If any of you have tried to use a strobe light to determine rotational
speed, you'll remember harmonics are a serious problem.

If there's a single mark on a shaft, it will appear stationary if the
flash rate is equal to the time it takes the shaft to turn once. It
will also appear stationary if its rate is half of that time, (it will
blink on the spot every other time) a quarter of that time, and so on.
It gets worse. If the blink rate is twice that of the shaft speed, the
spot will appear stationary, but only half as intense, since one flash
will 'lock' the spot, and the next will illuminate the opposite side
of the shaft at an unmarked place.

To make this relevant to aviation, replace spot with prop blade.


On Dec 4, 11:17 am, Harry K wrote:
On Dec 3, 7:21 pm, Harry K wrote:





On Dec 3, 2:45 pm, Just go look it up! wrote:

On Mon, 03 Dec 2007 21:52:04 -0800, Airbus wrote:
In article , says...

When observed directly under artifical light that "flickers", the most
obvious being a strobe light, but there are other types of artificial
lights that have flicker.

--

Fine - but which ones cause you to see the propellers turning in
apparent reverse? Do you frequently operate your airplane indoors?
Propellers are usually observed in natural light, which does not flicker. At
night, on the rare occasions where you actually see the props clearly, it is
from the aircraft's own lighting, which is DC. I have nbever seen the props
turning backwards on a real plane - see it frequently in movies though. . .

Night, near one of those big off-amber ramp lights, run the RPM up and
down, there's a range where it will look like it's going backwards. I
thought it was kind of interesting.

It's something similar to the poor-man's "is my RPM somewhat right"
test, it'll appear stopped at (I forget what RPM now) RPM and if your
tach is somewhat near, viola.- Hide quoted text -

- Show quoted text -

Been a long, long time but my rusty math skills says it would be about
3600 unless I am wrong (per wife that is my normal state). That is
the 1/2 harmonic of the rpm/flicker rate. 60 X 120 = 7200. The
phenomenon should appear at 1/2, 1/4, double rate etc intervals.

Harry K- Hide quoted text -

- Show quoted text -

After thinking that over...

It gets worse. There will be multiple rpm that will show the effect
under strobe conditions. With a 2 blade prop it can be sychronizing
every 1/2 rev. 3-blade prop every 1/3 or 2/3 rev, etc. in addition to
synching on the harmonics.

Harry K- Hide quoted text -

- Show quoted text -