wrote in message
...
On Fri, 09 Oct 2009 20:31:39 -0500, cavelamb
wrote:

RogerN wrote:
"Dan" 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 military used to use strobe lights on real helicopters to "stop"
rotors. Another neat gag was a large F shaped devise that had a strip
of
fabric across the opening. I think it was used to find out who was
crazy
enough to approach the tips of spinning main rotors with it.

Dan, U.S. Air Force, retired

I'm wondering if the F shaped device was to detect blade tracking, to
see if
one blade was flying higher than the other?

RogerN



That's right.
The blades have a small tab sticking out the ends which get marked up
heavily
with two different colored grease pencils.
Those tabs had a hole to hook your tiedown strap. Tail rotor was a
bit different. A grease pencil taped to a broom handle.

Now we didn't want to make up the F shaped device and have to replace the
tape all the time so we just used the cardboard tube from the center of a
paper towel roll attached with some foam inside the tube to a PVC pipe. My
wife ran the stick while I operated the helicopter. Two different color
grease pencils made the marks on the tips and they would indicate which
blade was tracking higher. Then an adjustment was made to lower the blade
and the test was repeated. Turning the carboard tube 90 degrees gave a fresh
place to "write" with the grease pencil marks. Kathy would just plant the
bottom of the PVC pipe on the ground and lean it in to make light contact
with the tip of the blade. Worked for us quite easy.

Stu