Which is better a 3 or 2 blade? I am having problems with a Thunder Tiger GP42 getting too hot while running. It has a 3 blade 9X6 and the engine is cowled, I am running 15% fuel. I have put pleanty of airflow holes in it. Could it be possible the 3 blade prop makes it work too hard? I am thinking of switching to a 2 blade. The plane is a Tower Voyager.
Thanks

Depends entirely on the model and type of flying.

Three blades look cooler (depending on the plane), but it is the general consensus that 2 blades are more efficient. Check your airflow holes again. Ideally, you should have twice the OUTLET as INLET. Good luck.

Best way to start an argument on RCU is to bring up the "3 blades or 2" question. I think both have their good points, but in terms of overall EFFICIENCY, 2 blades are better than 3. That being said, it may have no bearing at all on your overheating problem. Can you post a picture of your setup? "plenty of cooling holes" doesn't say where they are or how big, and you also have to let the warm air out as well as the cool air in. One larger hole in front of the cylinder head is better than several smaller ones near it, and the exit hole needs to be about 1-1/2 to 2 times larger than the entrance hole. The exit can be most anywhere behind the engine, but best locations are an entrance and exit that "channel" the airflow directly past the cylinder fins, as that's where the heat is being generated.

How to determine if your prop is overloading the engine requires fine-tuning your top end, and checking it with a tachometer. I'd think your engine should be able to run in the 11,000-12,000 rpm range with the correct prop, most .40 size engines can turn a 10-6 in that range (2 blade), so a 9-6 3 blade should be okay, but you might have to experiment. 3 blades will give greater thrust than 2 at lower rpms (diameter and pitch being the same), but lose some efficiency at high rpms because each blade is running in the turbulence of the blade ahead of it. I have a formula for prop efficiency somewhere, has to do with swept area, rpm (which help determine thrust), axial speed of the blades (which is tied to diameter) and helical speed (which is tied to pitch). Also figured in is aspect of the blade design (which helps determine induced drag), yada, yada, yada. It all boils down to more than just the # of blades.

I will try and post some pics tomorrow.

Here are some pictures of the cowl. seems to have pleanty of airflow now. I cut the holes in the side and top, and put on a 2 blade.