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STINGRAY USES AIR FOR ITS STRUCTURE
by Christopher HessA crowd in St. Stephan: At the end of May crowds of aviation enthusiasts and reporters make a pilgrimage to the remote village in the Bern Oberland, Switzerland, to witness the first public presentation of a new aircraft. An informed Zurich daily newspaper had already printed a first report on the Bern UFO, which in fact is a mixture between aircraft and airship.
The former military airport of the Swiss Air Force, which is situated closely to the village, seems to be the perfect background for the StingrayÕs premiere. When the heavy bunker doors of the hangar opened, the result of seven years of strictly confidential work were revealed. We wanted to work without disruption, the Swiss inventor and project manager Andreas Reinhard explains the work in secrecy.
In 1991 Reinhard took on the task of developing a technology bed, the main component of which was to be air. Client was Festo, one of the world's leading companies in the area of pneumatic applications headquartered in the South German town of Esslingen.
Why building with air? For Reinhard it is one of the world's most universal building materials. Apart from this, "air is available everywhere, it just needs the right wrapping." The main difficulty with conventional inflatable bodies is that they automatically assume a round shape, as soon as they are inflated.
The scientist found the solution to this problem in nature. Hollow birds' bones are a model for inflatable structures. Bone substance is only found where it is functionally needed for strength. Apart from this its only task is to keep the weight saving material air in place. The transfer of this bionic principle into a new technology needs light fabrics made of modern fibres, that are extremely strong.
According to Reinhard, aviation is the ideal application for this kind of construction. He is convinced that in no other area, the relationship between weight and strength plays a more important role. The decision-makers of this project decided at a very early stage that the intended technology carrier was going to be an aircraft.
The idea behind the technology bed was to build a pneumatic wing which is fixed onto a fuselage with undercarriage, piston engine and cockpit. The aim was to develop a flyable prototype which would be the basis for a second and much bigger pneumatic aircraft for passenger transportation.
Reinhard and the colleagues of his firm prospective concepts set to work. The first models of the wing were tested in the water tunnel of a research centre in Kiev. Wind tunnel tests were accomplished in Moscow. The research group was supported by Ukrainian experts in aerodynamics.
In the late autumn of 1995 the "Stingray" was finished. It has a wingspan of 13 meters, a length of 9.4 meters and a volume of almost 70 cubic meter of air. The wing with its individual segments is completely made of polyester with a nylon component and weighs roughly 80 kg. Only in the centre of the wing there is a small carbon structure, which serves as a fuselage attachment.
The Stingray has two rudders and a two-part elevator. The attached ailerons are positioned at the wing's two trailing edges.
The pneumatic system to inflate the wings works with two redundant compressors. The pressure in the wings is surprisingly small. In order to take its shape the skin needs a pressure of only 20 to 50 Milibar above the outside air pressure. This corresponds with one to two per cent of the pressure of a car tire.
However, the compressors are powerful enough to compensate for a 20 cm by 20 cm hole in the skin of the wing. Even when the pneumatic system fails, Stingray is reportedly able to stay in the air for another 20 minutes. Just in case the craft is equipped with a parachute recovery system.
Test flights took place in the remoteness of a former Russian military airbase in Hradcany, Northern Czechia. Gion Bezzola, test pilot and co-founder of the Aerobatics Team Patrouille de Suisse, was responsible for the flying part the test flight program.
At first, the craft was extremely nose heavy and was unable to take off. After the centre of gravity had been adjusted (initially achieved with mineral water bottles in the tail of the aircraft) the first flight was completed on 23 November 1995. During the winter months the configuration of the fuselage was modified. The Pilot's workplace, which had been open until then, was replaced by a semi-closed cockpit structure. The engine was also altered. Both Rotax-582 engines with an output of 47 kW each, now turn push propellers.
During further test flights, Bezzola reached a flight altitude of several hundred meters and speeds up to 130 km/h. Up to now Stingray has flown more than 180 times.
In 1997 the test campaign was moved to the Swiss military airport in St. Stephan. This venue is currently in a state of change. The Swiss Air Force is given it up, and is turning it over into a civil use airfield.
Prospective concepts plans to build a hangar on the airport. It will be the home for Singray's much bigger successor which will be built in the coming years. ReinhardÕs team is more than ready for this massive project.
With a wingspan of 26 meters Stingray II will have room for 12 to 14 passengers. The cabin and the engines, (probably water cooled diesel engines), will be integrated into the wing.
The Baby-Stingray, as the current technology carrier is called internally, is still made of a relatively conventional polyester material. However, Stingray II's entire pneumatic structure is going to be manufactured from new liquid crystal polymer fibres. LCP is ten times stronger than steel and two and a half times stronger than the super fibre Spektra.
Unlike common fabrics that are woven at right angles, the high-tech LCP threat can be used in multi-axis directions, allowing a true adaptive airfoil structure for the first time. The big "Stingray" will fly without conventional control surfaces, according to its designersÕ plans. With controlled pneumatic inputs in individual segments of the airfoil, Stingray II will be able change its airfoilÕs shape continuously, making it possible to steer around all three axis.
The airfoil of Stingray's big successor will most probably be partially filled with helium, which will generate a third of the required lift. A special starting device, the so-called "soft catapult" will accelerate Stingray II up to the necessary lift off speed. The wheels will not be rolling on the ground.
For Andreas Reinhard, all technologies, which he already tested on separate prototypes, are coming together in Stingray II, Baby-Stingray being only one of these. The function of the catapult was already demonstrated by his team as early as 1994 with the catapult device sting and the kangaroo aircraft, designed for extreme slow-flight.
The functionality of inflatable aileron and landing flaps, controlled via a closed pneumatic system has been successfully shown with "Pumpolino", also featuring a suspended, pneumatic wing that is mounted on the fuselage of an ultra-light aircraft.
The research scientist is convinced that the Baby-Stingray has fulfilled its purpose as a window to a new technology, which uses highly resistant fibre materials and air pressure. For Wilfried Stoll, Chairman of the Board of Directors at Festo, Stingray was first and foremost a learing exercise. All in all Festo has already invested more than DM10 million into the research of pneumatic structures.
It was important for Stoll to derive other commercial applications from the Stingray project. From approximately 400 so far, 20 are in near reach. For example, Festo was able to develop a pneumatic muscle for the automative technology which is planned to be presented in 1999 at the Hannover Industry Fair. Stoll is convinced: We would not have been able to do this without Stingray.
Neither Reinhard nor his German inventor is keen on actually producing aircraft. "We are only interested in selling technology, not planes," the Swiss inventor stresses. This does not change the immense public interest that his project has attracted. The small mountain village in the Simme Valley will be in the spotlight again, when the big Stingray is rolled out of his hangar.
From page 56 of FLUG REVUE 8/98
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