Originally Posted by
the King
I think you're on the right track with the change in prop pitch creating more work for the crank shaft and adding to manifold pressure. If there's another reason, I'd like to know as well.
Manifold pressure rises any time the prop is set to a higher pitch/lower speed. In our DA40s, I have my students reduce MP an inch lower than desired then reduce the prop. Works almost every time.
Something like that.
Key Concept: MP is the air pressure in the manifold and is determined by how much air goes into AND out of the manifold...change either one and MP changes.
-Throttle controls the plate which allows air INTO the manifold.
- The engine is an air pump...it sucks air out of the manifold with each intake stroke. The faster the engine runs, the more air is sucked out of the manifold.
What actually happens when you cycle the prop...
1) Prop control causes the governor to drive the prop to a deeper pitch.
2) The prop takes a deeper bite out of the air, moving more air and doing more work.
3) Since the throttle has not changed, the engine power did not increase. Since the prop is now doing more work, but the engine is making the original power, the extra load on the prop drags down the engine.
4) As the engine slows, it sucks less air out of the manifold.
5) But the throttle setting didn't change...
So we are allowing the same air into the manifold (throttle), but sucking less out (slower engine), so the MP rises slightly. This rise will also cause a slight increase in engine power, but not enough to overcome the high-pitch prop's drag.