B-29 Doc

[Pilot Lodkins]

I saw B-29 Doc on September 5th - Saturday



Engine Startup


https://www.youtube.com/watch?v=u6VJrr5aBcQ

[Buffy Foster]

Cool but I was confused about some of them changing direction? I didn't know they could do that, or are they geared?

[alan CXA651]

Hi Buffy.
The props are turning faster than they appear , the changing direction is a stobe effect i used to work with britannia aircraft in the RAF in the 70`s and you got the same effect on props appearing to change direction as RPM changes .
regards alan.

[Buffy Foster]

OIC, it looked like when the outboard ones were starting they were going counter-clockwise from our view, and when they idled later then clockwise and real slow. So that was strobig too? I guess using a lower framerate DSLR or something to film it? I guess still better than cockpit videos where cameras make the blades look curved.

[AKar]

OIC, it looked like when the outboard ones were starting they were going counter-clockwise from our view, and when they idled later then clockwise and real slow. So that was strobig too? I guess using a lower framerate DSLR or something to film it? I guess still better than cockpit videos where cameras make the blades look curved.

It is the high shutter speed used that makes the blades appear close to "stopped" in each frame. Then, depending on how they happen to sit in each frame relative to the previous one, the prop may appear to go to either direction. If the frame rate precisely matches any multiple of the time it takes of the propeller to advance a quarter of the turn (in case of four-blade prop), it appears standing still. If it only closely matches, the prop appears turning slowly. If it is a complete mismatch, the prop jumps so much in between the frames that the effect disappears. It is a rather obvious artifact in its origins, when you think about it. To avoid "stopped" propellers in both still and video photography, slower shutter speeds are used, motion-blurring the propeller.

The bent blades in the output of many digital video cameras are a result of something known as "rolling shutter effect". It happens, in simple terms, because the data is read from the image sensor sequentially, not at the same time instant, and the exposure 'rolls' with the readout to maximize the available exposure time. (Similar effect can also happen in stills with the DSLRs and film cameras, most visible when using shutter speeds higher than their flash sync speed, which is the maximum shutter speed where the entire shutter is open at once.) This effect can also be avoided by extending the shutter time enough to blur the blades out of the view.

Generally speaking, I am not aware of any aircraft capable of changing the direction of rotation of its propellers/rotors at will, nor can I invent any practical reasons to be able to.

-Esa

[Buffy Foster]

Oh thank you; now I learned lots! \o/

[Nick - A2A]

Just to compliment Esa's reply, there's a nicely done video on the 'Smarter Every Day' YouTube channel which explains rolling shutter effect.



Cheers,
Nick

[Buffy Foster]

That was a cool video and cool channel worth subbing to, thank you!

[BrettT]

Very Pretty! Wish my Connie was that reflective in P3DV5