I need help with variable pitch propellers
 

I've been with flight sims for a long time, but i still don't understand how to use VP propellers efficiently. I know that it works somewhat like the gears in a car; with a high propeller RPM, you get the highest power, but at a speed high enough the propeller pitch causes enough drag to overcome the engine power. The point is, how do i know when it's time to set a lower RPM?

I'm not an expert but the following should give you a rough overview:
In a dive lower RPMs (I usually set it to 70%) will help you to gain speed. In a climb you can always have it to 100%. I think in level flight 100% or 90% should get you the highest speed.
But maybe someone with a bit more knowledge has more accurate numbers.

I use it as Schroeder descriped. Except, I only use it on P51, AFAIK, in IL2 not many planes have any advantage of using prop pitch.

Really? I've seen plenty that can...

Be quiet Hurrijockey. :O:

Actually all planes with a constant speed propeller (I think that is the English name for that) will benefit from the tactics described above.

Schröder pretty much nailed it.

I use it quite similarily:
Take off and final approach: 100%
Level flight: The same as throttle
Climb: Usually the same as throttle, or 100%.
Dive: 100% up to 450-500 km/h IAS, then to 80%, then to 50%. When speed drops again after the dive I put it back to 100% around 600-650 km/h IAS. I've only really tested this with P-38s, P-47s, P-51s and the mid-to-late-war Navy rides, so no idea how it works on other aircraft.