Automatic manifold pressure regulator question.

[grafspee]

As we probably all know that DCS P-51 if climbed at climb power which is 46" at 2700rpm loses manifold pressure so  we need to advance throttle more and more to maintain 46" during climb until full throttle height is reached.

I was reading through many P-51 manuals including pilot notes, factory manuals and maintenance manual.

First is Flight handbook F-51D 20 January 1954

This is from Pilot's flight operating instructions for army model P-51D-5 to P-51D-30 from 5 august 1945

Another Pilot training manual 1945

And final P-51 maintenance manual

All of this manual states that automatic manifold pressure regulator can maintain set boost between 42 and 61 inch though out all alt up to critical alt.

And on top of that Maintenance manual states that differential unit permits manifold pressure regulator to operate throttles independently of cockpit throttle control.

Why in DCS things are different ? I would be very gratefully if someone explain it to me, why we have to manually compensate throttle while climbing from certain point.

Edited October 17, 2023 by grafspee

[=475FG= Dawger]

What do you mean its different?

It is maintaining constant manifold pressure due to changes in the propeller pitch. It isn't a FADEC throttle that time traveled back.

[kablamoman]

11 hours ago, grafspee said:

Why in DCS things are different ? I would be very gratefully if someone explain it to me, why we have to manually compensate throttle while climbing from certain point.

I am guessing you mean how when climbing at say, max continuous (46”/2700 RPM) there is a point where the manifold pressure starts to drop and you need to push the throttle forward to compensate?

I am wondering if maybe it’s just reaching critical altitude a little earlier as the engine RPM isn’t max and so is not driving the supercharger as fast? Perhaps the regulator is calibrated with a certain RPM in mind?

I honestly can’t remember if it behaves the same way at 3000 RPM. Maybe worth a test to see?

I would like to understand how it works a bit better myself.

[grafspee]

2 hours ago, kablamoman said:

I am guessing you mean how when climbing at say, max continuous (46”/2700 RPM) there is a point where the manifold pressure starts to drop and you need to push the throttle forward to compensate?

I am wondering if maybe it’s just reaching critical altitude a little earlier as the engine RPM isn’t max and so is not driving the supercharger as fast? Perhaps the regulator is calibrated with a certain RPM in mind?

I honestly can’t remember if it behaves the same way at 3000 RPM. Maybe worth a test to see?

I would like to understand how it works a bit better myself.

Yes exactly what i have in mind. Thing is if you say was true that opening throttle more would not increase MP and only increase rpm would ramp up MP. At beginning AMPR seems to work but above certain alt we need to advance throttle to maintain 46 inch until critical alt is reached. Manuals state that AMPR is capable to maintain any power setting between 42 and 61 inch through alt change within 1inch. Based on diagrams AMPR has no blower rpm reference. 

Only explanation to this is that throttle shaft can not move fully independently on throttle position in  cockpit but maintanance manual states it can.

8 hours ago, =475FG= Dawger said:

What do you mean its different?

It is maintaining constant manifold pressure due to changes in the propeller pitch. It isn't a FADEC throttle that time traveled back.

 Pls don't bring propeller pitch in here because it has nothing to with topic. My question is why when we climb p51 at 46inch at 2700rpm past certain alt we need to manually maintain MP by advancing throttle. We are below critical alt and AMPR does not work any more.

Edited October 18, 2023 by grafspee

[Art-J]

I used to wonder about it too. I recall there was be a very old post from Yo-Yo where he explained why the regulator was simulated the way it was, but it's borderline "mission impossible" to find it now.

An interesting note about necessity of advancing throttle during climb with reduced power can be found on the always-informative enginehistory.org website, but I admit I never investigated the rest of the article super closely.

https://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml

Edited October 18, 2023 by Art-J

[grafspee]

18 minutes ago, Art-J said:

I used to wonder about it too. I recall there was be a very old post from Yo-Yo where he explained why the regulator was simulated the way it was, but it's borderline "mission impossible" to find it now.

An interesting note about necessity of advancing throttle during climb with reduced power can be found on the always-informative enginehistory.org website, but I admit I never investigated the rest of the article super closely.

https://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml

 

Thing is that in this article they say that boost regulator can not move independently on cockpit throttle position. And it describe things based on Merlin III engine. For our engine manual states that boost regulator can operate independently on throttle position in cockpit, pilot just simply set boost via throttle and AMPR holds it up to engine's critical alt also called full throttle height. That is what exactly manual states.

Merlin III was used in hurricane I and early spitfires so things does not apply for late war engines.

Edited October 18, 2023 by grafspee

[saburo_cz]

This is from R-R Merlin manual.

[grafspee]

7 hours ago, saburo_cz said:

This is from R-R Merlin manual.

Is it from merlin 66 manual ? I have packard merlin v1650 manual before my eyes and there is nothing about that limitation, only that AMPR can operate throttles independently. 

Edited October 18, 2023 by grafspee

[grafspee]

Engine controls of Merlin 66

Engine controls of V-1650-7

And mysterious control shaft which i can not find in merlin 66, this is where V-1650-7 magic happens

It is obvious that they are completely different setups, i can read about AMPR limits of merlin 66 in merlin 66 manuals but i can't find a single info about this limit being in Packard merlin version V-1650-7/-3. I do agree that this is how DCS spitfire should behave but i don't agree that DCS P-51 V-1650-7 should behave the same.

Both merlin 66 and V-1650-7 are quite different engines in terms of accessories. 

Simple example the start up procedure in merlin 66 is quite different then from V-1650-7, i recall many topics about it, main difference is that merlin 66 starts up with mixture lever in run position with fuel pump off and V-1650-7 starts with mixture lever in cut-off and fuel pump on which was reported years ago and no hope for fix.

I don't think that this issue will be ever addressed too.

[Theodore42]

On 10/17/2023 at 11:17 AM, grafspee said:

As we probably all know that DCS P-51 if climbed at climb power which is 46" at 2700rpm loses manifold pressure so  we need to advance throttle more and more to maintain 46" during climb until full throttle height is reached.

All of this manual states that automatic manifold pressure regulator can maintain set boost between 42 and 61 inch though out all alt up to critical alt.

And on top of that Maintenance manual states that differential unit permits manifold pressure regulator to operate throttles independently of cockpit throttle control.

Why in DCS things are different ? I would be very gratefully if someone explain it to me, why we have to manually compensate throttle while climbing from certain point.

Describe how exactly you think the system should work in terms of the critical altitude and specific altitude relative to the position of the throttle lever...

[Critical Altitude is the highest altitude that you can achieve maximum MP. Specific Altitude is the altitude where Full Throttle is the maximum continuous engine setting.] Edit: I think the specific altitude is maybe technically the critical altitude for maximum continuous power but my point is the same.

When you take off and set your MP to max continuous, the butterfly valve is opened such that the pressure in the manifold is 46 inches. As you climb the air thins out and the butterfly valve has to open more to maintain 46 inches of pressure in the manifold. Therefore the butterfly valve automatically opens as you climb without you touching the throttle. That's what this is:

"All of this manual states that automatic manifold pressure regulator can maintain set boost between 42 and 61 inch though out all alt up to critical alt. And on top of that Maintenance manual states that differential unit permits manifold pressure regulator to operate throttles independently of cockpit throttle control."

When you reach the CRITICAL altitude the air is too thin for the butterfly valve to open all the way and achieve MAX MP. Even though you aren't at max MP but max continuous, this matters.

At this point you have to advance the throttle lever to open the butterfly valve enough to maintain the max continuous throttle setting up to your SPECIFIC altitude, which is where at Full throttle (butterfly valve fully open) the Manifold Pressure is set to the max continuous engine setting.

If the aircraft could advance the throttle lever on its own then this is where it would do that. But it can't.

I'm 99% sure if you guys tried to describe how you think the system works you would end up needing the throttle to move on its own.

I've attached some relevant pages from TM 1-407 Aircraft Induction, Fuel, and Oil Systems.

Edited October 19, 2023 by Theodore42

[grafspee]

@Theodore42

There in no point in that system which air is too thin to operate, because regulator acting pressure between supercharger suction and supercharger delivery pressure, only limitation factor is physical movement range of relay piston. And this is mentioned in merlin 66 manuals but not in V-1650 manual.

Edited October 20, 2023 by grafspee

[Theodore42]

The manual says the butterfly valve automatically adjusts up to critical altitude. Critical altitude is when you are at full throttle (the throttle lever is at 100%) and the manifold pressure is at max, 61 inches.

The manual says the automatic MP regulator functions:

If between 42 and 61 inches MP AND when altitude < critical altitude then the auto MP regulator is functioning. When between 42 and 61 inches and altitude > critical altitude, then the throttle behaves normally. Those are the rules as described in, and inferred from, the manual.

No matter your throttle setting, if you are above critical altitude, then the regulator CAN'T maintain manifold pressures at less than 100% of throttle setting. The pilot has to be able to increase the throttle up to 100% to whatever MP a fully opened butterfly valve is when above critical altitude.

I remember wasting a lot of time trying to think this out but I just had to do an experiment to understand, something like this:

When you are climbing at 100% throttle and speed and your MP starts drop, this is your critical altitude. You'll have to accelerate a minute to get your speed up because MP goes up the faster you are going due to ramjet. So critical altitude is max possible altitude at max manifold pressure AND at max SPEED. (The faster you go the higher you have to go too.) Write down the altitude and speed of the critical altitude.

Climb 2000 above that altitude and accelerate to max level speed. Write down your MP at full throttle. Now descend back down to the critical altitude, the one where at FT and max speed your MP just started dropping below 61 inches. Set your throttle to what the MP was at 2000ft above that altitude. Now don't touch the throttle. Climb up 4000 feet, only 500-1000 feet per minute to keep your speed up. What does your MP do as you climb? What is your MP at this new throttle setting stabilized at speed and altitude? Now go back down to the critical altitude and set your MP for what it was at 4000 feet above the altitude. Fly sensibly, like you're really doing an experiment, because speed and trim are factors 

Thinking won't help, it'll make sense if you just do it.

Edited October 20, 2023 by Theodore42

[grafspee]

@Theodore42 I would do other test. I will turn off ram and at 2700rpm i will find critical alt for 61inch. Then i will descent i will set 46inch and start to climb, it should drop that 46inch at exact same alt as when throttle was wide open, if what you say is true. Critical alt of the engine depends on engine rpm.

If AMPR carry out 46" up to 61inch crit alt then ok, i will be convinced.

Edited October 20, 2023 by grafspee

[Theodore42]

7 hours ago, grafspee said:

If AMPR carry out 46" up to 61inch crit alt then ok, i will be convinced.

It will, as long as your speed is stable at the engine settings. If you're still accelerating then it will look like critical altitude is lower than it really is.

[grafspee]

22 minutes ago, Theodore42 said:

It will, as long as your speed is stable at the engine settings. If you're still accelerating then it will look like critical altitude is lower than it really is.

I will shut off ram air this will rule out speed factor. 

Edited October 20, 2023 by grafspee

[TK-421]

Did anyone test this out? Seems to be a point of confusion with the Mustang.