Two for the Darwin awards...?
Joined: Oct 2002
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From: 1 Dunghill Mansions, Putney
Crab,
It's just an unusually smooth example of frame rate synch - their other vids (of aircraft that are clearly in flight) have a similar effect.
Anyway, just sit back and enjoy their factory tour guide. I mean, their factory tour.
I/C
It's just an unusually smooth example of frame rate synch - their other vids (of aircraft that are clearly in flight) have a similar effect.
Anyway, just sit back and enjoy their factory tour guide. I mean, their factory tour.
I/C
Last edited by Ian Corrigible; 10th December 2014 at 14:41.
Joined: Apr 2000
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From: EGDC
It's just an unusually smooth example of frame rate synch
I think we have all seen lots of video footage of helos with varying effects on the rotors but that first one is unlike any others I have seen. Usually the frame rate can make it look like the rotors are turning the wrong way or standing still but moving that slowly?????? Maybe a video guru can confirm or deny.
Joined: Jul 2002
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From: UK
Originally Posted by [email protected]
not convinced by that - although she could guide me anywhere, anytime, the end of that vid when she starts the rotors has a much faster blade passing rate than the alleged airborne with guys on the skids video.
I think we have all seen lots of video footage of helos with varying effects on the rotors but that first one is unlike any others I have seen. Usually the frame rate can make it look like the rotors are turning the wrong way or standing still but moving that slowly?????? Maybe a video guru can confirm or deny.
I think we have all seen lots of video footage of helos with varying effects on the rotors but that first one is unlike any others I have seen. Usually the frame rate can make it look like the rotors are turning the wrong way or standing still but moving that slowly?????? Maybe a video guru can confirm or deny.
Joined: Apr 2000
Aviation Qualifications: ATP+Mil
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From: EGDC
OK I give in, having looked at the end of the video when they shut down, I can see the blades change their relationship to the shutter speed/frame rate and appear to reverse direction.
Joined: Jul 2002
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From: UK
Joined: Nov 2003
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From: UK/OZ
The orange coloured one looked stable but the one in black has a twitchy tail.
Variable frame rate is used as a means of controlling exposure.
Pro cameras have a manual frame rate adjustment where the frequency can be dialled in to match the frequency of the movement of the subject. On Sony cameras its called "clear scan" and is adjustable in 0.2Hz steps.
Mickjoebill
Usually the frame rate can make it look like the rotors are turning the wrong way or standing still but moving that slowly??????
Pro cameras have a manual frame rate adjustment where the frequency can be dialled in to match the frequency of the movement of the subject. On Sony cameras its called "clear scan" and is adjustable in 0.2Hz steps.
Mickjoebill
Joined: Jan 2007
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From: At home
Not often I get to explain something to Crab. Plenty of the other way around. All assuming of course that this isn't a massive waaaaaa.
Someone much cleverer will give you a better explanation, but this is how my understanding of the frame rate works...
The video is made of a series of stills (30/s is typical). If the 'shutter' time of each of those stills is quick enough, the blades will not be blurred in each of those frames.
Take the example of a 3-bladed rotor doing 600rpm. That means 10 Rev per second or 0.1 seconds per revolution. With 3 blades, that means the time between blades going through any particular point on the disk's path is a third of that, or 0.033 seconds. If the video was at 30 frames per second (call it one frame every 0.033 seconds), the disc would appear stationary. This is because each of the stills of the disc would show the blades in the same place.
Now increase the Nr by a little and the video would show the blades turning slightly in the correct sense, because each blade is able to get a little bit past its predecessor in 1/30 of a second. Slightly below 600rpm and the blade will appear to be going backwards because each blade has not quite got to the position of its predecessor in 1/30 of a second.
This is also true for multiples of 600rpm. At twice the rpm, the blades appear stationary, but this time because in each frame, the position of a blade matches that of the one 2 ahead.
Simples. Also called Strobe Effect.
Someone much cleverer will give you a better explanation, but this is how my understanding of the frame rate works...
The video is made of a series of stills (30/s is typical). If the 'shutter' time of each of those stills is quick enough, the blades will not be blurred in each of those frames.
Take the example of a 3-bladed rotor doing 600rpm. That means 10 Rev per second or 0.1 seconds per revolution. With 3 blades, that means the time between blades going through any particular point on the disk's path is a third of that, or 0.033 seconds. If the video was at 30 frames per second (call it one frame every 0.033 seconds), the disc would appear stationary. This is because each of the stills of the disc would show the blades in the same place.
Now increase the Nr by a little and the video would show the blades turning slightly in the correct sense, because each blade is able to get a little bit past its predecessor in 1/30 of a second. Slightly below 600rpm and the blade will appear to be going backwards because each blade has not quite got to the position of its predecessor in 1/30 of a second.
This is also true for multiples of 600rpm. At twice the rpm, the blades appear stationary, but this time because in each frame, the position of a blade matches that of the one 2 ahead.
Simples. Also called Strobe Effect.
Joined: Feb 2010
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From: 'Stralia
Good explanation. The multiples will hold as fractions too remember - for your 3 bladed rotor one third, two thirds, 1 times, 4/3 times, 2 times etc. hold.
Pleased about the fractions because I have been imagining a 3 blade MR system at 600 RPM and feeling a bit queasy ....
Pleased about the fractions because I have been imagining a 3 blade MR system at 600 RPM and feeling a bit queasy ....
Joined: Jan 2007
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From: At home
The fractions don't work. I chose a worryingly high 600 rpm as the slowest that works with a 3 bladed rotor and 30 frames per second. An Nr any lower and 1/30 second isn't long enough for a blade to take its predecessor's place.
I'm no videographer, but I'll bet that these crappy videos where the blades appear to stop are from mobiles with slow frame rates.
I'm no videographer, but I'll bet that these crappy videos where the blades appear to stop are from mobiles with slow frame rates.
Joined: Jun 2008
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From: England & Scotland
With fractions in the next frame the preceding (etc) blade takes the place of the current blade. As the two blades look alike (at this resolution) the blade appears to be static.
ADDED BY EDIT
Say disc running at 480 rpm - so that's 8 RPS
3 blades - 24 blade passes per second
One of the standard for FPS is indeed 24 (actually often run as 23.976 FPS).
So, using a Nikon D7000 DSLR at 24fps to shoot a video clip would make a disc running at 480 rpm appear to have stopped still. It would make my 120 disc at 415 rpm (top of the green) look like it's turning backwards. Slowing the FPS to 20.75 would make the 120 disc at 415 rpm appear to have stopped
iPhone 5 video capture runs at 30 FPS.
For a 3-blade machine that's going to need 10 rps to have a blade always at the same point of shot
Hence the 600 rpm calculation above would apply to the crappy iPhone.
ADDED BY EDIT
Say disc running at 480 rpm - so that's 8 RPS
3 blades - 24 blade passes per second
One of the standard for FPS is indeed 24 (actually often run as 23.976 FPS).
So, using a Nikon D7000 DSLR at 24fps to shoot a video clip would make a disc running at 480 rpm appear to have stopped still. It would make my 120 disc at 415 rpm (top of the green) look like it's turning backwards. Slowing the FPS to 20.75 would make the 120 disc at 415 rpm appear to have stopped
iPhone 5 video capture runs at 30 FPS.
For a 3-blade machine that's going to need 10 rps to have a blade always at the same point of shot
Hence the 600 rpm calculation above would apply to the crappy iPhone.
Last edited by John R81; 15th December 2014 at 15:09. Reason: Added examples
