DoiDe
24th Sep 2023, 10:25
Hello,
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.
Thanks!
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.
Thanks!