"brian whatcott" wrote in message
...
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

I have one of the kind that read light through the blades, the kind I was
talking about was where you view the heli through a shutter that you sync
with the blades to read the in-flight rpm.

Such as this
http://www.heliproz.com/minairtach.html

RogerN