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Post by tibor on May 18, 2007 21:17:47 GMT 12
Interesting youtube video. Can anyone explain what is actually happening? |
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Post by stu on May 18, 2007 22:05:52 GMT 12
Probably find that it's the shutter speed of the video camera stopping the rotors.
Cheers,
Stu.
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Post by stu on May 18, 2007 22:26:05 GMT 12
Probably find that it's the shutter speed of the video camera stopping the rotors. Cheers, Stu. To be more specific ..... www.mediacollege.com/video/camera/shutter/Video Camera Shutter
The term shutter comes from still photography, where it describes a mechanical "door" between the camera lens and the film. When a photo is taken, the door opens for an instant and the film is exposed to the incoming light. The speed at which the shutter opens and closes can be varied — the faster the speed, the shorter the period of time the shutter is open, and the less light falls on the film.
Shutter speed is measured in fractions of a second. A speed of 1/60 second means that the shutter is open for one sixtieth of a second. A speed of 1/500 is faster, and 1/10000 is very fast indeed.
Video camera shutters work quite differently from still camera shutters, but the result is basically the same. (The technical difference is that, rather than using a mechanical device, the shutter speed is adjusted by electronically varying the amount of time the CCD is allowed to build a charge. If this means nothing to you, don't worry. It really doesn't matter how the shutter works, it's the effect it has that counts.)
The shutter "opens" and "closes" once for each frame of video; that is, 25 times per second for PAL and 30 times per second for NTSC. Thus, if a camera has it's shutter set to 1/60, each frame will be exposed for 1/60 second. If the speed is increased to 1/120, each frame will be exposed for 1/120 of a second.
The main effect of higher shutter speeds is that individual frames appear sharper, due to the minimisation of motion blur. Motion blur occurs when the subject moves within the frame while the shutter is open. The less time the shutter is open (ie. the faster the shutter speed), the less movement will take place.
One side-effect of higher shutter speeds is that movement appears more jerky. This is because motion blur tends to smooth consecutive frames together.
Higher shutter speeds are common in sports coverage. Watch any fast-action sport to see the "flickering" shutter effect. Notice how the slow-motion replays look, especially when they freeze the last frame.
Note: As a result of the reduced exposure time with high shutter speeds, the image may appear darker unless the iris is opened to compensate. |
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Post by Dave Homewood on May 18, 2007 22:28:07 GMT 12
Whatever the cause (and I agree with Stu, it's certainly not an autorotation) it makes for some very interesting footage. Cheers for pointing it out Tibor.
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Post by stu on May 18, 2007 22:33:02 GMT 12
it makes for some very interesting footage. Cheers for pointing it out Tibor. Agreed, fascinating to watch  Cheers, Stu. |
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Post by Bruce on May 18, 2007 22:44:02 GMT 12
The comments posted on that clip are pretty stupid, the strobe effect isnt really that difficult to understand - remember all the comments on the photo board about frozen props? video is just a number of similar frames, and if those frames were taken when one of the 5 blades was in the same position each time (It doesnt have to be one per revolution in this case 1/5 of each rev would work) you will have this effect. I'd say the Hind has an accurate main rotor governor, which gives the steady effect. A clue is the tail rotor is turning and the main rotor blades flex upwards. Unless there was a serious failure of the main transmission, the tail rotor is attached to the mains, and if it is rotating, so are they. (and in that case the Hind would not be doing the manouvers it is in the clip!) The same strobe principle is used when dynamically tracking and balancing rotor systems - any thing out of line becomes quite noticeable.
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Post by Dave Homewood on May 18, 2007 22:54:27 GMT 12
90% of comments made on YouTube are not worth reading no matter what the film IMHO
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Post by lumpy on May 19, 2007 14:53:15 GMT 12
Yes , its a fairly simple optical illusion . What I find astounding is how there is almost no change in rotor speed despite different loads ( climbing and decending ) - if there was it would riun the illusion . I wonder how hard it it would be to recreate this interesting clip on purpose !
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Post by ARU on May 29, 2007 23:53:19 GMT 12
I have actually got some footage of the seasprites and whenuapai with same thing going on,
I'll see if i can find it and post it later (by the way it is from a digital still camera about 5 years ago so the quality wont be that good)
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Post by ARU on Jun 15, 2007 20:33:46 GMT 12
I found the videos, my apologies for the poor quality   |
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Post by FlyNavy on Jun 15, 2007 20:54:23 GMT 12
Excellent demo. Thanks for that. :-)
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Post by turboNZ on Jun 20, 2007 11:55:33 GMT 12
I read somewhere that Hinds need forward motion to fly and they don't hover that well (if at all).
Don't know how true that is. If that is the case then an auto-rotation would be an interesting ride !!
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Post by Dave Homewood on Jun 20, 2007 12:38:51 GMT 12
Technically an auto-rotation is a forward motion - like landing a deadstick conventional aircraft. It's not straight down. (not how the RNZAF used to do it day in day out at Wigram anyway).
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Post by turboNZ on Jun 20, 2007 13:03:06 GMT 12
Granted, but I was referring to normal flight characteristics of the Mil Hind. I know that it was designed as a gunship and a troop carrier but didn't have enough grunt for both so became one or the other. Maybe I'm getting a little off-topic here. |
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Post by Dave Homewood on Jun 20, 2007 13:12:45 GMT 12
Maybe Ned can answer this, as he took those lovely shots of the Hinds in Afghanistan. They seemed to be hovering for him in the photos.
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Post by turboNZ on Jun 20, 2007 13:29:49 GMT 12
I guess I was right to some extent "The Hind is a difficult and unforgiving helicopter to fly. Unless a pilot is very careful, he can cause it to fly out of control or cause the rotors to collide with the tailboom. Although the Hind is fast, it is not very maneuverable, and this is a problem when flying at high speed close to the ground. In addition, a fully loaded Hind cannot hover and has to make a rolling takeoff. The Hind's performance suffered in the hot, thin air of Afghanistan." From www.centennialofflight.gov/essay/Rotary/Hind/HE18.htm |
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Post by corsair67 on Jun 20, 2007 13:48:31 GMT 12
That'd be fun - cutting your own tail off with the main rotors!  Great to have you back on board too, Turbo!  |
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Post by turboNZ on Jun 20, 2007 22:46:43 GMT 12
Thanks Corsair67.
I've missed being here.
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