Orla
Lv 6
Orla asked in Cars & TransportationAircraft · 4 months ago

Why do helicopter blades look like they're going slow and backwards sometimes ?

11 Answers

Relevance
  • 4 months ago

    Because they are, it's called magic.

  • Anonymous
    4 months ago

    Do you mean in person, or watching a video?

  • Fred
    Lv 7
    4 months ago

    The shutter speed of the camera compared to the speed of the rotation of the blades gives an illusion of the blades slowing down as they speed up to the point when the blades are turning quickly that they appear to be turning slowly backwards.

  • Anonymous
    4 months ago

    Only in the movies.  Does not happen with naked eye.

  • How do you think about the answers? You can sign in to vote the answer.
  • F
    Lv 6
    4 months ago

    This is mainly a film / TV effect where the frames per sec is close to but not exactly the same as the rotor speed. 

    Depending on the rotational speed , you can see this effect in real life. If you have ceiling fan, lie under it looking up. If you concentrate on one blade, and follow it round , you can actually make it seem to be still for a few seconds.

  • 4 months ago

    If there is any kind of pulsating light you get a strobe effect.   If it is at exactly the right speed,  the blades look like they are not moving.   At a slower or faster speed the blades look like they are turning slow or turning backwards.   You see the same thing in the movies with wheels on wagons and cars.    (You would think with all the high tech video stuff they have these days you would NEVER see the wheels turning backwards but that seems to be one effect they have never been able to fix.)  

  • Anonymous
    4 months ago

    Because of something called stroboscopic effect.

    If you saw those in real life, as opposed to video recording, you would not see this.

    The point is that a camera takes a series of quick snapshots, usually, the frame rate is 30 per second (but could be a multiple), and if the illumination is good, then the exposure of each frame could be so short, they capture the blade while it did not have time to really move much, and thus appears fixed.

    Now, if the rotor spins at something like 10 rotations per second, then each of those very short exposure will capture the same blade OR a different one (since they are identical, that makes them look the same anyway) at about the same apparent spot. If the rotor speed is just off a fraction of a second from the camera frame rate, then the rotor will seem to creep, potentially seeming to go backward.

  • Anonymous
    4 months ago

    Looks can be deceiving 

  • Anonymous
    4 months ago

    That only appears when you're watching a helicopter that has been filmed; you don't see it in real life. It is caused by the difference in blade speed vs frame rate of the camera. Airplane propellers, fans, car wheels, etc all can look that way on camera.

  • Barry
    Lv 6
    4 months ago

    The frame rate on the camera has that effect. We used to see it on wagon wheels in old films too.

Still have questions? Get your answers by asking now.