RPM decrease - manifold pressure

When you decrease the RPM shouldn't the manifold pressure increase? Is it the supercharger or the manifold pressure regulator? Maybe someone in the know can elaborate a bit the technology behind it.

The supercharger's flywheel is connected to the crankshaft. Higher RPMs make the flywheel spin faster, which makes the MP higher.

The high blower setting changes gears between the flywheel and crankshaft, causing the ratio to be more like 11:1 rather than 8:1. Which is why when the high blower comes on, the MP goes way up, the ratio between crankshaft and flywheel increased and you're getting more pressure per RPM.

You can test this by changing your RPMs and watching your MP follow (as long as you're below 41MP, otherwise the mustang will change the throttle setting to keep MP constant). So go up to high altitudes and try this. Also try turning off the high blower (set to LOW rather than AUTO).

When you decrease the RPM shouldn't the manifold pressure increase? Is it the supercharger or the manifold pressure regulator? Maybe someone in the know can elaborate a bit the technology behind it.

depends in what situation. p-51 has automatic boost control which kicks in in MP above 41" so if want test Mp rise, set MP to 30" then reduce rpm. But this mast happebn below crit alt.

For example, if you take off at 3000rpm'61" and you make transition to climb power that is 2700rpm and 46", so you drop MP to 46" but this is range where Auto boost control is working so when you start reducing rpm to 2700 boost control automaticly closing thottle even more to maintain 46" .

When you make cruise power setting and you drop MP to 36" and start reducing rpm there you will see thet MP will rise so you have to adjust throttle agai nafter droping rpm to 2400.

Supercharger speed is fixed with engine RPM so, If you are at 3000 rpm no matter whant you do with throttle Supercharger will spin at max rpm too.

And another interesting thing is when high blower kicks in and we can see sudden jump in MP, but engine power rise only for very short time, once engine inlet will heat up engine power will be exact as engien power at Low speed at at change altitude.

So for example when supercharge is switching at 16k ft engine develope the same ammount of power at Low and High blower speed at this alt.

This is a rule when supercharger speeds are changed. Becouse at 15k low blower has more power and at 17k high blower develop more power.

Pilot will feel increase of power while High blower kicks in becouse engine interia are cold, but after short ammount of time whne engine reach its temps power will drop.

V-1650-7 at 75"/3000rpm low blower SL develop 1860BHP at crit alt 1920BHP

V-1650-7 at 75"/3000rpm high blower SL develop 1560BHP at crit alt 1690BHP

There is crazy difrence in manifold air temperature

Ok, when you are at idle (and MP is less than ambient are pressure) and you cut the fuel to turn off the aircraft, then yes, the RPMs go down while the MP goes up.

But the MP is going up because of the ambient pressure you let in from the outside by opening the throttle, not from any air being compressed by the suprecharger.

Also, if you are interested in more info, grafspee and I have been here before:

https://forums.eagle.ru/showthread.php?t=237533

Ok, when you are at idle (and MP is less than ambient are pressure) and you cut the fuel to turn off the aircraft, then yes, the RPMs go down while the MP goes up.

 

But the MP is going up because of the ambient pressure you let in from the outside by opening the throttle, not from any air being compressed by the suprecharger.

 

Also, if you are interested in more info, grafspee and I have been here before:

https://forums.eagle.ru/showthread.php?t=237533

Try go from 46"2700 to 36" 2400 you will need to adjust throttlle,MP will want to rise while you will be reducing rpm

Just remember about main rule if you drop power setting MP first RPM latter

whne going up first RPM then MP

Ofc when you climb those things will change, at higher alt this behavior will change, reason is that supercharger will not have any surpluss pressure or air flow avilable so both rpm and MP will drop.

Same will happen to High speed just alt will be shifted up

Thanks! Very interesting!

BTW, together with the A2A P51d, just found out that DCS is the only simulator that correctly models this behaviour among the 3 combat flight simulation tittles where P51s are represented.

Very recently tested a version released for another platform, and just as in my goto WW2 simulation game - War Thunder in Simulator mode - it fails miserably to model the MP variation with RPM at low power settings or higher altitudes.

DCS is, after all, the "study sim" of the combat fligth simulators. The rest is "suburbs"... but I do like walking there :-) - Has Nice Gals !!!!

Curiously this very same question had been asked in a post of mine in 2012 :-)

https://forums.eagle.ru/showthread.php?t=99093

That's because DCS is the only Simulator :)

The others are all fudgers :)

That's because DCS is the only Simulator :)

 

 

The others are all fudgers :)

A2A is pretty serious about their planes, they're just hamstrung by a 16 year old platform. Hoping this changes with the new MS project...

A2A is pretty serious about their planes, they're just hamstrung by a 16 year old platform. Hoping this changes with the new MS project...

And although they arrived late ( maybe to late with the FS 2020 announcement ) they're also working on that highly detailled Spitfire for XP11, their first product for X-Plane.

So anyone did test at what alt rpm/mp behavior will change ??

At any altitude where you have throttle valve closed enough.

 

As far as I remember, it works at SL, if you set rpm max then advance throttle to get 2000 and then reduce prop governor to min (~1600).

By the way, increasing throttle from idle to 2000 rpm you can see reverse movement of the MP as the actual rpm increases, or as you throttle back from 2000 quite fast you can again see reverse movement.

 

Another effect that sometimes can confuse is a MP regulator related. If you climb at reduced MP settings you need to advance throttle lever to achieve real critical altitude. Otherwise, the throttle valve will not fully open.

Yes, during engine run up at 2300rpm this effect can be seen while recuding rpm and incresaing rpm.

But in flight, crit alt probably defining border when thic corelation will shift.