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What's the power source

  1. Nov 8, 2007 #1

    rcgldr

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    Radio control glider circling at up to 301 mph (current record is 309mph), for those that don't know, can you guess what is the power source (note, it's truly a glider, no motors except for surface controls).

    rc glider at 301mph.wmv
     
  2. jcsd
  3. Nov 8, 2007 #2

    mgb_phys

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    Ultimately gravity.
     
  4. Nov 8, 2007 #3
    Uh.. .. no. The sun's gravity is not powering the thermals, it only kick-started the thermo-nuclear reactions which are powering Earth's thermals. Earth's gravity is certainly not a power source.
     
  5. Nov 8, 2007 #4

    rcgldr

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    It not thermals. If this glider were put into an indefiniate veritcal dive, it's terminal velocity would be well below the 301mph it's acheiving in the video.
     
  6. Nov 8, 2007 #5

    mgb_phys

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    Gravity is powering the thermals - hot air rises because of the pressure of the cold air sinking around it, caused by gravity. You don't get convection without gravity.
     
  7. Nov 8, 2007 #6

    russ_watters

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    Gravity is conservative. Thermals are a thermodynamic cycle: they are powered by the sun.

    Anyway, that glider isn't thermaling, it is riding a ridge: a strong wind hits the side of the hill and is directed upwards. The link didn't say if the 309 mph is ground speed or airspeed, but I would suspect airspeed and the glider could be diving from a 120mph tailwind into a 120 mph headwind, with 60mph of its own speed.
     
  8. Nov 8, 2007 #7
    really what are you guys thinking
    aliens
    they have come here to tell us to stop sending shuttles to space
     
  9. Nov 8, 2007 #8

    rcgldr

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    You're getting close, the glider is circling on the downwind side of a ridge. Wind speeds at this ridge site can range from 45mph to 60mph.
     
  10. Nov 9, 2007 #9
    Remember our peregrine falcon friends, who dive at a speed of 270 mph!
     
  11. Nov 10, 2007 #10

    rcgldr

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    In case anyone is wondering, it's called dynamic soaring. Wind is blowing across a ridge line. Downwind of the ridge line, the wind above the ridge line is nearly horizontal, but below the ridgeline is turbulent but on average non-moving air. The shear boundary between the horizontal wind and the non-moving wind is fairly small.

    Gliders are launched on the upwind side of the slope to gain altitude, then they are flown past the ridge with a tailwind and flown across the shear boundary which increases the air speed. The gliders are circled back up into the headwind, gaining more airspeed. This is a gain of double the wind speed minus the losses from drag, especially when pulling high g turns as speeds increase. With 45mph to 60mph winds it's a lot of power, and the gliders can gain more speed than terminal velocity from a vertical dive. Early RC gliders used to break from the speed (flutter) and g forces, but newer ones designed for dynamic soaring with heavier carbon fiber used in the molded wings has reduced the rate of attrition.
     
    Last edited: Nov 10, 2007
  12. Nov 11, 2007 #11
    Very nice effect. :smile: (I'd wondered where this was leading - by definition gliders are powered "by the wind", but I'd not heard of exploiting a wind gradient like this.)
    Alas, apparently it's the albatross that's relevant!
     
    Last edited: Nov 11, 2007
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