RPM increase when cycling the prop? Why??

[gear goes down]

I came across an interesting question the other day and I (sadly) don’t know the answer, I have ideas, but don’t think they are right.

Anyways, why does not RPM increase when cycling the prop in the run-up?

 

[Workin'Stiff]

The RPM should be slowing down and increasing when you exercise the governor...

 

[wt219200]

Do you mean why does manifold pressure increase? If so it is because as you pull the prop lever to high pitch low RPM it creates more pressure on the engine because the blades are creating more resistance to air flow.

 

[GravityHater]

I think it (MP falls as pitch is coarsened) happens because when the blades meet more resistance (presented broadside to the air)...............

the engine is no longer sucking as much air - simply because it is slowing down.

 

[avbug]

Retarding the propeller RPM when cycling the propeller does not cause an RPM increase. It causes an RPM decrease, because by retarding the propeller toward "coarse" you are increasing the propeller angle of attack, increasing the drag on the propeller, and slowing it down.

The engine is a suction machine. It sucks air in. It acts just like your vacum at home. Put your hand over the vacum cleaner hose (shopvac) and what happens to the air pressure in the hose? It drops; it decreases. Your hand is the throtttle.

Remove your hand from the hose a little at a time, and as you increase the opening and allow more and more air to flow through the opening to the vacum cleaner hose, the air pressure rises in the hose. Your hand is the throttle, and the throttle is an air valve. It's function is to let in more air. At sea level on a standard day, the air pressure in that hose can't rise above 29.92" of mercury...it reaches it's highest value when the engine is shut down.

Why does manifold pressure increase when you retard the engine RPM? As the engine slows, it isn't drawing as hard, just as slowing down the vacum cleaner with your hand covering the hose would have an identical effect; pressure in the hose would increase because as the vacum cleaner slows down, it's not drawing as hard (creating as much suction), and subsequently the air pressure increases. The same thing occurs with the engine. Close the throttle, retard propeller RPM, and see a rise in manifold pressure because the engíne isn't turning as quickly, and therefore producing as much suction.

 

[siucavflight]

Um, yeah. The RPM does not increase when cycling the prop. The M.P. does, as the engine is sucking less air in. But if your aircraft is increasing RPMs as you are cycling the prop, you better have MX look at it.

 

[gear goes down]

Yeah, I typed that fast. I actually meant M.P. increase.

"IM SOFA KING WE TODD DID." Say that slow, that’s how i feel right now.

 

[svcta]

the engine is no longer sucking as much air - simply because it is slowing down.

Therefore M.P increases. Perefectly correct.

 

[time builder]

Your engine can only draw in the amount of air pressure available, you lose manifold pressure with friction in the air intake system. The higher the RPM, the faster moving air, the less pressure available. Hence, the highest amount of manifold pressure available is when the engine is not running.

 

[Princedietrich]

Maybe someone installed his prop governor upside down!

:laugh: