I have read some of the posts on here about the difference in the prop size and pitch and they were very helpfull. I have a Megatech P-51 Mustang and it came with a 4 blade 8x8 prop. I was wanting to change to a 2 blade prop for improved efficeney. I have caught on very fast to the way to crash a plane and have yet to get a flight off that didn't result in a bent gear shaft and or a bent prop. I was wanting to change to a 2 blade in hopes of gaining airspeed faster because my plane likes to stall out. My question is what size prop should I use in a 2 blade model. The motor on the plane is a speed 380 or speed 400 one of the two. (I have seen both of these motors listed on this plane before and am not shure which I have.) If it makes a difference the wing span is around 3 ft. (Just to give you and idea of the size of the model.) If anyone can help me with this I would greatly appreciate it. I am about to the point to throw the bird in the trash and move to another hobby.

Don't give up or you'll miss a large gob of fun. An 8 X 8 prop for a 350/400 motor is weird to say the least. It would take a .40 glow engine to throw that much lumber or plastic around fast enough to fly something. Two blades should certainly be enough even if they don't look as scale as a four. If the 4 blade had a pitch of about 2.5 it MIGHT work. A two blade7/8 X4 will probably work. I've used 8 X 4 power point wood on a 500 speed ungeared and even that was too much of a load. Another thing about two blades is they move in less disturbed air. The lower pitch will give you more RPM which is the name of the game if you stall a lot. You didn't say whether or not your setup is geared or direct. A geared motor will turn a larger prop albeit slower. I suggest you also check the CG which is too often a cause of stalls.
Stay with us. RC is the most fun you can have while standing on the ground.

ahem...I would to encourage you to stay in the hobby, but if you insist, please look me up before you toss the bird. I could always use a good first plane.