Red box

[Bob - S50]

can I add another dumb question?  I like to run my prop at 2400 rpm, even if I'm doing 24+MP.  they say lower rpm=increased cylinder pressure.  Is that true?  Because the formula for %HP is based on MP and RPM.  It seems that using a lower rpm would give you more red box clearance, but not if it's increasing cylinder pressure.  Am I missing something?

No dumb questions.  From my understanding, it does not matter what RPM you use when trying to avoid the red box.  At about 60-65% its no longer a problem, you can't generate enough power to hurt anything.

 

Also, as I understand, for any given power setting (i.e. 65%), yes, lower RPM should increase ICP but again, at the lower power setting it should not make enough difference that you need to worry about it.

 

Running at a lower RPM means the crankshaft will not have rotated as far as a higher RPM by the time the peak pressure is reached.  That means there will be less space above the piston which means higher pressure.  Depending on what your mixture is, it may move that peak pressure closer or further from the optimum connecting rod angle.  If you are running at about Peak to 50 ROP, the reduced RPM should move you further (earlier) from optimum.  If you are running LOP or well ROP, it should move you closer to optimum.  That's because either ROP or LOP create a mixture that is less than optimum for burning.  That slows the flame rate and extends the amount of time it takes to reach peak pressure.  So either LOP or well ROP move peak pressure to ...later... than optimum.  By reducing the RPM and moving the peak pressure earlier, you move it back toward optimum.

 

So mixture moves peak pressure later than optimum and lower RPM moves it earlier than optimum and they tend to offset.  It is just a matter of how LOP/ROP and how slow the RPM is as to whether you end up at early, perfect, or later than optimum.

 

Running at lower RPM means less internal friction.  That means a higher percentage of power goes to moving the airplane and less goes to keeping the engine turning.  That's why if you look at your POH power settings and for any given altitude, and any given power setting (say 65%), fuel flow is lower at lower RPM even though MP is higher.  Notice that the speed doesn't change for the power setting, only RPM, MP and fuel flow.

 

How's that?

 

Bob

[MooneyCFII]

Lovely Bob. Well done!

 

So many variables, so little time. :-)

 

I find it helps to remember that MAP dominates when it comes to controlling ICP.