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3 Answers

RPM and boost

Asked by: 3880 views Aerodynamics, Aircraft Systems, General Aviation

CFI quoting a poorly-done old POH, or perhaps

just regurgitating what his CFI taught him -- has told you that the first power reduction after

takeoff should be to 25 inches and 2,500 RPM. Leaving aside the issue of whether this is really a

good procedure, let's assume you take off and dutifully pull the throttle back from about 29

inches, to exactly 25 inches. Then you pull the prop control back from 2,700 to 2,500.you are 

surprised to see the MP rise to 26 inches or so as the RPM comes down. please explain the phenomena behind it

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3 Answers



  1. Brian on Aug 30, 2012

    Rather than rewrite or copy and paste I will direct you here:

    http://www.avweb.com/news/pelican/182081-1.html

    This article covers the topic in question as well as any other questions you might have regarding this system. It truly is a must read for anyone moving into constant speed systems in my opinion.

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  2. Jon on Aug 30, 2012

    Interesting that the question was pulled directly from the article that was pointed to as an answer. Khoda, let me reiterate Brian’s suggestion, though – read and try to understand the entire article. It’s a great article.

    In short, though, the manifold pressure gauge shows the amount of suction behind the throttle plate. When the engine is not running, the pressure behind the throttle plate is the same as open air, and MP will read ambient pressure. When the throttle plate is closed and the engine is running, the pistons are sucking as hard as they can, so there’s a big vacuum behind the plate and the MP reads low pressure. Key point: it’s the pistons pulling air into the cylinders that creates the vacuum and lowers the MP from ambient to whatever the MP gauge reads. To answer your question, when you pull the prop control back, you’re telling the prop to slow down (by increasing pitch and taking more bite out of the air). When the pistons move slower, they’re not creating as much of a vacuum behind the throttle plate, so the MP rises. It may help to think of it as ‘generating less vacuum’ rather than ‘increasing manifold pressure’.

    Hope that helps.

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  3. Brian on Aug 30, 2012

    Whoever thought the thumbs down was necessary, here is a quote from the article. This is the 4th paragraph, and first question the author poses and later answers:

    “Then you pull the prop control back from 2,700 to 2,500. Are you surprised to see the MP rise to 26 inches or so as the RPM comes down? Do you understand clearly why that slight rise occurs?”

    …You might notice it’s nearly identical to the one asked here. Sometimes reading before smashing a button proves useful. Oh well, I know so many people like to be spoon fed, god for bid someone gives you 2 pages to read.

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