The idea sprung from the brain of Robert D. Hunt, a theoretical physicist and inventor who founded Hunt Aviation to develop his patented "gravity powered hybrid aircraft" concept which operates on the principles of buoyancy, aerodynamic lift, and gravity. It uses a cycle of climbing and descending to maintain its lift and forward speed, mimicking the behavior of the bodies of warm and cold air which make up the weather.
In order for the GravityPlane to become airborne, gas bags inside a pair of rigid, zeppelin-like structures are filled with helium from storage tanks inside the vehicle. This causes the aircraft to become lighter-than-air, and it rises from the ground. Compressed-air jets on the sides of the craft add further propulsion, pushing the vehicle skyward and decreasing the craft's overall weight by releasing the stored air which acts as ballast. Once the craft reaches the altitude where the helium is no longer lighter than the surrounding air-- theoretically as high as ten miles up-- it is unable to climb any further. Some of the stored compressed air is then expanded into the dirigible areas, decreasing the buoyancy effect of the helium and starting the aircraft's descent phase.
As gravity pulls the plane towards the earth, the long wings are moved to the swept-back position to reduce wind drag, and air turbines mounted on the top of the craft capture some of the forward momentum and use it to drive air pumps which can refill the on-board compressed air storage tanks. In this gliding mode, the aircraft achieves aerodynamic lift for a gradual descent at high speeds, and can travel in this configuration for about 400 to 600 miles. At the end of the gliding phase, the wings are redeployed. The compressed air can once again be forced out through the compressed air jets, pushing the vehicle upwards and increasing the vehicle's buoyancy to lighter-than-air once again, beginning the cycle anew. This process can be repeated as many times as needed to cover the required distance.
Considering the GravityPlane's simplicity, its environmentally friendly propulsion, and its freedom from heavy and expensive fossil fuels, this concept could completely revolutionize aircraft design in the coming decades if it proves viable. And using non-flammable helium means that a Hindenburg-style disaster is not a risk. Can Hunt Aviation deliver the sparkling, rigid-airship future that zeppelins promised us so long ago? Time will tell.