Prop RPM vs MAP....

[Anatoli-Kagari9]

I know the p51d has a supercharged engine and that it's regulator tries to keep a constant MP, but on ground, say durin run.-up, with MAP settings around 30-32 mmHg, reducing Prop RPM causes an increase in MP, and increasing Prop RPM reduces MP. This is at least the effect I see in the P51d.

 

Above which power settings, density altitude, etc... should I see the MP going on pair with RPM changes?

 

Anyway, I believe that just as with any other CS prop engine I should allways reduce throttle 1st, then Prop RPM, and the way around - increase Prop RPM first then throttle....

[Yo-Yo]

I know the p51d has a supercharged engine and that it's regulator tries to keep a constant MP, but on ground, say durin run.-up, with MAP settings around 30-32 mmHg, reducing Prop RPM causes an increase in MP, and increasing Prop RPM reduces MP. This is at least the effect I see in the P51d.

 

Above which power settings, density altitude, etc... should I see the MP going on pair with RPM changes?

 

Anyway, I believe that just as with any other CS prop engine I should allways reduce throttle 1st, then Prop RPM, and the way around - increase Prop RPM first then throttle....

 

On A2A forum you have found a brilliant explanation. I just can add my own earlier explanation that is almost the same.

 

UPD

OMG! :) I feel like a worm getting out from soil and having one hour of nice conversation with another worm always being in accord... as they wished good-by the truth broke that it was his own tail... the quotation is my own text translated in English. :D

 

Concerning the MP behavior I can add that it depends on two nonlinear functions that have the slope depending on absolute rpm. The blower pressure ratio function rises with rpm has the slope increasing with rpm. The MP function for the after-throttle pressure goes down with the rpm, and its slope decreases with rpm. It means that there are two areas of rpm - the lower area where the second function slope is greater then the first one and the upper with vice-versa situation. So we can see that at low rpm MP drops as rpm rises and at higher rpm it grows.

 

To avoid MP regulator effect these tests are better to perform at the altitude higher than the critical for the highest rpm we want to test.

Edited December 19, 2012 by Yo-Yo

[Anatoli-Kagari9]

Yes Yo-Yo, but I believe that DCS is doing it closer to real because, as you also point out on another thread:

 

"The automatic regulator does not work throughout the entire performance envelope of the engine. In the V-1650-7 model engine [...], it begins to function at 40 in.Hg. Below this value, manifold pressure is controlled exclusively using the throttle handle and all of the effects described above can be witnessed..."

 

meaning that, bellow that limit increasin Prop RPM lowers MAP and the opposite aplies as well...

Edited December 19, 2012 by jcomm

[sobek]

I have no empirical data but i feel that in the whole RPM governor range from 1400 RPM to 3000, you won't be seeing the inverse MP behaviour, it's only really apparent on the ground.

[Yo-Yo]

I have no empirical data but i feel that in the whole RPM governor range from 1400 RPM to 3000, you won't be seeing the inverse MP behaviour, it's only really apparent on the ground.

 

"I feel" - it's not an engineer's argument :). Especially in adiabatic area. Do you remember the equation for the pressure ratio of a blower vs its rpm?

[sobek]

"I feel" - it's not an engineer's argument :). Especially in adiabatic area.

 

I wasn't trying to make an argument. :)

 

Do you remember the equation for the pressure ratio of a blower vs its rpm?

 

No, i'm an audio engineer, the only adiabatic equations i know are for sound pressure and velocity propagation. ;)

[Anatoli-Kagari9]

Adiabatics are with me :-) - Meteorology ;-)