That's what i initially found out and it's confirmed by the manual for the real aircraft, the Mk.IV had a propeller system similar to the DH hurricane and the early Mk.I Spit we have in the sim with only fine and coarse positions. In the sim however it's possible to move the pitch sliders gradually but only the fully back position switches to coarse and anything above that switches to fine.
On the other hand some people say that while the animation is that of a two-stage prop, it actually works like a fully variable one. I think we need to do some more testing to clarify this.
Xpupx's write up is still good advice though, minus a small inaccuracy: there is no water temp on Blenheim. The engines are air cooled so the gauges display cylinder head temperature
Other than that it's pretty sound advice, especially the part about how each engine control affects certain parameters. I still do some things differently but it doesn't mean it's the only way. For example i don't keep the carb heaters on unless i'm flying close to clouds, because heating the air lowers its density. This means that the mixture is also altered (less air volume for a given fuel volume, it's like running a richer mixture) and i don't know if the semi-automatic mixture systems corrects that.
What i've found out is that you can easily fly up to 10000ft without any carb heat at all. I just turn it on along with the pitot heater when i get fog/icing on my windshield, then turn it off once i'm out of the layer of humidity.
Now, i don't know how close this is to reality, but the current Blenheim in the sim can fly fine with reduced or no carb-heat usage when in clear skies, getting a bit more performance in exchange for having to keep an eye out for potential icing effects.
It's easier to just set carb heat at a certain alt and leave it there but you'll get a bit reduced performance, so it's up to us to individually decide on how to do it.
However, in the real one they probably left the carb heaters on so i guess i should start doing it like xpupx does
The mk.IV manual has no recommended settings about carb heat, but the Mk.V manual does. Since the engines are the same (the main difference is that the Mk.V has constant speed props while the Mk.IV has two-pitch props), maybe we can follow the same guidelines. Here's what it says:
1) Warm intake should be used:
a) For all flying at less than +3.5 psi boost, unless atmospheric temperature is more than +15 degrees Celsius. In that case (more than +15 C), Cold intake should be used regardless of boost.
b) For all flying (regardless of boost used and ambient temperatures), when in conditions of high humidity, in or just below clouds, in rain, snow or sleet. Warm intake can also be used to help warm-up the engines in very cold weather.
2) Cold intake should be used for all other conditions, including:
a) Starting at all times
b) Take-off
c) Landing, unless the bad weather conditions of 1b) apply.
So, i guess it's safe to turn it on and keep it there at the altitude where the ambient temp drops below 15 C. Since we don't yet have the dynamic weather, i guess the sim uses a standard atmosphere model. In that model, the ambient temp on sea level is considered to be 15 C and drops 6.5C for every 1km of altitude (or roughly 3000ft). In other words, if CoD uses an ISA atmosphere and the carb heaters on our Blenheim are similar to the Mk.V carb heaters, we can turn on carb heat as soon as we have completed takeoff, assuming we climb at less than +3.5psi boost. I usually set it to +1 boost for a normal cruise climb, so it would definitely apply to my method.
The easiest way to see what kind of ambient temps the game uses is to spawn a Blenheim in the cross country mission in the QMB without starting the engines, accelerate time for a while to let the engines and oil get up to ambient temperature and see what kind of temps the instruments indicate.
I think that last time i checked this CHT temps were 10-12 C, so it definitely looks safe to turn on carb heat as soon as you climb 500-1000ft after take-off and settle into reduced power settings.