How Low MP Is Too Low?

[johnggreen]

This morning I went out to shoot some approaches in the Bravo.  As you fellows who have corresponded with me probably know, I fly my Bravo at relatively low power settings, even cross country.  Once at "cruise" I usually set 2200 rpm simply because it is unbelievably quiet and smooth and well, it just feels good.

When I am just flying around setting up for approaches, holding patterns etc. I usually set 2200 rpm and 26" MP.  Today, I pulled the MP back to 22 or 23 where I can still indicate almost 130 knots and burn only 10-11 gallons per hour.  At that manifold pressure, I had trouble maintaining 2200 rpm.  It wanted to drop off a little.  Everything was normal as far as temps, but the OAT was already at 90 F and climbing.  I just pushed the power up to 25" and all was well.

Just curious, has anyone else found that they can't maintain rpm's below 24"?

I have no reason to think there is anything wrong.  I just never noticed it as an issue.  I routinely bring the power back to 22" for pattern and approach maneuvering, but never remember the rpm's falling off.

Jgreen

[carqwik]

Interesting post...I know mine do fall off at lower MPs but whether or not it maintains 2400 rpm at 22" I can't recall. 

[sreid]

Wasn't it just on the flat pitch stop?

[Skywarrior]

Quote: sreid

Wasn't it just on the flat pitch stop?

[johnggreen]

Sreid,

I assume you mean wasn't the prop control fully forward?  No, it was set at 2200 rpm's and no where clsoe to full flat pitch.

Jgreen

[Piloto]

I know that on the Ovation at FL 170 you can go down to 15"MP and maintain 2400RPM easily with fuel flow of 10gph or less.

José

[johnggreen]

To update my original post.

I have flown twice since the post.  The first was a cross country, fighting thunderstorms, meaning, avoiding thunderstorms, and I didn't take time to experiment.

Yesterday, I went out to check out a couple of things for a long cross country later this week and I tried low power under various RPM settings.

Right after take off, I leveled off at 1200' and pulled the power back to 22".  The prop rpm initially lowered slightly from 2400.  After moving the prop setting to 2200 and then back to 2400, everything went back to normal.  Then, I could lower my MP to the mid teens without an issue.

I'm thinking that I may not be exercising the prop enough before takeoff, perhaps?  I could understand this issue at 40 degrees but not at 90.

Also, I never noticed the issue until I had the prop overhauled 25 hours ago.

Jgreen

[Piloto]

John I don't think you have a prop problem but an airspeed situation. If you lower the MP on the climb the prop will turn slower due to lower airspeed drive. But if you lower the MP on descent the RPM will remain constant due to a higher airspeed drive. If you do not see an RPM change on descent your prop is ok.

José

[johnggreen]

Jose',

I think you are right.  I just never noticed it before.  Kind of like flying over water.  You hear bumps and misses you never heard before.

Jgreen