Question for the Motor Heads

[ATAG_Dutch]

No, not Lemmy's gang!

I do have a bit of knowledge about engines and how they work, but an aspect of engine behaviour in CoD is baffling me a bit.

It's my understanding that superchargers are driven mechanically, as opposed to turbos driven by exhaust gasses, so my question is this;

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?

Any answers or links to articles would be appreciated!

Thanks in advance!:)

[Babi]

I was wondering the same thing myself. Perhaps something connected with fluid dynamic losses increasing with increased speed in the air intake. Waiting for an answer too.

[Armatian]

Superchargers have lower rpm peak performance unlike turbos, they start to loss capacity

[raaaid]

yes interesting stuff and asking about game engines whats 109 post combustion

hell i got a degree in english philology and they incapacitated me for english now im four subjects from a degree on nautic engines and all i have is phobia to thermodinamics

[choctaw111]

I can't speak for all aircraft but usually the RPM refers to the engine RPM.

[kimosabi]

Superchargers have lower rpm peak performance unlike turbos, they start to loss capacity

Bingo. However, where your superchargers peak rpm is depends on how you gear it and how much air it is designed to feed at any given supercharger rpm.

I could expand on that for a few pages but you got the essence of it, "capacity" is the key word. A supercharger tuned/geared to give the engine max boost at 2600rpm will be unable to provide air to uphold that pressure at 3000rpm. Think of the engine as a pump. The higher the rpms, the more air it "pumps" through, and the supercharger can only keep up with it up to a certain point.

As for the British planes, they show prop rpm. Not sure about ze Germans.

[louisv]

People, you need to go to A2A Simulation and watch the video on props. Find a way to study the CEM on the web...

http://forum.1cpublishing.eu/showthread.php?p=280560#post280560 top of the page.

The RPM is the engine RPM
The Boost is the manifold pressure (look for it), not the supercharger.
When you use a Constant Speed Propeller, pushing the throttle has the effect of changing the propeller's blade angle to keep the RPM constant, the manifold pressure will show the change as the propeller bites more agressively in the air, and you WILL go faster EVEN if the RPM does not change...This is NOT like a car.

The problem is for people who have never taken a flying course, you cannot guess the CEM...And you generally don't turn with the rudder either as I have seen in many videos. Look up "coordinated turn", the "slip" indicator should stay centered during a turn, you use the rudder to keep the needle straight while you bank the wings with the ailerons and pull on the stick...its quite a trick really to keep the turn level while holding the slip needle centered...Practice makes perfect, but you must to know what to practice.

Again go to A2A Simulation and you will finf the help you need.

PS: I am not affiliated in any way to A2A, its just a great place to learn. Especially look at the two videos on propellers and manifold pressure...study then practice...Otherwise keep away from the CEM.

[TeeJay82]

this easy to understand, but very hard to explain in text :/

[ATAG_Bliss]

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.

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!

[ATAG_Dutch]

Thanks for the replies chaps, all good information!:grin:

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!:confused::grin:

[DK-nme]

No, not Lemmy's gang!
Thanks in advance!:)
Hmmm they are good though...
:grin:
Also is the RPM gauge telling you prop RPM or engine RPM?
Thanks in advance!:)

RPM is for the engine, I think. And this is why. A quick calculation:

If prop lenth is 1.5 meters (too long, too short?)
And max RPM is 2400-2500 r/min (too much?), then, the speed of the propeller tip will be:

2*radius*Pi (m/r) * 2400 (r/min) * 60 (min/h) *1/1000 (km/m) = 1357 (km/h), which is a bit above speed of sound.

Edit: Oooops, a small bug in the calc. Now it has been fixed

The propeller should not exceed the speed of sound, cause the compresability will eliminate the lift (of the propeller, that is) and thus, the RPM must logically represents the revolving of the engine...

[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? ;)

[DK-nme]

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

Yup, I know, but the idéal speed is just slightly beneath the speed of sound - I think, that A2A actually stated the same, in the video about prop pitch and constant speed propeller...

[ATAG_Dutch]

Hmmm they are good though...
:grin:

First saw them in 1979, about 25 times in total, and will be going to the Bloodstock festival this year. Played pool with Lem, Philthy and Brian Rob once in Sheffield.

As much an English institution as Fish n Chips.

Lemmy for PM!:grin:

So anyway, manifold pressure and prop pitch......;)

[MadBlaster]

I read somewere a while back that the constant speed prop is based on oil hydraulics and aerodynamic forces on the prop from airspeed. Basically, the faster you go, the prop wants to run fine pitch from the aero forces, so to counteract that, there's a governor mechanism that runs off oil pressure to make it run course pitch. So you have the two forces constantly balancing to keep the rpms in range so the engine doesn't overspeed. So whether your running fine or course gears, its the same principle. It's a passive control more or less. Not like a variable pitch prop in manual mode ala 109.

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).

[Seeker]

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]

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. :-P

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]

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]

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).

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.

[MadBlaster]

Don't disagree with anything said. Backpressure plays a role in the deal too (e.g., number exhaust valves, header/manifold/cat). Each engine is engineered out for a specific task. I guess I was thinking about the example more literal. The plane is on the ground with the chocks on no aero forces on prop, the throttle valve on carb is fixed at some position and the only thing being done by the pilot is toggling the prop pitch from fine to course to increase the thrust load and lower the rpms. If the rpms on the supercharger pump are directly proportional to turns on the driveshaft (i.e., the belt drive gearing is a fixed ratio), what would make the manifold pressure increase if the rpms of the engine and supercharger are both proportionally decreasing from the increased prop load and fuel being metered is unchanged? I think what blackdog is saying makes sense. Backpressure on the exhaust side holds the air in the intake manifold longer for that particular engine at that range of rpm/load change and fuel rate. Another thing to consider, the supercharger being belted to the driveshaft. I guess is a variable load on the engine. It takes energy to pump air. So while the prop is increasing load to lower engine rpms, the lower rpms on the supercharger pump are decreasing load (still assuming fixed fuel meter). That's intuitively what I was thinking about above where cranking that supercharger pump at certian rpm range with the increase in load that it brings to do that work doesn't pay off verses normal aspirated. Jeez, what a brain drainer this one is.:-P

[ATAG_Bliss]

what would make the manifold pressure increase if the rpms of the engine and supercharger are both proportionally decreasing from the increased prop load and fuel being metered is unchanged?

Manifold pressure is going to increase when you compensate for the loss in RPM's through engine load and throttle increase. But that same metered fuel that required 50% throttle to maintain the RPMs you wanted to achieve, is now doing it at less RPM's. This creates positive pressure - boost (but only until the engine is normalized) then the boost guage (depending on the engine / charger combo) will remain fairly steady, but again, this is under a load. Superchargers do take HP to turn and cause a load on themselves, but they are generally designed for the application it's being used on.

A boat is the easiest way to see a supercharger in action with regards to boost, because the prop is always submerged in water at any RPM.

As far as back pressure is concerned, I think you meant to say exhaust manifold instead of intake, but your points are exactly right. That all factors into the equation of a boosted combustion engine.

[Babi]

Maybe i don't undestand because english is not my language, but i don't really grasp the fact that pressure output from the supercharger is dependant on "load"; probably because i dont understand the term.

The pressure should just depend on the engine RPM (wich determine the supercharger's RPM) and the throttle position (and of course external pressure). In the example of the car, the supercharger itself couldn't care less if the car is going uphill or downhill. Then if more "load" means that the engine RPM decrease, this will affect pressure output from the supercharger, but it's not DIRECTLY tied to load.

Then again maybe i undestand because of my bad english.

I think the discussion was about the fact that with constant throttle position and constant altitude manifold pressure drops with RPM

[ATAG_Dutch]

I understand the 'load' part.

Back in the late 80's, I owned a Kawasaki GPZ750 turbo (what an animal!) and the boost gauge rarely got up to full boost unless I gave it 'loads' of throttle in a higher gear. :D

Keeping it at or near the rpm red line resulted in lowish readings on the boost gauge, not that I could look at it too much at those speeds, as it was mounted on the tank. :)

But I think I've got it now. Put simply, at lower rpm, the engine isn't sucking as hard so the pressure in the inlet manifold goes up, and with no alteration to fuel/air metering would result in a dangerously weak mixture.

Conversely, increasing rpm makes the engine suck harder, lowering the pressure in the manifold, and without alteration of fuel/air metering would result in an overly rich mixture.

Have I got it?

Thanks for all the posts if so!

[swiss]

I understand the 'load' part.


Not really, turbos need exhaust pressure to spin up the turbine.
So flooring it is the same action but for a different purpose.

How engineload and sc-pressure on a mechanical driven SC are related is beyond my understanding - that is, unless the Sc engines have the pressure sensor behind the TB, or SCs use a clutch(?).


http://www.superchargersonline.com/content.asp?id=21
Boost builds exponentially with engine rpm, meaning that boost comes on very quickly in the upper half of the powerband.


confused,
swiss

[ATAG_Dutch]

I think I'm getting it, if you have a look at my edited post. :)

Increased rpm results in lower manifold pressure, but a mechanically driven supercharger would compensate for this pressure loss to an extent by raising the level of forced induction exponentially.

I think!

Also, yes I know turbos need exhaust velocity, hence the dreaded 'lag' which superchargers don't suffer from too much, but it's still driving a turbine (impellor?) to force-feed air into the manifold. :)

[ATAG_Bliss]

Maybe i don't undestand because english is not my language, but i don't really grasp the fact that pressure output from the supercharger is dependant on "load"; probably because i dont understand the term.

The pressure should just depend on the engine RPM (wich determine the supercharger's RPM) and the throttle position (and of course external pressure). In the example of the car, the supercharger itself couldn't care less if the car is going uphill or downhill. Then if more "load" means that the engine RPM decrease, this will affect pressure output from the supercharger, but it's not DIRECTLY tied to load.

Then again maybe i undestand because of my bad english.

I think the discussion was about the fact that with constant throttle position and constant altitude manifold pressure drops with RPM

Well what you just said "throttle position" is in effect partial engine load. To give you an idea, you could generally redline (max RPM/rev limiter) an engine with the transmission in neutral far before you ever reached 100% throttle. It's quite easy to spin that engine up with no load on it. This means that it doesn't take as much fuel or the atmospheric "air" to go to max RPM. In this case the engine is running at max RPM (say it's 10,000 RPMs) yet will be fed the same amount of fuel and air that an engine lugging or loaded @ 2000 RPMs will.

When you have a huge load on the engine, it may require full throttle, meaning much more fuel and air, than the same engine turning the same RPMs or it may be such a load that the engine simply can't rev up that high. This creates more boost simply because the inlet is not restricted and more air can be pumped into the intake manifold.

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.

Another way to think about it - drag racing. There's obviously many boosted cars both supercharged/turbocharged cars in high HP racing. Well when you get to a certain limit, most people prefer an automatic transmission, but hear me out. If those guys racing, simply raised their engine RPMs to max while the car was in neutral waiting for the go light to come on, they would have no boost built up. Because it doesn't require hardly any throttle to make an engine go to max RPM's without a load on it. That's the reason there are things such as tranny brakes. In a sense, it locks 2 gears together (generally 1st and reverse) and puts a massive load on the engine, which in turn cranks the boost up, so when they finally release the tranny brake, their car shoots off like a rocket. This allows you to go WFO (wide f'n open) or floored on the throttle. Without any boost that car would launch horribly. So basically you can sit there and rev the engine all day long and won't gain hardly any boost until it's under a load. This helps much more with a turbocharger as it's even more dependent on base engine and engine load. The more load, the hotter the exhaust, the more the impeller will spin to spool up the turbo.

There's guys that will use a line lock method (a solenoid that will hold the brake pressure - say you press on the brakes and you have 1200lbs of brake pressure, if you press the solenoid it now holds the brakes for you - don't need to press the brake pedal anymore) to create a load for boosted apps. All you do is lock the brakes and press in on the gas (load it up / viola - boost!) The good thing about superchargers is the power is available almost instantaneously. So, many guys will run a blower car with a 2 step module. This will electronically kill the spark at a certain RPM so you can keep the throttle pedal held down without blowing the engine up, and therefore create boost. But those that do this usually are the one's that have some pretty nasty blow off / bypass valves, because the pressure will be enormous (WOT at almost max RPM help constantly) and fuel delivery is not stopping. Supercharger explosions aren't fun.

I'm sure if you just googled superchargers or engine load with superchargers it will say the same thing.

Edit: I just did. http://www.superchargersonline.com/faq.asp

@swiss - you need the part about understanding how a SC works though :)

Is the supercharger always working?

Answer: While the supercharger is always spinning and moving air, it is not always producing boost in the engine. Boost is a function of engine load and RPM. The majority of the time your supercharger will not be producing boost. The supercharger produces boost under high load conditions which may include heavy acceleration, going uphill, passing another vehicle or under towing conditions. Superchargers offer the power you need on demand, the reminder of the time the engine is working just like a normally aspirated engine.

What exactly does a supercharger do?

Answer: A supercharger forces additional air and fuel into the engine. This occurs when the engine is under full throttle or under load, not at normal cruise or most normal driving. A large displacement engine makes more power than a small displacement engine because it can convert larger amounts of fuel and air into energy. A supercharger allows a smaller engine to do the same thing but only when extra power is actually needed.

In an airplane (supercharged engine), your engine load is determined by how much air the prop is pulling. That is why less RPM because of increased load will = more boost. If you want the physics of the engine, I think Blackdog_kt's links will give you a thorough understanding of that.

I haven't played flown IL2COD all that much because I'm deployed, but I'm assuming with full CEM switched on, you're probably not able to run at full throttle very long with an incorrect prop pitch correct? If that's the case, load is modeled. But I'm not saying it's modeled correctly. It's just if you can hammer the throttle wide open and set a bad prop pitch (ie - less load / higher RPMs) I bet the engine will be damaged in a short while, which is what would really happen. Does it work like that in game?

[ATAG_Dutch]

Does it work llike that in the game?

Yes it does as far as I can tell.

And thanks hugely for the input Synbliss, and Blackdog for the links, and Thor et al.

Some of this stuff was in my brain somewhere already, it just needs a good prod now and again to bring it to the fore!

:D

[Babi]

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]

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 :grin:

[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]

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]

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!

[Babi]

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.

Ok, i'm starting to get what you mean!

Found my notes on propulsion class, and i feel extremely embarassed for the fact that i completely forgot that in an aspirated engine, intake pressure drops with RPM because of the increased friction losses in the intake (higher RPM, higher air speed in the intake, more friction which cause a dissipation of energy and ultimately a decrease in static pressure when the air is brought to rest). Or if you want to think it more simply, more suction, less pressure.

Now, in a supercharged engine you have to take another effect into consideration, which is the fact that the compression ratio (p after sc/ p before sc) the sc is able to deliver is dependant on its RPM, which in turn is directly linked to engine RPM.

So generally the boost the sc is able to deliver increases with RPM, but also the friction losses increases with RPM, so it's all a matter of what is the most important effect. pressure loss in the intake because of friction should be proportional to RPM^2 if i'm not mistaken, hence very important at high RPM. The boost the supercharger is able to deliver as a function of its RPM will depend on the specific type i think, i will try to find some data.

In the end at higher RPM, if you apply more load and the RPM drop, probably the reduction of losses caused by slower air flow, can compensate for the modest reduction of boost by the supercharger; and the pressure slightly rises.

I think however that at slower RPM it should be the other way around (RPM goes up, pressure rises, because the losses are smaller and the predominant effect is the supercharger), can anyone test it in game?

[Blakduk]

Just had a reply from an engineer who works on superchargers all the time- the boost gauge should not show higher boost when RPM drops regardless of whether the RPM decrease is due to higher loading of the engine. It relies entirely on what the supercharger is doing and it is directly linked to engine RPM.
It seems CoD is wrong.

[ATAG_Bliss]

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?

But the exhaust side of the equation has nothing to do with anything in the intake manifold. The exhaust valves only open during the exhaust stroke and only push exhaust out the exhaust manifold/header.

The intake valves start to open up as the piston is at TDC (top dead center) and as the piston goes down it fills the cylinder with air and fuel. This step is the only suction coming from the intake manifold where the supercharger is supplying forced induction. Next that same cylinder compresses and at a certain point in the ignition cycle during this compression stroke, the spark plug fires and creates an explosion. This causes the piston to go down again (cylinder is now full of exhaust gas) and on it's trip up the exhaust valve is open to let it leave the cylinder head through the exhaust manifold/header.

There's ongoing exhaust back pressure in almost virtually every combustion engine. If you didn't have back pressure, you would easily burn valves amongst other problems. To give you an idea about the Merlin (from memory) the exhaust manifold design was changed at a backwards angle because of the sheer amount of thrust it created. The exhaust gases are flowing either 700 to 1400mph out of the exhaust manifold (don't remember), and it didn't take long before someone realized that if they angled the manifold a certain way, that it would actually increase top speed, help with propulsion.

So, the more the load jump and rpm drop, the higher the boost effect. Correct?

Not necessarily. It depends on your starting RPM, the output of the supercharger, the engine etc etc. But generally if you are spinning 75% of max RPM, and significantly increase load to decrease RPM on a s/c'd engine boost will increase. There will be a point where this won't happen anymore. An engine creates vacuum. So at a certain point you will not have any positive pressure anymore because the blower will not keep up, unless****, while more load is added you are opening the throttle. Again, this all depends on so many variables and specifics of the engine/sc combo that I don't think anyone can give you a good answer. But if you remember that increased load and decreased RPMs will give you boost (in a boosted app) but only to a certain point.

[ATAG_Bliss]

Just had a reply from an engineer who works on superchargers all the time- the boost gauge should not show higher boost when RPM drops regardless of whether the RPM decrease is due to higher loading of the engine. It relies entirely on what the supercharger is doing and it is directly linked to engine RPM.
It seems CoD is wrong.

Well no offense to your engineer buddy but he is wrong m8. I'm an engine builder (drag racer) and a mechanical engineer.

Take this analogy: Easiest way to test this would be with a supercharged car with a manual transmission. Say you are in 2nd gear winding up around 5000 RPMs - you are accelerating at the time, not crusing (with a 6500 redline). Now without changing the throttle, press on the brakes (create a load) that not only stops your acceleration but decreases your RPM's by say, 500. Your boost will increase. Now just for the fun of it, keep pressing the brakes even harder to really lug it down and floor the throttle. Boost is still not dropping, but RPM's are falling off. Now if you press the brakes hard enough while you are floored to essentially kill the engine, you will maintain boost up until a certain point (even more so than what you originally started with) until the engine dies. That scenario just shows you that boost (just like the definition of how a supercharger works) relies on both engine load and RPM.

If you don't believe me. Please go to a car dealership and test drive a supercharged car, borrow a supercharged car, steal a supercharged car:), do w/e you want to get one. But do just what I said and you'll see the same thing. I don't claim to know anything about an aircraft engine, but if it's a V configuration, combustion engine, that has a gear driven supercharger going through a carb, it can't be that much of a different principle than a similar setup on a car.

[Blakduk]

Bliss- unfortunately i live in an area where supercharged hotrods are rare so my opportunities to steal one are very limited. Also i'm a wuss and extremely scared of getting caught. Basically i have to take other people's word for how the real thing works.
Your argument makes sense, it seems plausible but.... i haven't seen the proof.

Basically i've got two engineers telling me different things, and two sims displaying different things (this could turn into another 'plane on a conveyor belt' thread)

[winny]

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!

Just for you.. Alex Henshaw in action over Castle Bromwich 1941
The 1st Min of the video is him.


http://www.youtube.com/watch?v=nCmzYccyBYM

[Blakduk]

Winny- I've been googling Alex Henshaw this evening. I didn't realise he was such a celebrity!
I basically just said 'G'day' to him as he was surrounded by other pilots & engineers- he was having afternoon tea in the hangar my brother was working in (i was helping by shifting boxes of gear in a ute, filling the esky with beer and getting food etc). They were working on Spitfire that was in bits- it was owned by a New Zealander and i think it ended up at Wanaka. They were pointing at bits and nodding knowingly at each other- about the only thing i contributed to was a discussion about the bolts being made of magnesium. I commented they would react with the aluminium and disintegrate over time- i got an approving nod and went to get a beer (it was after 4pm after all and i thought i better quite while i was ahead). I do recall him saying the Spitfire was 'only a 1000 hour airframe, so it's a wonder any are still flying today'.

[Sutts]

Bliss- unfortunately i live in an area where supercharged hotrods are rare so my opportunities to steal one are very limited. Also i'm a wuss and extremely scared of getting caught. Basically i have to take other people's word for how the real thing works.
Your argument makes sense, it seems plausible but.... i haven't seen the proof.

Basically i've got two engineers telling me different things, and two sims displaying different things (this could turn into another 'plane on a conveyor belt' thread)

If boost rises when rpm falls (at a fixed throttle setting) and boost falls as rpm rises then the sim is correct.

As mentioned earlier, an engine is just like a big suction pump, sucking air fuel mixture out of the inlet manifold and thereby reducing the pressure in that manifold (which shows on the boost gauge).

The higher the RPM (at a fixed throttle setting), the faster the air is pumped out of the manifold and the lower the manifold pressure becomes. On the other hand, when RPMs are reduced (at a fixed throttle setting), less air is being sucked out and the pressure rises.

When the engine isn't running, the suction pump stops and the pressure in the manifold returns to the atmospheric pressure at the current altitude being flown.

On a non-supercharged / non-turbo charged engine, the highest boost pressure available is the atmospheric pressure at the current altitude.
On a supercharged / turbo charged engine, the highest boost pressure available is the max pressure provided by the compressor at the current altitude.

That's my understanding anyway.

[Sutts]

But the exhaust side of the equation has nothing to do with anything in the intake manifold. The exhaust valves only open during the exhaust stroke and only push exhaust out the exhaust manifold/header.

The intake valves start to open up as the piston is at TDC (top dead center) and as the piston goes down it fills the cylinder with air and fuel. This step is the only suction coming from the intake manifold where the supercharger is supplying forced induction. Next that same cylinder compresses and at a certain point in the ignition cycle during this compression stroke, the spark plug fires and creates an explosion. This causes the piston to go down again (cylinder is now full of exhaust gas) and on it's trip up the exhaust valve is open to let it leave the cylinder head through the exhaust manifold/header.

There's ongoing exhaust back pressure in almost virtually every combustion engine. If you didn't have back pressure, you would easily burn valves amongst other problems. To give you an idea about the Merlin (from memory) the exhaust manifold design was changed at a backwards angle because of the sheer amount of thrust it created. The exhaust gases are flowing either 700 to 1400mph out of the exhaust manifold (don't remember), and it didn't take long before someone realized that if they angled the manifold a certain way, that it would actually increase top speed, help with propulsion.



Not necessarily. It depends on your starting RPM, the output of the supercharger, the engine etc etc. But generally if you are spinning 75% of max RPM, and significantly increase load to decrease RPM on a s/c'd engine boost will increase. There will be a point where this won't happen anymore. An engine creates vacuum. So at a certain point you will not have any positive pressure anymore because the blower will not keep up, unless****, while more load is added you are opening the throttle. Again, this all depends on so many variables and specifics of the engine/sc combo that I don't think anyone can give you a good answer. But if you remember that increased load and decreased RPMs will give you boost (in a boosted app) but only to a certain point.

Agreed, boost can only rise as high as atmospheric pressure on a non-blower engine and as high as the blower output pressure on an engine fitted with a blower.

[ARM505]

Again, I must ask/clarify:

In the following situation - constant everything (ie altitude, throttle setting, mixture setting, airspeed etc), a change in RPM via the prop control lever will result in (and I have tried both the A2A model and CLOD in similar density alts/airspeeds, for what it's worth):

a) a slight decrease in indicated boost (ie the A2A model Spit)

b) an increase in indicated boost (ie the CLOD model)

One is correct, the other not. So....which is it?

[Sutts]

Again, I must ask/clarify:

In the following situation - constant everything (ie altitude, throttle setting, mixture setting, airspeed etc), a change in RPM via the prop control lever will result in (and I have tried both the A2A model and CLOD in similar density alts/airspeeds, for what it's worth):

a) a slight decrease in indicated boost (ie the A2A model Spit)

b) an increase in indicated boost (ie the CLOD model)

One is correct, the other not. So....which is it?


You haven't said whether you're increasing or decreasing RPM with the prop control.

Increased RPM.....lower boost
Reduced RPM....higher boost

[ATAG_Dutch]

You haven't said whether you're increasing or decreasing RPM with the prop control.

Increased RPM.....lower boost
Reduced RPM....higher boost

He must be reducing rpm as the CoD model would show an increase.:)

[Sutts]

He must be reducing rpm as the CoD model would show an increase.:)

Sounds like CoD is modelled correctly then, thanks.

[ARM505]

You haven't said whether you're increasing or decreasing RPM with the prop control.

I'll just quote my original post again quickly since it somehow didn't get through....I'll use the exact same wording since I just can't put it any other way :) :

...a change in RPM via the prop control lever will result in...

And here's the question, since I am indeed asking a question:

One is correct, the other not. So....which is it?

ie which is correct, the A2A model, or the CLOD model, since they show the exact opposite result?

[Sutts]

I'll just quote my original post again quickly since it somehow didn't get through....I'll use the exact same wording since I just can't put it any other way :) :



And here's the question, since I am indeed asking a question:



ie which is correct, the A2A model, or the CLOD model, since they show the exact opposite result?

Still don't get it I'm afraid....you just say a change. A change UP or a change DOWN in RPM??

[winny]

Winny- I've been googling Alex Henshaw this evening. I didn't realise he was such a celebrity!


I thought you didn't really realise who he actually was :)

He's a bit of a legend Spitfire wise, you were lucky to meet him.


Anyway I don't want to interrupt the thread too much.

Now what is it again? More throttle to go faster, less to go slower, ignore gagues - repeat till engine explodes?

[Blakduk]

Back on topic, a slight reframe of the query:
- throttle steady, maintain altitude (and therefore air pressure is same)
- change pitch to coarse, increasing load on engine, dropping prop & engine RPM
- drop in engine RPM drops RPM of supercharger impeller, decreasing amount of air/fuel forced into inlet manifold
- drop in engine RPM decreases amount of air/fuel being sucked into cylinders.

Question- is the decrease in air/fuel negative pressure at inlet to cylinders more or less than the decrease in the positive pressure of air being forced into the inlet manifold by the supercharger?
If the decrease at the cylinder end of the manifold is more, the boost gauge will read higher as the boost gauge measures increased air pressure in the inlet manifold as there is a backup of air/fuel.

[ARM505]

I'll ask my question another way.

I'm comparing two different models (FSX vs CLOD), both of a Spit MkII with a constant speed propeller:

1) The A2A simulations Spit Mk II running in FSX. In this model, whilst flying at a constant altitude, constant airspeed, constant throttle setting, the indicated boost will DECREASE when you move the prop control lever to DECREASE RPM - the A2A docs mention that since the supercharger is now being driven at a lower RPM, the indicated boost will decrease. The decrease is however very slight.

http://shockwaveproductions.com/

2) The CLOD model, which (under the same conditions), will show an INCREASE in indicated boost when the RPM is commanded to a lower value via the propeller control lever, ie the opposite of the A2A model in FSX.

Since they display the opposite behaviour, one is therefore correct, and the other incorrect. My question is thus which is most true to life. The A2A Spit is widely recognised as a very well modelled aircraft in FSX terms. I have in the past accepted it's behaviour (with the exception of the rapid wear issues) as well modelled. I would like to find out if this is true in this respect.

[Babi]

If boost rises when rpm falls (at a fixed throttle setting) and boost falls as rpm rises then the sim is correct.

As mentioned earlier, an engine is just like a big suction pump, sucking air fuel mixture out of the inlet manifold and thereby reducing the pressure in that manifold (which shows on the boost gauge).

The higher the RPM (at a fixed throttle setting), the faster the air is pumped out of the manifold and the lower the manifold pressure becomes. On the other hand, when RPMs are reduced (at a fixed throttle setting), less air is being sucked out and the pressure rises.

When the engine isn't running, the suction pump stops and the pressure in the manifold returns to the atmospheric pressure at the current altitude being flown.

On a non-supercharged / non-turbo charged engine, the highest boost pressure available is the atmospheric pressure at the current altitude.
On a supercharged / turbo charged engine, the highest boost pressure available is the max pressure provided by the compressor at the current altitude.

That's my understanding anyway.

It's true that pressure drops with increased RPM in a normal (aspirated) engine because of "suction". But you completely miss the other important factor: the supercharger is not some magical box that gives its nominal boost output regardless of what the engine is doing. Its mechanically driven by it, and the faster the engine RPMs, the faster the supercharger spins and the more boost is able to deliver. The pressure ratio the supercharger is able to achieve does not vary linearly with its RPM, and depends on the specific type of compressor installed. So you see? the pressure tends to go down because of increased RPMs, but the supercharger tends to deliever more boost because of this RPM increase. What is the predominant effect? difficult to say, depends on supercharger and intake design, and also if the engine is running at low or high RPM.

Read my post on top of page 5.

In the end we can't say anything about the accuracy of the model, until we have specific engine test data or supercharger characteristics.

[MadBlaster]

But the exhaust side of the equation has nothing to do with anything in the intake manifold. The exhaust valves only open during the exhaust stroke and only push exhaust out the exhaust manifold/header.

The intake valves start to open up as the piston is at TDC (top dead center) and as the piston goes down it fills the cylinder with air and fuel. This step is the only suction coming from the intake manifold where the supercharger is supplying forced induction. Next that same cylinder compresses and at a certain point in the ignition cycle during this compression stroke, the spark plug fires and creates an explosion. This causes the piston to go down again (cylinder is now full of exhaust gas) and on it's trip up the exhaust valve is open to let it leave the cylinder head through the exhaust manifold/header.

There's ongoing exhaust back pressure in almost virtually every combustion engine. If you didn't have back pressure, you would easily burn valves amongst other problems. To give you an idea about the Merlin (from memory) the exhaust manifold design was changed at a backwards angle because of the sheer amount of thrust it created. The exhaust gases are flowing either 700 to 1400mph out of the exhaust manifold (don't remember), and it didn't take long before someone realized that if they angled the manifold a certain way, that it would actually increase top speed, help with propulsion.



Not necessarily. It depends on your starting RPM, the output of the supercharger, the engine etc etc. But generally if you are spinning 75% of max RPM, and significantly increase load to decrease RPM on a s/c'd engine boost will increase. There will be a point where this won't happen anymore. An engine creates vacuum. So at a certain point you will not have any positive pressure anymore because the blower will not keep up, unless****, while more load is added you are opening the throttle. Again, this all depends on so many variables and specifics of the engine/sc combo that I don't think anyone can give you a good answer. But if you remember that increased load and decreased RPMs will give you boost (in a boosted app) but only to a certain point.

I understand what your saying and agree, but I think about it differently. Consider that this is a multi-piston situation, air being a fluid and the rpms being so high. So at any moment in a cycle, one piston has intake valve opening and sucking air (creating negative force flow on the downstroke), another piston has exhaust valve opening pushing spent gas (creating positive force flow on the upstroke) and the other cylinders have their valves closed because they are either compressing fuel/air or driving the shaft from ignition. So if the load increases and rpm drops, the engine cylinders are moving less air through the "system" (negative and positive flows from the system are reduced). In my mind, this creates the air "backstop" that allows the supercharger to build up pressure in the intake manifold, not just the longer time that the intake valves are closed because that's only half the system that is moving air through the engine. :-P

[Blakduk]

Just had a reply from a pilot who often flies old warbirds (including Spitfires, P40's, Wirraways etc) as well as many other types of aircraft. He stated the boost gauge goes down when the revs drop on supercharged engines, regardless of whether it's due to increase load on the engine.
It seems CoD is wrong and A2A is correct.

[ARM505]

Thanks very much for that info.

I've also done a bit of asking (could only find a Mustang pilot though, and it seems the P51 has some sort of automatic boost control, so it pretty much stays where you set it, within limits - it was a short convo, and I didn't want to press too hard?), and this confirms what I've suspected and the info I've seen.