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Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
That's amazing, looks rather amusing as well
I just wasted about 10 minutes reading the comments, would highly recommend it to anyone who wants a laugh! It seems most people commenting on there are barely conscious...
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
IntraWinding wrote ... Can you tell the frame rate from this? Whatever it is, assuming the camera frame rate is quartz locked, the rotor speed is surprisingly constant!
If the frame and blade rate are 30/second (NTSC television) then rotor is turning 360 rpm. If the frame and blade rate are 24/second (standard in film and digital cinematography) then rotor is turning at 288 rpm. What is the frame rate in PAL television? I think it's unlikely that two or more blades are passing in each frame interval (google helicopter blade rpm).
I think big helicopter engines are normally operated at a constant, optimum RPM; collective pitch control provides the "continuously variable transmission". [edit] Looks like the aliased speed is zero plus or minus a few RPM during the clip - you can see the variation. That would translate to plus or minus a few hundred RPM in turbine engine.
Registered Member #2161
Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247
Klugesmith wrote ...
I think big helicopter engines are normally operated at a constant, optimum RPM; collective pitch control provides the "continuously variable transmission". [edit] Looks like the aliased speed is zero plus or minus a few RPM during the clip - you can see the variation. That would translate to plus or minus a few hundred RPM in turbine engine.
I also noticed the small variation on the rotational speed. I would venture and say that the small variation in blade speed resulted from the varying load put onto the turbine engine, increasing and decreasing with the amount of blade pitch used by the pilot for the manoeuvres. The engine and throttle system was still flying a fixed rpm setting, however was not "fast"/"powerful" enough to compensate and ensure 100% constant rotational speed.
Thanks for the link richie, probably the best thing I have seen on break.com...got a good laugh from the comments as well. :D
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
My aviator friend Jack says: "Mil Mi-24 'Hind' attack helicopter, 300rpm (typical main rotor RPM) as a guess."
That would be consistent with an ordinary 25-fps PAL video recording.
Here are many details about the helicopter type. But the only "300 RPM" value given there is associated with its 30mm cannon. I wonder what -that- looks like, frequency-aliased in a video? Consecutive frames would cycle between 5 phases, one of which might capture muzzle flashes or even projectiles in flight.
Thanks for showing us this video, Richie. I also found the comments amusing. Similar ignorant comments and arguments can be found at other host sites, for example
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A neat display of visual aliasing...
