Spinner strobing as a "Bird Strike Countermeasure"

[Note: nonstandard attribution layout.]
Beryl wrote:
Entertaining thread this turned into. Anyway, here's a nice illusion.

http://img408.imageshack.us/img408/8067/imagegm1.gif

Concentrate on the + in the middle.
An illusory green dot soon appears.
Moments later, pink dots seem to disappear.

Very interesting and neat.

cavelamb himself wrote:
I'll repost this one here - so it's easier to find?
HEHEHE!

http://www.sonnyradio.com/spinninglady.html

It took some doing, but by glancing to one side (and sometimes briefly
closing my eyes and trying to imagine the reverse spin) I was able to
switch the perceived rotation direction. However, I have it on good
authority that I do not have an IQ of anywhere near 160. :-)

A couple times I had a shift in perceived rotation direction without
looking away.

cavelamb himself wrote:
I think this one is awesome.
Can these really be the same color???

http://www.home.earthlink.net/~cavel....htm#samecolor

Hard to believe indeed. I decided to do an image copy from the browser and
pasted the copy into the MS Windows Paint program. Copied two square chunks
of each of the two labeled board squares and pasted them side-by-side. Sure
enough they are the same color.

On Dec 4, 9:48 pm, Airbus wrote:

And the front

fan
on a turbofan is turning at around 40-50 revolutions per second, not
thousands.

Guess again.

Well, I think he made a pretty good "guess" - the GE90-115B tha powers the
777 turns (front fan) at 2550RPM, or 42.5 RPS. He probably didn't ahve to
guess much, as these facts are easily and irrefutably verified. I "guess" you
haven't learned that. . .

Not too many folks bother to do a little research. It gets
easier all the time, what with Google and all that. Things like fan
speeds are easily available, and with a tiny bit of arithmetic the
rotational speed per second is easily calculated. Some posters just
diss anything someone else says, mostly because they didn't know it
first or perhaps becuase of who is saying it.
There's a well-known phenomenon that can incapacitate pilots. A
propeller spinning so that it cuts through the sun's glare and causes
a stroboscopic effect on the pilot's eyes can result in confusion. A
frequency of less than between 25 and 55 Hz, depending on the
individual, can cause dizziness or nausea. It's called "flicker
vertigo" and the fact that it happens tells me that the human eyeball
(or at least the visual cortex) does have a frequency function of some
sort. 25 Hz on a two-blade prop would be 750 RPM and 55 would be 1650.
Some tests have found that frequencies as low as four to 20 Hz will
induce it rapidly.
See this: http://www.flightsafety.org/hf/hf_mar-apr04.pdf
When I get the prop between the sun and my eyeballs I have to
increase the RPM to get rid of it. It's painful.

Dan

Here's an illusion I can't expain at all.

http://amiedotcom.blogspot.com/2005/...hat-i-see.html


been told it has to do with resolving frequencies in space, not time,
but it's way past my number crunchiing skills.


On Dec 4, 8:47 pm, Jim Logajan wrote:
[Note: nonstandard attribution layout.]

Beryl wrote:
Entertaining thread this turned into. Anyway, here's a nice illusion.

http://img408.imageshack.us/img408/8067/imagegm1.gif

Concentrate on the + in the middle.
An illusory green dot soon appears.
Moments later, pink dots seem to disappear.

Very interesting and neat.

cavelamb himself wrote:
I'll repost this one here - so it's easier to find?
HEHEHE!

http://www.sonnyradio.com/spinninglady.html

It took some doing, but by glancing to one side (and sometimes briefly
closing my eyes and trying to imagine the reverse spin) I was able to
switch the perceived rotation direction. However, I have it on good
authority that I do not have an IQ of anywhere near 160. :-)

A couple times I had a shift in perceived rotation direction without
looking away.

cavelamb himself wrote:
I think this one is awesome.
Can these really be the same color???

http://www.home.earthlink.net/~cavel....htm#samecolor

Hard to believe indeed. I decided to do an image copy from the browser and
pasted the copy into the MS Windows Paint program. Copied two square chunks
of each of the two labeled board squares and pasted them side-by-side. Sure
enough they are the same color.

On Dec 4, 8:14 pm, Tina wrote:
Here's an illusion I can't expain at all.

http://amiedotcom.blogspot.com/2005/...hat-i-see.html

been told it has to do with resolving frequencies in space, not time,
but it's way past my number crunchiing skills.


Looks to me more like a detail issue, with the resolution
that diminishes with distance causing the illusion.

Dan

On Dec 4, 9:55 am, wrote:
In rec.aviation.piloting 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.

It isn't that bad.

Synchronization only occurs on integral fractions and engine RPM is
usually 1000 and about 2700 RPM so the possibilities are limited.

For a 2 bladed prop:

7200/2= 3600 - 1800 RPM

7200/3= 2400 - 2400 RPM and 1200 RPM

7200/4= 1800 - 1800 RPM and 900 RPM

etc.

I leave it to someone else to show how many blades you see at each RPM.

--
Jim Pennino

Since the prop appears stationary, you would see all the blades.

Harry K

wrote:
On Dec 4, 8:14 pm, Tina wrote:
Here's an illusion I can't expain at all.

http://amiedotcom.blogspot.com/2005/...hat-i-see.html

been told it has to do with resolving frequencies in space, not time,
but it's way past my number crunchiing skills.


Looks to me more like a detail issue, with the resolution
that diminishes with distance causing the illusion.

Could be. Or the optic nerve does something equivalent to a two-dimensional
Fourier transform on images and filters the high frequency components out.
(I believe many lossy image compression algorithms use an equivalent
technique. E.g. JPEG.)

Nice call, I can understand that now.


Thanks


On Dec 4, 10:20 pm, wrote:
On Dec 4, 8:14 pm, Tina wrote:

Here's an illusion I can't expain at all.

http://amiedotcom.blogspot.com/2005/...hat-i-see.html

been told it has to do with resolving frequencies in space, not time,
but it's way past my number crunchiing skills.

Looks to me more like a detail issue, with the resolution
that diminishes with distance causing the illusion.

Dan

On Dec 4, 5:47 pm, Jim Logajan wrote:
[Note: nonstandard attribution layout.]

Beryl wrote:
Entertaining thread this turned into. Anyway, here's a nice illusion.

http://img408.imageshack.us/img408/8067/imagegm1.gif

Concentrate on the + in the middle.
An illusory green dot soon appears.
Moments later, pink dots seem to disappear.

Very interesting and neat.

cavelamb himself wrote:
I'll repost this one here - so it's easier to find?
HEHEHE!

http://www.sonnyradio.com/spinninglady.html

It took some doing, but by glancing to one side (and sometimes briefly
closing my eyes and trying to imagine the reverse spin) I was able to
switch the perceived rotation direction. However, I have it on good
authority that I do not have an IQ of anywhere near 160. :-)

A couple times I had a shift in perceived rotation direction without
looking away.

cavelamb himself wrote:
I think this one is awesome.
Can these really be the same color???

http://www.home.earthlink.net/~cavel....htm#samecolor

Hard to believe indeed. I decided to do an image copy from the browser and
pasted the copy into the MS Windows Paint program. Copied two square chunks
of each of the two labeled board squares and pasted them side-by-side. Sure
enough they are the same color.

I was curious if one-eyed people see the spin. Yep. My wife is, and
I am 1/2 vision in one. Of course a simple experiment by closing one
eye (either of mine) and it still rotates. I can see it either
direction, blinking usually changes it.

Back in the 60s I was at a remote site co-located with a radar site.
It had one big antenna formed from open meshwork. I could not tell
which direction it was rotating even though I _knew_ it was clockwise.

Harry K

In article ,
cavelamb himself wrote:

Entertaining thread this turned into. Anyway, here's a nice illusion.

http://img408.imageshack.us/img408/8067/imagegm1.gif

Concentrate on the + in the middle.
An illusory green dot soon appears.
Moments later, pink dots seem to disappear.



Hey, I was about to post that.


I'll repost this one here - so it's easier to find?
HEHEHE!

http://www.sonnyradio.com/spinninglady.html


I think this one is awesome.
Can these really be the same color???

http://www.home.earthlink.net/~cavel....htm#samecolor

Yes, they really can and really are the same colour..

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling four feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect if you
sit in the bottom of that cupboard."

In rec.aviation.piloting Harry K wrote:
On Dec 4, 9:55 am, wrote:
In rec.aviation.piloting 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.

It isn't that bad.

Synchronization only occurs on integral fractions and engine RPM is
usually 1000 and about 2700 RPM so the possibilities are limited.

For a 2 bladed prop:

7200/2= 3600 - 1800 RPM

7200/3= 2400 - 2400 RPM and 1200 RPM

7200/4= 1800 - 1800 RPM and 900 RPM

etc.

I leave it to someone else to show how many blades you see at each RPM.

--
Jim Pennino

Since the prop appears stationary, you would see all the blades.

Yes, but how many virtual blades will you see?

Simple example: You are illuminating a 2 bladed prop running at 1000 RPM
with a strobe light pulsing at 4000 pulses per minute.

In the time between pulses, the prop makes 1/4 of a turn.

The prop is therefor lite up every 1/4 turn and it appears as though
the prop has 4 blades.


--
Jim Pennino

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