Manifold pressure vs RPM

[CisTer-dB-]

Hi guys, I want to touch a subject for a while back but I assume that all where pretty busy with the release of Kuban.

 

I think that now that it is released it might be a good time to reach out.

 

The RPM and Manifold relation that we have in game are not the way it is on an aircraft. The way it should work is once you reduce the RPM via the prop lever your manifold pressure should increase and when we increase RPM with the propeller pitch lever, we should see a decrease on manifold pressure. As for now ingame we have to opposite. 

 

Not that is a big issue but knowing that you guys aiming to get it as close as the real thing, I decide to share it.

 

 

 

Edit: I've decided to include a PDF file that explain the relation between both

 

https://www.advancedpilot.com/downloads/prep.pdf

Edited May 11, 2018 by ATAG_dB
PDF

[JtD]

5 minutes ago, ATAG_dB said:

The RPM and Manifold relation that we have in game are not the way it is on an aircraft. The way it should work is once you reduce the RPM via the prop lever your manifold pressure should increase and when we increase RPM with the propeller pitch lever, we should see a decrease on manifold pressure.

 

Err, no? Why would that be the case? Please walk me through step by step with any of the BoX aircraft.

 

As it is, manifold pressure is either regulated by an automated valve, in which case rpm doesn't mean jack, or the valve is controlled directly with the throttle lever or wide open, in which case the supercharging depends directly on the engine rpm which means manifold pressure goes up as rpm go up and vice versa.

 

Outside of odd control mechanisms which to my knowledge none of the modelled aircraft have, only naturally aspirated engines would have manifold pressure decrease with increasing rpm. We don't have naturally aspirated engines in game either.

[Ehret]

18 minutes ago, ATAG_dB said:

The RPM and Manifold relation that we have in game are not the way it is on an aircraft. The way it should work is once you reduce the RPM via the prop lever your manifold pressure should increase and when we increase RPM with the propeller pitch lever, we should see a decrease on manifold pressure. As for now ingame we have to opposite.

 

Most in game airplanes have supercharges driven by engine's crankshaft - when you lower the RPM you lower the compressor's RPM as well thus reducing the boost. This effect seems to be much stronger than reduced suction by cylinders.

Edited May 11, 2018 by Ehret

[CisTer-dB-]

1 hour ago, JtD said:

 

Err, no? Why would that be the case? Please walk me through step by step with any of the BoX aircraft.

 

As it is, manifold pressure is either regulated by an automated valve, in which case rpm doesn't mean jack, or the valve is controlled directly with the throttle lever or wide open, in which case the supercharging depends directly on the engine rpm which means manifold pressure goes up as rpm go up and vice versa.

 

Outside of odd control mechanisms which to my knowledge none of the modelled aircraft have, only naturally aspirated engines would have manifold pressure decrease with increasing rpm. We don't have naturally aspirated engines in game either.

I wish I could give you the exact reason JtD, but I don't have an engineering background strong enough to find the proper words. The only thing I can says is that on all constant speed prop type aircraft I flew, all of them it's like that. I of course never had the change to fly most of the planes we have in BoX but I flew the Mk IX and like all other plane I flew the relation between the manifold pressure and the RPM are the same. I can spam this post with a bunch of video that concur that. The why I'll like to know

 

  

The follow up the discussion here are some reference materials.

 

https://aviation.stackexchange.com/questions/13723/why-does-manifold-pressure-increase-with-power

https://www.avweb.com/news/pelican/Pelicans-Perch-15-Manifold-Pressure-Sucks-182081-1.html

https://forums.jetcareers.com/threads/manifold-pressure-vs-rpm.192033/ 

 

I hope it help in understanding the situation

Edited May 11, 2018 by ATAG_dB

[JtD]

Well, sounds as if it is as initially suspected. Your experience and all the evidence appears to be solely based on naturally aspirated engines. That's all true, but we don't have a single naturally aspirated engine in game. How things are for supercharged engines, has already been put it into a nutshell by Ehret.

 

So it's no bug, it's correct.

[CisTer-dB-]

Not quite, I have flown quite few with supercharger. Just be sure I understand you right are you saying that a Mk Ix doesn't have a supercharger?  

Edited May 11, 2018 by ATAG_dB

[JtD]

Are you referring to a Spitfire Mk IX?

[CisTer-dB-]

Yes I do.

Not sure I follow you (I am sorry) but airplane engines can be have a carburetor "naturally aspired" and have a supercharger or turbo at the same time. 

 

Although not related to WW2, If my memory recall right, I would had that it's also the same for turboprop engines.   

Edited May 11, 2018 by ATAG_dB

[JtD]

Naturally aspirated means no turbo, supercharger or anything else. Air gets in at outside pressure, and is sucked into the cylinders, where it gets burned. The faster the piston move (the higher the rpm go), the more they suck and manifold pressure goes down.

 

Engines with superchargers or turbochargers are different. The air gets compressed before it gets to the manifold and is not just sucked in, put also pressed into the cylinders. As superchargers are directly couple to the engine, they turn faster as the engine turns faster. As superchargers turn faster, they produce more pressure. This way they press a lot more air into the manifold than the pistons can suck out, and manifold pressure goes up.

Turbos are more complex, but similar in effect. They need a while to speed up. So you'll see first lowered, then increased manifold pressure as rpm go up.

 

The only regime I can think of, where a Merlin engine (supercharged) would reduce manifold pressure with increased rpm, are extremely low rpm, like idling on the runway. At these rotations, the supercharger is basically too slow to actually work. Unfortunately I could only find charts with 2200rpm+, where boost and manifold pressure go straight up as rpm increase. The relevant part would be lower than this.

I have a chart for a DB605 which also includes low rpm, showing that manifold pressure goes down up to 1600rpm, and then goes on to rise as rpm increase. The DB605 produces about 200hp at these 1600rpm, so they are not really relevant for flying.

[ZachariasX]

I think the error is made somewhere else in the logic. What the OP is describing is tha balance between MAP and rpm to produce an assumed equal net output of power.

 

In theory, it doesn‘t matter if you use half the air (and fuel) at twice the rate (rpm) or double air (and fuel) at half the rate (rpm), you still are burning an equal amount of carburant and are producing the same amount of net power. Assuming equal/steady efficiency over the given range of rpm, the OP’s assumption holds true.

 

The problem is that this is not how our in game engines are controlled, turbo or not. All engines (except maybe the Kommandogerät equipped ones, 190, 109) adjust directly or indirectly the carburator opening (cock plug, right?) and in consequence are directly and exclusively adjusted by varying manifold pressure.

 

There are various kind of feedback mechanisms to keep rpm steady at a set rate, but this is always, even with the Kommandogerät, it is realized as a feedback loop to adjust the airflow to the cylinder.

 

This is why the OP got it wrong. In a system as we have it in our aircraft, reducing rpm at a given MAP setting implicitly instructs the system to produce less power because we are setting the airflow with the throttle lever. The „constant speed machine“ doesn’t offset that. If you reduce rpm at similar throttle opening, there will be less power produced at similar MAP and the aircraft will slow down and MAP has to lower as well and vice versa. This happens fast as you make the prop windmill as soon as you produce less power instantly, so you will see MAP lower instantly.

 

It has nothing to do with the superchargers, it is theory vs. applied practise.

[CisTer-dB-]

11 hours ago, JtD said:

Naturally aspirated means no turbo, supercharger or anything else. Air gets in at outside pressure, and is sucked into the cylinders, where it gets burned. The faster the piston move (the higher the rpm go), the more they suck and manifold pressure goes down.

 

Engines with superchargers or turbochargers are different. The air gets compressed before it gets to the manifold and is not just sucked in, put also pressed into the cylinders. As superchargers are directly couple to the engine, they turn faster as the engine turns faster. As superchargers turn faster, they produce more pressure. This way they press a lot more air into the manifold than the pistons can suck out, and manifold pressure goes up.

Turbos are more complex, but similar in effect. They need a while to speed up. So you'll see first lowered, then increased manifold pressure as rpm go up.

 

The only regime I can think of, where a Merlin engine (supercharged) would reduce manifold pressure with increased rpm, are extremely low rpm, like idling on the runway. At these rotations, the supercharger is basically too slow to actually work. Unfortunately I could only find charts with 2200rpm+, where boost and manifold pressure go straight up as rpm increase. The relevant part would be lower than this.

I have a chart for a DB605 which also includes low rpm, showing that manifold pressure goes down up to 1600rpm, and then goes on to rise as rpm increase. The DB605 produces about 200hp at these 1600rpm, so they are not really relevant for flying.

Thanks JdT to take the time to refresh my memory, It's been a long time since last time I flew props. I assumed naturally aspirated being carburated engines, my bad.

Again I can't recall the why but all the aircraft, including naturally aspirated, injected, supercharged, turbocharged or turbo-prop,  that have a constant speed prop all the 17 different type that I flew, when you reduce the prop pitch (RPM) the MP/boost increase and vise versa. That is why there is a procedure into witch one you reduce first or increase depending of the stage of flight.

 

       

Edited May 12, 2018 by ATAG_dB

[SCG_Darbzy]

Sorry I am too lazy to read all the posts above, but in principle I concur with DB in that all naturally aspirated engines get an increase in MP with a decrease in RPM and vice/versa unless they have MP/boost regulators. I was also fortunate enough to fly a real Spitfire Mk IX. The Spitfire has a boost regulator that keeps a constant boost with changes in RPM but I'm not sure what the other Allied aircraft had. Were the Russian machines that sophisticated? I'm also not sure about the P-40/P-39 but I suspect they had no MP/Boost regulation either. 

 

My only comment would be that I suspect that the effect of adjusting RPM/MP differs between aircraft depending on whether they have boost/MP gauge regulators or not. Some do, some don't. I'm not sure which as I only have a limited knowledge of most of the fighters in this game.