Do I really need to see MAP INHG?

[Piper J3]

I'm going to fly again on Weds. Maybe I'll cover the tach with a sticky note and give it go. Could be I'm letting the best in life past me by....

[Mel]

Originally Posted by Piper J3

I'm going to fly again on Weds. Maybe I'll cover the tach with a sticky note and give it go. Could be I'm letting the best in life past me by....

I wouldn't cover the tach. It's still a good reference.

It's just that the MP reacts more quickly.

[uk_figs]

Originally Posted by Mel

Manifold pressure is handy for setting power. When changing power settings, RPM takes a while to settle down. Once you are used to your airplane, you can adjust power by using MAP much faster.

Mel
Is there a chart for the 0-360? I have never really looked at MP as I thought it was not relevant with a FP prop. Learn something new every day.
Figs

[Sam Buchanan]

There is the old rule of thumb:

24 square = 75% power

The application is any combination of rpm and map the adds up to 48 is 75% power.

No doubt an engine-appropriate chart will refine that rule but it might be a starting point in absence of a chart.

[bbaggerman]

Originally Posted by Mel

You can still set power faster with MAP than with RPM. MAP is definitely useful with F/P prop. I flew my RV-6 with F/P for over 25 years making power adjustments with MAP.

On my RV-12 a previous owner installed a 3-bladed Whirlwind prop. I understand the 3-bladed prop is supposed to be smoother and quieter. I don't have any experience with the 2-bladed standard version so I am not sure of the pros and cons.

One of my issues with this prop is that since it was installed it was setup such that on the ground WOT results in 4900 RPM. I think that is lower than the usual 5200-ish that you get with a standard prop. Takeoff performance is great and it doesn't have any problem reaching higher RPMs in flight so I am disinclined to try to adjust it. So my quandary has been how to set power. With a standard prop high cruise is 5500 RPM and low cruise is 5000 RPM. But 5500 RPM "seems" too high for me. Fuel flow, MAP, and air speed all seem higher than expected.

My approach then has been to look at MAP pressure rather than RPM for setting power. Like Mel, my experience has also been that it is easier and quicker to set power using MAP rather than RPM.

Which raises another question I've been meaning to post. What MAP do you get with a standard 2-bladed prop setup by the book? What pressures do you see at 5000 and 5500 RPM? The Mooney I used to fly was very happy at 23" but the Rotax seems to want to run higher. If I don't exceed 5500 RPM any idea what max continuous MAP is for the 912 ULS?

[Tommy123]

MAP with a fixed pitch sort of like RPM in a car with a automatic transmission but it still does tell you something. Mel is correct, MAP is quicker and isn't flopping around like RPM can do.

[seagull]

Originally Posted by Sam Buchanan

There is the old rule of thumb:

24 square = 75% power

The application is any combination of rpm and map the adds up to 48 is 75% power.

No doubt an engine-appropriate chart will refine that rule but it might be a starting point in absence of a chart.

Does this rule apply with a geared engine? The 912ULS is 2.43 to 1.

I use RPM in the -12, I find the resolution to be more precise changing only 10 - 20 (engine) RPM than trying to set .1 of manifold pressure. Actually it is 2.43 times more precise than using propeller rpm on non-geared engines.

In the 172 I use manifold pressure because it is more precise than propeller (engine) RPM.

[Amadeus]

Originally Posted by seagull

Does this rule apply with a geared engine?

Of course not.

Sam Buchanan was replying to the question from uk_figs asking for a chart for the O-360.

[bobnoffs]

well.....lets get to the elephant in the room since it brought up the original question. what map numbers for the 12?

[seagull]

Originally Posted by Amadeus

Of course not.

Sam Buchanan was replying to the question from uk_figs asking for a chart for the O-360.

sorry, being the -12 forum, I assumed