minimum R.P.M. permissible at maximum boost

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Zacke
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minimum R.P.M. permissible at maximum boost

Post by Zacke »

Hey guys,

I'm quite new to the Spit and trying to successively dig into all that important stuff for handling her (the MkI) correct. I realize I'm pretty much a prisoner of the concept of constant props and have some difficulties to get the hang of the DH 2-pitch-prop.

My question is concerning proper cruise settings. I'm used to have charts for manifold-/rpm-combinations for various power settings at different altitudes. That's not given in the case of the Spit (with the 2-pitch screw) so my concern is to threaten the engine with wrong boost-/rpm-combinations.
But: The P.O.H. says: "The minimum R.P.M. permissible at maximum Boost is: Mk I - 2080 R.P.M." (p. 65). So did I get that right that I don't harm anything when I hold her above 2080 rpm in cruise regardless of the boost setting (even at lower cruise altitudes with high boost)?
Furthermore: On page 88: "Recommended Settings for Cruise: RPM 1800; Boost sufficient to maintain 180-200mph (but +4psi MAXIMUM)". So can I handle that one as the recommended cruise setting for all altitudes (except the very high ones as there the engine can't deliver that much power with low rpm).

By the way: On page 88 it's recommended to clear the engine from time to time when running low rpm in cruise to prevent plug fouling. Is that simulated too and do you actually perform this routine?

Thanks a lot in advance for your help,
Zacke

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Killratio
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Re: minimum R.P.M. permissible at maximum boost

Post by Killratio »

Zacke wrote: 24 Feb 2020, 11:51 Hey guys,

"The minimum R.P.M. permissible at maximum Boost is: Mk I - 2080 R.P.M." (p. 65). So did I get that right that I don't harm anything when I hold her above 2080 rpm in cruise regardless of the boost setting (even at lower cruise altitudes with high boost)?


"Recommended Settings for Cruise: RPM 1800; Boost sufficient to maintain 180-200mph (but +4psi MAXIMUM)".


clear the engine from time to time when running low rpm in cruise to prevent plug fouling. Is that simulated too and do you actually perform this routine?

2080 rpm .. yes, you can run any boost you like with 2080. any less than 2080rpm at full boost and you are either driving the prop with the airflow and placing strain everywhere it should not be back through the system, or putting too much power forward through the system for the amount of rotation, putting strain on everything, the blades taking big chunks of air that they can't efficiently send rearward. Think of you hand out of the car window...at 20mph you can easily keep it flat on to wind but at 200mph you may break your hand if you try to do it.


1800 cruise rpm... yes. The 200 comes from best mpg and will give roughly the same range at all altitudes (200asi being different TAS at all altitudes)

Yes, definitely good for long term engine health to practice this. You won't foul up in a normal flight to point of engine quitting..but it does cause extra wear and tear if you run dirty for long periods.
<Sent from my 1988 Sony Walkman with Dolby Noise Reduction and 24" earphone cord extension>


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