Supercharger

[Zimmerdylan]

As a casual flier, I don't always take a lot of time to learn the specifics of many of the functions of these planes.

I fly, and if everything seems to work OK, I don't usually take it much further than that.

However, I really want to pull as much power and agility out of the Mustang as I can. And was wondering how the supercharger effects the performance of the plane. Does it help while in a dogfight? And when is the best time to set it to high or low performance?

I know that many of you guys use it, and understand it's properties very well. So could anyone shed some light on this subject for me??

 

Thx!!!

[Nerd1000]

As a casual flier, I don't always take a lot of time to learn the specifics of many of the functions of these planes.

I fly, and if everything seems to work OK, I don't usually take it much further than that.

However, I really want to pull as much power and agility out of the Mustang as I can. And was wondering how the supercharger effects the performance of the plane. Does it help while in a dogfight? And when is the best time to set it to high or low performance?

I know that many of you guys use it, and understand it's properties very well. So could anyone shed some light on this subject for me??

 

Thx!!!

 

The supercharger is a crank driven compressor that increases pressure in the intake manifold of the engine. It is always on and is a vital part of the engine, so it definitely helps in a dogfight :).

 

Essentially the supercharger exists for two reasons: 1, it shoves more air and fuel into the cylinders, resulting in bigger explosions and thus more power. 2, it helps compensate for the decreasing density of the air as you climb.

 

Because it's intended for use at altitude the supercharger is oversized for sea level, and if it were to run at full speed at sea level would provide far too much boost (the increase in the pressure in the intake manifold), causing the engine to knock. This can be countered by throttling back, but that reduces power. The Merlin 60 series engine found in the Mustang reduces this issue by driving the supercharger through a 2-speed gearbox. The 'low' speed is for use at low to medium altitude, while the 'high' speed is for use at high altitude. There is an aneroid switch (in other words, a barometer) in the engine compartment that switches the supercharger speed automatically at the appropriate altitude.

 

There is also an automatic boost controller that throttles back at low altitude, because the supercharger low speed still produces too much boost when you're flying down low. What this means is that the engine has a 'sawtooth' pattern of power vs altitude, where power increases to a maximum as you climb (due to lower throttling losses), starts to reduce as the plane goes above the 'full throttle height' of the 1st gear, and then power starts to increase towards a second maximum after the engine changes into 2nd gear, before falling away again above the full throttle height of second gear. The plane will perform best at altitudes around the two peaks in power output, and rather badly in the 'dip' between them.

 

Look in the manual for those best altitudes. I don't know them off by heart because I don't fly the mustang- I'm just an aviation nerd who likes Rolls-Royce Merlins.

Edited November 28, 2015 by Nerd1000

[Anatoli-Kagari9]

Very good explanation Nerd!

 

One question, because it was raised at another forum due to the system used on a model of the Fw190 - do you know if the aneroid takes density altitude into consideration, or is it a simple barometer ?

 

I ask because if it's a simple barometer - based system, then in cold / denser Winter weather, and provided such effect is simulated in DCS ( which appears to me it is... ), the effcetive / real height above MSL at which the change occurs should be lower than in Summer.

 

Just curious, because this has been extensively discussed at that other forum, but I never really understood very well how it works...

[Crumpp]

do you know if the aneroid takes density altitude into consideration, or is it a simple barometer ?

 

It uses density altitude, jcomm.

 

It is really a silly idea to think an engine would change supercharger gearing based off pressure altitude only. The confusion results from the fact the supercharger aneroid switch in the BMW801 are used for different purposes from the Merlin system.

 

The physics and the engineering just would not work.

 

The Merlin aneroid switch is exposed to the carbureted air at the intake. It is the same temperature as the charge air. Because it is at the same temperature, it only needs to measure the pressure differences to get the correct density.

 

The supercharger aneroid on the Merlin is the "brain" of the engine. The fuel metering system is linked and uses a calibrated differential unit to adjust the mixture IAW the supercharger intake density.

 

 

The confusion results because the BMW801's supercharger aneroid is dumb. It is only used to maintain a fixed relationship with the linkage and is simply a more accurate control shaft differential unit. That is also why it is adjusted at ambient pressure and temperature. It does not require an elaborate test stand to calibrate.

 

Calibrating the Merlin aneroid switch It is a rather involved process to set it and it requires a special test stand. The maintenance manual has instructions for building one in the field.

 

Basically you find Pressure altitude (29.92inHG) for the specific conditions the calibration is being performed at and adjust the aneroid to switch at 14,500 + or - a few feet depending on the type of aneroid. Pressure altitude = density altitude on a standard day. When exposed to charge air at the charge air temperature, the aneroid will then trigger the gear change at 14,500 (+ or -) ft density altitude for those specific atmospheric conditions and charge air density.

 

The BMW801 system uses a different aneroid as the systems "brain". That one cannot be calibrated in the field and simply gets replaced IIRC. :thumbup:

[Blech]

Hi,

 

According to the flight manuals 1945/1954/DCS the supercharger´s three positions are

 

AUTO - for normal operations

LOW - provides better range at high altitudes*), which can be used for long-range flights

HIGH - makes it possible to test the high blower mode on the ground

 

I can´t remember no further pilot instructions concerning the use of the supercharger as AUTO and LOW

 

*) The questions is - high altitude begins at ??? ft. ?

 

It seems to be that 10,000 ft is high altitude

In the 1945 manual is on page 95 an example for a flight at 10,000 ft and supercharger LOW

[Crumpp]

The Pilots Operating Instructions:

 

 

The Maintenance Manual mirrors those instructions:

 

 

Basically leave it in "Auto". "High" Is for testing before flight and "Low" is for an emergency if you lose the aneroid switch controller.

[Zimmerdylan]

All great explanations. Thanks!

SO, if I am understanding correctly. The supercharger is more for better effifiency at different altitudes? And, if I am on the deck, it is unwise to manually kick the supercharger into high for any kind of boost as it would have the opposate effect on the engine.

I was thinking of binding the supercharger settings to my X52 but it seems there would be no real benifit to it because it's not really something that you do on the fly. And "auto" would seem to be the best place to keep it unless I'm in need of the extra power to climb between the 12,000 to 16,000 ft range or something like that? The Mustang always seems to struggle when made to function in this area.

Crumpp: Got your post just now. So the high should never really be used?? Unless I'm on the ground testing?

[Crumpp]

Crumpp: Got your post just now. So the high should never really be used?? Unless I'm on the ground testing?

 

That is it!

[Art-J]

Just to get more data on this "density modelling in DCS" issue, I've just made a few P-51 flights in standard summer map conditions (31 degrees) and edited winter map conditions (-20 degrees).

 

In hot test the supercharger changeover happened usually at 16000-16100 ft, with throttle lever pushed full forward to maintain 46" (actually, before changeover it drops to 42'ish).

 

In cold test changeover happened at 17000-17500 ft, 46" maintained easily even with throttle lever moved to about 3/4.

 

And while we're at it, I've noticed:

 

a) in these freezing conditions contrails are already there at 10 000 ft (it was an airstart mission), so guys who complained about narrow range of altitudes at which contrails showed up in 1.2.xx should be happy now;

 

b) contrails disappear when engine is throttled all the way back - I saw a bug report some weeks ago about contrails being always there even with engine off, so it's been probably fixed lately I presume.

[Anatoli-Kagari9]

Thx Crumpp,

 

I'll try to assimilate your detailed explanation tomorrow - I really have to understand how it get's the right density, and not simply the pressure.

 

BTW: I did some tests setting different Winter vs Summer scenarios in DCS, and I believe it actually models air density in Winter time.

 

For a same airfield QNH, I reached the same altitude according to my altimeter reading, but using F2 there was a very noticeable differenve in height between the Summer and Winter scenarios :-)

[Crumpp]

From what ARt-J is saying, DCS does model density altitude effects.

[Anatoli-Kagari9]

Oops!

 

I had missed Art-J's post, just before mine...

 

 

Thx for posting your results Art-J - They confirm what I also observed.

 

The altitudes can't ne measured in the aircraft altimeter, but rather on the legend at the footer of the F2 view, which reads "true alt." / height....

 

See Crumpp... you argued so much about it in another sim, and this is one of the reasons I am now 100% again here too - it's called DETAIL! :)

[ShuRugal]

Thx Crumpp,

 

I'll try to assimilate your detailed explanation tomorrow - I really have to understand how it get's the right density, and not simply the pressure.

 

pressure is density, the two terms are interchangeable when talking about the same gas.

[Crumpp]

pressure is density, the two terms are interchangeable when talking about the same gas.

 

They are not interchangeable independent of volume. You must know both to find density.

 

Pressure is pressure.

 

Density is pressure and temperature. Unfortunately science has no "density gauge" so density must be derived by measuring both pressure and temperature.

 

However, if temperature is constant or our pressure measurement is at the same temperature, pressure can be used to find density.

 

But only in the context of temperature AND pressure.

 

This derivation of the Ideal Gas Equation allows us to characterize the relationship between the pressure, density, and temperature of the gas sample independent of the volume the gas occupies; it also allows us to determine the density of a gas sample given its pressure and temperature, or determine the molar mass of a gas sample given its density.

 

Source: Boundless. “Density Calculations.” Boundless Chemistry. Boundless, 28 Jul. 2015. Retrieved 29 Nov. 2015 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/gases-5/the-ideal-gas-law-52/density-calculations-258-1026/

 

https://www.grc.nasa.gov/www/k-12/airplane/eqstat.html

 

jcomm is a meterologist and is familiar with density altitude. Another discussion it was claimed by other individuals that the German Kommandgerat engine controller on the BMW801 changed supercharger gearing based on pressure altitude. If you understand pressure altitude vs density altitude and how engines work....

 

It would not work very well based on pressure altitude alone on anything but a standard day.

[ShuRugal]

They are not interchangeable independent of volume. You must know both to find density.

 

Pressure is pressure.

 

Density is pressure and temperature. Unfortunately science has no "density gauge" so density must be derived by measuring both pressure and temperature.

 

However, if temperature is constant or our pressure measurement is at the same temperature, pressure can be used to find density.

 

But only in the context of temperature AND pressure.

 

Right, I'm familiar with Boyle's law, It just seems to me that variations in the volume of the atmosphere would be small enough to be negligible in that equation. There may be some other variable I am missing here (probably is) but I would still expect a strong positive correlation between pressure and density.

[Crumpp]

Right, I'm familiar with Boyle's law, It just seems to me that variations in the volume of the atmosphere would be small enough to be negligible in that equation. There may be some other variable I am missing here (probably is) but I would still expect a strong positive correlation between pressure and density.

 

It would make sense but the problem is the atmosphere is so large the fixed volume mathematical assumptions break down.

 

That is why we have weather....

 

Low pressure/High pressure/Cold/Warm Fronts.....

 

jcomm can tell you more about that! :)