Constant speed prop/Tach question

[flhrci]

So the 182 I got checked out in yesterday with constant speed prop has a green arc for the tach guage. I understand the redline is bad but CFI said to keep RPM's in the green arc and do not go below. So, have to ask, what happens if it goes below the green line?

David

 

[Skylane81E]

You go slower

 

[Henning]

You end up 'lugging' the engine at higher MP which greatly increases the susceptibility to detonation. Typically the governor will not let the RPMs get below the green arc until you seriously reduce the manifold pressure to below where this is a risk.

 

[denverpilot]

There's no documentation anywhere that I can find if your flying behind a Continental in a 182 where either Continental gave Cessna a number and said, "below here is bad a full power" and Cessna appears to have quite literally made it up in the 70s unless there's a doc I haven't found yet. Nothing in the TCDS. No Continental service bulletins, nada. Maybe it's out there but I haven't found it.

That said, I have no intentions of being a test pilot and if you study the power charts there is very little to no gain to going below the green arc in cruise in an O-470 powered 182. It doesn't buy you anything other than a really slow flight.

Do note that there is NO LIMITATION on running the O-470 at redline. I've seen people up at this altitude on a hot day trying like mad to get the prop back after takeoff into the top of the green and they end up with an anemic 100 fpm climb rate. Just leave the prop full forward and climb if you need to. Yes it's a noise problem for the airport neighbors. No there's no reason to climb out completely flat and have no options if the engine actually does quit.

On a good performance day or down near sea level I get the prop back "as soon as practicable". On a hot high day? Screw it. I'm making noise. I didn't buy a 230 HP engine to de-rate it on a crappy climb rate day.

 

[MAKG1]

There is also the possibility of an internal resonance (such as valve float or a crankshaft mode) at a certain RPM. Some engines have an avoid range for this reason.

 

[Skylane81E]

There is also the possibility of an internal resonance (such as valve float or a crankshaft mode) at a certain RPM. Some engines have an avoid range for this reason.

True, but those get marked for it. In this cas the green is just a "normal operation" range. And in this cas it is 2100-2400, not much need to operate out of it.

 

[denverpilot]

There is also the possibility of an internal resonance (such as valve float or a crankshaft mode) at a certain RPM. Some engines have an avoid range for this reason.

No possibility on the O-470. Doesn't exist. The thing is a giant vibrating 7.7L Harley engine. There is nothing but "internal resonance". Heh.

 

[Tom-D]

There is also the possibility of an internal resonance (such as valve float or a crankshaft mode) at a certain RPM. Some engines have an avoid range for this reason.

Actually no, the slower you run any engine the less chance of valve float, or they would float at idle.

the reason for the low RPM limit is simply the high pressures the cylinders will encounter at low speeds and high MAP.

 

[NineThreeKilo]

I know in the IO550 you don't want to spend much time above 25 squared or below 2300RPM. Prolonged ops outside of there can turn out poorly.