ballpark to go fly and make a fine tuned adjustment to suit your needs.
I would shoot for 5600-5650 at YOUR average cruise altitude. This can be different for a guy living in Florida at sea level versus a guy that lives in upper California at 7100 and always flys around 11-12K. You are relegated to 5 min. between 5500-5800 rpm. I set the props to get 5600-5650 at WOT in level flight then you'll cruise at a reduced rpm from there.

From what I observe in the field is that most pilots now days cruise between 5100 - 5300 rpm. Yes there are a few lower and some higher at cruise rpm, but this is what I see as an average. For me personally flying over 5300 rpm in cruise just uses more fuel without any real airspeed gain, but that choice is up to each pilot to make. If you like 5400-5500 cruise then go for it. Do a few cruise test and see how rpm and fuel correlate to get your best balance performance. Do not fly this 914 down in the 4K rpm range it's entire life.

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

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Re: Rotax 914 Static/Cruise RPM

by JEFF A. » 7 years ago

JEFF A.

10 10 2

Thanks Roger,
My configuration currently leads to the following table: basically the static RPM (* means turbo is on so 115HP) is 5160 (with turbo off I did not write it down) and the pitch is optimized for 6000 ft where WOT (turbo off) leads to 5500 RPM. Above that I have to back off the power (as seen by MAP going down from 36 in).
So the 914 is rarely operating below 4800 though at higher weight and higher pitch during climb out I guess it could for a few minutes. Does that mean the pitch could be more coarse and still be safe?
Also, since you call out 5600 RPM cruise, if fuel consumption is not a concern, would it be safe to cruise at 5600 so above the recommended 5500?
Thanks again.
Jeff

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Re: Rotax 914 Static/Cruise RPM

by Roger Lee » 7 years ago

Roger Lee

126 5.14k 4.58k

You're overall numbers are fine. You won't be cruising above 5500 rpm. 5500-5800 is for 5 minutes.
You can and should get over 5500 WOT in level flat and flight not climb. You have the option to increase the power with the turbo over a 912 pilot.

Are your numbers at 6K at WOT? My personal preference is If that's all you can get I would reduce pitch .25 degrees more.

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

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Re: Rotax 914 Static/Cruise RPM

by JEFF A. » 7 years ago

JEFF A.

10 10 2

Thanks Roger, yes it is WOT up to 6000', perhaps slightly less than WOT because MAP is 35.4" instead of closer to 36". Above 6000' then it is progressively less and less than WOT, as shown by the diminishing MAPs.
Basically my pitch is optimized for 6000', I suppose if I wanted to optimize it for 8000' then I could go faster at 8000' (albeit it would climb slower)? Is there a RPM below which it is undesirable to climb? With my current setting and at full MTOW I believe my climb RPM (WOT with turbo off, so 100 HP) is always above 4700 RPM now (obviously it would climb so much better if it could be at 5800 :).
Thanks.
Best regards,
Jeff

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Rotax 914 Static/Cruise RPM

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