RE: What is the max prop RPM?
Might it be the HB 21 Blitz? A very hot motor in its day-competitive with the K&B 21. APC do a number of props suitable for 19-21's: 8x4m 9x4, 9x3, 8.636 x 3.75 etc depending on intended use. For sport R/C you might want to go down in dia a bit and up in pitch to 4.5"-5"-5.5"
ChrisM
'ffkiwi'