The Following Article is from the December 2003 Issue of the World and I Magazine by Science Editor, Glenn Strait

Neither a blimp, nor a glider, nor a fuel-propelled airplane, a new aircraft promises to translate gravitational attraction into both lift and forward motion, capture wind energy with a radical new wind turbine, and usher in the era of fuelless flight.

Editor's introduction: Thinking totally out of the box and utilizing private funds, independent inventor Robert D. Hunt is pushing forward with a project that could transform air transport in the twenty-first century--if it proves successful. While Hunt's gravityplane quite reasonably could have been conceived of in the twentieth century, the materials and technologies for building it may only be available now. Even today, the craft is pushing the envelope, requiring the most advanced of lightweight, fiber-epoxy materials and a radical wind turbine design that has yet to emerge onto the commercial market. Hunt envisions a craft that will control its weight and center of gravity by balancing partial vacuum, helium, and compressed air in interior chambers. The craft will rise when it is lighter than air, and, when it is heavier than air, tilt and glide forward, or float level and sink straight down. All this is to be achieved through a computer-controlled system of pumps, valves, piping, interior chambers, and polyester-reinforced nylon balloons.
        In this aircraft, compressed air serves as a source of both power and weight. The compressed air is produced through energy captured from the high-speed airstream passing by the craft as it glides downward. The energy capture is achieved by two of Hunt's new vertical-axis turbines, which sit like two counter-rotating eggbeaters atop the craft's center section.
        The concept is simple, but the engineering is difficult. Every additional mechanical component adds deadweight to a structure that must weigh in as a lightweight if the craft is ever to fly. Hunt is clear that a Cessna-sized gravityplane could never fly. It would be too small. The weight of the aircraft structure and machinery would overburden even the greatest possible gravitational lift, which would be achieved if the plane's interior were evacuated to hold a vacuum.
        To fly, then, the gravityplane must be big, gaining the advantage of volume over surface area as its size increases.
        Will the gravityplane ever become a reality? Only a lot of engineering, model making, and testing can decide. Hunt is betting that he can make it a reality. Already a test module, a pod 100 feet long, is taking shape in a high-tech shipyard on the outskirts of New Orleans.
        In the following article, Hunt writes directly about the principles, technology, and developmental process of the gravityplane.

any people do not see the fingerprint of gravity in floating battleships, buoys, and bottles. Yet a little thought reminds us that buoyancy occurs because gravity exerts a greater pull on more-dense materials than on less-dense materials. An air bubble in water and a helium-filled balloon in air, for example, both rise because of the force of gravity--because they are less dense than the surrounding fluid.
        Similarly, it is easy to overlook the fact that glider aircraft can fly only because earth's pull accelerates them downward as their forward-moving wings produce a counteracting aerostatic lift. The new gravityplane simply harnesses both forces of gravity--the upward force of buoyancy and the downward pull of gravity acceleration--so that it can rise into the sky using a gas such as helium, and then glide downward like a glider using the earth's gravitational pull.
        Before shifting into the glider mode, however, the aircraft must change from being lighter than air to being heavier than air. The weight change is achieved by drawing in and compressing air from the surrounding atmosphere. This moderate-pressure compressed air may be thought of as a primer load of potential energy needed to initiate the downward glide through which the craft's wind turbines will capture high-pressure compressed air.
        Aboard the gravityplane, high-pressure compressed atmospheric air at about 1,500 pounds per square inch, or roughly 100 atmospheres, is a treasure store of potential energy of both height and pressure. The gravityplane is designed to take advantage of both of these. The compressed air's weight (gravitational potential energy of height) enhances downward glide speed, while its pressure is a potential energy reserve, a fuel, to run the plane's machinery. As the gravityplane operates in a cycle of rising and falling (gliding), rising and falling, it depends on high-pressure compressed air for changing between the two modes, so it must always keep a minimal supply in its tanks. In its original start-up, the gravityplane will require an injection of high-pressure compressed air from an outside source; or it may be able to capture its own high-pressure air as it sits on the ground, if the wind is adequate to drive its wind turbines (20 MPH or more).

More truly an airplane

t could be argued that the gravityplane will have a greater claim to the name airplane than today's jet planes, which take off with a heavy load of fossil fuel that their engines burn throughout the flight. The conventional airplane uses air for two crucial flight elements: aerodynamic lift and forward thrust. In comparison, the gravityplane uses air for four
The gravityplane's inventor, Robert D. Hunt (center), and his two chief associates at Hunt Aviation, vice president Joe Chomko (left) and president Gene Cox (right), announced the gravityplane vision at the National Business Aviation Association's annual convention in October 2003. Here, outside the convention, they express their anticipation of a bright future for fuelless flight.
crucial flight elements: aerodynamic lift, onboard power, aerostatic lift (buoyancy), and ballast (weight distribution within the craft).
        In both the jet airplane and the gravityplane, air flowing past the wings yields aerodynamic lift. Here the similarity ends. Yes, the jet airplane's thrust is from air, but that's only after the air is heated and forcefully expelled by the fuel-burning jet engine. The fuel provides the energy behind air's thrusting force.
        In the gravityplane, highly compressed air is the fuel providing all the plane's energy needs not met by gravity. Thus not only does the gravityplane fly through the air like a conventional plane, it also gulps in and captures huge volumes of the air. The plane will use high-pressure compressed air to drive its internal system of pumps and pipes, generate electricity, and power two outboard turbines (used for vertical propulsion at takeoff and landing and controlling the plane's direction).
        The gravityplane's wind turbines hearken back to windmills in the preelectric era, when a windmill's spinning shaft would have been mechanically linked to a mill wheel for grinding grain. In the gravityplane, the wind turbines' spinning shafts are linked directly to high-pressure air compressors for capturing compressed air at 100 atmospheres. High winds from the plane's glide drive the turbines. Through being compressed, air becomes the craft's fuel and is readily converted to rotary motion when it is released through a pneumatic motor. The rotary motion in turn drives the plane's pumps and moderate-pressure compressors.
        Just as water is the pervasive component of the human body, compressed air is the pervasive component of the gravityplane. The plane maintains two separate confined-air systems: a high-pressure system and a variable-pressure system. The high-pressure fuel system is contained within a limited network of high-pressure storage tanks and delivery lines. In contrast, the variable-pressure system is coextensive with the entire interior of the plane's two large pontoons, which are each subdivided into five great chambers that can hold a vacuum. Operation of the plane will require that the pressure of each chamber varies from perhaps one-half to three atmospheres (7.4--44 psi). Each chamber contains a partially inflated helium-filled balloon that expands to fill the chamber under the partial vacuum and is compressed to be nearly flat under the weight of three atmospheres of air.
        If the craft needs to add weight, it pumps outside air through a moderate-pressure compressor into the appropriate large interior chamber. The compressor, of course, is driven by a pneumatic motor powered by high-pressure compressed air. The spent fuel (the previously high-pressure air) is simply added to the air supply in the large chamber.

Follow the flight cycle

nderstanding the basic concept of the gravityplane, we can begin to grasp the overview of its flight cycle and the way it manages air for flight advantages never before achieved. By capturing high-pressure compressed air during its descent, for example, the craft naturally increases its weight and thereby increases its glide speed. "Increased weight implies increased glide speed" is a well-known principle of glider flight, but no previous gliders have had the option of adding weight once they were launched. With its full embrace of air as both fuel and ballast, the gravityplane opens a new era of the high-speed, long-distance glider.
        Combining properties of a glider and a blimp, the gravityplane must rise or fall to be productive. Only in falling does it advance forward, and only in rising does it gain the altitude it must have if it is to fall fruitfully. When rising, the
When executing a steep dive or floating on water like a seaplane, the gravityplane will fold its wings back.
craft gains potential energy that in its falling is converted to kinetic energy of motion. This energy gain is in direct relationship to the height attained.
        If in its ascent the craft fails to gain sufficient altitude before the glide begins, then in its gliding descent it will fail to store enough high-pressure compressed air to complete another ascent-descent cycle. To complete the cycle, it needs to halt its descent by pumping a partial vacuum into its chambers so each is filled with its low-pressure helium balloon. Then the gravityplane must rise again to an effective altitude, carrying sufficient high-pressure air to run the compressors that take in the air that allows it to become heavy enough to fall again. The aircraft would normally land with a significant load of compressed air--its fuel for later use, including thrust for vertical takeoff.
        While the craft is on the ground or floating on water, its wind turbines can generate power so long as the wind blows with a sufficient velocity. This allows the supply of compressed air to be fully charged after short flights that do not produce sufficient compressed air to resume high-altitude flight. On the ground, operation of the wind turbines also provides high-pressure compressed air that can be used to produce electricity if a pneumatic motor is connected to a generator.

Building the gravityplane

reating this radical glider-blimp hybrid stretches the imagination of designers and engineers alike. It also challenges the most advanced manufacturing facilities. How can we begin to construct such a craft?
        Once the basic design plans are in place, the materials search leads to such strong, lightweight materials as carbon fiber or Kevlar bonded with epoxy resin. These will be used for constructing a rigid frame and outer skin, and the helium bag in each chamber will be made of lightweight, nonporous polyester-reinforced nylon. Studies already completed for Hunt Aviation give the green light for construction to begin. They show that a lighter-than-air craft with internal chambers each occupied by a helium-filled balloon can be built of these new ultralightweight materials.
        A single layer of Kevlar bonded with epoxy resin can cover an area of a square yard while weighing as little as three ounces. Built up into multiple layers, the Kevlar composite becomes rigid and strong. The studies affirm the value of making a lightweight rigid aircraft by coupling a Kevlar composite shell with a rigid, carbon-fiber framework. Such a craft, the studies show, could weigh as little as 16 ounces (one pound) per square yard of surface area.
        Realizing the potential of the gravityplane requires that it be made big. This size prerequisite is apparent from a property of enclosed spaces that is well known by balloonists. Bigger balloons hold more gas per square foot of surface area than smaller balloons do. The gravityplane follows the same principle even though it is more complicated, with its rigid shell requiring a rigid frame and internal balloons as well. Calculations show that to carry the same load, the gravityplane would need to be about 50 percent larger than the 747.
        Building a craft that flies based on the gravityplane vision is the task of years and a small army of experts. Hunt Aviation has already assembled many of these:
        Century Aerospace: Bringing together experience from major aircraft manufacturers of both large and small planes, including Boeing, Cessna, Lear, Lockheed, and Rockwell, Century's design team will help direct conceptual design of the prototype gravityplane. At later stages, Century will be instrumental in the gravityplane's certification process.
  • The Mississippi State University Aerospace Engineering Department: Faculty in the department are conducting a nine-month evaluation of the gravityplane's technological feasibility under the title "Systems Analysis of the Hunt Aviation Gravity Powered Airship."
  • United States Marine: A builder of lightweight and strong vessels for the U.S. Special Forces, United States Marine is in the early stages of building the first, 100-foot-long pontoon.
  • Raven Industries: The main supplier of strong, lightweight, high-altitude balloons to NASA, Raven will supply the polyester-reinforced nylon gasbags designed to go into the pontoons and wings of the gravityplane.*
            The gravityplane embodies so many new ideas and design concepts that testing the concept requires starting with one part of it: the pontoon (lifting body). Each plane will have two pontoons, but testing begins with one. It will be 100 feet long by 20 feet in diameter and have five internal chambers, each 20 feet long and containing a polyester-reinforced nylon balloon. Both the exterior and the dividing walls of the chambers will be strong enough to hold a vacuum, and the entire unit must be built out of materials that are of the lightest weight possible.
            As this article is being written, engineers are finalizing plans for a prototype of the lifting body to be constructed by United States Marine within the next few months. The full-size pontoon shell constructed of the lightweight composite materials will weigh just 712 pounds, and the polyester reinforced nylon collapsible gasbags plus the cell dividers will add 152 pounds. Thus the pontoon's total weight should be only 864 pounds. When that weight is balanced against the lift provided by helium in its bags, the pontoon's net lift is 1,108 pounds.
            Maximum lift will be obtained by the use of a partial vacuum outside the gasbags, allowing the helium within them to expand without opposition from atmospheric pressure at sea level and expelling air from between the gasbags and the pontoon shell. In this way, the helium pressure within the gasbags can be lowered to substantially below 1 atmosphere (14.7 psi).
            Once the pontoon is completed, it will be moved like a giant helium-filled sausage on a string to a test site; there, it will be tethered with lines connected to scales for measuring lift. The pontoon will also be hooked up to umbilical hoses managing the flow of gases (helium and air) into and out of the chambers. When all is in place, every aspect of the pontoon's function will be tested. Aspects to be tested will include:
    • lift capacity
    • filling the balloons with helium
    • drawing a partial vacuum in the chambers
    • compressing air into the chambers
    • expelling air from a chamber through downward-facing exhaust ports to produce upward thrust
    • raising one end by adding air to the end chamber and removing it from the other locking the pontoon back and forth by alternately sinking one end, then sinking the opposite end (the three central chambers would remain constant)

          These tests will provide data that will be crucial for developing the complex flight-control systems needed to manipulate the gravityplane's constantly changing weight and weight distribution. As the gravityplane flies, it will need to constantly monitor and adjust each cell's lift characteristics through the use of a computer program. This program will be constructed from scratch, starting with the results of the tests on this first module.

    Manned but tethered

    n the foundation of successfully testing and modifying the first pontoon, we plan to build the first prototype gravityplane, starting by building the second pontoon. The two pontoons will be bridged together and fitted with wings and other aircraft flight-control structures, such as ailerons and a rudder. The bridge section will be fitted with a minimal pilot's cabin, a wind turbine, and a small thruster-propulsion turbine.
            Although the unit will be manned in flight, it will be tethered to the ground at very low altitude (probably less than 500 feet). With both wings and pontoons, the prototype will provide valuable data about the gravityplane's fundamental premise that aspects of buoyant lift and aerodynamic lift can both be beneficially incorporated into the same craft.
            In many ways, this tethered device may be considered a flying wind turbine that employs both aerodynamic lift (like a kite) and aerostatic lift (like a helium balloon). When the wind is blowing, power will be generated and stored as high-pressure compressed air within pipes inside the pontoons. A portion of the compressed air will drive a pneumatic motor that will power an electrical generator to provide onboard power and charge the craft's batteries.
            One option will be to store so much high pressure (and hence heavy air) that the gravityplane's weight will overcome the combined upward force of the wings' aerodynamic lift and the pontoons' aerostatic lift, causing the craft to sink to the ground. Descent and landing could be controlled more precisely by using the propulsion turbine powered by high-pressure compressed air producing a downward thrust. Likewise, the craft can practice vertical takeoff by downward thrust that also reduces its weight as the air is exhausted through the propulsion turbines.
            After the gravityplane passes all its tests and gains experimental flight-class certification, it will take its first free flight. Ascending and descending runs in the aerostatic lift mode will eventually lead to the aircraft's initial gliding descent, in which it will attain substantial velocity of perhaps 100 MPH. The craft's relatively small size will limit its maximum altitude and hence its maximum speed.
            Assuming no insurmountable obstacles, the first prototype will likely be followed by a larger, more complete model capable of long-distance, sustained flight through the gravityplane's distinctive series of cycles alternating between buoyancy-lifted ascents and gravity-driven, gliding descents.
            Bringing the gravityplane into commercial operation as an alternative to fossil-fuel-powered aircraft will require a decade or more. The craft will be so different from any existing flying machine that we probably cannot begin to imagine all the forms it may take and the uses to which it could be put.
            For now, however, dreams of future flights must wait. The next step is clear: build and test the first pontoon.

    On the Internet

    Hunt Aviation
    Video of fuelless gravity-powered flight

    Robert D. Hunt, the founder and president of Hunt Aviation, is an independent inventor holding more than 50 patents. Earlier in his career, he worked as a nuclear designer for Newport News Shipbuilding, a division of Tenneco Oil Company.

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