11
$\begingroup$

I was watching this youtube video and I noticed that at 3:17 seconds they show a coast guard helicopter rescuing a man from a ship. Whats shocking about this is that the main rotor of the helicopter seem to be going incredibly slowly and the tail rotor has completely stopped.

How was the helicopter able to stay flying with such a low rotational speed? Was this some optical illusion or was the rotor really spinning that slowly?

Also, how was the helicopter not spinning? From what I understand the main purpose of the rear rotor is to counteract the rotational force of the main rotor. Since the rear rotor wasn't moving, shouldn't the helicopter developed a spin counter to the spin of the main rotor (even though it was slow)?

Improve this question
$\endgroup$
11
  • 24
    $\begingroup$ It's almost certainly just a frame-rate effect. The rotors won't actually be spinning that slowly. $\endgroup$
    – David Richerby
    Commented Apr 10, 2016 at 5:53
  • 6
    $\begingroup$ I meant that that it's the usual thing where, because you're looking at a sequence of still images shot at a particular frequency, things that are rotating quickly often seem to be moving slowly or even backwards. You can often see this in videos of car wheels. For example, it's impossible to tell the difference between the blades rotating 10 degrees per frame and 100 (=90+10; it's a four-rotor machine) degrees per frame or even 190 degrees per frame, because the resulting sequence of still photographs would look identical. $\endgroup$
    – David Richerby
    Commented Apr 10, 2016 at 6:01
  • 10
    $\begingroup$ It's a simple optical illusion. A common, and well known strobe effect. Nothing more, nothing less. $\endgroup$
    – Simon
    Commented Apr 10, 2016 at 7:24
  • 6
    $\begingroup$ The tail rotor doesn't appear to be stopped. you can't see the blades at all; they are just a blur. What you are looking at are the flow-straigtening stators in the fenestron tail. They don't move $\endgroup$
    – TomMcW
    Commented Apr 10, 2016 at 9:38
  • 4
    $\begingroup$ By the way, that's not a rescue, they are simply picking up a pilot (the naval kind). $\endgroup$
    – Jörg W Mittag
    Commented Apr 10, 2016 at 11:13

3 Answers 3

Reset to default
36
$\begingroup$

It's an illusion that the blades appear to be going slowly. It's actually a well known effect called the wagon wheel effect.

Essentially the rotor is spinning at close to an even multiple of the camera's framerate divided by the number of rotors.

This means that between frames the blades have moved a full quarter rotation (or a multiple of that). Creating the illusion that the rotor is moving more slowly that it actually is.

Improve this answer
$\endgroup$
3
  • 5
    $\begingroup$ You could add the point about the fenestron brought up by @TomMcW in the comments above. To quote part of his comment here: "you can't see the blades at all; they are just a blur. What you are looking at are the flow-straigtening stators in the fenestron tail." $\endgroup$
    – J W
    Commented Apr 10, 2016 at 12:53
  • 2
    $\begingroup$ "This means that between frames the blades have moved a full quarter rotation": Or any number of quarters (if there are 4 blades on the rotor). $\endgroup$
    – mins
    Commented Apr 10, 2016 at 18:34
  • 1
    $\begingroup$ The shutter effect can even make the rotor to appear to not turn at all $\endgroup$
    – Dohn Joe
    Commented Dec 15, 2017 at 12:22
9
$\begingroup$

Life is full of illusions and many things give similar illusions. Strobe lights are popular with dance and and other entertainment venues. One game was to adjust the strobe timing so a spinning wheel would look like it was going backwards, then forwards, then standing still, and so on until you grew tired of the game.

In old movie westerns wagon wheel spokes sometimes look like they are going backwards.

In the days of old CRT monitors TV shows had to use special electronics with TVs filmed on show sets to avoid the video appearing to break up or doing other funny things.

You can get a simple strobe effect with many modern LED Christmas light strings. Move your eyes rapidly so your vision crosses the lights and you may well see the lights appear to flash. Actually, they are always flashing. It is just that they are flashing faster than the eye can normally notice. If you don't want to get dizzy, you can shake a handheld mirror to see the same effect.

Edit: 2023-Feb-06: more and more I'm noticing wheels on cars, etc., on online videos, with there lower refresh rates and higher compressions, can get very noticeable with how uneven they jerk and spin.

Improve this answer
$\endgroup$
2
  • 5
    $\begingroup$ And they were also used to adjust ignition timing on cars, when ignition was still mechanical. Also on turntables. Turntables are not well known yet, but are top hype :-) $\endgroup$
    – mins
    Commented Apr 10, 2016 at 18:27
  • $\begingroup$ Also very common-- rear-view mirror in car vibrates slightly so you can get a strobe effect-- if a vehicle behind you has exposed tires with large treads (e.g. tractor, road grader, high-clearance truck with oversized tires), they may appear to be stationary or slowly rotating backwards -- $\endgroup$
    – quiet flyer
    Commented Feb 6, 2023 at 20:23
4
$\begingroup$

That's an illusion, the result of the frame rate of the video camera being a multiple of the rotor speed. It so happens that when the video takes an individual frame, the rotor blades are at or near the same position, so the rotor appears to be turning very slowly or in some cases, to be stationary. If you look up recent videos of large prop planes being started, you can often see the prop appear to become stationary (the prop blades also appear to be bending) as they rev the engine up... same basic effect. You don't see this in old films, because the film was of such a slow speed with a long exposure time for each frame that a moving rotor would be blurred.

However... there was one helicopter whose main rotor ran at a very leisurely 88 rpm... the Hughes XH-17. It was cobbled together from a variety of sources, including a cockpit from a Waco CG4 glider, to test the feasibility of a very large helicopter with rotors driven by jets at the tip of the rotor blades. Fuel was sprayed into the tip jets and ignited to provide rotor propulsion, hence the very small tail rotor - very little torque was produced by this system.

The two turbine engines are there only to supply compressed air to the jets at the ends of the rotors. The XH17 has to date, the largest rotor blades ever fitted to a helicopter, with a 129 foot main rotor diameter.

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .