A gravityplane from Hunter Aviation changes its density using
helium and vacuum to make it lighter or heavier than air.
Current plans call for the plane to use variable-geometry
wings, swinging out to 90° for a maximum wingspan and
good glide performance, or tucking in for high-speed dives.
Stephen J. Mraz
Here's a good trick: The gravityplane, brainchild of inventor
Robert Hunt, will be able to change its density from
lighter-than-air to heavier-than-air. The aircraft, still in
development, will be similar to a submarine that changes its
buoyancy, a form of gravity, to float on the surface of the
sea or cruise 300 ft below it. If the design pans out, the plane
won't need any fossil fuel and will have a virtually unlimited
Hunt, an aviation enthusiast and Chairman of Hunt Aviation, Pass
Christian, Miss. (www.fuellessflight.com), says the proposed
plane will use helium or vacuum to make it lighter than air
and rise into the sky. (At sea level, helium's lifting capacity
is 0.0628 lb/ft3; vacuum lifts 0.0755 lb/ft3.)
Once at a sufficiently high altitude, vacuum is released or
the helium is compressed and stored for later use, making the
plane fall. High-aspect-ratio wings give the plane a glide ratio
in the range of 40:1, letting it glide 40 miles forward for
every mile it falls vertically.
Hunt expects to take the aircraft up to 10 miles, giving it
a range of 400 miles for each up-and-down cycle.
As the plane falls, the air powers two turbines that compresses
the air. Compressed air is stored at 1,000 to 1,500 psi and
powers pumps, valves, generators, control surfaces, and runs
two external turbines for vertical propulsion on take-off and
directional control in flight. Taking on compressed air also
increases aircraft weight, and this boosts its speed during
the downward glide. The plane might need an initial charge for
its high-pressure tanks for take-off, but if managed correctly,
the gravityplane should always land with its tanks fully pressurized.
Even if the tanks are empty, however, a 20-knot wind on the
ground is enough to turn the turbines and build up a supply
of compressed air.
Geometry dictates the plane must be large to be practical.
(Larger structures hold more lifting gas or vacuum per square
foot of surface area.) Hunt estimates a gravityplane that can
carry the same payload as a Boeing 747 would be roughly 50%
larger than the current 747. He envisions the airplane consisting
of two large pontoons, each containing several chambers. The
pontoons will be multiple layers of Kevlar and epoxy, which
weigh as little as 1 lb/ft2, around a rigid carbon-fiber
Chambers in the pontoons will have polyester-reinforced nylon
bags that can be individually filled with helium. Or the chambers
can be pumped out to maintain a vacuum, giving the craft a backup
lift system. The twin-pontoon design lets Hunt control the plane's
attitude, a task that would be more difficult with a single,
tubular fuselage. The finished plane will also be able to rise,
fall, or hover at the pilot's discretion.
The plane will use wind turbines invented and patented by Hunt.
The vertical-axis turbines change their drag profile using collapsing
blades, letting the turbine more efficiently harness the wind.
The turbines are said to be four times more efficient than conventionally
bladed horizontal-axis versions (20% compared to 5%, respectively).
Hunt's turbines are also reversible, letting them collect and
store energy or serve as propulsion units to control aircraft
attitude and possibly steering.
The biggest challenge in building the gravityplane, according
to Hunt, will be building an airframe strong enough for high-speed
gliding while carrying a significant payload, but light enough
to be lifted by helium or vacuum. To help test and refine his
designs, Hunt plans on building a scaled-down, three-man submarine
version of his gravityplane over the next five months. The craft,
a 30-ft-long sea glider, will change its density using compressed
air to rise and fall in the water, gliding forward as it rises
and falls, and deploying hydroturbines
to extract energy from the water it moves through.
The prototype will have to dive and submerge at perhaps 20
knots to generate speeds the turbines need to work efficiently.
Hunt plans on testing his prototype in a "water tunnel"
rather than a wind tunnel. "Everyone agrees if the concept
works in water, it will work in air, which is merely a more
dilute lifting fluid," he says. "It will just be far
easier to do in water."