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Determination of rpm @ certain power settings

  • jan kerstens
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1 month 2 days ago - 1 month 2 days ago #17012 by jan kerstens
jan kerstens created the topic: Determination of rpm @ certain power settings
Hello,

Goodevening.
Could not find a reference regarding the determination of a rpm @ certain power setting. For instance: what is the rpm at 75% power setting. For a ROTAX912S the rpm=5000 @ 75% power setting.

Best regards Jan
Last Edit: 1 month 2 days ago by jan kerstens.

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1 month 1 day ago #17017 by Roger Lee
Roger Lee replied the topic: Determination of rpm @ certain power settings
Are you looking for a chart with all the percentages and rims?
The answer is predicated on your prop pitch. If you are over or under pitched the rpm vs % of power will be different. I believe the manual takes into account that you can achieve 5800 rpm. For instance; Most of us don't get 5800 with a ground adjustable prop. What type of prop setup do you have? In flight adjustable or ground adjustable?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 month 1 day ago - 1 month 1 day ago #17020 by jan kerstens
jan kerstens replied the topic: Determination of rpm @ certain power settings
I have a ground adjustable prop: DUC Swirl.

The following article gives the answer:
How To Determine The Part-Throttle RPM of a Fixed Pitch Propellor At a Given Horsepower by Stan Hall, EAA.
He has determined the following table:
Power Setting 75% Multiplier=0,91
Power Setting 65% Multiplier=0,87

First you have to determine your WOT level flight RPM: RPM1 then your RPM @given Power Setting is RPM2=RPM1*Multiplier!

Example at 75% Power Setting:
Suppose @WOT level flight we obtain RPM1=5800 RPM then RPM2=5800*0,91=5278RPM.
RPM1 depends as Roger said on over or under pitching and at what cruise level we fly.
Last Edit: 1 month 1 day ago by jan kerstens.

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1 month 22 hours ago - 1 month 22 hours ago #17023 by Bill Hertzel
Bill Hertzel replied the topic: Determination of rpm @ certain power settings
That is just the textbook "Power vs.Velocity" Formula.
Velocity is proportional to the Cube Root of the Power.

The cube root of 75% is 91%
The cube root of 65% is 87%
The cube root of 50% is 79%

This works reasonably well within a few hundred feet above Sea level but way off at altitude.
At 8000ft 75% Sea level Power is closer to 97% Max rpm.

RPM x MAD (Manifold Air Density) will be real close throughout the range.
Last Edit: 1 month 22 hours ago by Bill Hertzel.

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1 week 13 hours ago #17232 by Craig Dingwall
Craig Dingwall replied the topic: Determination of rpm @ certain power settings

Roger Lee wrote: Are you looking for a chart with all the percentages and rims?
The answer is predicated on your prop pitch. If you are over or under pitched the rpm vs % of power will be different. I believe the manual takes into account that you can achieve 5800 rpm. For instance; Most of us don't get 5800 with a ground adjustable prop. What type of prop setup do you have? In flight adjustable or ground adjustable?


Hi Roger,

Can I just clarify this a little more. I am new to the 912 having upgraded from a 582 and so am trying to build a picture of how mine should be performing.
You say that with a ground adjustable prop most do not get 5800RPM and I would guess that is at the standard ops manual prop setting?

I am running a 912UL on an Airborne XT Trike with a Bolly ground adjustable prop. If I set my prop pitch as per the maint manual I am lucky to get around 5000-5200 when I go full throttle on the runway. By going 3deg fine on the props I can get that to 5500 under the same conditions (my airfield is sea level).

I considered something wrong due to this and interestingly in comparing with another machine very similar to mine with prop set to the maint manual this one actual indicates 5800 on the runway!

I am sure this subject has been laboured a bit but perhaps you could offer some additional comments based on the above?

Cheers Roger
Craig

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1 week 5 hours ago #17235 by jan kerstens
jan kerstens replied the topic: Determination of rpm @ certain power settings
Hi Craig,

Just to summarize Roger above and beyond.

At ground level you should get around max 5000-5200 RPM.

At WOT (Wide Open Throttle) at cruising altitude you should get around 5800 RPM. You have to check this.

Cruising at 75% is then around 5200 RPM.

Cheers

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1 week 3 hours ago - 4 days 2 hours ago #17237 by Roger Lee
Roger Lee replied the topic: Determination of rpm @ certain power settings
Hi Craig,
Sorry if I didn't explain it very well. Those rpms are in flight numbers and not on the ground for static. 2 blades, 3 blades and some prop Mfgs can have a slightly different WOT setting in static on the ground from each other. If you want a well balanced prop setting for your ground adjustable prop forget the static because you're already close. Go fly and you should get around 5600-5650 rpm at WOT in level flight. At take off you may only see about 5000 rpm at WOT. If you want a better climb over cruise prop then set the in flight max WOT rpm to get 5750-5800 rpm. With the higher rpm setting you will loose a tad in cruise and some fuel milage. The manual says they would like to see 5200 rpm at take off, but ground adjustable props may not see this. 4900-5100 seems to be the norm at take off with ground adjustable or fixed pitch props.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last Edit: 4 days 2 hours ago by Roger Lee.

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4 days 15 hours ago #17257 by Craig Dingwall
Craig Dingwall replied the topic: Determination of rpm @ certain power settings
Thanks Roger, appreciate that.

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