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rosaenrico
07-22-2004, 07:30 AM
Hi all,
maybe some of you people with good knowledge about aircrafts and their mechanics can advice me about this:
I would like to understand what "manifold pressure" represent in the planes we have in the sim (eg, some gauges show negative or positive values); which kind of devices did determine it, apart obviously the throttle lever http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif ?
recommended URL are welcome
Enrico

rosaenrico
07-22-2004, 07:30 AM
Hi all,
maybe some of you people with good knowledge about aircrafts and their mechanics can advice me about this:
I would like to understand what "manifold pressure" represent in the planes we have in the sim (eg, some gauges show negative or positive values); which kind of devices did determine it, apart obviously the throttle lever http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif ?
recommended URL are welcome
Enrico

Kuikueg
07-22-2004, 07:56 AM
Go here:

http://www.avweb.com/news/columns/182081-1.html

Enjoy

S!

Kuikueg

http://img78.photobucket.com/albums/v293/Queequeg/IL2FB/jg27_gunsynch2.jpg
That's the only way I can hit anything.

rosaenrico
07-22-2004, 04:20 PM
Thanks Kuikeug, a reading plenty of info.
In the meantime I read all the stuff (1-2 weeks), I'll try to make my question more practical:
here is the gauge of the A6M2:

c:\manifold.jpg

what does the red part represent? Why negative (black part) and positive labels ?

hop2002
07-22-2004, 06:48 PM
Manifold pressure is simply the air pressure at the inlet to the engine. The higher the pressure, the more air goes into the engine at each stoke. The more air goes in, the more fuel you can put in, which means you get more power out.

Some countries, like the US and Germany, measured the air pressure as an absolute value. So for a German engine, switched off at sea level, it would show almost exactly 1 ata. As the altitude increased, the pressure would drop, but it could never reach 0 because 0 would be a perfect vacuum, and the engine would stop operating long before the pressure got that low.

Some countries, like Britain, measured the pressure as a value above standard sea level pressure. So with the engine off at sea level, a British plane would read 0 lbs/sq in. The supercharger would compress the air to well above sea level pressure, in the case of some Spitfires 18 lbs above. Above a certain altitude, the supercharger couldn't provide that much pressure because the air pressure had dropped, so the boost gauge would read less. At really high altitudes, the supercharger couldn't even maintain normal sea level pressure, so it would read a minus value, eg -2 lbs/sq in. -2 lbs would still be positive on a German or US gauge though.

To convert between the British, German and US measures:

1 lbs/sq in = 2.036 in hg = 0.068 ata (approx, the German ata wasn't quite the same as a standard atm, but not far off)

However, when converting between British measurements and the others, you have to add 14.7 lbs/sq in to the British figures to allow for the difference between absolute and relative pressure measurments.

I can' see your picture of the Japanese boost gauge, but if it shows negative values then they must have measured pressure as a relative, ie above sea level pressure, like the British did.

WTE_Galway
07-22-2004, 09:31 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>
I can' see your picture of the Japanese boost gauge, but if it shows negative values then they must have measured pressure as a relative, ie above sea level pressure, like the British did.<HR></BLOCKQUOTE>

Probably ... though a normally aspirated engine with no supercharger would also show negative man. pressure I believe.

hop2002
07-23-2004, 05:45 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Probably ... though a normally aspirated engine with no supercharger would also show negative man. pressure I believe.<HR></BLOCKQUOTE>

Only if it's using relative, not absolute pressure.

With absolute pressure, like the US and Germans used, a manifold pressure value of 0 indicates a perfect vacuum. It's not physically possible to get lower pressure than that. It's also not possible for the engine to run at pressures anywhere near 0.