4 ms·

Does it not include balloons? They want surface area, slow movement, and the ability to operate on little to no energy. Wouldn't a large, permanent balloon fill

• upofadown 10y ago
  >slow movement But not too slow. You need to be able to stay on station when there is a wind. You would need some sort of powered airship. To be better than a wing it would have to have less drag.
  • abakker 10y ago
    I was wondering about that. Is 25mph cruise speed really going to be enough to overcome wind anyway? I suppose it could be both. Maybe inflatable fixed wing, if it had the right dimensions.
    • anotheryou 10y ago
      Maybe they have some safety altitude so they can drop to lower altitudes with other winds and come home when much wind is expected. Clouds often move in another direction than the wind near the ground. (is that some eddy current?)
      • mikeash 10y ago
        It's wind shear. Wind at altitude tends to travel at right angles to the pressure gradient, as the coriolis force does its thing. Wind near the ground tends to travel directly from high pressure to low pressure, because of friction with the ground. That friction also tends to slow it down. Net result, it's common for wind to turn and increase in speed as you go up.
    • Someone 10y ago
      picture #14 in the last slideshow claims "less than 80mph". Chances are it has to be faster at altitude, as it will need to push the same amount of air down there, while the air is less dense.
      • mikeash 10y ago
        The article says that it cruises at an altitude where the air density is 10x less than at sea level. Lift is proportional to density and to the square of speed, so it would need to go about 3x faster to generate the same amount of lift at altitude, or about 75MPH.
        • mveety 10y ago
          That the stall speed at altitude (give or take). 75 mph winds aren't uncommon at altitude, but that might work to their advantage to loiter. I've hovered my dad's Cessna before to take pictures and such when the headwind is there, so they should be able to take advantage of that as well.
    • bigiain 10y ago
      I don't think that's it's "cruise speed", I think that's it's minimum flight speed. In the context of Challenge #3, they talk about Aquila's "25mph" speed comparing it to a commercial airliner at "200mph" - that's about takeoff speed for a 747, which then goes on to cruise at somewhere around 700mph, so if the Aquila has a similar operating flight envelope you'd expect a "cruise" speed of perhaps 80-90mph. Also, it's entirely possible the Aquila is designed to have a wider efficient flight envelope that a 747, which a) was mostly designed using the best aerodynamics available in the 50s/60s, b) is limited by the sound barrier at the top of it's speed range (it'd hit Mach1 at ~750mph and things go strange aerodynamically there), and c) needs to be highly optimised for air-breathing jet engine fuel efficiency - which requires some tradeoffs electric power isn't constrained by (the thin cold air at 70,000feet doesn't change the power output of an electric motor the way it does a kerosine-burning jet).
• njharman 10y ago
  Helium balloons are far from permanent. Helium leaks/leeches out.
  • amelius 10y ago
    How about hydrogen? Yes, dangerous, but perhaps acceptable for an unmanned aircraft.
    • xkcd-sucks 10y ago
      Hydrogen really isn't all that bad. It's used lots in industry/academia as a process gas, in normal rooms, in processes that involve high temperatures, mixtures of chemicals etc.
    • njharman 10y ago
      It leaks out too, even more so I believe.
• amelius 10y ago
  Yes, I'd like to see an evaluation of different designs. Why not multiple balloons attached to the ground (to overcome wind drift), at not too high altitude, perhaps interconnected by line-of-sight communication.