The Airfoilboat Story
What is it that floats like a ship, glides over land and water like a hovercraft and flies like an aeroplane? It is unlikely as yet that many people would come up with the only correct answer to this riddle - the 'Airfoilboat'. Brainchild of VFW-Fokker affiliate, Rhein-Flugzeugbau GmbH (RFB), it opens up new prospects in the field of military and civilian aviation. Thanks to its favorable aerodynamic properties, fuel consumption is approximately 50% lower than for comparable aircraft types.The Airfoilboat glides over the surface of water, resting on an air cushion created by the streamlined wings. However, although it functions on the same principle as the hovercraft, its commercial utility is four or five times greater. Initial testing carried out this spring over the Baltic, with experimental model X-114 fully confirmed the high expectations of the Monchengladbach development team.
The Airfoilboat is a brainchild of no less a personage than Dr. Alexander Lippisch, world-famous in the aerodynamics field until his death in 1976. In the early 'thirties, he developed his first delta-winged aircraft, far ahead of its time. This model was the foundation stone for the rocket aircraft Messerschmitt Me 163 and for further development of the slim delta constructions of smaller caliber with which Lippisch established the basic principles of supersonic flight during the 'forties.
After the war, this aviation pioneer worked principally in the USA where he devoted his energies to solving hydrodynamic as well as aerodynamic problems. In the early 'sixties, he designed for Collins Company a very fast motorboat which was initially tested in the towing basin. The trials showed that the dual-float principle (catamaran) was well suited to high-speed performance. However, on attaining a certain speed the models tended to unstick, like racers. This brought a typically laconic comment from Lippisch: "If the things insist on flying, let them fly".
Around this period, NASA published some test readings on the performance of the thrust wing patented by Finnish designer T. J. Kaario in 1935. Lippisch decided to investigate ground effect, both in theory and in practice. And so it came about that a simple, two-seater airfoil type, the X-112, was built in the hydrodynamics laboratory of Collins Radio Co. A series of test flights, carried out through the autumn of 1963, served to confirm the validity of the principle. In order to distinguish the ground effect craft from the hydrofoil craft, Dr. Lippisch called it Airfoilboat.
The early model was already equipped with the reversed delta wings, in negative V-shape, spanning the hollow enclosed by the trailing edges of the wings. The dynamic pressure created by the air streaming into this hollow not only serves to stimulate thrust, but constitutes an air cushion which reduces resistance and acts as an automatic stabilizer when the machine is flying close to the ground or gliding over water. An important factor here is coordination between the horizontal stabilizers and the lift wings, upon which stability depends. Until then, this had been one of the unsolved problems of the airfoil principle.
In 1966, Dr. Lippisch met Hanno Fischer, Technical Manager of RFB. Lippisch realized almost immediately that this company in Mönchengladbach (now a member of the VFW-Fokker group), would be the right partner for further development of the Airfoilboat: RFB were not only specialists in the light aircraft sector, but had also shown considerable talent for improvisation.
Another factor which attracted Lippisch was his search for a solution to the raw material problem. The X-112 prototype was built of balsa wood which, in contact with water, becomes useless within a few weeks. For some time RFB had been successfully using plastic reinforced with glass fiber (GFK) for aircraft construction purposes and were pioneers in this sector. Thanks to this combination of circumstances, a highly satisfactory partnership was formed whereby Dr. Lippisch became Scientific Manager for Airfoil Technique with RFB.
During the same year (1967) RFB received an order from the Federal Defense Ministry to carry out model testing. The background to this request was the US Navy 100-kt requirement (still unsatisfied) for an amphibious craft, capable of flying under a radar screen at a speed of roughly 180 kmh. The prospect of achieving this goal by means of a ground effect craft aroused the interest of the military authorities and led to granting of government subsidies.
The RFB team commenced intensive research on free-flying remote-controlled models, principally in order to study take off performance from the water and longitudinal stability. At the moment of take off, it is a question of transition from watercraft to free-flying aggregate, via the ground effect stage. Performance tolerances here are three times more stringent than for ground effect flight alone. Constructional design must, therefore, center on the requirements at take off. Tests were carried out with both catamaran and trimaran types weighing up to 15 kg. Results showed that the trimaran concept, consisting of a fuselage and two supporting floats, was better suited to operations in calm waters.
These conclusions led to building of a single-seater, experimental model, the X-113 Am. According to Hanno Fischer, the main problem was to design a construction with the tensile strength required for a superfast speed-boat, but which would also conform to the stringent weight specifications for aircraft. The final choice was the GFK sandwich construction which can be adapted to conform with the various tensile specifications for individual elements. RFB developed a tube construction system, which has since been patented, intended specifically for the carrying elements. The tubes are manufactured, according to a process developed by RFB, in the shape of square profiles which are individually sealed to make the craft unsinkable if damaged.
Project Leader D. Schoenfelder and his team started testing the X-113 Am over the Lake of Constance in October, 1970. The tests went remarkably well. To quote Mr. Schoenfelder: "The fact that pilots were able to operate the machine without difficulty on the basis of a short briefing prior to taking over, confirms how simple and problem-free it is. Not only were they able to steer the craft over the surface of the lake, but had no trouble in taking off for free flight from the transitional ground effect stage".
The success of these initial tests made RFB decide to undertake a further series in troubled waters, although this did not form part of the original program. This took place in the autumn of 1972 on the estuary of the Weser river and the first obstacle to be overcome was the skepticism of the shipping authorities who looked upon this 'flying fish' as a navigational danger. The remarkable maneuverability and precision of the experimental craft soon caused them to revise their opinion and lift the initially imposed restriction. "If necessary" said Mr. Fischer, "we could fly up and down the Niagara Falls".
Flights were carried out up to a height of 800 m as well as close to the surface of the mud banks in the shallows. On the basis of this test program with the X-113 Am, it was possible to reach the following conclusions:
The machine is inherently stable in ground effect and automatically stabilized when in the air. Consequently, the pilots do not have to cope with the orientation problems which normally occur when flying low over water.
The Airfoilboat can land automatically from free flight: it engages in ground effect and glides down. This makes for excellent landing chances in emergencies, on a similar principle to that of gliders.
The very favorable lift-drag ratio of approximately 1:25, with an aspect ratio of 1:7, is a considerable improvement from the commercial angle. The lift-drag ratio corresponds roughly to that of a glider, so that the Airfoilboat requires only about half of the propulsive output needed by an aeroplane.
However, certain questions of principle still remained unanswered because the size of a craft has to be in proportion with the anticipated height of the waves. In order to test ground effect performance as well as landing in heavy swell, it was necessary to develop a larger Airfoilboat: the X-114. An order was received from the Federal Defense Ministry in February 1975 and the basic requirement for the new model was capacity to perform efficiently under typical Baltic Sea conditions with waves of 1.5 m in height, corresponding to a 3-4 swell.
The six-seater X-114 is 12.8 m long with a wingspan of 7 m. It can develop a maximum speed of 150 kmh with a flight duration of roughly 20 hours and a range of approximately 2,000 km. The maximum take off load is 1,500 kg. A four-cylinder Lycoming engine activates a cap propeller on the rear section of the fuselage. When fitted with a special, detachable landing gear, it can also serve as an amphibious vehicle. The successful GFK construction was taken over from the X-113 Am and the X-114 can take off and land equally well on grass, concrete, ice or snow.
Test pilot Dr. Volkmar Wilckens took off on April 15, 1977 for his first flight and, in the course of the next few weeks, the steering mechanism, stability, maneuverability and gliding capacities were exhaustively tested. From the technical point of view, the Airfoilboat can be compared neither to a hydroplane nor to a ship. Consequently, none of the existing building specifications and local calculations were applicable. This meant that particular attention had to be focused on testing various structural ratios. A newly developed measuring device proved helpful in determining the exact height and length of waves. It can also be used as a precision altimeter, with a maximum deviation of +/- 5 cm.
Dr. Wilckens, who had also flown the X-113 Am over the Weser estuary, was scientific leader of the flight test program. He carried out approximately 25 starts and completed a total of 7.5 flight hours. All test readings were meticulously registered on measuring devices and pre-amplifiers on board. Since the new amphibious vehicle constitutes a fast and economical means of transport it lends itself to a variety of purposes. Examples in the civilian sphere are as a police patrol boat, customs cruiser, ferry or freighter, as well as purely for sport.
Apart from the fuselage, the delta wings can also be used for seating passengers or stowing freight. To take an example, a craft with a 20 m delta span has a wing thickness of 2 m at its base. The Airfoilboat is furthermore ideally suited for bridging the gaps in the infrastructure of underdeveloped countries: waterways are natural traffic arteries which present no obstacles. The latest model already exists on paper as the RFB-215 and countries such as Canada and nations in South East Asia with their island realms have shown interest in it.