75% power???

by Eugene Klimenko in General Tech Discussion 6 years ago

Every LSA manufacturer out there will give us cruise speed at 75% power.

I would like to know at what RPM they want you to fly on ULS engine to get this power setting?

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Re: 75% power???

by Garrett Wysocki » 6 years ago

Garrett Wysocki

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https://www.cps-parts.com/cps/912_ePerf100.jpg

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Re: 75% power???

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 8k

Power delivered by a fixed pitch propeller will be proportional to the Cube Root of the RPM.
The Cube Root of 0.75 (75%) ≈ 0.91 (91%)
Assuming your WOT Full Power RPM is the 5800rpm Max of the engine, 5800 * 0.91 ≈ 5275 RPM.

75%hp = 91%rpm = 5275rpm
70%hp = 89%rpm = 5150rpm
65%hp = 87%rpm = 5025rpm
60%hp = 84%rpm = 4900rpm
50%hp = 79%rpm = 4600rpm

At 1800rpm you are producing <3hp and can just barely taxi.
The RPMs vary just over 3:1 (5800:1800) while the power varies over 30:1 (100:3)

The ≈ 4300rpm shown in the engine performance charts show that an engine operating at WOT and loaded down to 4300 rpm will be producing 75% of full power.
This would be a severely overloaded engine and not something you would ever want to do except on a Dyno Test stand, and only for a very brief time at that.

For comparison sake, if you removed the prop and ran the engine up to 5800 without a propeller to load it, you would be just barely over idle HP and not anywhere near the 100HP the performance chart would seem to indicate.

The Engine Performance charts only apply to an engine running at Wide Open Throttle (WOT) and loaded to the RPM levels indicated.
The Torque value is quickly measured by the Dynometer and along with the RPM indicated at that point, the Power is calculated.
The Marketing department asked the Engineers for something impressive to print in the sales brochures and this is what they got!
It is an Engineering Design Planning Chart and not something to refer to for daily operational information.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Garrett Wysocki, Eugene Klimenko

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Re: 75% power???

by Eugene Klimenko » 6 years ago

Eugene Klimenko

12 74 1

Wow!
I can see now how much I didn't know this morning! But I know now! Thank you!

So, we can say that ULS with WOT 5600-5650 RPM will be at 75% power at about 5200 RPM.

What will be fuel consumption at this power setting? Can we use Rotax chart to find out? Is it accurate?

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Re: 75% power???

by Roger Lee » 6 years ago

Roger Lee

131 5.38k 5.44k

Fuel will be around 4.7 - 5.0 gph depending on a few other factors. These numbers are based on real time flight, but like I said other factors could weigh in on this some.

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

Thank you said by: Eugene Klimenko

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Re: 75% power???

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 8k

... So, we can say that ULS with WOT 5600-5650 RPM will be at 75% power at about 5200 RPM...

5200rpm would be 75%hp of 5715rpm.
5100rpm would be 75%hp of 5600rpm.
5275rpm will be real close to the 75%hp of the engines Full Rated 5800rpm.

If you can only make 5600rpm at WOT, you are limited to 90% of the full rated power.
The question is, do you want 75% of full rated power? 5275rpm
Or... Do you want to run at 75% of your 90% power (68%hp)? 5100rpm
I would recommend just setting anything between 5200 and 5300 for ≈ 75%

This is not a Lycoming or a Continental that is limited to 75% Max Continuous power.
The Rotax 912 is rated for 90% Max Continuous Power at 5500rpm and 27" MAP.
75% power is just an arbitrary number in the Rotax universe.

This means that for many installations, once you are above than 3000' MSL you will not be able to make 27"MAP and may not be able to reach 5500rpm even at WOT.
You could run at Wide Open Throttle all day long and never be over MAX Continuous Power.

The 912ULS Operators Manual quotes 75%hp = 5000rpm @26"MAP.
There are a lot of variables involved that are not taken into account.
5500 RPM on a Cold Winter day needs more power than 5500rpm on a Hot Summer day.
5500rpm at sea level requires more power than 5500rpm at 10,000MSL due to the lower air density at altitude.
You will never see 26"MAP at 10,000MSL. You will be lucky to see 20"MAP.

Stay between 5200 and 5500 rpm and you will be just fine 99% of the time.
RPMs well below 5000rpm can be hard on the Gearbox.
Don't be surprised if you are running Wide Open while cruising at altitude.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Eugene Klimenko

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