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XyZspineZyX
08-05-2003, 04:45 PM
We've all been there before. A hit to the oild radiator. Now as you are trying to get back to home base, you have your throttle wide open, your RPMs are high yet your manifold pressure is decreasing and you are losing power.(That's what happened in original Il-2 at least). If your throttle fully open, shouldn't your manifold pressure be high? But a decreasing manifold pressure shows that the engine is not getting enough air... What is going on?

XyZspineZyX
08-05-2003, 04:45 PM
We've all been there before. A hit to the oild radiator. Now as you are trying to get back to home base, you have your throttle wide open, your RPMs are high yet your manifold pressure is decreasing and you are losing power.(That's what happened in original Il-2 at least). If your throttle fully open, shouldn't your manifold pressure be high? But a decreasing manifold pressure shows that the engine is not getting enough air... What is going on?

XyZspineZyX
08-05-2003, 06:00 PM
Manifold pressure is the difference between the air pressure outside and the pressure in your intake manifold. And throttle controls airflow into the intake manifold. (not trying to be condescending, just saying so anyone who reads my post will follow) If your throttle is wide open then air can enter freely and the pressure will equalize and the MP gauge will go down. As the throttle closes the pistons get 'thirsty' and air is being sucked in. This creates low pressure in the intake manifold and the manifold pressure gauge goes up. (I know this seems backward, maybe they should call it 'manifold suction'.)

From closed to full open the MP gauge should rise from equalization and descend back to equalization. MP cannot, by itself indicate power, when used in conjunction w/ the tachometer you can gauge 'relative' engine power.

What it's really good for is gauging engine efficiency. You want the desired RPM's w/ the highest MP. If the MP is too low (on a high throttle setting), then you're wasting fuel cuz your sending more fuel into the pistons than is necessary for that power setting. You want the pistons to be 'thirsty' and give them only the fuel that they absolutely need w/o loosing power. What I do is set my prop pitch to a desired RPM, then use the throttle to get the max MP I can w/o loosing RPM's. This is efficient.

You can also determine some engine damage w/ MP. If the intake system is damaged then you will show low MP at all throttle settings, because air is leaking into the system and throttle losses it's authority.

I have a limited understanding of all this and am prob wrong on many points. Feel free to correct my assessment as I would like to enjoy a better understanding.




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"Altitude, speed, maneuver, fire!"-The "formula of Terror" of Aleksandr Pokryshkin, Three times awarded the rank of Hero of the Soviet Union

XyZspineZyX
08-05-2003, 06:18 PM
If you received damage to the supercharger/turbo, or the plumbing system. You will lose boost (manifold pressure). You should try and baby it back to base. don't go full throttle.

Da Buzz
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XyZspineZyX
08-05-2003, 10:18 PM
If you are talking about the Bf109 it has a compressor with barometricly controlled hydraulic clutch, which of course needs oil to operate. Without oil, the compressor will slow down and conseqently manifold pressure will drop.

Scorpius

XyZspineZyX
08-05-2003, 11:49 PM
KrasniyYastreb, I'm not sure what level engine damage is modeled to in the sim, but what you describe sounds like damage to the supercharger and/or intake manifold. Georgeo76, what you describe is pretty close to how non-supercharged aero engines work, though the manifold pressure gauge shows absolute pressure, not vacuum. With these engines it will show less than sea level static pressure, because the air is being restricted as it's sucked into the engine thru the throttle valve/carburetor. This may be what confused you. The rule of thumb for these engines is don't let MP (in in.Hg) exceed RPM (in 100's). Setting 1700 RPM with 25 in. MP would put a lot of stress on a non-supercharged engine. Most of the engines on the flyable aircraft in the game have at least an engine mounted supercharger, so manifold pressure on these can be quite a bit higher than atmospheric pressure, and they're built to take the added stress (the Jug has a compound turbo-supercharger setup, but until the patch comes out, its modeling is kind of porked.) If MP drops, but RPM stays high, then there's either a hole in the intake manifold letting air out, a busted supercharger not compressing air like it should, or both. Assuming a constant speed prop, MP and not RPM, is a good indicator of power produced, since the prop will try to hold RPM constant regardless of how much power the engine is actually putting out. Also, with two speed blowers it may be that it's stuck in low speed if you're at high altitude (where you'd normally have high speed selected.)
I have seen the prop governor damaged, too. In this case, the MP and the RPM both go down (if the governor defaults to full coarse), though the engine keeps running and doesn't trail smoke. As the sim is now, 0% prop is essentially feathered, so if this happens, you're a glider pilot. The Hurri's governor apparently goes full fine when damaged, which will usually result in the engine revving very high. MP should stay high in this case, too.
Finally, oil or coolant system damage leads to heat, a rough running engine, and eventually nasty noises and a stationary prop. RPM and MP will go away as the noise gets louder, and the associated oil/coolant temps will go up, and pressures will go down. Two schools of thought on how to handle this. One is nurse the engine by reducing power and going to L/D max (best climb speed, usually about 250 kph.) The other school of thought is that the lump is going to quit soon anyway, get everything out of it you can. Full power, climb, and get as much of your fuel turned into altitude (and glide distance) as possible. Open the radiator full, and go full rich on the mixture to try to keep as much heat out of the engine as possible. Again, I don't know how much of this is modeled in the sim.
More than you ever wanted to know, I'm sure, but I hope this helped.

Blotto

"Only the spirit of attack, born in a brave heart, will bring success to any fighter craft, no matter how technically advanced." - A. Galland

"Look, do you want the jets, or would you rather I slap the props back on?" - W. Messerschmitt

XyZspineZyX
08-06-2003, 01:02 AM
There's nothing left for me to add. Good explaination, Blottogg.

It would be interesting to learn just how much of this is modelled into FB. I suppose if they told us, that would create new arguments and complaints on what was done wrong...

XyZspineZyX
08-06-2003, 01:15 AM
yep and if the prop governers hit it will rev its nuts off