If you only visited Aspen during ski season, you might be forgiven for thinking the place is snowy all year round. Similarly, a third grader might well imagine his teacher lives at school, since that’s the only place he ever sees her. Any time a periodic observation is synchronized with the event it measures, things get screwy. This movie makes the problem clear.
No matter how surreal it looks, this helicopter isn’t doing anything strange. The rotor turns at a certain rate, and the camera happens to be snapping frames for the movie at exactly the same rate (or a multiple of the same rate, but that complicates the explanation). So the camera is catching the rotor blades at exactly the point as they whirl around. As a result they look like they’re motionless. But if you were on the ground watching, nothing would look amiss.
Here’s another instructive video of a more prosaic system: the wheel of a bike.
A notable application of this kind of synchronization was the Red Baron’s machine gun. In World War I, if you mounted the guns where they were most conveniently operated by the pilot (just over the nose of the plane), you had the unfortunate side effect of shooting off your propeller. The first solution to this problem, introduced in 1914 by Saulmier, was to use armored blades, so bullets did minimal damage when, inevitably, they struck the prop. But the cleverer solution by far was strobing: use an interrupter gear that only lets the machine gun operate when the prop is safely out of the way. Just as with the helicopter video, from the gun’s point of view, the propeller is motionless.