Would someone explain Boost to a American Pilot?

[curtis008]

If at an airport on a standard day of 29.92 while in a P-47. the Manifold pressure reading of 30" hg pressure would be equal to the outside air pressure.
On the Spitfire is 0 Boost the same as 30" hg. Plus 4 lbs of Boost would be (something like 34" hg, -4 lbs something like 25" hg.?) I read the Accu-Sim manual, on page 28 it talks about manifold pressure. I am not sure how to wrap my brain around this to understand.

Thanks in advance for someones help.

[Roadburner426]

I will take a try at this. Granted I understand it in my mind, but lets see how this comes out..... If you have ever had a turbocharged, or supercharged car with a real boost gauge (not the junk ones the car manufacturers put in) you would see a gauge very similar to the one in the Spit. While on the P-47 (and all American birds for that matter) you have a measure for MP which is the total boost pressure across the manifold in Hg (standard atmospheric pressure). The boost gauge in the Spit is merely measuring the output of the supercharger itself whether it be positive, or negative boost. Not the engine as you are thinking. So when you are watching the red gauge you are only seeing whether the supercharger itself is making negative, or positive boost. Normally you should not take the engine over 4psi of boost, but with the cutout switch you can have a maximum of 12psi boost.. I imagine in the Merlin (just like a regular car engine) if you sustain these type of boost pressures you start burning pistons, and backing out head studs. Not to mention creating a lot of blow by on the cylinder rings (which is probably why they gave us the new compression test).. Your real only guess is to how much power/torque you are making is though RPM, and throttle position if you do not have a positive boost reading.

I guess a shorter version of the above is that you really do not know for certain. Except for when you are at the high end and reading positive boost on the gauge. Which that is really the only area you truly need to worry about anyway so you don't blow the engine up (even though that doesn't stop me).

[rc flyer]

Well, I'm an American pilot just like you, but I think you're basically correct. The way I'm looking at it, we're used to seeing a total pressure unrelated to the atmospheric pressure at the altitude of the plane and this is a differential reading relating to the atmospheric pressure at the altitude of the plane. That's just my guess without any research so I'm probably as lost as you .

Steve

[JoeS475]

Thanks, roadburner, I was trying to figure out a way to ask the same question! Your answer is helpful indeed!

Joe

[curtis008]

Thanks Roadrunner.
I did an internet search for superchargers and turbochargers and a Boost gauge. negative numbers mean the charger is not compressing air. and positive numbers the charger is compressing air going into the engine. the values -4 to +4 is a scale of some kind. The pilots just needs to keep it under +4 to keep flying. (my answer in a nut shell of sorts).

[CPDST]

What I would like to know, is what is the conversion from PSI of boost to InHG of manifold pressure. They are the same thing, I just would like the conversion factor

[DHenriques_]

What I would like to know, is what is the conversion from PSI of boost to InHG of manifold pressure. They are the same thing, I just would like the conversion factor

Try ( Multiply by 2.03625437 to convert from psi to inches mercury. )

Dudley Henriques

[r4y30n]

Thanks Roadrunner.
I did an internet search for superchargers and turbochargers and a Boost gauge. negative numbers mean the charger is not compressing air. and positive numbers the charger is compressing air going into the engine. the values -4 to +4 is a scale of some kind. The pilots just needs to keep it under +4 to keep flying. (my answer in a nut shell of sorts).

Not quite... The turbo- or supercharger is always compressing air, but in the case of a turbo engine at low power the throttle plate is blocking off the flow of air to the manifold creating a vaccuum while pressure between the turbo and the throttle is still above ambient. In the case of supercharged aircraft engines, where the throttle is predominantly upstream of the supercharger, the closed throttle creates a lot of vaccuum immediately behind the plate and then the supercharger 'boosts' this to slightly less vaccuum (but still below ambient).

The Spitfire's "Boost" gauge is really just a rescaled manifold pressure gauge where 0 PSI = approximately 30" Hg and every pound of boost is about 2" Hg.

[CPDST]

Thanks Dudley!

[Solareagle]

80 inHg = +25 lbf/in² boost
67 inHg = +18 lbf/in² boost
61 inHg = +15 lbf/in² boost
46 inHg = +8 lbf/in² boost
44.5 inHg = +6 lbf/in² boost

[Norforce]

Look at a2a's B-17 training video part 5 here hope this helps

http://www.youtube.com/watch?v=C49vFMcd ... re=related

Ron