VIDEO: An amusing strobe effect with radial aircraft engines

[230RN]

https://youtu.be/8kDEsKDTQWY

I go all boogeyshit over radial aircraft engines and sometimes just browse the web for videos of them, especially with their usually dramatic startups with their smoke and flame and bellowing like the demons of hell.

I found this one of starting and running one of those rotary aircraft engines (Gnome, Rohne, etc) where the propeller is attached solidly to the engine casing and the whole engine rotates with the prop.  

In this video, the difference between the two blades of the prop and the seven cylinders of the engine make for some weird stroboscopic effects.  

Even though they're both necessarily rotating at the same RPM, sometimes it looks like the engine is running opposite to the prop rotation and at other RPMs, it looks as if there's a loose connection between the engine and the prop.

I laughed like hell over this one.

Enjoy !

Terry, 230RN

[French G.]

Landing one of those fixed prop radials is a time you really hope your mags work.

[griz]

That is pretty funny.  Besides being able to shift in to reverse with respect to the cylinders it's attached to, I like the way the number of blades is a variable as well.  

[230RN]

Landing one of those fixed prop radials is a time you really hope your mags work.

I am told that on most (all?) of them, there was no throttle and speed was regulated by cutting the magneto ignition on and off.  That's why they made that weird brapp-pause-brapp-brapp-pause-pause-braaaaaaap noise on landing.

There's some of that in that video.  I also read somewhere that because of the huge gyroscopic effect of that spinning engine they were difficult to make turns in one direction or the other, depending on the direction of rotation.

That is pretty funny.  Besides being able to shift in to reverse with respect to the cylinders it's attached to, I like the way the number of blades is a variable as well.  

Yeah, me too.  The whole thing is kind of weird, strobe-wise.

Terry, 230RN

[AJ Dual]

This one is my favorite.

https://youtu.be/2IghwseoISc

[230RN]

^ Some hell of an updraft, eh?

[TommyGunn]

This reminds me of those old westerns when you would be watching the stagecoach go buy....the spoked wheels looked to be rotating backwards because of the stroboscopic effect you'D wind up with on motion picture film.
Video, and movie film, both have a "frames per second" rate.
That old airplane engine was a pretty cool effect.

[BobR]

Digital video makes for bending prop blades, seemingly going in reverse and the sync rate also has some effect.

I have heard you can make it go away by shooting through a polarized lens, even a pair of polarized sunglasses.

http://www.aircraftowner.com/m/blogpost?id=6511841:BlogPost:26836


bob

[griz]

Digital video makes for bending prop blades, seemingly going in reverse and the sync rate also has some effect.

I have heard you can make it go away by shooting through a polarized lens, even a pair of polarized sunglasses.

http://www.aircraftowner.com/m/blogpost?id=6511841:BlogPost:26836


bob

That's from a rolling shutter.  Instead of taking lots of individual consecutive photos like they do with film, they sample different parts of the screen at different times.  It can make moving things appear to be in different places.  Here is an example:

https://www.youtube.com/watch?v=ecV7oo68vAc

[TommyGunn]

That's from a rolling shutter.  Instead of taking lots of individual consecutive photos like they do with film, they sample different parts of the screen at different times.  It can make moving things appear to be in different places.  Here is an example:

https://www.youtube.com/watch?v=ecV7oo68vAc

Now THAT was SUPER-COOL!  Thanks for linking it!   

[RocketMan]

You post some pretty neat round engine videos, 230RN.  Thanks.

[230RN]

https://youtu.be/Hq78ZocOAkY

(Sopwith Camel)

Sounds like Di-Gel was needed.

You can see the oil trail.  According to one book I read ("This Was Air Travel"), they used castor oil in the fighters and the pilots had almost constant diarrhea from its fumes.  Which, it is said, why they imbibed lots of brandy between sorties.  Or so it is said.

That book also mentioned that the rotary engine's popularity was largely due to the high power to weight ratio, but they were very delicate and had to be overhauled every ten or so hours.  According to that book, the pistons were so lightly built that a man could crush one between his fingers.

<Edited for spelling error>

[Hawkmoon]

https://youtu.be/8kDEsKDTQWY

I found this one of starting and running one of those rotary aircraft engines (Gnome, Rohne, etc) where the propeller is attached solidly to the engine casing and the whole engine rotates with the prop.  

In this video, the difference between the two blades of the prop and the seven cylinders of the engine make for some weird stroboscopic effects.  

Even though they're both necessarily rotating at the same RPM, sometimes it looks like the engine is running opposite to the prop rotation and at other RPMs, it looks as if there's a loose connection between the engine and the prop.

I don't understand how the apparent reverse rotation is possible if the engine is attached to the propeller.

[230RN]

^
Normally I just take things like that on a "that's just the way things are" basis without doing the calculations.

And I don't actually don't want to do the numeration now, but somebody can attack it if they want.

But remember that with a certain frame (or camera scan) rate, AND a certain RPM, what appears to be a single one of the seven cylinders is not actually the same cylinder in each frame.

Similarly, nor is it necessarily the same prop blade.  

Figuring, with numbers plucked out of the air*, a 1000 RPM rotation rate, AND a 1000 per second frame rate, everything would seem to be standing still.

The same would be true, just looking at the prop, if the frame rate were 500 per second, but in each frame, it would be a different (though identical) prop blade, so it would still look as if the prop were standing still.

Similarly, the cylinders would seem to be standing still, but it may be a different, though visually identical, one of the seven cylinders in each frame.

But the frame rate always stays the same in the camera as we change the RPM from, say, 500 RPM to 1000 RPM, so the camera would be capturing each of the identical successive cylinders in a different relative position with each frame, making it look as if it were advancing at some RPM and going backwards at another RPM.  This advancing and retarding, with each cylinder being  51.42° away from the next cylinder in line, may or may not  coincide with the advancing and retarding of the prop blades, which are 180° apart.

In our imaginary situation, at, say, 750 RPM it may look as if there were four prop blades.

So, at varying RPMs but a constant frame rate, the cylinders may seem to be standing still while the prop looks like it is slowly rotating, or one may look like it's turning one way while the other is rotating oppositely.

The thing to keep in mind is that while either the prop blades or the cylinders may appear to be standing still in each frame of the camera, they may or may not actually be the very same prop blade, OR the same cylinder in each successive frame.

The simple math involved is left to the simple student.  

Corrections and comments invited.

Terry, 230RN

* "plucked out of the air" is frequently abbreviated in high level scientific discussions as a "PFA" or "plucked from the air" number.    The PFA frame rate and RPM both being 1000 were PFA-chosen to make the concepts somewhat simpler.

<Edited for minor grammar corrections.>

[Hawkmoon]

Good explanation.

Those videos are a lot of fun.

[230RN]

Major correction:

"Figuring, with numbers plucked out of the air*, a 1000 RPM rotation rate, AND a 1000 per second frame rate, everything would seem to be standing still."

Should read,

"Figuring, with numbers plucked out of the air*, a 1000 RPM rotation rate, AND a 1000 per minute frame rate, everything would seem to be standing still."

Glad nobody caught that.  However, I was just trying to show the concept numerically.

As a happy accident, 1000 per minute frame rate corresponds closely to the 16 per second frame rate of
old silent movies.

Terry "I ain't perfect, despite what Mom used to say," 230RN

REF (Additional complications if you feel particularly daring today):
https://en.wikipedia.org/wiki/Frame_rate