The first truly revolutionary hull since the ironclad is on the horizon. It is the application of aerodynamic ground effects principle to water-based vessels.
Ships and boats employing the ground effects phenomenon would scoot over the water on a cushion of air. Hulls would be as dragless relative to the water as a low flying airplane. Some startled observers see them as opportunity unlimited; some less startled observers see them as another gimcrack, not amounting to much. Time and many engineering manhours will arbitrate.
Ground effects machines do not have wings, or rotor blades of the helicopter type. A blister of air is trapped between the machine and the ground or water, causing the vehicle to lift free of the earth. Air escaping the blister is resupplied, maintaining ground or sea clearance. Once in position off the ground, there exists a true frictionless bearing with respect to the ground, and propulsion methods such as airscrews or the controlled positioning of blister chamber vanes provide the thrust to move. One machine weighing several tons, built in England, can be propelled back and forth by hand like a medicine ball, since ground drag is near zero.
As may be expected, the question has already come up: what kind of a beast is it, an aircraft, a sea vessel, or a ground vehicle? At this time the answer depends upon who is doing the talking. Technical nomenclature for these machines is just as confused. They are referred to as “ground effects machines,” “air cushion machines,” “ground proximity machines,” and “ground cushion machines.” One thing is certain: they work. Models are known to be operating in many places, both in the United States and abroad. Thus, since feasibility over both land and water is a fact, it is safe to assume that through methods from “cut and fit” to starchy academic analyses, the search is on for the best practical commercial and military applications.
The term ground cushion is not new, having been observed and tagged by early helicopter pilots who noted that a marked change in lift occurred when rotor blades were operating close to the ground. In fact, ground effects have been noticed almost since man’s first successful ventures with powered flight.
One of the earliest attempts at operating models using air cushion principles was in Finland in the middle 1930’s where a speed of twelve knots over ice was attained with a machine of eight by twelve-foot proportions. A Finnish patent was issued. This development, and perhaps others, was interrupted and delayed by World War II, and it was not until the late forties and early fifties that substantial work was resumed.
Of the several schemes for machines whose operation is dependent entirely upon ground effect induced lift, two are of primary interest. One is the annular jet in which a downblast of air is pumped through a peripheral jet which creates lift and seals the main air blister. Shapes for the annular jet machines tend to the circular. The other general type is the labyrinth seal machine which keeps the cushion air in position under the vehicle through recirculation by means of fans in series. Shapes for the labyrinth vehicle may or may not be circular.
Little is known about how water surfaces behave under the downblasts of air, or about the heights ground effects machines must operate relative to high or changing sea states. Saunders Roe, Limited, of England, builders for Hovercraft, Limited, have tested a machine of about four tons over water. Of course, they are most secretive about data obtained. News pictures of the machine show that heavy generation of spray might be a problem. Designers, however, feel that with larger machines, forward speeds of perhaps 100 knots would preclude gouging of the sea surface and its attendant spray.
In this country, since 1954, so many Government technical agencies and offices have actively studied the ground effects phenomena that a special Department of Defense Ad Hoc Committee has been established to co-ordinate the programs. The Committee was also fitted to ajudicate the avalanche of design proposals from industry and do-it- yourselfers.
For the Navy, the Bureau of Naval Weapons, David Taylor Model Basin, the Office of Naval Research, and the Bureau of Ships have been most active in exploring both feasibility and application of the machines. While most of these projects have been in the vein of pure research studies and calculations, the Model Basin has built a machine which has successfully operated over land. The Navy also has a contract with a commercial vendor to obtain a one-man flyable test vehicle.
Other representative study and limited “hardware” contracts let by Navy include studies for very large oceangoing ground effects vessels, applications of the catamaran design to air cushion machines, and the investigation of varied hovering, forward flight, and maneuvering characteristics of small test machines.
Since the ground effects principle works for all size vessels, practical application appears to be most flexible. Proposed uses have run from extremely small automatic unmanned devices to serious proposals for 100-knot aircraft carriers.
On the military side, both land and water, many ideas have come to the front.
For amphibious assault vessels, the design criteria dictated by the mission calls out a low silhouette and a medium load-carrying ability. It is thought that ground effects vehicles could meet the requirement, with added advantages. They could carry a payload comparable to present amphibious craft plus speeding up the ship to shore movement from mother vessels many fold. They would do up to forty-five knots, be able to negotiate breakers of respectable height, and maneuver on beaches of up to 15% slope.
A smaller “boat” might be useful as a “flying jeep” or “recon car.” Such an air cushion car may attain 100-knot dash speeds and zoom altitudes of fifty feet by juggling its weight-lifting capacity to attain maneuverability. One- or two-man vehicles with observation gear and 105-mm. recoilless rifles look practical.
It is also proposed that any mobile military force could use an all-purpose airborne truck. A machine operating at ground clearances of three to ten feet, and carrying loads comparable to heavy trucks or small barges, is one scheme. Although it would be prohibitive in very tough terrain or jungle, it could whiz along at sixty knots over unimproved roads, riverbeds, rivers, swamps, bays, and other terrain of minor irregularity.
Perhaps the most attractive over-sea application for these machines is in anti-submarine warfare. The ground cushion principle makes available a highly mobile automatic platform which could land on the water, listen, takeoff, track, and destroy the submerged submarine. Detection devices and kill stores could be readily accommodated since speed requirements would be scaled down to slight advantages over known underwater speeds of the hunted sub.
The most advanced thinking along the lines of very large oceangoing warships or transports using ground effects has come from abroad, from England and Switzerland. Both the English and the Continental press have described as feasible air cushion supported platforms of 1,000 feet in diameter capable of 150 knots. Since the closer to the water these machines operate the greater their weightlifting efficiency becomes, the larger designs gain in attractiveness because they would cruise at maximum efficiency. That is, for the large machines, “close” to the water would be about fifty feet, giving adequate wave clearance in heavy seas. In proportion, smaller machines would find their most efficient height from inches to two or three feet, making turbulent seas a real problem for them.
Most of the large machine designs show external prop or jet propulsion systems on the weather deck, independent of the fans which create the cushion underneath. One such circular dream scheme from Switzerland is over 300 feet in diameter, has passenger accommodations for 1,000 persons, weighs 40,000 tons, and would cruise at 100 knots. Incidentally, the magic figure of 100 knots speed for large transports is the figured minimum to make a clear cut revenue advantage over existing conventional oceangoing transports. Atlantic crossings would be one and one half days.
On the commercial side, just about everything from the above mentioned passenger liners to swamp buggies has been talked about. There is no way to tell how many basement and backyard shops will launch small ground effects machines. There will be many.
One of the first public mentions of the air bearing plan in this country was the proposal for a wheelless air cushioned automobile. Working with very close tolerances, the same idea has been applied to rail transportation which has railroad rolling stock riding a mat of air sealed between the rails and the carrier. Other cargo and personnel carriers would convert every plain, icefield, riverbed, and swamp into ready avenues of transportation, say the ride-on-air apostles.
Both in the Department of Defense and in industry, those who have looked at the machines “have seen enough” to consider seriously more sophisticated hardware applications. But this does not minimize the many problems to unravel and the many questions to answer. Much investigation of the dynamics of the air blister, the sea cavitation caused by downblasts, hovering and forward motion, and the tailoring of platforms to particular missions is yet to be done.
Answers to these problems will in general govern the extent of application. The need or requirement for machines of such speed and mobility over a variety of surfaces is apparent. Navy is in on the ground floor of development activity. It is anybody’s guess whether the skeptics or the zealots will have the last word.