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:

 

[svcta]

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.

Say huh?

By this line of reasoning you would make lowest M.P. at takeoff power. The engine will make ambient pressure in the intake manifold when the flow is unrestricted by the throttle. The atmosphere is pushing as much air as it can without restriction in to the engine(all the way into the cylinders).

At a constant RPM you reduce the M.P. by restricting the ambient pressure's access in to the intake manifold by blocking it with the throttle plate. The engine keeps a suckin', but the atmospehere(ambient press.) cannot reach the cylinders. Therefore you create a vacuum, or lower M.P.

SO! If you maintain a constant throttle(M.P.) setting and change the RPM, in this case reduce the RPM, the manifold pressure has to increase because your big sucker(the engine) is asking for less air(not sucking so much) and not vacuuming the M.P. down. Simplest terms: The RPM decreased at a given Throttle position will result in increased Manifold Pressure.

I hope this will settle it.

 

[VNugget]

Say huh?

By this line of reasoning you would make lowest M.P. at takeoff power.

He said RPM, not power.

 

[svcta]

It took me a while, but I think I understand the point Time Builder was trying to make. At a given throttle setting(with the prop in govern but less than full power) an increase in RPM would certainly decrease MP. But the engine will make equal MP while @ full power (combination of RPM and MP) or while shut down. It has to. Friction? I think if you suffered a slow down in the air flow after the throttle that would cause an INCREASE in MP. This because the world is still trying to force more air in to the manifold regardless of throttle position, and if friction slowed it down before the cylinders press. would have to increase because you would have more air trapped in the manifold. This does not happen. MP is controlled by choking the engine, literally, and then compensating for the lack of air with reduced fuel flow(accomplished at the metering device) to keep the engine running properly. Clear as mud?

 

[time builder]

But the engine will make equal MP while @ full power

The point is that the engine doesn't "make" MP unless you're turbocharged. You can suck as much as you want, but you'll still only get the atmoshperic pressure minus whatever you lose in friction.

It's the same thing with your lungs, breathing faster won't increase the amount of air pressure, instead, its better to take slow deep breaths.

 

[svcta]

okay, so you're stuck on the semantic of "making" power. It's just an expression, don't get wrapped around the axel. In so much you are correct, and it has been my point all along if you read at all that you will only get what good old Ma Nature is throwing at you. But I think we need to visit this idea of friction. How do you lose anything because of friction? Again, just to entertain the theory if you had a slowing down of air inside the manifold you would get an increase in MP. I don't get it.