I-16 engine, MP and Prop RPM....

[Guest deleted@50488]

Shouldn't, at least at lower power settings, a variation in RPM cause a noticeable opposite variation in Manifold pressure ?

 

What I see is MP increasing a bit when Prop RPM is increased, and the way around, but I would expect a drop in MP as I increase Prop RPM and the opposite too ?

[Finkeren]

I'm rather ignorant about the finer workings of aircraft piston engines. Can you explain to me, why MP should drop when you increase prop RPM?

 

I thought MP was just the pressure (or rather suction) in the induction system, which is used as a measure of current power output of the engine. When RPM setting is changed, the propeller governor in the spinner changes the prop pitch dynamically to adjust to the desired RPM (IIRC most CSPs in WW2 was based on a system where oil was pumped back and forth through the prop shady to create specific pressure in a piston that controlled the pitch or something like that)

 

At no point should the engine have to work any harder (or the opposite) unless I'm overlooking something.

[SCG_Space_Ghost]

I'm rather ignorant about the finer workings of aircraft piston engines. Can you explain to me, why MP should drop when you increase prop RPM?

 

I thought MP was just the pressure (or rather suction) in the induction system, which is used as a measure of current power output of the engine. When RPM setting is changed, the propeller governor in the spinner changes the prop pitch dynamically to adjust to the desired RPM (IIRC most CSPs in WW2 was based on a system where oil was pumped back and forth through the prop shady to create specific pressure in a piston that controlled the pitch or something like that)

 

At no point should the engine have to work any harder (or the opposite) unless I'm overlooking something.

 

My understanding is that more suction (MP) is required to cycle the propeller through the air when it is at a lower RPM - thus MP climbs.

 

High RPMs on the prop equates to less suction required to cycle the prop - thus MP drops.

[Finkeren]

My understanding is that more suction (MP) is required to cycle the propeller through the air when it is at a lower RPM - thus MP climbs.

 

High RPMs on the prop equates to less suction required to cycle the prop - thus MP drops.

That's not my understanding of how a constant speed propeller works. A CSP controls RPM solely through variable pitch. When you change RPM to a different setting the engine should perform the exact same amount of work (as long as the RPM is set within the limits of the pitch range for the current power setting)

 

As I said, I'm no expert, so it's quite posible that I'm overlooking something. If so I'd like to be enlightened.

[Guest deleted@50488]

Still, as RPM increases, the additional suction will decrease MP. 

 

A well known saying by "John Deakin", a well known Aviation texts author is "Manifold Pressure Sucks"...

 

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

[unreasonable]

That article, I believe, is for normally aspirated engines. Would the supercharger make a difference?

[Guest deleted@50488]

That article, I believe, is for normally aspirated engines. Would the supercharger make a difference?

 

 

It certainly would, and you bring a valid point unreasonable, but it had to have kicked in already. On the ground, or in the air at lower power settings ( low MP values ) I guess it should work as if it was a normal engine.

 

But it may well be that my tests weren't run at the correct power settings.

 

Also, I thought the I-16 modeled in IL2 BoS was aspirated / carburetor based ?

Edited July 18, 2015 by jcomm

[unreasonable]

Type 24 had a supercharger, at least according to Wiki

 

I just wondered if it would act to smooth out the changes in MP mentioned in the article.

 

Engines are all a bit of a mystery to me: I am still half convinced that there are hundreds of hamsters in there frantically turning little wheels.

[JtD]

If you increase engine rpm, you increase supercharger rpm and increase manifold pressure. The only way to not increase manifold pressure is if it controlled by an automatic device that prevents pressure from rising, for instance if engine limits are reached.

[fjacobsen]

Regardless wheter a supercharger is fitted or not, ifthe Throttle is set to a certain setting and left untouched, then reducing the Prop RPM should make the maifold pressure increase.

 

The engine is simply an airpump...

The faster it runs (RPM) the more air is pumped through and with throttle set to the same position, higher RPM would mean lower MP, while lower RPM would mean higher MP.

 

All the supercharger does is to increase the MP for a certain throttle setting, even to a value above ambient pressure, which is the highest reading a normal aspirated engine would give on the MP gauge.

 

What we see in IL-2 BOS/BOM is that MP drops with lower RPM, which isn´t correct.

 

The above anly holds true for aircraft with normal engine controls and not for aircraft with more complex controls like the "komandogerät" for the FW-190 or the automatic throttle/ prop unit of the BF-109 (not called "Kommandogerät" as far as I know).

 

 

Finn Jacobsen

[JtD]

The above anly holds true for aircraft with normal engine controls...

Wrong. It's true for naturally aspirated engines. Not for any engine driving his own supercharger.

 

Maybe you guys can stop thinking 'naturally aspirated engines' and start educating yourself about supercharged aero engines? For instance, the throttle control does not control the supercharger, throttle only controls the throttle, hence the name.

[fjacobsen]

Please read my post again...

 

I never said that the throttle controls anything else but the throttle plate in most aircraft, but for the FW-190 and BF-109 Prop RPM and mixture are governed as a funtion of the throttle position, thus they work differently.

The FW-190 and BF-109 also doesn´t have normal constant speed props, at least they function differently.

 

A supercharger is simply a compressor coupled to the engine via a gearbox. Some have serveral stages. It´s output is dependent on engine (and also Prop) RPM, but so is the amount of air that flows through the engine and therefore they relate to each other. Nevertheless the superchargers function does not change the fact that with reduction of prop (engine) rpm, the manifold pressure should rise.

 

Finn Jacobsen

[JtD]

, for instance Merlin engine characteristics.

 

At a constant throttle setting, chart shows full throttle, you'll increase manifold pressure from 1lb to 9lb if you increase rpm from 2000 to 3000 at 11000feet in M.S. gear.

[Finkeren]

Yay! Charts! Finally

 

Seriously though, I think I see why I was mistaken in believing that the MP would remain constant.

[Guest deleted@50488]

Here you go, for when I asked the same question long ago for another module :-)

 

http://forums.eagle.ru/showthread.php?t=99093&highlight=reduce+rpm+increase+mp

 

or... the Magic of a Regulator, and when it kicks in :-)

 

If the I-16 had one, we should see the usual MP / Prop RPM relationship before the regulator comes to play. The same applies to the Yak...

Edited July 19, 2015 by jcomm

[Finkeren]

My misunderstanding was due to the faulty assumption, that MP actually is a direct measurement of current engine power output.

[Guest deleted@50488]

And just to add to this interesting subject, the same question asked at the A2A P51d Civil forums ( a great FSX add-on that I used to be a fan of, before staring to use IL2 BoS and DCS World... )

 

http://a2asimulations.com/forum/viewtopic.php?f=93&t=33030&hilit=prop+rpm+mp+regulator

 

The answers, referred to by Yo-Yo of ED in the link I mentioned above, was given to explain the behavior in the A2A P51 Civil, but it was latter fine tuned in one of the updates, and at low power settings, on ground and in the air, we could then also see the usual RPM up --> MP down and the way around relationship, before the regulator kicked in.

[unreasonable]

And just to add to this interesting subject, the same question asked at the A2A P51d Civil forums ( a great FSX add-on that I used to be a fan of, before staring to use IL2 BoS and DCS World... )

 

http://a2asimulations.com/forum/viewtopic.php?f=93&t=33030&hilit=prop+rpm+mp+regulator

 

The answers, referred to by Yo-Yo of ED in the link I mentioned above, was given to explain the behavior in the A2A P51 Civil, but it was latter fine tuned in one of the updates, and at low power settings, on ground and in the air, we could then also see the usual RPM up --> MP down and the way around relationship, before the regulator kicked in.

 

OT for this discussion, but the video/chart in that thread about how supercharger gear change affects MP, HP and speed is very well done - simple and clear.

[JtD]

OK, that's true, if you have the throttle almost closed, increasing rpm reduces MP.

[Guest deleted@50488]

OK, that's true, if you have the throttle almost closed, increasing rpm reduces MP.

 

Or:

 

"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."

 

But in any of the Russian fighters in Il2 Bos, I am unable to see this variation.

 

In the P51d, flying at that altitude does show the effect - MP going noticeably down as you push your RPM lever... and the way around...

 

I've been testing it in the Yak-1, La5 and LagG3, and now the Rata too.

 

But I also don't have the base to say this is wrong, because I do not know the details of the engines used on these fighters....

Edited July 19, 2015 by jcomm

[JtD]

Or:

Wrong. It's "and".