I agree, it is apples and oranges. The efficiency of a prop is relative. Relative to what you are wanting to get out of the prop. Are you wanting speed or vertical lift? Do want fast acceleration and high rpm like for a pattern plane or gut wrenching torq to hang a 3D on at 1/4 throttle. How efficient the prop is, depends on what you want it to do. The comparison I like to use for 2 vs 3 or more blades is the difference between a gas engine vs a diesel engine. A 300hp gas engine and a 300hp diesel engine have the same hp but are two very different animals. The gas engine has fast acceleration and high rpms and the car goes fast but if you hook it to a 2000 pound trailer it stuggles. The diesel has slower acceleration but more torq at lower rpms. It will pull the trailer without breaking a sweat but its not going to win any races. With 3 & 4 bladed props you have more blades digging in to the air so more torq or pulling power at lower rpms and a little slower throttle response. That is what H9 wanted for their trainer. Now the downside is the at higher rpms 3 & 4 blade props start to kind of catch up to themselves. The blade behind is passing through the disturbed air from the blade in front and becomes less "efficient". There is not enough time between blade passes to let new air in between to bite into. Which keeps the top speed down, something else H9 wanted on the PTS. First you have to decide what you want your plane to do, then you can start making engine and prop choices. Actually you need to first decide what kind of flying you want to do and then decide what plane to get. The P-51 PTS is a great trainer and intermediate plane and will do basic aerobatics but the dehydril keeps it from doing them as clean as a 3D or pattern plane. For that kind of flying I fly my Twist 40 or my Kaos 40. Choose a plane for the type of flying you want to do. Trying to get a plane to fly in a way it wasn't designed for is asking for heartache.

In Showtime100's case, he wants the cool look of a 4 blade prop without sacrificing a lot of performance. The problem with a 40 size plane is the choices are few on 3's and almost non-existent on 4's. There seem to be more multi-blade prop choices for larger planes. I'm looking for a good 4 blade for my PTS as well. A nice black one so I can dip the tips into yellow paint for that cool authentic P-51 look. I don't care if it goes fast or has a lot of verticle. I just want it to look as good on the ground as it does in the air. As for choosing an engine, the argument for 2-stroke vs 4-stroke is just as fierce. But in my own opinion good rule of thumb is for 4-stroke you need to go up 2 to 3 sizes from a 2-stroke. Correct me if I'm wrong guys, I've never used a 4-stroke. I have been pleased with the evolution engines so far. I have three in my planes and they start easy everytime and run flawless. Most of the others in my club use O.S. and they seem to just as reliable. So, if you have your heart set on a 4-stroke then I would go with the biggest one you can shoe horn in at least a .60. Go buy the only 40 size 4-blade prop your LHS has in stock (if they have one) I'd say a 10x4x4 or 10x6x4 but thats just a guess. Your choices are going be very limited on props but no your not barking up the wrong tree.

-Andy

PS: Sorry for the long post. I tend to ramble. (too much free time not flying)