Official Fulqrum Publishing forum

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1. Does using the radiator, in fact, help prevent overheating?

Even with radiators wide open you can overheat the engine - throttle back a bit, or reduce engine revs by more coarse Prop-Pitch (Lower %)

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2. Does using the radiator (assuming it helps cooling) have a negative impact on performance and, if so, how much?

Yes.. depends on aircraft and how much radiator you use.

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3. I assume that by "opening up" the radiator, the higher percentage it's open, the greater the cooling effect, correct?

Yes - but again is depends on how fast you're going and your altitude

Settings I've used..
Fw190: 50% throttle and 60% Prop-Pitch (PP) with radiators closed = cruise forever.

Me109: 45% Throttle + 50% PP with radiators closed = cruise forever.

Both settings (altitude dependent) produce fairly high cruising speeds and engine is cool on entry into combat

If in an emergency escape you can push the throttle to around 70% (same PP) with radiators at 1st setting and get away fairly easily - Just watch you Temp gauge and adjust radiator or throttle.
;)

I find the inline allied planes too touchy on the temperature side.. they heat up very quickly