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Homework Help: Helicopter Rescue Device - real application

  1. Jan 28, 2007 #1
    I'm doing background research on a real application. At high altitudes (and/or when people are shooting at you) helicopters can't stop and hover, but could snag a survivor or critically injured litter patient at around 30 knots (15 meters per second) A similar device (Fulton Recovery system) has been used from airplanes at over 100 knots (see James Bond/Green Beret movies).

    Just to check my math, this would be the same equation as a ballistic pendulm. A 15 M/sec speed would result in the survivor swinging to a maximum height of 11.48 meters, with the maximum G load based on the cosine of the length of the line. A line 23 meters long wouls result in a 2G max load.

    The design would be trainling a line with a hook, rather like a fighter's tail hook, which would snag a loop held horizontal above the survivor.
  2. jcsd
  3. Jan 28, 2007 #2
    Your post did not make any sense. Are you trying to figure out what happens, under high altitude conditions, when people shoot at a hanging survivor?
  4. Jan 29, 2007 #3


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    I believe that one should rather see the situation as a pendulum with the kinetic energy of the survivor converted into potential energy as he rises upwards. With the ballistic theory one would have a rather massive hook and a less massive survivor, which is not the case here.

    By the load I guess that you are referring to the tension in the cable? Which at liftoff will be given by

    [tex]T = m(\frac{v^2}{L} + g)[/tex]

    if one views it as a pendulum again.
  5. Feb 3, 2007 #4
  6. Feb 5, 2007 #5


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    I would think that one should rather sit with one's back towards the up line!

    Yes, one can regard the swing up via an energy conservation principle - which gives a rising height of 11.5 meters for a 23 meter line. I would think that the drag on the person being lifted will determine by how much he will initially (plane in level flight) rise though since much of the energy will be lost due to drag (upwards speed decreases).

    The speed that you mention - 30 knots is quite low (55.5 km/h). I saw a value of 120 knots on the navy seal pages. The g-force seems to be around 1 for 30 knots.
    Last edited by a moderator: Apr 22, 2017
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