constant speed prop question

[capt. skidmark]

dear fellow instructors,

i have had a question from a student why exactly the manifold pressure increases when the prop lever is pulled backwards and vice versa.

i tried to approach this answer with the total pressure formula etc.
or that the pressure builds up since the prop gives more resistance to the engine when the prop is in high pitch, but i started to doubt if this is correct.

i cant find anything in my books about this, can any of you give a simple (american style) answer?

thanks a lot,

sm

[rotorfossil]

When the RPM reduce, the velocity of the air through the carburettor butterfly reduces, hence less pressure reduction and the indicated MAP goes up.

[Brooklands]

Its probably worth having a read of the John Deakin articles on the subject. They're in his Pelicans Perch (http://www.avweb.com/news/columns/182146-1.html) colum on Avweb. This one (http://www.avweb.com/news/columns/182081-1.html) and this one (http://www.avweb.com/news/columns/182082-1.html) may be relevent.
Brooklands

[Send Clowns]

As RPM increases you are drawing more air in to fill the cylinders more frequently. There has to be a greater pressure drop in order to draw in more air, so the pressure drops until sufficient differential is achieved.

Try it at high power on a turbocharged engine. The opposite happens, because as you reduce RPM the turbocharger is slowing down too, and boost drops.

[Islander2]

i have had a question from a student why exactly the manifold pressure increases when the prop lever is pulled backwards and vice versa

A good question, which has not been matched by good answers!

With the engine running, the manifold pressure gauge is actually a vacuum gauge. It is measuring the suction created by the pistons on their induction strokes drawing in air against a partially-closed butterfly valve.

With the throttle full open, the butterfly valve offers little resistance and there is almost no suction ... the manifold pressure gauge then reads only slightly less than the surrounding air pressure (this small difference being accounted for by filter and other intake losses).

With the thottle fully closed, the butterfly valve is also almost fully closed, the pistons are sucking against its resistance and cannot move much air ... so the manifold pressure drops to a substantially lower value than the outside air.

Now, increase the RPM against a partially closed throttle and the pistons (the suction pump, if you like) are pumping much faster, creating more suction ... so the manifold pressure drops.

Correspondingly, reduce the RPM with a partially closed throttle and the pistons pump less hard and create less suction, the pressure drop from the outside air reduces ... and so the manifold pressure thereby increases.

Does that help?

[FlyingForFun]

As Islander2 says.

When thinking about these things, I always find it easier to think about the case with the throttle partly closed. That way, there is some real "sucking" going on. The engine is trying to suck air in from the inlet manifold, and the throttle, being partly closed, prevents the air from being completely replaced, hence a manifold pressure which is lower than atmospheric pressure.

The slower the engine turns, the less it sucks air out of the manifold, therefore the higher (and closer to atmospheric pressure) the manifold pressure. As an extreme, when you stop the engine the manifold pressure goes right up to atmospheric pressure.

(The same is, of course, true with the throttle open, but I find it harder to visualise with the throttle open.)

FFF
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[capt. skidmark]

thank you all for your explanations, its clear to me now!

thanks,

sm