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ICARÉ 2 WINS SOLAR PRICE AT ULM

With an extraordinary technical achievement the team from the University of Stuttgart won the Berblinger Prize of the city of Ulm. Icaré 2 (the name is a combination of Icarus and Ré) was the only solar aircraft of all participating teams which could take off twice, test pilot Werner Scholz at the controls, from the army airfield in Laupheim. This meant a superior victory and a cheque of DM 100.000.

With the Berblinger Prize, Ulm is honoring every two years outstanding innovations in the area of General Aviation. This time the task was especially difficult and the participating teams were given four years to accomplish it. Along with being able to carry one person, the solar aircraft was supposed to fulfill the following requirements:

• Horizontal flight at 500 watt per square meter of solar radiation
• three-axis controlmaximum speed greater than 120 km/h
• stall speed less than 60 km/h
• glide ratio (without propulsion) greater than 20
• climb altitude of 450 m within 225 seconds after take-off.

These requirements called for extraordinary technological efforts. The aerospace faculty of the University of Stuttgart decided three years ago to participate in the competition for the Berblinger Prize. The requirements for the solar aircraft were first analyzed in the scope of several dissertations, revealing that they were at the edge of the technical feasibility. Some core technologies had to be newly developed:

• Integration of the solar cells into the flexible wings,
• development of a highly efficient propulsion system,
• extreme lightweight structure to maintain the day-to-day utilization of the construction,
• design of an aircraft configuration with the lowest possible thrust requirement.

The construction phase began with the evaluation and selection of different possible aircraft and propulsion configurations. While the decision to use a conventional aircraft design was soon made, the selection of the propulsion system was more difficult. Following some detailed studies, a motor driving a pusher propeller, located in the crossing point of the T tail seemed to be the optimum configuration.

In the spring of 1994, the team decided to build a proof-of-concept aircraft. The only purpose of this IcarÈ 1 was to glide in low altitude over the runway. Some construction methods of the real solar aircraft had been tested with this first airframe. The work on IcarÈ 1 was terminated soon after its first flight in May of 1995 to concentrate the team's capacities on the solar aircraft.

All the efforts of the University of Stuttgart were however limited by he available funds. The situation cleared only after Artur Fischer decided to support the aircraft project. The generous gift from the honary senator of the university was the initiation for more sponsors to join the project, thus securing the realization of IcarÈ 2.

The aircraft was built as much as possible in accordance to JAR 22. The solar aircraft features a three-part wing with a span of 25 meters and a wing area of 26 square meters. The entire aircraft is made of a sandwich construction, with a core of nomex honeycomb, covered by carbon fibre. The mold of the ASW 15 glider was used for the front fuselage. The construction team developed a special wing airfoil which was to allow for the specified airspeed range, but at the same time was not to have to much camber, in order to facilitate the integration of the solar cells.

The propulsion system is IcarÈ 2's technological heart. The solar generator is comprised of eight strings with 360 monocrystaline silicon cells each. The team used cells with 17 percent efficiency, costing a total of DM 150.000. Each percent efficiency increase would have effectively doubled the costs.

Each string is individually governed by a so called maximum power point tracker. The energy supplied by the solar cells is accumulated in 376 nickel cadmium batteries having a total capacity of 747 watt hours.

Newly developed electronics allowed for optimum control of the electric motor. The motor is a linear motor with transverse flux generating 12 kW of power and weighing 15 kg including the electronics. It was specially constructed by Professor Weh from the University of Braunschweig and has the exceptional efficiency of 92 percent.

The two flights of Icaré 2 on June 7 showed that the aircraft truly deserves its name. Considering that the aircraft is indeed offering the day-to-day usability required in the specifications of the competition, makes the aircraft a real technological sensation.

The project was accomplished in almost 50.000 working hours by a team of 40 people, as well as sponsor funds totaling 1,4 million Marks. The students from the University of Stuttgart now have a solar aircraft available that may be the basis for further developments. Two directions seem possible. The university sees the most important application in a weather platform, operating at an altitude of 20.000 meters and having a nearly unlimited station time. This platform could possibly even take over duties from satellites. Another application may be the development of a new generation of environment friendly solar/electric-powered motor gliders.

From page 58 of FLUG REVUE 9/96