Yesterday's post about how to actually build a Cloud Nine.
Ways of achieving lift. Plus one that's never been used.
Rocket lift. One of the oldest forms of lift. Stay on one side of a long-lasting explosion and you will find yourself flying.
Airfoil lift. Again, very old. Kites use this. Essentially, a combination (to a relative degree depending on airspeed, wing chord, wing loading, etc) of downward deflection and an upward-pulling low-pressure area above the wing. Used in gliders, powered aircraft, ekips, rotorcraft, frisbees.
Hotter than air lift. Relatively recent invention. A form of lighter-than-air lift. Heating normal air lowers it's pressure relative to the surrounding air, making it lighter. Requires a heat source. Is highly dependent on the relative temperature of the surrounding air. Doesn't work well in warm weather.
Light gas lift. More recent still. Helium or hydrogen has a lower molecular weight than the air thas surrounds it. Doesn't need extra heating, works regardless of relative temperature. Economical for use or large airships. Low cost way of staying aloft for a long time.
Accelerated air lift. It's debatable whether this deserves to be a category of its own. Like rocket lift but works by moving the air, not by controlling an explosion. Insects use a version of this. Insects (with the exception of butterflies and their ilk) don't use airfoil lift.
Eh, okay. Electrostatic lift. Electrically inducing an ionic flow. Check youtube.
And then their's one more that has never been exploited before.
Low-pressure lift. It's a form of lighter-than air lift. Instead of heating the air to lower its weight by lowering its pressure, or simply using a lower weight gas to begin with, it is possible, though very difficult, to achieve lift by lowering the pressure.
Essentially, by removing air from the lift volume, you make the lift volume lighter. But you can't do that with a balloon. A balloon would just get smaller, equalizing the pressure difference as it did. You need a rigid wall. The more air you remove, the more rigid it needs to be.
With Cloud (see previous post), which requires a rigid wall to begin with, it would be possible to to pump a few percent of the air out of the inside of the volume. This would put positive pressure, pushing inward, on all parts of the structure simultaneously (though not equally- an approximately 17% difference from top to bottom due to differences in atmospheric pressure).
Each percentage point of air removed from the interior would translate into more than a hundred tons of additional lift.
This method can also be used in concert with light-gas lift and hot-air lift.
But it's a risky approach because, if you breach the envelope, you lose the extra lift. Instead of relying on low-pressure lift at all times, it could be used as an emergency back-up plan. Powerful pumps could quickly remove some of the air to boost altitude. It would be used as a first resort- particularly in areas where dropping a hundred tons of water might not be particularly neighborly. It could also be used after water ballast had already been dropped, as a last-resort emergency maneuver.
Actually, low pressure lift is an old idea. Otherwise known as vacuum lift, it dates to 1670 when a brilliant Jesuit priest named Francis Lana conceived of an airship held aloft by airtight copper spheres from which the air had been pumped out. To make a copper sphere strong enough, however, would require making it too heavy to fly. In fact, no such metal is strong enough on the scale Lana had in mind (about seven meters in diameter).
By building extremely large, and by lowering one's expectations of acheiving near-total vaccuum, it's actually conceivable.
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