MAP variation with DA

[gisurvey]

Hi all,

 

Maybe a dumb question, but...

 

Rotax912ULS

What MAP do I have to expect for cruise flight, 5000rpm, 10000'DA?

 

I know that in standard conditions ISA, sea level, for 5000rpm is expected to have 27.2inHg.

 

Thanks,

Alec

[coppercity]

Hi Alec,

 

I usually see between 18-19 in/Hg at those DA's?

[Ed Cesnalis]

It depends on your prop pitch.

[gisurvey]

Thanks Eric & CT.

I know that MAP depends by prop pitch. In fact, I have a variable pitch prop and I want to know what MAP is "ideal" (to be achieved/maintained) for another conditions than ISA. And I'm not quite agree with the rule of thumb to subtract the lapse rate of 1"inHg/1000' from MAP at ISA conditions.

[Roger Lee]

You should set your prop to see 5500-5600 rpm at your average altitude since you can't fly at that exact altitude all the time. The MAP isn't a great help with the 912 and is better suited for the 914 turbo or constant speed prop. Trying to use the MAP pressure to set the prop will have you chasing a pitch. Go fly (the best way) at your average altitude and fly it at WOT. This will help remove any guess work. Come back and I can tell you what how many degrees to add or remove. Since this is a normal aspirated engine MAP isn't real important. It will always be 1 - 1.5 in. lower.

[gisurvey]

...

I know that MAP depends by prop pitch. In fact, I have a variable pitch prop and I want to know what MAP is "ideal" (to be achieved/maintained) for another conditions than ISA.

...

 

For clarification: is about variable "in-fly" pitch prop, european version CTLS.

[Ed Cesnalis]

The ideal mp at 10,000 feet is whatever you get pitched for 5,500 rpm and at wide open throttle. Ideal for realizing maximum power that is.

 

If you want to fly around at 65% power or less at that altitude you would use a lower throttle setting and get a lower mp and it would be ideal for 65%. In other words the ideal mp depends on your mission.

 

Most of us are in the US and have fixed pitch props so MP means nothing to us when flying our CTs.

[Roger Lee]

I'm with Charlie Tango. We have had this discussion and tested this back in early 2007.

Ed flies out of high altitude and fly's between 9K'-13.5K' in normal everyday flight. He lives in Mammoth, CA. home to high altitude flying. He's been there and done that. So have the other 100 LSA aircraft owners that I have re-pitched there props. Just fly to 10K' (if that's where you normally fly) and you want to see 5500-5600 rpm WOT and you don't care what the MAP is.

[gisurvey]

ok, thanks for your answers.