Manifold Pressure vs Propeller Speed

[Blarney]

I'm having trouble understanding how additional manifold pressure, past what is needed to maintain a 3000rpm prop speed, affects speed.

 

If the propeller produces thrust, it makes sense that the faster the prop spins, the more thrust is produced. So, for a constant speed propeller, it would take a certain manifold pressure to achieve that maximum prop speed at a given altitude.

 

However, If I'm flying at 2000ft, my prop speed is 3000rpm, and my manifold pressure is 46in HG, why would I need to increase to 61in HG? Or, even further, why would I need WEP?

 

I have done experiments in the sim where my airspeed is ~250mph with ~45in HG of manifold pressure. My prop speed is 3000rpm. Then I increase manifold pressure to 61in HG and I begin to accelerate.

 

WHY?!? My prop speed has stayed the same at 3000rpm.

 

Similar situation during take off. Why do I accelerate faster down the runway at a higher manifold pressure even though my prop speed is the same (3000rpm)?

 

Thanks.

[Yo-Yo]

The answer is - PITCH.

The rpm is maintained with the automatic regulator that changes pitch to make prop dissipated power equal to the power engine can provide.

Any time you change MP this regulator works for you changing prop pitch.

The same thing is for speed changes.

[Ich666]

What will happen if you add MP? The propeller will try to hold the speed at 3000 RPM, thus will increase its pitch to do this. This increases thrust, as the propeller now moves more air due to its increased pitch, while still having constant RPM.

 

EDIT: Yo-Yo sniped me there. Should really type faster next time ;)

[Blarney]

That makes sense.

So is it fair to say that as good as the Merlin engine was, it was not able to take full advantage of what the propeller was capable of, even at 1400+ HP?

If so, engineers must have been constantly trying to balance engine power with propeller design.

[sobek]

So is it fair to say that as good as the Merlin engine was, it was not able to take full advantage of what the propeller was capable of, even at 1400+ HP?

 

No. You need to keep in mind that as speed increases, the prop has to increase its pitch so that RPM stays constant. You need a margin for speed and torque.

[peter]

Think of it this way. The propeller maintains a CONSTANT speed, yes, but the pitch varies depending on your demand (read Manifold Pressure). Look at what happens when you reduce manifold pressure enough, the prop RPM falls off, why? Because it has fine'ed out the pitch to the point of maximun fineness so it can only fall below the RPM of what you have selected. The engine is doing everything it is supposed to do. :)

[Chill31]

Think of it this way. MP is your torque control. Increase MP, you increase engine torque. HP is Torque X RPM. Increase Torque, even though RPM is the same, you must have increased HP.

[Yo-Yo]

Think of it this way. MP is your torque control. Increase MP, you increase engine torque. HP is Torque X RPM. Increase Torque, even though RPM is the same, you must have increased HP.

 

Not accurate explaining - increasing MP you increase POWER at current rpm at current prop pitch setting. If the pitch was constant the rpm would increase. Prop governor increase the pitch to maintain constant prop speed so the resulting effect can be seen as torque.

[Chill31]

Not accurate explaining - increasing MP you increase POWER at current rpm at current prop pitch setting. If the pitch was constant the rpm would increase. Prop governor increase the pitch to maintain constant prop speed so the resulting effect can be seen as torque.

 

Its accurate. We are saying the same thing in different ways. I was just providing an alternate explaination for anyone it might help understand.

 

I think you are referring to torque effect where I am referring to mechanical torque in the engine.

[Yo-Yo]

Its accurate. We are saying the same thing in different ways. I was just providing an alternate explaination for anyone it might help understand.

 

I think you are referring to torque effect where I am referring to mechanical torque in the engine.

 

I meant the same but I only extended your explanation - MP changes torque if the prop governor keeps rpm. Maybe it's a kind of purism though...