DARPA selects competitors for its Liberty Lifter seaplane project• https://newatlas.com, By David Szondy
In 2022, DARPA announced its project to develop an aircraft, called the Liberty Lifter, with the size and capacity of a C-17 Globemaster III transport aircraft, yet could lift over 100 tonnes of payload. That's pretty impressive, given that a C-17 can only manage about 77 tonnes on its best day, and that the Liberty Lifter is supposed to be a seaplane with a ferry range of 6,500 nm (7,500 miles, 12,000 km). That's enough to fly from the North Pole to the Equator with a bit to spare.
The secret of this performance is what is called "ground effect" or "wing-in-ground effect," which is an esoteric aerodynamic phenomenon that was at the center of one of the great mysteries of the Cold War.
In the late 1960s, American spy satellites watching the Soviet Union saw a strange, very large aircraft tearing about the Caspian Sea. Dubbed the Caspian Sea Monster by the intelligence community, it had analysts scratching their heads because this monster of an aircraft weighing over 500 tonnes had thick, stubby wings that couldn't possibly support it in the air.
It turned out that the mystery craft was an ekranoplan, which were a series of ground effect vehicles being developed by the Soviet military that could evade radar detection while carrying a heavy missile load by flying at very low altitude.
It was the very low altitude that was the key. Ground effect occurs when an aircraft is flying very close to the ground or, preferably, water. Without going into too much technical detail, when an airplane is moving forward at low altitude, it acts as if a cushion of air is trapped between it and the ground. As a result, drag is reduced and lift is increased, so the aircraft can either have smaller wings, carry a heavier load, or some combination of both.