Ptuey:

Pitch and diameter are totally independent of each other. But the blade angle depends on both.

Pitch is the distance the prop will screw itself through the air in one turn. At a 10" diameter the tip is traveling a much greater distance with each turn than a 5" diameter tip, because the 10" diameter has a lot greater circumference. The 10" tip travels 31.4" per turn static, and the 5" has only 15.7" for one turn static. I specify static because as the prop travels forward the tip distance per turn will increase, but we don't need to get that complicated here.

A 10x5 propellor will have the tip at an angle of a hair over 9 degrees, while the 5" prop, to get the same pitch, has a tip angle just a little under 18 degrees. But on the 10" prop, if you check the blade at the 5" diameter point guess what you will find? Right. The blade angle is 17.66 degrees, the same as the tip angle on the 5" prop.

The propellor blade is a wing, all considerations for wing shapes apply to props. The most efficient wing shape is an ellipse, but it's also the most expensive to build, Because of the cost, very few production aircraft have been built with them. The Republic P-47 was one of the more well known, another was the Supermarine Spitfire, at least in most marks. There were some Spits that weren't elliptical. Because of the way prop blades are made, there's no great difference in price from one shape to another, and the ellipse shape being the most efficient, is also the most common.

Wing tip plates, Hoerner tips, and winglets are all ways to make a relatively square tip approach the efficiency of the elliptical wing. Most simply put, the inefficiency comes from the tendency of the air to go around the tip from the lower surface to the upper instead of following the airfoil shape. The added tip thingies are attempts to stop or limit this flow around the tip. However, as air speed increases, the air has less "Time" to leak around the tip, so tip shape becomes less important with increasing air speed. The propellor tip will always be traveling at a much higher airspeed than the wing tip. I wont go into the mechanics of it here, but give it a little thought and you'll see why.

So back to prop tips. Again, no explanation, but as the tip speed goes up you reach the point where a blunt tip, as on your cut down prop, approaches the aerodynamic efficiency of the elliptical tip. And when the larger blade area is factored in, the blunt tip can actually be more efficient than the ellipse shape. Again, explanation would take too much time for this discussion, but it works.

Or, to put it in the simplest way, when you cut the diameter of a prop running at model plane rpm, rounding the tip is more trouble than it's worth. Just be sure both tips are cut the same, and the prop is balanced.

Bill.