Naked Science Forum

Non Life Sciences => Technology => Topic started by: Mariana on 06/06/2019 09:55:42

Title: Why do aeroplane propellers appear curved?
Post by: Mariana on 06/06/2019 09:55:42
Michael asks:

Why do aeroplane propellors appear curved when filmed from behind with an HD camera?

What do you think?
Title: Re: Why do aeroplane propellers appear curved?
Post by: evan_au on 06/06/2019 12:10:07
It has to do with the way the image is scanned out of the image sensor. The aeroplane propeller moves between the time the  image is scanned from one side or the other of the sensor.

In this video, turning the camera by 90 degrees changes the order that the image is scanned, producing a very different image of the propeller spinning.

I am sure that the propeller doesn't have 20 blades - this is a stroboscopic effect.

Title: Re: Why do aeroplane propellers appear curved?
Post by: alancalverd on 07/06/2019 09:41:00
Video images are processed and displayed by serial raster scanning, in the same way that you read a printed page: one line slowly left to right, then "fly back" quickly to the left and move down a line, until you have covered the whole page, at which point you fly back a "frame" (or turn over the page).

A propellor will have rotated a small amount between line scans, so each sample of the image will be slightly displaced with respect to the previous one. If the frame flyback is nearly synchronised with the prop rotation, the resultant image will be of a slowly rotating ("strobed") curved propellor.

There is cunning software available that reprocesses the image to present an integrated  "prop disc" with an image of a stationary or blurred straight propeller inside it. Much favored by stills photographers but I don't think it is used in video.

You don't see the curve on cine film because the entire image is captured at once.

And just to make life complicated, some "scimitar" props really are curved, and some have 6 or more blades!
Title: Re: Why do aeroplane propellers appear curved?
Post by: Bored chemist on 08/06/2019 20:31:14
You don't see the curve on cine film because the entire image is captured at once.
But you sometimes see the prop turning backwards.
Title: Re: Why do aeroplane propellers appear curved?
Post by: alancalverd on 08/06/2019 23:04:03
That's because of the stroboscopic effect.

Small props spin at 2000 - 3000 rpm in cruise, and idle at 500 rpm. At 3000 rpm each blade is in the same position 50 times per second. TV frame rate is generally a multiple of 50 Hz, and cine film runs at 24 fps with a shutter duration of about 0.01s or less, so if you are watching cine direct or via TV, the image of a prop blade may appear to move slowly forwards or backwards as the blades move in and out of sync with the sampling frames, and at some intermediate speed, say 2639 rpm, that movement itself becomes a blur and we see a "prop disc".

Interestingly, although each frame is seriously blurred by the prop movement, the eye and brain integrate the data to give an image of a sharp blade with a bit of "grey fuzz".  There is another thread somewhere  discussing whether other species are interested in television. I suspect that such interest depends on  the ability to integrate a raster or flicker frame display into a smoothly moving sharp image, and it is possible that some animals, particularly predators, have a very short integration period and see either the individual blurred frames or even the moving dot, which doesn't relate to reality at all.

Single engine props are usually painted matt black so the pilot's forward view is through a clear disc about 3 - 5% darker than the surrounding area.
Title: Re: Why do aeroplane propellers appear curved?
Post by: chris on 08/06/2019 23:47:38
Fascinating discussion. Taught me quite a bit. Thanks guys!
Title: Re: Why do aeroplane propellers appear curved?
Post by: Bored chemist on 09/06/2019 09:31:12
It would be an interesting (albeit impractical) "weapon" to point a strobe light at a single-prop driven plane. You could seriously worry the pilot.
Title: Re: Why do aeroplane propellers appear curved?
Post by: alancalverd on 09/06/2019 15:45:25
"I say, chaps, the prop seems to have stopped but we are still flying"

"Praise the Lord"