Understanding relationship between RPM and manifold pressure

[HOKUM52]

Hi

 

I've been flying the P-51D for ages and i know how to get the best performance out of the engine but i don't understand the science behind what im doing. In other words i just learnt a procedure.

 

This is really what i cant get my head round...

 

In the P-51 you set the propeller pitch using the prop pitch control lever, but you can't actually read its pitch from the cockpit, instead my understanding is that you are indirectly controlling the engine RPM, which you can read from the cockpit instrument. correct??

 

I do understand that the MP is the pressure of the fuel air mix in the induction and thus the higher the pressure provided to the piston heads the more power the engine can produce.

 

But, say I set an RPM of 2400 the engine can maintain this speed whether the manifold pressure is at say 50 or as low as 30 etc. So if the engine is operating at the same speed regardless of the manifold pressure, why does the aircraft still speed up or slow down when MP is changed. Is something happening to the pitch?

 

I am aware that to little MP and an engine RPM cannot be achieved but once you have set the MP high enough and it can be achieved any increase in MP is just excessive (i think im starting to answer my own question) but why then does it result in an increase in speed.. it must have something to do with prop pitch right ???

 

 

 

 

Can anyone explain in comprehensive detail or perhaps direct me to a webpage that will really clear up my understanding of whats going on.

 

Thanks

[sobek]

In the P-51 you set the propeller pitch using the prop pitch control lever, but you can't actually read its pitch from the cockpit, instead my understanding is that you are indirectly controlling the engine RPM, which you can read from the cockpit instrument. correct??

 

You control a governor that sets the pitch so that the RPM you dialed in is held, regardless (within limits) of how much torque the engine produces.

 

 

I do understand that the MP is the pressure of the fuel air mix in the induction and thus the higher the pressure provided to the piston heads the more power the engine can produce.

 

MP is the pressure inside the induction system after the throttle right before the inlet valves, thus it gives you an indication of how much fuel/air is sucked/forced into the cylinder, therefore it is (in conjunction with RPM) an indicator for power.

 

 

But, say I set an RPM of 2400 the engine can maintain this speed whether the manifold pressure is at say 50 or as low as 30 etc. So if the engine is operating at the same speed regardless of the manifold pressure, why does the aircraft still speed up or slow down when MP is changed. Is something happening to the pitch?

 

Yes, as i mentioned, the RPM governor changes the pitch to hold a constant RPM. When you increase MP, the engine produces more torque, so the RPM governor increases the pitch which results in more thrust.

[WildBillKelsoe]

This is best coupled with animations..

[HOKUM52]

You control a governor that sets the pitch so that the RPM you dialed in is held, regardless (within limits) of how much torque the engine produces.

 

Which RPM are you referring to that I dial in? Engine or prop. The cockpit instrument displays engine RPM.

 

 

Yes, as i mentioned, the RPM governor changes the pitch to hold a constant RPM. When you increase MP, the engine produces more torque, so the RPM governor increases the pitch which results in more thrust.

 

This suggests to me that the governor is maintaining the engine RPM at a constant speed rather than prop RPM. In other words as the MP is increased the engine generates more force/ torque, this would result in an increase in engine rpm but before it does the governor increases the prop pitch. ?

 

Am I correct in saying the following? : Though called a constant speed propeller you can select the speed you wish it to maintain. It just seems to me that we from the cockpit can only select an engine speed to be maintained and thats what determines the propeller RPM. Why is it called the propeller control lever and not the engine RPM lever. Is therefore the engine rpm directly linked through a gearbox to the Prop RPM?

[sobek]

Which RPM are you referring to that I dial in? Engine or prop. The cockpit instrument displays engine RPM.

 

The difference is of no consequence, the engine runs at prop RPM or at least at a fixed ratio, i can't remember which for the mustang.

 

There is no torque converter between prop and engine, in fact, the prop itself is the torque converter.

 

It is called constant speed because the governor maintains the speed you set regardless of MP and aircraft speed. If you were, say, in the Bf-109 and you turn the auto prop control off, you still have a variable pitch prop, but no longer a constant speed prop. That is, you still have control over pitch, but it will just maintain the pitch you set, not a RPM setting.

Edited December 22, 2014 by sobek

[HOKUM52]

:doh: Thanks its clear now!!

 

My question arised after flying the 109.

 

I have another question therefore but no rush to answer this one

 

How does the 109's engine automatically change the RPM?

[sobek]

I don't know the exact mechanism for the 109, but i translated some docs for the dora that dealt with the engine control system.

 

Essentially, you still have both levers (not physically, but the mechanisms are the same), only now they are driven by a common lever. To make sure that the RPM/MP ratio is always optimal, one of them rides on an excentric tappet with a certain input curve to achieve the forementioned optimum setting.

[Art-J]

As a sidenote, engine-to-prop RPM ratio of reduction gear in V-1650-7 was 2.08:1.

 

Actually, it was around 2 for most of the WWII planes, with exceptions for some silly big props, like these on B-29s (~2.85:1)

[Crumpp]

Is therefore the engine rpm directly linked through a gearbox to the Prop RPM?

 

Sometimes....basically the tachometer reads engine rpm but the propeller is controlled by the governor. The governor uses spring pressure and engine oil pressure to keep the engine rpm constant by adjusting the propeller blade pitch. Some have their own oil pumps as is the case with the Hamilton Standard so it uses spring pressure, engine oil pressure from the propeller gear housing, and its own governor oil pressure to keep the engine at a constant rpm.

 

It can only do this over a set range before manifold pressure changes and rpm changes so you have to keep in tolerances while maneuvering.

 

The basic rules for CSP rpm and manifold pressure are:

 

1. Increase power by first increasing rpm, then adjusting manifold pressure

 

2. Decrease power by reducing manifold pressure and then reducing rpm.

 

Here is the illustrations from the Mustang Maintenance Manual. It explains the propeller operation for both the Hamilton Standard and the Aeroproducts propellers.