AviationBanter - View Single Post - Spinner strobing as a "Bird Strike Countermeasure"

**cavelamb himself[_4_]** · December 6th 07, 04:56 AM posted to rec.aviation.piloting,rec.aviation.homebuilt

Airbus wrote:
In article ,
says...

On Dec 5, 2:05 pm, wrote:

In rec.aviation.piloting Harry K wrote:

On Dec 4, 8:25 pm, wrote:

In rec.aviation.piloting Harry K wrote:

snip

Harry K- Hide quoted text -

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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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Nope. The prop appears stationary and you see only the two.
What you see is the complete image (frame), not a composite of 4
'frames'.

I'm talking about eyeballs and strobed light here, there are no "images"
or "frames" involved.

What you see is a prop blade in whatever position it is in when
the light goes off.

If the light is going off every quarter turn, you see a particular
blade end in four different places each a quarter turn apart.

If the light is going off every sixth of a turn, you see a particular
blade end in six different places each a sixth of a turn apart.

--
Jim Pennino

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Yes, but you will see them (along with all the others) one at a time.
Not the entire sequence at once or even a composite. It is the same
as taking a snap shot with shutter speed fast enough to stop the
motion, it will show all blades, in this case it will show two blades
and no 'virtual' blades. You can prove it with a fan and a variable
strobe light but I suspect that will not be a common kitchen
appliance .

Don't agree - I agree with Jim, and I do frequently use scientific strobe
analysers for decomposing motion. With such a tool, one must be very careful to
avoid harmonic illusions. Obvjects may appear to be moving in reverse, and
indeed phantom members may appear - a fan or prop or spoke for example may
easily appear to have more blades than it really has. Our eyes do not
immobilize instantaneous images beyond a certain speed - this makes it
possible for us to watch movies - or to be fooled by stroboscopic images.

One of my favorite stunts while checking out a new engine installation
was to use a cheap variable rate party strobe to see if anything was
moving. Amazing was a little variable rate stop action looking can spot.