Better late than never.

I just want to agree with most of what has been said. Yes there is a point that extra rpm really doesn't get you much. Both have been talked about. One, the prop tip speed. I try to keep mine down around mach .5 for the noise and safety reasons. Remeber if it is loud it is draggy. As was stated before around mach .8 you really start to pay a big price in terms of power for any additional thrust. Two, the plane/prop relationship.

The first issue is pretty easy to address, how much do you value your safety and ears. Since your limited to an 8 inch prop, this is 2/3 of a foot and the curcumfrence would be 2/3*pi or 2/3*3.14 or a little over 2 feet. The speed of sound is about 1100 fps. So if we want to keep it below mach .8 we have 880/2 or 440 rps or 26400 rpm. Say we didn't want to push it quit this high so we limited our prop to mach .6....660/2=330 rps or almost 20000 rpm.

I've attached two graphs, both using 8 inch diameter, 3.5 pound wieght plane with a wing area of 5.5sqft (the wing area wasn't stated). I also assumed a somewhat draggy airframe, gliders have a lot of frontal area and surface area.

The first graph is of an 8x4 and 8x8 both at 15000 rpm. By my calculations the 8x8 would produce around 50oz thrust static and the 8x4 would produce about 38oz of thrust static at 15000. The big story is that the 8x8 prop would require something around 620 Watts of input power and the 8x4 would only require 220 Watts. The 8x8 would have a max climb rate of about 25 fps and the 8x4 about 12 fps. Notice you will never reach the speed needed for max power output for the 8x8 but with the 8x4 you will. Your top speed would be about 80fps (56mph) for the 8x8 and 60fps (42mph) for the 8x4.

The second graph is of and 8x4 at 20000 rpm and the 8x8 still at 15000. The 8x4 at 20000 rpm would need about the same input power as an 8x8 at 15,000 but the 8x4 would produce about 64 oz of static thrust. The max climbrate for the 8x4 is 35fps and again the top speed is the same for both props..80fps.

Now any relationship to actual reality is only an accident, and I don't fly electric so draw your own conclusion just thought a graph might clear up what everyone is talking about.

Power in the graph must be above the min power line (pink), this line is the minimum power for level flight. Climb is substainable climb.

Happy Flying.