why cant you use a scale 4 blade prop on say a P-51 for instance? why are all the props 2 bladed?

The engines we have are not powerful enough to turn a true scale prop such as you have mentioned.

On a 1/7th scale P-51 Mustang, a scale 4-blade prop would be roughly 19" in diameter. A 1.20 size 4-stroke can't even swing a 2-blade prop at that diameter, let alone 4 blades. The RCV 120-SP, which is designed to swing large multi-bladed props, can't even swing a 4-blade prop that big.

Our model engines make their torque at much higher RPM than a Merlin, say. The prop was also geared on most warbirds, so they were only turning 1500 or so RPM (for 3000 crankshaft RPM). As such, they can use much larger diameters/more blades, relatively speaking. If we could get a 1.20 4 stroke to make 2 hp at 3000 rpm (instead of 12000) and then gear the prop 2:1, we could probably use close to scale props.

could always use a a REALLY big engine or buy a smaller prop thats "close" to scale. also, would it help to buy a large diamiter prp with a very small pitch? that should help the engine turn it right?

You can experiment with large fine pitch props, but then you run into other problems. Such as forward speed, engine cooling, torque and and other issues.
If we could have turned scale props on our models someone would have done so already. The closest that anyone has gotten was Byron's Purr Power system that he developed for his line of warbirds. Funny name for it as the engine was really cranking in order to produce the thrust. And it provided just enough thrust if you didn't go overboard with the model wieght. There were other prop drive systems (i.e. reduction gear or belt drive) tried and developed during the eighties. Most have been forgotten due to the improvement in engines. As well as other trends. Plus they where heavy as well.
Remember our models don't fly in scale air. [&:]

Another factor that has not been mentioned is the fact that the warbirds such as the P-51 had variable pitch propellors that were controlled by the pilot.

Hobby Lobby used to sell a variable pitch prop for the pattern fliers. Was variable from the transmitter much like the throttle. Was developed in Europe. Couldn't turn a scale prop though. After a couple of seasons they figured out it was a bit difficult changing pitch threw the manuvers. And it didn't always work well. I'm sure there are a few guys out there that may have seen them. [&:]

I think someone was working on a variable-pitch prop for use in a turboprop powered model plane-Wagstaff Extra maybe? Seems like it worked.

even merlin had reduction gears.
why do real war bird need reduction gear? because prop will reach sound barrier if didn't.

some interesting pix for you.

OS engines attemp at reduction gears

http://www.osengines.com/history/ostimeline09.html

Vario Prop - may be solution to your quest?

http://www.aircraft-world.com/prod_d.../varioprop.htm

I came close..

A 24" 3 blade flies my Spit.

The plane will fly on a G 62.. I used a Q 100 to spin this prop. Should be a 26" prop though. Pretty close.