Question for the Motor Heads

[Babi]

Originally Posted by SYN_Bliss

Once you see that a loaded engine at 2500 RPMs can use more air and fuel than an unloaded engine at 10,000 RPMs - meaning RPM's are just part of the equation, then you can start to understand the load part of it.

I think we are expressing the same thing with different approaches. To me it's more intuitive to think that the intake pressure is dependant on "throttle" (throttle butterfly valve position) which causes a pressure drop, and engine RPM because it mechanically drives the supercharger (the relation between supercharger's RPM and its compression ratio may depend on the type of sc, not sure though).

A loaded engine at 2000 rpm achieves more boost (and consumes more fuel) than an unloaded one at 2000 rpm just because in the unloaded one the butterfly valve (throttle) is "more closed" (otherwise RPMs would go up) which causes a bigger pressure drop.

the point is that, ignoring the external pressure dependance, the intake pressure should depend on 2 variables, not three.

Do we agree on this?

Regarding the game, at constant altitude and throttle, if the prop pitch is coarsed (load increases, RPM decrease) the intake pressure goes up, which is not what one would think since the supercharger spins slower.

[swiss]

this is exactly how i understand it.

The output of the SC is only dependent on rpm but the pressure(boost) measured behind the butterfly on load.

Regarding the game, at constant altitude and throttle, if the prop pitch is coarsed (load increases, RPM decrease) the intake pressure goes up, which is not what one would think since the supercharger spins slower.

I just tried that on the FW190 1.65.
If you decrease pp(lower rpm) boost decreases too.

Not sure on the turbo planes, I'll try later.

[ATAG_Bliss]

Originally Posted by Babi

I think we are expressing the same thing with different approaches. To me it's more intuitive to think that the intake pressure is dependant on "throttle" (throttle butterfly valve position) which causes a pressure drop, and engine RPM because it mechanically drives the supercharger (the relation between supercharger's RPM and its compression ratio may depend on the type of sc, not sure though).

A loaded engine at 2000 rpm achieves more boost (and consumes more fuel) than an unloaded one at 2000 rpm just because in the unloaded one the butterfly valve (throttle) is "more closed" (otherwise RPMs would go up) which causes a bigger pressure drop.

the point is that, ignoring the external pressure dependance, the intake pressure should depend on 2 variables, not three.

Do we agree on this?

Actually the actual intake pressure of a particular engine will depend on so many variables that I could write a 10 page essay about it. You can use RPM and throttle position as the variables, but without knowing the particular case of which you using the engine, what engine, what percentage of load you are using, it's hard to make any sort of analogy to what's actually going on in a given instance of operation.

Regarding the game, at constant altitude and throttle, if the prop pitch is coarsed (load increases, RPM decrease) the intake pressure goes up, which is not what one would think since the supercharger spins slower.

1st you have to understand the principles between the 2 (engine and supercharger) to realize that you will always have positive manifold pressure in that situation.

Think about it this way. Lets say we have a supercharger that creates a nominal output of boost of 8lbs. That means @ full load and full throttle situations you are going to pump out 8lbs of boost / positive manifold pressure of 8lbs. Lets say the operating RPM range of this engine is 600 RPMs to 6000 RPMs. So when you are maintaining 5000 RPMs with 50% throttle and jump the load up significantly to bog the engine down to 4500RPM's (500 revolutions per minute) all while maintaining the same throttle position, you have effectively increased boost because the engine has slowed down it's air intake significantly while the supercharger has only done this marginally. This creates a boost situation. This is exactly why a supercharger is used. Because now, when you jab the throttle, you not only have the fuel to feed it and increase RPMs, you also are exponentially increasing the amount of air the supercharger is positively charging the intake to feed the monster / increase RPMs / go faster / etc.

So the supercharger as you know, is always supplying air, but the engine (say a V12/V16) decreasing 500RPMs with the same throttle position, will significantly decrease the air it draws in (16 pistons/connecting rods all with w/e stroke - basically one big huge vacuum pump (the engine) being slowed down) while that 500 RPMs on little Mister super charger only slows down it's output marginally. The supercharger outlet is probably only slightly bigger in diameter than a single piston of said 16 piston engine. On most supercharged cars it's usually smaller in diameter than a single piston. And with a max output of 8lbs of boost (for instance) slowing down 500 RPMs of that monster vacuum pump is not going to effectively create a 1:1 vacuum pressure with relevance to each other (engine air sucking to supercharger air blowing). It's going to exponentially increase or decrease depending on the situation. That is why at full load and full throttle (say jabbing the throttle at 2000RPMs) you are going to maintain maximum boost and virtually identical boost completely through out the RPM range that you jabbed the throttle at (2000-6000 rpms)

That make sense? I've tried to type it out as best as I can explain it through text lol. I'm running out of analogies!

[louisv]

Boy, I learned in this thread today !!! thanxs everyone, especially for the three links...lots of detail here

[Blackdog_kt]

I think i'm starting to get a grasp on what SYN_Bliss is saying, i will still have to re-read it with a clearer head tomorrow (major lack of sleep today) but there's some good analogies in there that make it simpler to understand.

On another note, what surprised me is that the thread dived right into boost and superchargers. Maybe it would be much easier to explore and/or explain the basic effect of "RPM reduction leads to manifold pressure increase" if we had started with something simpler: a non-supercharged engine (say, a Lycoming on a Cessna) and absolute units of measure for manifold pressure.

In any case, very interesting and informative thread, cheers to all for their input

[MadBlaster]

So when you are maintaining 5000 RPMs with 50% throttle and jump the load up significantly to bog the engine down to 4500RPM's (500 revolutions per minute) all while maintaining the same throttle position, you have effectively increased boost because the engine has slowed down it's air intake significantly while the supercharger has only done this marginally. This creates a boost situation.

And it has also significantly reduced outake of spent gasses too. In other words, the exhaust valves are cycling less now from the engine rpm drop and effectively create a temporary "backstop" while the supercharger pushes the air in and so the intake manifold increases like a balloon blowing up with a small hole in it instead of a large hole where most of the air would go right through it. So, the more the load jump and rpm drop, the higher the boost effect. Correct?

[DK-nme]

Originally Posted by kimosabi

Propeller tips are known to reach the speed of sound but not the whole blade. Ever wondered what the "flapping" sounds from helicopters are?

Ahem, this is a quote from the following link, just to confirm my earlier statement about prop tip speed, not exceeding the speed of sound:

"You will not see prop tips moving much faster than 600 knots, as this is getting too close to the speed of sound, and all sorts of nasty aerodynamic things start to happen in the vicinity of the prop tips. The noise alone is bad enough!"

http://www.avweb.com/news/pelican/182082-1.html

[Blakduk]

So CoD is correct and A2A is wrong????

Syn_Bliss makes a very convincing argument for the accuracy of CoD- but all good theories must be proved by experiment. Does anyone have a Spitfire they can take for a spin to check this out?
I could ask my brother's mate who occassionally flies a MkV at Temora, but i feel really stupid asking him questions like this.

[ATAG_Dutch]

Originally Posted by Blakduk

but i feel really stupid asking him questions like this.

Why? I'm sure he'd be only too happy to give you the benefit of his experience.

[Blakduk]

Dutch- He makes me feel like i'm a 12yr old playing games while he's a grown-up doing things for real (which is all very accurate, except i'm well past 12).
I'll humiliate myself and ask him, i can handle his disdain.

Back a few years ago i met a guy called Alec Henshaw. He was a test pilot for Spitfires during WW2 and flew hundreds of them- now he would have been the one to ask!