There have been many attempts to understand the principles of high air pressure below hulls and wings. To a great extent, the majority of these can be termed “ground effect” or “water effect” vehicles rather than hovercraft. The principal difference is that a hovercraft can lift itself while still, whereas the majority of other designs require forward motion to create lift. These active-motion “surface effect vehicles” are known in specific cases as ekranoplan and hydrofoils.
The first mention in the historical record of the concepts behind surface-effect vehicles that used the term hovering was by Swedish scientist Emanuel Swedenborg in 1716.
In 1915, Austrian Dagobert Müller (1880 – 1956) built the world’s first “water effect” vehicle. Shaped like a section of a large aerofoil (this creates a low pressure area above the wing much like an aircraft), the craft was propelled by four aero engines driving two submerged marine propellers, with a fifth engine that blew air under the front of the craft to increase the air pressure under it. Only when in motion could the craft trap air under the front, increasing lift. The vessel also required a depth of water to operate and could not transition to land or other surfaces. Designed as a fast torpedo boat, the Versuchsgleitboot had a top speed over 32 knots (59 km/h). It was thoroughly tested and even armed with torpedoes and machine guns for operation in the Adriatic. It never saw actual combat, however, and as the war progressed it was eventually scrapped due to lack of interest and perceived need, and its engines returned to the Air Force.
The theoretical grounds for motion over an air layer were constructed by Konstantin Eduardovich Tsiolkovskii in 1926 and 1927.
In 1931, Finnish aero engineer Toivo J. Kaario began designing a developed version of a vessel using an air cushion and built a prototype Pintaliitäjä (Surface Soarer), in 1937. Kaario’s design included the modern features of a lift engine blowing air into a flexible envelope for lift. Kaario never received funding to build his design, however. Kaario’s efforts were followed closely in the Soviet Union by Vladimir Levkov, who returned to the solid-sided design of the Versuchsgleitboot. Levkov designed and built a number of similar craft during the 1930s, and his L-5 fast-attack boat reached 70 kn (130 km/h) in testing. However, the start of World War II put an end to Levkov’s development work.
During World War II, an engineer in the United States of America, Charles Fletcher, invented a walled air cushion vehicle. Because the project was classified by the U.S. government, Fletcher could not file a patent.
When the war ended a number of groups took up development of low friction water vessels including hydrofoil and water effects. The Soviets, specifically Rostislav Alexeyev and his Central Hydrofoil Design Bureau, returned to the ground effect design pioneered by Levkov, and produced a wide variety of such craft over the next 30 years. However, Alexeyev’s systems were always experimental, and never entered production. Famous among these is the Lun-class ekranoplan, a massive missile-firing boat powered by eight jet engines. Unlike Levkov’s design, however, these boats generally lacked a lift engine, using power and short wings to create lift at speed. This represented a unique evolution of craft but were also not classed as true hovercraft.
In the 1950s and 1960s in Canada, John Carver Meadows Frost at Avro Canada started experimenting with the Coanda effect and noticed that he could produce a ring of airflow by blowing the air down over a convex surface. He originally intended this to produce VTOL lift, and then use the same basic effect to provide for high-speed, high-altitude flight. This work led to the development of the Avrocar with the US Army, a design of much more modest performance more typical of a modern helicopter. In testing it proved incapable of flying more than a few feet off the ground and at speeds greater than about 45 km/h, and after a lengthy period of testing the program was abandoned in 1961.
The idea of the modern hovercraft is most often associated with Sir Christopher Cockerell. Cockerell came across the key concept in his design when studying the ring of airflow when high-pressure air was blown into the annular area between two concentric tin cans, one coffee and the other from cat food. This produced a ring of airflow, as expected, but he noticed an unexpected benefit as well; the sheet of fast moving air presented a sort of physical barrier to the air on either side of it. This effect, which he called the “momentum curtain”, could be used to trap high-pressure air in the area inside the curtain, providing lift based on pressure, not airflow. In theory, only a small amount of active airflow would be needed to create lift and much less than a design that relied only on the momentum of the air to provide lift, like a helicopter. In terms of power, a hovercraft would only need between one quarter to half of the power required by a helicopter.
Cockerell built several models of his hovercraft design in the early 1950s, featuring an engine mounted to blow from the front of the craft into a space below it, combining both lift and propulsion. He demonstrated the model flying over many Whitehall carpets in front of various government experts and ministers, and the design was subsequently put on the secret list. In spite of tireless efforts to arrange funding no branch of the military was interested, as he later noted, “The Navy said it was a plane not a boat; the Air Force said it was a boat not a plane; and the Army was ‘plain not interested.'”
SR.N1 General Arrangement
This lack of military interest meant that there was no reason to keep the concept secret, and it was declassified. Cockerell was finally able to convince the National Research Development Corporation to fund development of a full-scale model. In 1958 the NRDC placed a contract with Saunders Roe for the development of what would become the SR.N1, short for “Saunders Roe, Nautical 1”. The SR.N1 was powered by a 450 hp Alvis Leonides engine powering a vertical fan in the middle of the craft. In addition to providing the lift air, a portion of the airflow was bled off into two channels on either side of the craft, which could be directed to provide thrust. In normal operation this extra airflow was directed rearward for forward thrust, and blew over two large vertical rudders that provided directional control.
SR.N1 made its first hover on 11 June 1959, and made its famed successful crossing of the English Channel on 25 July 1959. In December 1959, the Duke of Edinburgh visited Saunders Roe at East Cowes and persuaded the chief test-pilot, Commander Peter Lamb, to allow him to take over the SR.N1’s controls. He flew SR.N1 so fast that he was asked to slow down a little. On examination of the craft afterwards, it was found that she had been dished in the bow due to excessive speed, damage which was never allowed to be repaired, and was from then on affectionately referred to as the ‘Royal Dent’.
Skirts and other improvements
Testing quickly demonstrated that the idea of using a single engine to provide air for both the lift curtain and forward flight required too many trade-offs. A Blackburn Marboré for forward thrust and two large vertical rudders for directional control were added, producing the SR.N1 Mk II. A further upgrade with the Armstrong Siddeley Viper produced the Mk III. Further modifications, especially the addition of pointed nose and stern areas, produced the Mk IV.
Although the SR.N1 was successful as a testbed, the design hovered too close to the surface to be practical; at 23 cm even small waves would hit the bow. The solution was offered by Cecil Latimer-Needham. In 1958 he suggested the use of two rings of rubber to produce a double-walled extension of the vents in the lower fuselage . When air was blown into the space between the sheets it exited the bottom of the skirt in the same way it formerly exited the bottom of the fuselage, re-creating the same momentum curtain, but this time at some distance from the bottom of the craft.
Latimer-Needham and Cockerell devised a 4 foot (1.22 m) high skirt design which was fitted to the SR.N1 to produce the Mk V, displaying hugely improved performance, with the ability to climb over obstacles almost as high as the skirt . In October 1961, Latimer-Needham sold his skirt patents to Westland, who had recently taken over Saunders Roe’s interest in the hovercraft. Experiments with the skirt design demonstrated a problem; it was originally expected that pressure applied to the outside of the skirt would bend it inward, and the now-displaced airflow would cause it to pop back out. What actually happened is that the slight narrowing of the distance between the walls resulted in less airflow, which in turn led to more air loss under that section of the skirt. The fuselage above this area would drop due to the loss of lift at that point, and this led to further pressure on the skirt.
After considerable experimentation, Denys Bliss at Hovercraft Development Ltd. found the solution to this problem. Instead of using two separate rubber sheets to form the skirt, a single sheet of rubber was bent into a U shape to provide both sides, with slots cut into the bottom of the U forming the annular vent. When deforming pressure was applied to the outside of this design, air pressure in the rest of the skirt forced the inner wall to move in as well, keeping the channel open. Although there was some deformation of the curtain, the airflow within the skirt was maintained and the lift remained relatively steady. Over time, this design evolved into individual extensions over the bottom of the slots in the skirt, known as “fingers”.
With these improvements the hovercraft became an effective transport system for high-speed service on water and shallow land, leading to widespread developments for military vehicles, search and rescue, and commercial operations. By 1962 many UK aviation and ship building firms were working on hovercraft designs, including Saunders Roe/Westland, Vickers-Armstrong, William Denny, Britten-Norman and Folland. Small-scale ferry service started as early as 1962 with the launch of the Vickers-Armstrong VA-3. With the introduction of the 254 passenger and 30 car carrying SR.N4 cross-Channel ferry in 1968, hovercraft had developed into useful commercial craft.
Another major pioneering effort of the early hovercraft era was carried out by Jean Bertin’s firm in France. Bertin was an advocate of the “multi-skirt” approach, which used a number of smaller cylindrical skirts instead of one large one in order to avoid the problems noted above. During the early 1960s he developed a series of prototype designs, which he called “terraplanes” if they were aimed for land use, and “naviplanes” for water. The best known of these designs was the N500 Naviplane, built for Seaspeed by SEDAM. The N500 could carry 400 passengers, 55 cars and 5 buses. It set a speed record between Boulogne to Dover of 74 knots (137 km/h). It was rejected by its operators who claimed it was unreliable.
Another discovery was that the total amount of air needed to lift the craft was a function of the roughness of the surface it traveled over. On flat surfaces, like pavement, the needed air pressure was so low that hovercraft were able to compete in energy terms with conventional systems like steel wheels. However, as the hovercraft lift system acted as both a lift and very effective suspension, it naturally lent itself to high-speed use where conventional suspension systems were considered too complex. This led to a variety of “hovertrain” proposals during the 1960s, including England’s Tracked Hovercraft and France’s Aérotrain. In the U.S., Rohr Inc. and Garrett both took out licenses to develop local versions of the Aérotrain. These designs competed with maglev systems in the high-speed arena, where their primary advantage was the very “low tech” tracks they needed. On the downside, the air blowing out from under the trains presented a unique problem in stations, and interest in them waned in the 1970s.
By the early 1970s the basic concept had been well developed, and the hovercraft had found a number of niche roles where its combination of features were advantageous. Today they are found primarily in military use for amphibious operations, search and rescue vehicles in shallow water, and sporting vehicles.