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Could you pull down a helicopter?

  1. Jan 24, 2007 #1
    Me and a friend are having a dispute after watching the movie Underworld : Evolution.

    There's a scene in the movie where a character literally pulls a helicopter out of the air by pulling its fast rope straight down. Now I know it's Hollywood, but I started laughing at the reality of such an act.

    The friends I was watching it with got defensive about the reality of such an event occuring. So, movie magic notwithstanding, could it happen, provided the character was strong enough?

    Conditions:

    Helicopter generates enourmous thrust - say ~30,000 pounds downward

    Superhuman character isn't using any 'extra' or 'supernatural' powers - straight physics

    Character weighs around 200 pounds

    Character is not anchored to the ground in any way

    Character is enourmously strong - could lift the helicopter off the ground if he wanted to

    Character is not pulling suddenly, so no aerodynamic or airflow issues to deal with


    My thought is that anyone pulling on a rope in this situation would simply pull themselves up the line, since the helicopter is generating far more thrust than the actual tug that they could exert due to their physical weight.

    My friends believe that if the character had enough strength, he could pull it down regardless.

    Who's right?
     
  2. jcsd
  3. Jan 24, 2007 #2

    NoTime

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    Haven't seen the movie, but...

    A hovering helicopter is neutrally balanced.

    Crudely speaking it is a large mass on a frictionless surface.
    Adding 200lbs to it will make it come down.

    If the force is applied off the helicopters center of gravity then it will tip, possibly sending it into a region of instability, where it will crash.

    To a large degree what happens in real life it would depend on how quick the pilot can react.
     
  4. Jan 24, 2007 #3
    Understood. Let's assume the rope was at the exact center of gravity.

    Maybe a good analogy would be a blimp - let's say a line was tied to the blimp at the central mass, if anything of sufficient strength (but again, weighing 200 pounds) pulled that rope, could they pull it down?

    Thanks,
    xeed
     
  5. Jan 24, 2007 #4

    russ_watters

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    Theoretically, maybe, but in real life there is a pilot who can adjust for that and small changes in wind require bigger control inputs to counter than adding 200 lb would unless the helicopter is very small.

    Xeed, the answer to your question is no, it isn't possible for someone to pull a helicopter down (with the iffy caveat above). The most any person can do is increase the weight the helicopter can carry by their own weight.

    To put it another way, even if you can do a 300lb lat-pull, if you weigh 200lb, 200lb is all the force you add continuously by lifting yourself up.
    Well, unlike a helicopter, a blimp really is roughly neutrally buoyant, and adding 200lb would significantly change that.....however, a blimp still has a pilot who still has a significant amount of input into the lift of the blimp. Those fans on the sides are easily strong enough to handle an unexpected extra 200lb.

    So like the helicopter, you could only pull down a blimp if the pilot stopped flying it.
     
    Last edited: Jan 24, 2007
  6. Jan 24, 2007 #5
    Yes, as NoTime said, technically if the pilot does not adjust the thrust then the helicopter/blimp will accelerate downward because of the force of the character's weight. However, that's assuming the pilot is sleeping or something and doesn't adjust the thrust. Assuming that the pilot would increase the thrust to compensate for this since the acceleration would be slow, the forces would once again cancel out and the character would just be lifted off the ground.

    With this assumption, the person would not be able to pull it down by weight alone since the helicopter is able to provide enough upward thrust so that the helicopter lifts the character's weight. I would probably say that in the situation you're describing, if the character is not anchored to the ground, it is not possible.

    However, with that said, I think it may be theoretically possible to pull the helicopter down by a certain amount if the character pulled very very fast.

    Say the helicopter weighs about 10000 kg and the character weighs 100 kg. If we think about the forces acting on the helicopter (thrust force upward and weight downward) cancelling out, and neglecting the force of gravity on the character, we can use conservation of momentum to get an idea of how fast the character would have to pull.

    Let mH = mass of helicopter = 10000 kg.
    Let mc = mass of character = 100 kg.
    Let vH = velocity of helicopter vertically.
    Let vc = velocity of character vertically.

    mc vc = mH vH

    The velocity with which the helicopter would come down if the character pulls on the rope would then be

    vH = (mc vc) / mH.

    That means that if the character were able to pull down on the rope at 10 m/s (about the fastest human running speed ever recorded), the velocity of the helicopter downward would only be (100 kg * 10 m/s) / 10000 kg = 0.1 m/s. It would only budge 10 centimeters, about the length of your index finger, for every 10 meters the character climbs up!

    At this impressive rate, to lower the helicopter just 10 meters from its initial position, the rope would have to be 10 m/s * (10 m / 0.1 m/s) = 1000 meters long...1 kilometer (more than half a mile), lol. Probably not going to happen. And even then, the helicopter will still be high in the air, so this method won't work in bringing the helicopter to the ground...only decreasing its altitude a bit, since the character has to climb up the rope a much greater distance than the helicopter comes down.

    Realistically, even if the character is extremely strong, it would not possible to bring the helicopter down with strength alone unless he/she is anchored to the ground.
     
    Last edited: Jan 24, 2007
  7. Jan 24, 2007 #6
    Thanks

    Thanks for all the input on this question - it has been definitively answered.

    -xeed
     
  8. Jan 24, 2007 #7

    NoTime

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    I think there may be a caveat here.
    Helicopters have a max weight at which they can fly.
    If that weight is exceeded they stop flying.
    For a small 2 place helicopter 200 lbs is about half of the carrying capacity.
    So if the thing is already carrying max load the addition of 200lbs could get interesting.
     
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