WOT RPM @ 5100

by jan kerstens in 912 / 914 Technical Questions 12 years ago

During TO of my FK9ELA with a Rotax 912 UL engine the WOT RPM is about 5100. Is this acceptable or should the pitch of the blades be reduced?

BR Jan

Re: WOT RPM @ 5100

by Roger Lee » 12 years ago

Roger Lee

128 5.28k 4.98k

First question: Was this 5100 rpm in climb or WOT flat and level flight?
If it is in climb then it is okay and if you wanted could add back a tad of pitch to drop it back to 5000 rpm in climb. If you are at 5100-5200 WOT in climb then you must be around 5800 rpm WOT flat and level.

General info:

Here is a blog from this forum on understanding your ground adjustable prop and rpm relationship.

http://www.rotax-owner.com/rotax-blog/item/9-understanding-the-ground-adjustable-prop

You are absolutely over pitched if 5100 is WOT in flat and level flight and this causes a lot of undesirable stress on the engine and its components. This can cause a crankcase to crack from all the undue strss. You are also loosing climb, cruise speed and fuel economy from the over pitch.
Reduce the prop pitch approximately 2.0 - 2.25 degrees and shoot for at least 5500-5600 rpm WOT in flat and level flight at your average flight altitude. If you live at sea level and never fly over 2'K then set it for that. If you fly an average altitude of 8'K-10'K then set the 5500-5600 rpm there. When you get up to let's say 5600 rpm WOT flat and level you are going to think someone added a super charger to your engine with a gain of 500 more rpm. You will roll out faster, you will climb faster, you will have a higher top end speed, you will be able to pull the throttle way back in cruise and still go faster than you did before. you will save about 1.5- 2.0 gph. I prefer to be closer to 5600 rpm and cruise at 5200 rpm at around 4'K.

When you do this come back and let us know how it went and what you think.

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

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Re: WOT RPM @ 5100

by jan kerstens » 12 years ago

jan kerstens

34 131 5

Just made a flight without changing the pitch of the propellor.

During climb 4.4 m/s @ 5100 rpm
During flat and level flight in no time the FK9ELA reaches 200 km/hr @ 5800 rpm @ 3300 ft AMSL(indeed as Roger said a supercharger feeling)

So, all good values as given by Roger.

I was confused by a test report of the FK9ELA where it was stated that during climb 5500 rpm was attained. This obviously an error.

Thanks Jan

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WOT RPM @ 5100

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