Manifold Pressure vs. RPM

Aceninja

Aceninja

Well-Known Member
I recently started flying a Dutchess which comes with Constant speed props. Regarding the relationship between the RPM and Manifold pressure, I would like to know why as the RPM is reduced, the manifold pressure increases?

Ex: To enter cruise flight, we reduce the manifold pressure to 18", then reduce the RPM to 2300, which brings up the manifold pressure to approx 20". Can anyone comment on this inverse relationship?

Any help would be sincerely appreciated.
 
That's the link I was going to supply, I know a lot of people have no idea why MP increases with a decrease in RPM. Here's another link to a bunch of small articles, not necessarily on that topic but this guy does know what he's talking about and I've learned a fair bit from his short blurbs: http://www.pittspecials.com/articles.html
 
Now a real-world example: Assume you're cruising at some low altitude (say 4,000 feet), throttled well back to about 20 inches MP and 2,000 RPM. (Remember, this means the throttle plate is somewhat cocked, restricting induction airflow.) Now reduce the RPM to 1,200 without changing anything else, and you'll see the MP rise sharply. Why? Simple: The ambient pressure hasn't changed; the throttle plate hasn't changed; the only thing that has changed is the speed at which the pistons are pumping the air. Since they are moving much more slowly at the lower RPM, they are not sucking nearly as hard — not creating as much of a vacuum — so the MP goes up, towards ambient pressure. The natural extension of this experiment is to reduce the RPM to zero, when the MP will rise all the way to outside ambient pressure (about 25 inches at 4,000 feet).
Click to expand...

Great article!
 
You must log in or register to reply here.
Back
Top