Question for the Motor Heads

[ATAG_Bliss]

Originally Posted by Dutch_851

Why is it that when you coarsen the prop pitch and RPM's drop, the boost gauge goes up? Conversely, when you reselect fine pitch, boost goes down?

If driven mechanically, surely supercharger boost should increase with RPM.

Or does a waste gate in the manifold close in relation to prop pitch setting?

Also is the RPM gauge telling you prop RPM or engine RPM?

Thanks in advance!

Boost is relative to both engine RPM and engine load. Typically speaking, say in a blown (supercharged) car, holding a steady 2000 RPM's (with the transmission in neutral) will hardly create any boost. But take that same car and lug it (put a load on it) at 2000 RPM's and you'll see an exponential amount of boost displayed or a huge difference between the amount of boost seen when the engine is at the same RPM.

Changing the prop pitch can induce more of a load (lowering RPM and increase boost) depending on what you were running before hand. Just think of it this way. Say you are in a huge diesel truck with a manual transmission. You take off from a stop sign/light and are at high RPM's in 1st gear just holding steady (like you are on the governor/ready to shift). There's not much load on the engine and it's easy to maintain that speed given the low gear ratio and high RPMs. Now take off from that same stop sign in 3rd gear. It'll take much more fuel needed to burn to achieve the same result, even though you'll get to that same speed you were in 1st gear with much less RPM's. But because you are using twice the fuel, (lets say you are at a stoichiometric fuel rate of around 14.7:1), you are also going to be using much more air. Now if think about it, you are burning more fuel and sucking in more air in the last scenario using less RPM's. So the load of the engine (easily calculated by fuel consumption or air flow at any RPM) is a huge driving factor of your base engine with regards to how much manifold absolute pressure (boost) you have.

I'm not that familiar with aircraft engines, but all combustion engines are based off the same principles. I would assume that the RPM gauge in-game only measures engine RPM, but I not 100% sure.

[T}{OR]

Nicely put Bliss.

Just to simplify even more on your example: this is equivalent to driving up the hill vs. driving on the straight and level road. To achieve the same RPM, or lets say speed (in the same gear) - you will "flour" the pedal when driving up the hill vs. gently press it when driving on a normal and level road.

Originally Posted by SYN_Bliss

I'm not that familiar with aircraft engines, but all combustion engines are based off the same principles. I would assume that the RPM gauge in-game only measures engine RPM, but I not 100% sure.

Unless there is a gearbox between the engine and the prop, I should think so too.

[ARM505]

Funny that A2A is mentioned, because I seem to remember that LOWERING RPM in the A2A Spit LOWERS boost (ie, lowering prop RPM's via the prop control lever at a constant throttle setting results in less boost indicated). This is explained in the A2A docs - the supercharger, being directly linked to the engine, is now being turned at a lower RPM, hence a lower manifold pressure - the opposite to unforced induction, where lowering RPM at a constant throttle setting will result in higher manifold pressure, the 'car going up a hill in a high gear' analogy.

Yup, just checked the A2A Spit, and that's what happens. COD seems to behave in the opposite way - lowering RPM's lowers manifold pressure. One is right, the other wrong I suppose. Funnily enough, once again we seem to have a multitude of inputs explaining why COD is correct. Oh well. I'll just go and fly my Spit IIa IRL to check it out quick

To be honest, my vote goes to the A2A Spit, which is frankly awesome, even though the engines tend to wear too fast!

[Seeker]

Originally Posted by ARM505

Funny that A2A is mentioned, because I seem to remember that LOWERING RPM in the A2A Spit LOWERS boost (ie, lowering prop RPM's via the prop control lever at a constant throttle setting results in less boost indicated). This is explained in the A2A docs - the supercharger, being directly linked to the engine, is now being turned at a lower RPM, hence a lower manifold pressure - the opposite to unforced induction, where lowering RPM at a constant throttle setting will result in higher manifold pressure, the 'car going up a hill in a high gear' analogy.

Yup, just checked the A2A Spit, and that's what happens. COD seems to behave in the opposite way - lowering RPM's lowers manifold pressure. One is right, the other wrong I suppose. Funnily enough, once again we seem to have a multitude of inputs explaining why COD is correct. Oh well. I'll just go and fly my Spit IIa IRL to check it out quick

To be honest, my vote goes to the A2A Spit, which is frankly awesome, even though the engines tend to wear too fast!

There's a thread about the Spít I and II in the FM forum where people are beating each other over the head with docs and charts as usual.

In the thread there's a comparative test between 109 and Spit, and the RAF docs clearly state that the Spit pilot reduced revs to 2600 which raised the boost...

[ATAG_Dutch]

Originally Posted by Seeker

There's a thread about the Spít I and II in the FM forum where people are beating each other over the head with docs and charts as usual.

In the thread there's a comparative test between 109 and Spit, and the RAF docs clearly state that the Spit pilot reduced revs to 2600 which raised the boost...

If you watch the A2A vid referenced by louisV, the A2A spit is provided with three different props, so it would depend on which one was being used. From what I can make out, the spit in the comparison would have the constant speed in order to set revs at 2600. I think!

[Blackdog_kt]

The efficiency of direct-drive supercharges is tied to the RPM. This is true.

The above effect is not always linear and probably each supercharger has its own "powerband" so to speak.

There's also another thing to consider. RPMs=amount of combustion cycles per minute. The engine is essentially a vacuum pump, it sucks air, mixes it with fuel and burns it to produce power.

If you lower the RPM you essentially lower the amount of combustion cycles during a given time frame. This means that for the same throttle position, less air is being "used up" by the engine.

Where does this air go then? I guess it stays in the manifold for a while longer because at one end (the intake) air is still being forced in, while at the other end (the actual engine/carbs/pistons) less air is being drawn out of the system. This would easily result in an increase of pressure in the intake manifold system and since this is what the boost/ata/manifold pressure gauges measure (just with different units), it shows up in the instruments.

As another interesting bit of information, the less amount of stress on the manifold is not with the throttle closed. At low throttle settings the intake "tube" is trying to implode, because the inside pressure is less than the outside pressure. In fact, the lower amount of stress on the intakes occurs when running throttle that gives a manifold pressure equal to the outside (ambient) air pressure: at sea level this would mean running full throttle on a non-supercharged engine.

Just goes to show how things are not that much set in stone but there's a lot of inter-dependency between different conditions.

If any of you want some pretty long winded explanations that cover everything, check out the following links. Someone else posted them here and i didn't miss a chance to bookmark them after reading, very useful stuff.


Manifold pressure: http://www.avweb.com/news/pelican/182081-1.html

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

Mixture: http://www.avweb.com/news/pelican/182084-1.html

[Skoshi Tiger]

Originally Posted by Blackdog_kt

If any of you want some pretty long winded explanations that cover everything, check out the following links. Someone else posted them here and i didn't miss a chance to bookmark them after reading, very useful stuff.


Manifold pressure: http://www.avweb.com/news/pelican/182081-1.html

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

Mixture: http://www.avweb.com/news/pelican/182084-1.html

Blackdog,

Thanks for re-posting those links. I've only managed to get through the first one but it was really informative.

Cheers!

[ATAG_Bliss]

Originally Posted by MadBlaster

Manifold pressure, I'll take a guess as I'm not a pilot either. I think it's just like a car. Natural aspirated, the engine creates vacume on the piston downstroke and sucks in the fuel/air mix. The higher the rmps, the more sucking power and the higher the manifold pressure. In supercharger setup, pump is belted somehow to the drive shaft and forces air/fuel into the manifold via a pump. I guess what the guys are saying up above to explain your observation about manifold pressure going up when prop pitch is changed to 'course' is that at high rpms, output of the supercharger pump somehow hinders the engine vacume (ala "law of diminishing returns"), causing it to be lower than it would be if the engine was naturally aspirated. So, it's a trade off when you use a supercharger. What you lose in power at the high end of the rpm scale you gain on the low end of the scale (i.e., higher manifold pressure at course pitch setting where the load on the prop in static state (no acceleration or deceleration) is highest).

Originally Posted by Blackdog_kt

The efficiency of direct-drive supercharges is tied to the RPM. This is true.

The above effect is not always linear and probably each supercharger has its own "powerband" so to speak.

There's also another thing to consider. RPMs=amount of combustion cycles per minute. The engine is essentially a vacuum pump, it sucks air, mixes it with fuel and burns it to produce power.

If you lower the RPM you essentially lower the amount of combustion cycles during a given time frame. This means that for the same throttle position, less air is being "used up" by the engine.

Where does this air go then? I guess it stays in the manifold for a while longer because at one end (the intake) air is still being forced in, while at the other end (the actual engine/carbs/pistons) less air is being drawn out of the system. This would easily result in an increase of pressure in the intake manifold system and since this is what the boost/ata/manifold pressure gauges measure (just with different units), it shows up in the instruments.

Superchargers are Load Sensitive. This means that if the engine is not under load the superchargers will not make boost REGARDLESS of engine RPM. This is something I think many people are missing.

Even though your 1st thoughts are to think that a supercharger is pumping air directly linear with how fast the the supercharger screw(s) are turning (engine RPM), you gotta realize that at even high RPM's unless you are under a load and needing to be heavy on the throttle (aka opening up throttle plate and allowing the carb jets more fuel flow) you are not creating much boost. The engine needs to be working to get ANY boost at any RPM. That same throttle plate limits the amount of air that can enter the manifold to build up positive pressure (boost). It's the same reason why increasing engine load (with the same throttle setting applied) will lower RPMs and increase boost.

It would be like cruising down the interstate at 70mph on a flat road then trying to go up a hill without giving it any more gas (throttle pedal position). You are going to slow down (because of load) unless you increase throttle which in turn, is going to increase fuel consumption, to be able to maintain 70mph. In one instance you are at, say 10% throttle while driving on a flat road. In the other instance you could be at 50% throttle while driving up a hill to maintain the same speed as the 1st instance.

[ATAG_Dutch]

Thanks for the replies chaps, all good information!

So going back to fixed pitch props for a sec, presumably the throttle then controlled an air/fuel valve in the carb, so boost/manifold pressure and rpm would both go up or down according to throttle setting?

The constant speed prop set up is a) set required engine revs and b) set throttle to achieve faster or slower airspeed at those revs, making the throttle effectively the pitch control, and there's some automation in making sure enough fuel/air is fed to the engine for those settings?

The two speed prop's throttle then controls what?

Sorry, I always was a bit slow with this stuff!