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Does anyone happen to know a rough estimate of a typical light turboprop aircraft propeller's compression ratio? Something with 6-12 seats like a Cessna Caravan or Piper Malibu. Doesn't need to be that accurate but something spinning around 1800-2000 rpm at sea level ISA?

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  • $\begingroup$ Welcome to aviation.SE! I edited your question to (hopefully!) make it a little clearer. If I got it wrong then you can just roll back my changes or edit it again. Your title mentioned the Malibu, which is the PA-46, but you had tagged it PA-32, which is the Cherokee Six. I guess you intended the PA-46 because I think you're asking about light turboprops but again, don't hesitate to correct it if I'm wrong. $\endgroup$ – Pondlife Nov 12 '20 at 20:58
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    $\begingroup$ @Pondlife Turns out they really meant propeller compression ratio. $\endgroup$ – Sanchises Nov 13 '20 at 9:49
  • $\begingroup$ Mikey, please edit your question per your comment below. $\endgroup$ – Michael Hall Nov 13 '20 at 18:26
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By first principles, one simply expects the pressure difference to be thrust $T$ divided by disc area $\pi r^2$. Due to the fact that the propeller "sucks" ahead and "blows" behind itself, we'd expect to see about half of this pressure difference as a pressure drop ahead of the propeller and the other half as a pressure increase behind the propeller. The effective compression ratio $n$ in ambient pressure $p_0$ is then

$$ n=\frac{p_0+T/(2\pi r^2)}{p_0 - T/(2\pi r^2)}$$

To see what thrust is created for a given amount of power, you can use this excellent answer by Peter Kämpf.

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  • $\begingroup$ We need an expression to relate thrust to power, as power is known, not thrust. $\endgroup$ – Efe Ballı Nov 13 '20 at 12:45
  • $\begingroup$ That equation is for static thrust. I guess the OP wants to get the power in flight, which is given in the linked answer. Thanks for your comment! $\endgroup$ – Sanchises Nov 13 '20 at 12:47
  • $\begingroup$ Couldn't get the equation in the proper form, so I removed it. You're right, it is for static thrust, but to me it is really unclear what the OP asks in terms of flight conditions. $\endgroup$ – Efe Ballı Nov 13 '20 at 12:49
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Welcome.

https://en.wikipedia.org/wiki/Pratt_%26_Whitney_Canada_PT6

From Wikipedia, the compression ratio of the PT6 varies between 7 and 10.8, depending on the version. Please note the trend between compression ratio and engine power, due to this, I think the Caravan's engine (-114A) has a compression ratio of 7. The Malibu's engine (-42A) has a compression ratio of 8. In off-design conditions (part-power, low RPM), the PT6 has a pre-swirl system that uses high pressure bleed air to change the direction of inlet air, thus widening the operational range of the engine. When compressed air is bled back to the inlet, the compression ratio obviously drops.

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  • $\begingroup$ thanks for this but I was just wondering about the propellor not the engine. I just want to know the rough pressure behind the propellor. It can be from any light aircraft, even piston engined. $\endgroup$ – Mikey Patterson Nov 12 '20 at 22:03
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    $\begingroup$ Engine compression ratio refers to the pressure ratio between the combustion chamber and inlet. For the pressure difference/ratio between the two sides of the propeller. I will refer you to momentum theory, en.wikipedia.org/wiki/Momentum_theory $\endgroup$ – Efe Ballı Nov 12 '20 at 22:07

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