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Fixed pitch propeller question

Old 24th Sep 2023, 11:25
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Fixed pitch propeller question


While I was flying the Evektor SportStar( Rotax 912 + Fixed pitch propeller) a question came to my mind and I really can't find an answer.

How can the RPM stay constant during a climb from 3000 ft to 8000 ft at constant airspeed (65 KIAS) since the manifold pressure is decreasing constantly (25 inHg to 20 inHg)

From what I understand, on fixed pitch propeller, the throttle controls the manifold butterfly valve, which controls the manifold pressure.
I know that the main indicator of power % in fixed pitch propeller is RPM.
From this, I concluded that, for the fixed pitch propeller, in reality, I control the manifold pressure that will give me a certain RPM.

For example: at 3000 ft I achieve 4000 RPM at around 19-20 inHg
at 8000 ft I achieve 4800 RPM at around 19-20 inHg, but with more throttle pushed in.
KIAS for both cases is 65
TAS at 3000 was around 67-68
TAS at 8000 was around 74-75

Does it have anything to do with the TAS?

Or maybe with the density and the pressure of the air at higher altitudes.

Or maybe it happens because the Rotax leans the mixture by itself?

I also fly constant-speed propeller aircraft and I understand very well the relation between MAP, RPM and propeller pitch, but in this case I really can't find what I am missing.

DoiDe is offline  
Old 25th Sep 2023, 08:48
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Off the top of my head

It ties in with Static boost RPM. You know when you get in and look at the MAP gauge it reads a particular figure? If you start the engine and open the throttle until you get back to this "Static Boost" figure, the engine should be at "reference RPM" this RPM should be written down somewhere in the aircraft FCOM or engine manual.

So loosely, as you climb and power reduces, so does the resistance to turning of the propeller due to decreasing air density, so RPM doesn't drop, it isn't controlled, it's just the combination of lower power and lower turning resistance.

This is why at static boost, you should always be able to see Reference RPM and if it's lower, your engine is inefficient. Hope that all makes sense. 😊
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