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[Herra Tohtori]

Neat. Does this support simulating water cycle, ie. presence of water vapour, liquid water droplets, or ice crystals?

[asheshouse]

I think the odd's of predicting the national lottery numbers are lower than the odds of predicting any journey on the UK rail network.   

Must file that quote away for future use

[Stainless]

Neat. Does this support simulating water cycle, ie. presence of water vapour, liquid water droplets, or ice crystals?

Not just the water cycle, I have a term in the equations that handles dust in the atmosphere. 

[Herra Tohtori]

Can the weather system update itself during gameplay? What kind of resolution can it provide? Like, let's say you wanted to create clouds and precipitation based on the weather system, would it be practical to use the weather system to determine whether a particular air cell, or "block" should have cloud, rain, or snow effects visible on it?

You mentioned the system can provide global weather information for any point on a planet's atmosphere. Do you have plans on modeling microclimate phenomena, like thermal columns, landwind/seawind, wind interacting with terrain to create things like updrafts when wind meets a hill or a ridgeline?

If that's possible, it would make it possible to create really interesting integration for maps - for the basic source of thermal convection, you'd just need to have an "albedo map" of the terrain, something that allows the game to track the surface temperature based on how much sunlight it's absorbing... Which would then transfer heat to the air cells near those points.

[Stainless]

Not written the weather system yet, the atmosphere model is very complex and can feed in data to many systems.

Conditons for icing, contrials, fog, all sorts of things.

[Stainless]

Ok, nearly got jet engines in, but I've realised I've missed something.

I don't have any code in for compressor stalls.

I know that they happen at high AoA and it's obviously because the air flowing through the engine is less than some critical value, but I don't know what critical value.

I'm really busy at work and don't want this to be sidelined because I don't have the time to do some research, so any chance you guys could do some research and get me some data?

[Herra Tohtori]

http://en.wikipedia.org/wiki/Compressor_map

Basically all jet engines have their individual characteristics and to complicate things, inlet shapes can change things too. Especially in supersonic flight where you have to slow the airflow to subsonic before it enters the compressor.

I am not an expert in jet engines so I don't think I can be of much help. But basically, I think it can be reduced to simple principles of flow dynamics: Gas always flows from high pressure to low pressure, and takes the path of least resistance.

The compressor produces a pressure differential and makes air flow through it, up to a certain pressure ratio. If the pressure differential between inlet air and combustion chamber is suddenly increased (by disrupted airflow on the inlet, like turbulence caused by high AoA or foreign objects) the pressure ratio can suddenly increase above the critical pressure ratio, which causes already compressed, high-pressure air (or fuel-air mixture) from within the engine to flow back through the compressor.

[VF111Sundowner]

I don't have any code in for compressor stalls.

I know that they happen at high AoA and it's obviously because the air flowing through the engine is less than some critical value, but I don't know what critical value.

http://en.wikipedia.org/wiki/Compressor_map

Basically all jet engines have their individual characteristics and to complicate things, inlet shapes can change things too. Especially in supersonic flight where you have to slow the airflow to subsonic before it enters the compressor.

Herra is right, it depends on the engine and inlet design to when a compressor stall will happen.
Take the F-14 for example with the TF-30 engines which were slightly under powered and extremely prone to compressor stalls.
When replaced with the GE-110s the stalls stopped and performance improved greatly, The inlet remained the same.

Second also true is supersonic airflow, which reverses properties as it exceeds the speed of sound. (IE  pressure decreases as speed increases in sub sonic, while supersonic air pressure increases as speed increases.)
That's why most super sonic fighters have variable inlet ramps to slow the air flow down before entering the engine. and Aircraft like the F/A-18C and F-16 can only just barely touch the mach two realm, they have fixed inlets.

Third ...

I think it can be reduced to simple principles of flow dynamics: Gas always flows from high pressure to low pressure, and takes the path of least resistance.

  Also true.
This problem is solved in a few ways, when an engine doesn't have enough airflow entering the compressor in at enough speed or volume it tends to follow the least resistance and reverse back out the front, (Compressor stall) There are a few ways to alleviate this.
Pratt and Whitney's idea is to bleed off all the excess air overboard until the engine spool up enough to allow proper flow.
General Electric likes to put variable inlet guide vanes, (little movable doors/fins) in front of each compressor section to limited airflow entering the compressors.
The Russian's add blow in and vent doors to some of their aircraft inlet to allow the engine to draw in more air at low speeds to keep the pressure entering the engine high prevent in the back flow.

Stainless what data are you looking for individual aircraft stall points or a general overview?

[Stainless]

I need hard data so I can add parameters to the engine definition.

Something like min and max flow rates, or min and max air pressure at the input to the compressors.

Something I can measure and apply based on engine design.

[VF111Sundowner]

What you want sounds like EPR (Engine Pressure Ratio) the difference between the air pressure going in to air pressure coming out.

http://www.grc.nasa.gov/WWW/k-12/airplane/epr.html

The unfortunate fact is every engine model has a different EPR.
http://en.wikipedia.org/wiki/Overall_pressure_ratio

Here is alittle info on the Stall it self.
http://en.wikipedia.org/wiki/Compressor_stall

[Stainless]

Every engine having a different value is perfect, I want something that we can put in on an per engine basis.

So if you fly an early version of the f16, you have to be aware of the engine limitation, but if you fly a later version you don't
 

[Herra Tohtori]

Ducted fans have pressure ratios too, by the way, and technically propellers do as well but since they are not surrounded by a shroud, the airflow through the propeller disk isn't as easy to define...

Are you going to model turbofan engines with separate fan and turbine core pressure ratios (each producing a certain amount of thrust), or as a single, integrated entity?