RogerN wrote:
"cavelamb" wrote in message
m...
Stu,

Have you seen this one?

http://www.wimp.com/impossiblehelicopter/

There are optical tachs used on R/C heli's that you adjust the speed of a
shutter to make the blades appear still then read the rotor RPM.

RogerN


The usual optical (digital) tach sold by RC hobby sources reads through
a prop blade to a light background - can use mirror tape or a white
paint dab too. No synchronizing needed. About $25. I have one.
Switchable to read two blade or three blade.

Brian W