Rcm 112 poh

[jukelemon]

Hey Carl.

What am I missing here...

There is a clearly stated warning to not go over 24" MP if less than 2400 RPM.

Yet, the power setting graph states a 75% setting at 2400 RPM could be at 27" to 25" depending upon altitude.

How can that be? At 2399 RPM you have to immediately reduce to 24"??

I must be reading/looking at this wrong.

Thanks

 

[carllee]

Re: Rcm 112 poh

Hey Carl.

What am I missing here...

There is a clearly stated warning to not go over 24" MP if less than 2400 RPM.

Yet, the power setting graph states a 75% setting at 2400 RPM could be at 27" to 25" depending upon altitude.

How can that be? At 2399 RPM you have to immediately reduce to 24"??

I must be reading/looking at this wrong.

Thanks

Why would you ever need to run it below 24? Maybe in descent, but you would be reducing MP too, so I don't understand the conflict?

 

[jukelemon]

Re: Rcm 112 poh

Why would you ever need to run it below 24? Maybe in descent, but you would be reducing MP too, so I don't understand the conflict?

Hi.

The conflict is that it doesn't make sense that you can be at 27" MP at 2400 RPM yet then have a warning below 2400 - which can be 2399 or 2350. So at 2390, you have to be at 24" or below yet at 2400 it is OK to be at 27".

 

[carllee]

Re: Rcm 112 poh

So at 2390, you have to be at 24" or below yet at 2400 it is OK to be at 27".

Yep, or the engine will detonate, and the airplane will fall from the sky!

 

[jukelemon]

Re: Rcm 112 poh

Yep, or the engine will detonate, and the airplane will fall from the sky!

Carl: how about a real answer? I did not ask the question for sh-ts and giggles and because I had nothing better to do

I am trying to understand why things are stated as such.

Thanks

 

[mcgrathm]

Re: Rcm 112 poh

Think of manifold pressure as torque. The RPM is set with the prop. The prop will decrease and increase pitch as necessary to maintain that set RPM. Your throttle now is used to apply "MP" aka "power" or "torque". As increase MP the engine want to increase in RPM. The prop will sense this and change the pitch of the prop to give you the power you want at that set RPM. At some point you can request more power then the prop can handle at a given RPM like 2400. Those are the settings they are referring to in the manual.

That said, it should not be an issue if you are reducing RPM in the published manner of reducing MP first then RPM.

In my case I takeoff and full throttle and prop full forward. After takeoff I reduce MP to 25" and RPM to 2400 for economy climb I guess you say. Then at Cruise I go to 23" MP until I am ready to descend for landing. I tend to drop my MP to about 20" as I don't need the power and fuel economy making sure my temps don't drop to much.

If I am coming in too fast I will move my prop lever forward a bit which will increase RPM but put an aero dynamic drag with the prop pitch.

At least that is my take on it. It is the same as the C130's I use to work on in the Air Force. The engines were at constant speeds and the power setting would supply more fuel to the engine trying to over speed and the prop would increase pitch to prevent it and give you more thrust.

 

[jukelemon]

Re: Rcm 112 poh

Think of manifold pressure as torque. The RPM is set with the prop. The prop will decrease and increase pitch as necessary to maintain that set RPM. Your throttle now is used to apply "MP" aka "power" or "torque". As increase MP the engine want to increase in RPM. The prop will sense this and change the pitch of the prop to give you the power you want at that set RPM. At some point you can request more power then the prop can handle at a given RPM like 2400. Those are the settings they are referring to in the manual.

That said, it should not be an issue if you are reducing RPM in the published manner of reducing MP first then RPM.

In my case I takeoff and full throttle and prop full forward. After takeoff I reduce MP to 25" and RPM to 2400 for economy climb I guess you say. Then at Cruise I go to 23" MP until I am ready to descend for landing. I tend to drop my MP to about 20" as I don't need the power and fuel economy making sure my temps don't drop to much.

If I am coming in too fast I will move my prop lever forward a bit which will increase RPM but put an aero dynamic drag with the prop pitch.

At least that is my take on it. It is the same as the C130's I use to work on in the Air Force. The engines were at constant speeds and the power setting would supply more fuel to the engine trying to over speed and the prop would increase pitch to prevent it and give you more thrust.

Thanks Mike. I appreciate the details.

But... that still doesn't explain why there is a warning to not go over 24" below 2400 yet you can run it at 27" AT 2400.

Follow? Essentially, it is setting you up to think that at 2390 RPM, you would have to very quickly reduce your MP to 24" or below. Yet, if you increase the RPM by 10 i.e. to 2400, then you can go back up to 27" MP. That sounds odd to me.

Again...maybe I am missing something but I have yet to hear why the above is true...which it seems to be if looking at the warning and performance settings.

 

[mcgrathm]

Re: Rcm 112 poh

Thanks Mike. I appreciate the details.

But... that still doesn't explain why there is a warning to not go over 24" below 2400 yet you can run it at 27" AT 2400.

Follow? Essentially, it is setting you up to think that at 2390 RPM, you would have to very quickly reduce your MP to 24" or below. Yet, if you increase the RPM by 10 i.e. to 2400, then you can go back up to 27" MP. That sounds odd to me.

Again...maybe I am missing something but I have yet to hear why the above is true...which it seems to be if looking at the warning and performance settings.

You are exceeding the limits of the prop to handle/maintain that low of RPM, or in another term, over torquing the engine. I am still not sure why you are reducing Prop before MP?

 

[jukelemon]

Re: Rcm 112 poh

You are exceeding the limits of the prop to handle/maintain that low of RPM, or in another term, over torquing the engine. I am still not sure why you are reducing Prop before MP?

Hi Mike.

Sorry...to be clear...I am not doing what I am calling out. I am just thinking through it and trying to reconcile the 2 things

Frankly, I am never under 2400 in flight so I am not worried about it.

I think you explained it well. Thank you.

 

[carllee]

Re: Rcm 112 poh

Carl: how about a real answer? I did not ask the question for sh-ts and giggles and because I had nothing better to do

I am trying to understand why things are stated as such.

Thanks

Sorry, It's just like the redline on your car tach. It doesn't mean that is the exact point where the pistons are going to start denting the underside of your hood, or your flywheel is going to come up through the floor, but they have to put a limit somewhere. 2400 is just the number they chose.

As mentioned above, probably not going to be running it outside of that box anyway.

 

[bradmc]

Re: Rcm 112 poh

Frankly, I think you're over-thinking this a bit. Yes, in your example if you increase by 10 RPM you can go back to 27", but that's the case for all limits/restrictions in place for prop/MP/gear/etc. Just think of the gear speed of 130kts. At 131 it's not safe, but at 129 it's safe to extend the gear; that's only a two knot difference. How can two knots difference be unsafe????? A limit is really just a number picked by the engineers to help us NOT get into trouble. Follow them, stay safe, and don't over-think them.

 

[Pete]

Re: Rcm 112 poh

I don't run my 114HotShot that way in cruise. I'm a lean of peak guy