The tacho has arrived and I have a free day for once, so I have been having a go with the lift engine (Robin EC25PS).

It's a 9/9 set up, in a 550mm duct.

Their 3Z Nylon blades, so the max tip speed is 168m/s

So the radius is 0.275m

so 0.275*2*pi = 1.727m circumference

so 168m/s / 1.727m = 97/s

97/s*60s = 5833

so the fan should be going at about 5800rpm at most.

However this would probably produce >90dba @25m,

so using the regs' recommended limit of 122m/s, that would be 4236m/s.

I will set the blades up so full throttle max's the revs at 5750rpm for a bit of peace of mind, then can use the tacho to keep the revs at 4200 for quiteness.

I just put some old petroil into the for testing and it seemed to produce a lot of smoke, and had oil coming out of the exhaust joint - does the oil separate from the fuel after a bit (i did shake it) or can the fuel go off or something?