Stephen J. Mraz
Senior Editor

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 range.

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 airframe.

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."