Manifold Pressure

[Straiga]

In AHII when I reduce prop RPM with the minus key the manifold pressure decreases also. Well this is not right. Just by changing the RPM only changes the oil to the prop govener and prop dome this should not effect manifold pressure at all, which is the throttle.
The manifold pressure shows the given amount of air in the manifold for a given day and density altitude. As altitude increases manifold pressure decreases unless there is a turbo or supercharger keeping the pressure up. The prop RPM will still stay constant as manifold pressure decreases.


Straiga

[hitech]

Straiga: You are dealing with a supercharged engine. Less RPM causes the  supercharger to spins slower, hence less availible mainifold pressure.

HiTech

[icemaw]

Which planes govern supercharger speed via prop rpm? On these planes are the superchargers driven hydraulically via the prop hub? Never heard of this before. Not like I am any kind of expert mind you.

I really just dont know.

[hitech]

All supercharges are gear or belt driven to the engine. Props are
connected directly to the engine. Hence change Prop rpm, changes engine rpm, which changes the super charger RPM.

HiTech

[icemaw]

Ya OK so I am stupid. LMAO
For some reason I had prop pitch in my mind. Of coarse rpm will effect man pressure DUH what a dope I am.

[Casca]

In reference to the original post in the thread:  Decreasing RPM (with the propeller control) in a naturally aspirated engine will increase manifold pressure.

Edit:  At  other than full throttle.

[hitech]

Yep have noted that in my RV casca.

HiTech

[Captain Virgil Hilts]

Originally posted by hitech
All supercharges are gear or belt driven to the engine. Props are
connected directly to the engine. Hence change Prop rpm, changes engine rpm, which changes the super charger RPM.

HiTech

Turbosuperchargers (P-47, P-38, U.S bombers) are neither gear or belt driven. They are exhaust driven, and manifold pressure is more affected by load than by RPM. My turbocharged engines will reach maximum boost at less than half of maximum RPM.

[Casca]

Superchargers are by definition mechanically driven.  Turbochargers (or turbosuperchargers if you prefer) are, as you indicate, gas driven.  The aircraft you mentioned had supercharged engines that had turbochargers in addition to the mechanical superchargers.

BTW Virgil:  Sorry about your dad (saw it in another thread).

[Straiga]

Turbosuperchargers (P-47, P-38, U.S bombers) are neither gear or belt driven. They are exhaust driven, and manifold pressure is more affected by load than by RPM. My turbocharged engines will reach maximum boost at less than half of maximum RPM

So the RPM drop with a large Manifold pressure drop this should not happen.

Straiga

[Captain Virgil Hilts]

Originally posted by Straiga
So the RPM drop with a large Manifold pressure drop this should not happen.

Straiga

Correct. To drop MAP in a turbocharged plane you have to cut throttle. Increasing load by increasing the pitch will raise MAP on a turbocharged engine.

[Captain Virgil Hilts]

Originally posted by Casca
Superchargers are by definition mechanically driven.  Turbochargers (or turbosuperchargers if you prefer) are, as you indicate, gas driven.  The aircraft you mentioned had supercharged engines that had turbochargers in addition to the mechanical superchargers.

BTW Virgil:  Sorry about your dad (saw it in another thread).

Technically, any mechanical device that increases the amount of air forced into an engine, regardless of what drives that device, is a supercharger. It is what you put in front of the word supercharger that describes the design and what drives it.

[hitech]

Normal aviation nominclature.

Turbocharge Gas driven.
Supercharge Mechanicly driven.

Capt. But I do agree on you technical definition.

And turbochargers I belive can also effected by rpm.  

Basicly the turbo will spin based on the volumn of gas (not fuel) exiting the engine. More Throttle, more expansion, more gas more turbo rpm.

Like you state.

But also, Same gas per stroke, higher RPM,more gas leaving cylinder hence more turbo rpm.

Now some  of the planes we have also had a turbo rpm control to not over speed the turbo. Basicly a type of waste gate before the turbo.

HiTech

[Casca]

Originally posted by Captain Virgil Hilts
Technically, any mechanical device that increases the amount of air forced into an engine, regardless of what drives that device, is a supercharger. It is what you put in front of the word supercharger that describes the design and what drives it.

I agree with you...technically.  In common usage a supercharger is mechanically driven and a turbocharger is not.

As far as turbocharging relates to manifold pressure at full throttle operation (part throttle is a different case) on an engine without a supercharger.  At lower altitudes part of the induction airflow is sent overboard through the waste gate.  The waste gate can be contolled by differential pressures, a simple spring, or occasionally manually in the case of some older turbonormalized equipment.  As the plane ascends the wastegate progressively closes.   At full throttle the altitude at which the waste gate is all the way closed is known as the critical altitude.  Above the critical altitude a decrease in RPM will result in a decrease in MAP (due to the decreased mass airflow though the engine).  An RPM reduction below the critical altitude will result in no change in MAP (the wastegate closes some more to make up the difference).

[hitech]

Casca: AH models it exatly as you describe.


HiTech