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]

Originally Posted by Armatian

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/showthr...560#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]

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.