LOTTE airship

LOTTE airship is the world's first autonomous solar-powered airship and has been certified under the registration number D-UISD, which was developed in close cooperation with the Federal Aviation Authority in Braunschweig, Germany, as there were no guidelines for the certification of solar-powered airships at the time of its construction. The world's first autonomous solar-powered airship was built in Stuttgart (Germany) by Prof. Bernd Helmut Kröplin and his team. The LOTTE airship has been operated since 1994 by Airship Technologies GmbH, a company founded specifically for this purpose.

The Institute of Computational Structural Mechanics for Aeronautical Engineering of the University of Stuttgart (Stuttgart University) started the research program of "LOTTE-1" airship in 1991, and the airship was successfully test-flown in April 1993. In 1996, the University of Stuttgart started a three-year research program to carry out relevant theoretical research and technical tests using the "LOTTE" airship as a test platform. In 1996, the University of Stuttgart began a three-year research program to carry out theoretical research and technical tests with the "LOTTE" airship as a test platform. Historical design concepts for rigid or non-rigid airships could not produce the small airships that people wanted. Rigid airships have the disadvantage of high structural weight, whereas non-rigid airships cannot generate such high internal pressures and require less energy to stabilize the cladding. The result of this work is a new design concept, the quasi-rigid airship.

"LOTTE" was officially handed over to the Zeppelin Museum on November 27, 2014.

Development history
At the end of 1991, a team of students, scientists and teachers was formed, and about 20 months later the first flight of a airship called LOTTE 1 was successful. Basic experience was gained with the design and flight characteristics of the airship.

Unfortunately, a severe thunderstorm in the summer of 1993 destroyed the airship. Over the next six weeks, the airship was rebuilt with some minor modifications, LOTTE 2 was mainly reduced in structural weight, with new cladding and new fins , and its payload was increased to 8 kg. In November 1993, it participated in the World Solar Challenge in Australia and flew about 330 kilometers in four days. On the fourth day of that challenge, the radio control system interfered with the ILS locator at a nearby military airfield, and the airship lost ground control, disabled its internal safety systems, and finally landed in a tree. The high tail winds at the time (10 - 15 knots) resulted in severe damage to the airship, which could not be repaired in the Australian bush.

The structural components of the LOTTE 3 airship are almost identical to those of the previous two generations, but with new electronic systems, advanced radio control and energy conversion systems. The solar generator and batteries are now modular in design, which allows the maximum payload to be increased to 12-15 kilograms, depending on flight requirements. LOTTE 3 completed certification testing in 1995.

Range of tasks that can be applied
Aerial photography, traffic monitoring, urban planning tasks: geological monitoring, accompanying inland waterway transportation, port entrance monitoring, environmental monitoring (smoke measurement), air pollutant detection, early warning systems for marine pollution, forest fire monitoring border surveillance, pipeline control.

Practical applications of LOTTE
Within the scope of the measurement program EVA (Emission Data Set calculated by evaluating the city of Augsburg as an example), the proportions of various gases in the ambient air were measured using the solar-powered airship LOTTE, which is the only type of vehicle capable of precisely following a specified route at a given altitude without emitting pollutants on its own. At the same time, LOTTE records its own position and speed, as well as wind speed and direction. The solar-powered airship LOTTE was used for research purposes at the University of Stuttgart and as a measurement platform for other institutes. Within the framework of the Integrated Research Group-Loft (FOGL), LOTTE was further developed as a test bed for multifunctional measurement projects. Two measurement instruments - platforms with their own power supply and interfaces to the on-board data infrastructure - are able to perform very different measurements on the airship.

Design concept
The LOTTE is controlled by a pilot on the ground via two redundant radio links that control the aircraft even if one frequency is disturbed. In order to ensure the safe operation of the airship, it is essential to be able to continuously monitor all important parameters of the solar airship. This control is carried out autonomously on board through the on-board electronics and the data downlink to the ground station For the data of the so-called "payload", i.e. the measuring instruments and cameras on board, another downlink exists, so that in addition to the on-board recordings, the data on the ground can be immediately controlled and processed.

The electrical power supply of the airship must always be ensured, not only for the propulsion system, but also for all other components such as the steering gear, balloon ventilators, balloon valves, the payload supply and the complete on-board electrical system for the control and monitoring of the ship, and must therefore also be monitored from the ground. The photovoltaic energy supplied through the solar cells provides constantly changing power due to variations in radiant power (cloudiness, time of day) and the constantly changing orientation of the solar cells towards the sun during flight. In some cases, the power output fluctuates constantly, even between zero and maximum values. As a result, the power is fed into an accumulator and is temporarily stored there. As a result, the solar airship LOTTE can fly even in absolute darkness until the accumulators are exhausted.

The propellers have electronically commutated motors and the propellers are made of fibers to enhance their structural strength. Current from the boat's batteries provides propulsion to the electric motor at the stern. The motor's thrust can be rotated around both the transverse and longitudinal axes. This thrust vector control is used for maneuverability at low airspeeds, where the effectiveness of the tail unit and rudder is reduced. The arrangement of the propulsion system and the tail unit in the aft area inevitably leads to the arrangement of the heavy flying accumulator in the bow area, since the overall horizontal balance of the airship must be maintained.

The buoyancy gas is helium, which is stored inside the airship's bladder. Dynamic pressure compensation is achieved through two inflatable balloons as the gas volume expands or contracts due to changes in air pressure and temperature. These also allow adjustment of the flight attitude. In case of emergency, a helium valve is installed which opens automatically when the airship goes out of control. The airship was built with a semi-rigid structure. The hull acts as a pneumatic support structure supported by rigid elements, today known as Tensegrity.

The rigid structural components of the airship are the nose structure, the system shaft, the center shaft, the aft unit cross shaft and the stern pan with engine base. The load bearing stresses in the hull are generated by internal pressures. In the bow region, a rigid bow cap with additional trusses provides the necessary stiffness. In the stern region, the unit consisting of the stern unit cross, connecting rods and the stern pot forms a rigid unit that can introduce propulsive forces due to the thrust vector and inclined to the centerline as well as the stern unit forces into the hull.

Quasi-rigid structure
The structure of the solar airship LOTTE is based on a quasi-rigid structure. The internal pressure is stabilized by two balloons, which are also used for the static fine-tuning of the airship. The pressure is generated by an electric blower and measured electronically.

Ultralight airship envelope
The envelope material is a composite foil using Tedlar as the outer skin and Mylar as the gas barrier film. The different parts of the envelope are bonded together. In order to determine the reliability, extensive tests were carried out on the material and the adhesive, including high temperature tests, UV radiation tests and fatigue tests. The results of the tests showed that the envelope was very thin but had very good structural properties.

Thrust vectoring propeller
The propellers have electronically commutated motors, the propellers use fibers to enhance their structural strength, and the motors and propellers can rotate around a vertical axis.

Solar cell-integrated airship hull
As the world's first solar-powered airship, this airship leads the future development direction of new energy airships. It is of great significance for the development of new energy airships.

Design significance
The autonomous, unmanned solar-powered airship is equipped with a photovoltaic energy supply that provides sufficient capacity for the deployment of the airship. A number of missions in recent years have led to the optimization of the technology used in the airships and to significant new team experience in their operation. The airship has been optimized by a large number of research activities in the field of airship technology and by its use as a flight measurement platform in many scientific and security fields. This type of solar airship is one of the best tested airships in the world. The solar airship is able to fly precisely at a certain altitude without emitting any emissions.

Impact on the Worm Blimp (Airworm)
Based on LOTTE, the research team proposed a segmented Airworm concept (shown in the figure) to solve the structural weight problem of high altitude airships, starting from the hull structure to realize the structural lightweighting. Airworm divides the whole hull into a number of interconnected bladders, with spherical secondary airbags inside the bladders, and realizes the control of airships by controlling the segmented bladders individually. control of the airship through individual control of the segmented bladders. This design solution reduces the internal and external pressure differences required to maintain the shape of the bladder.

Research team
Flying with light - that was the vision of Prof. Dr. Bernd Helmut Kröplin and his students and assistants, who designed and built the world's first solar-powered airship in 1992. Even today, the solar-powered airship "LOTTE" is still regarded around the world as a model for floatation technology. After many missions in the service of science and participation in solar races in Australia, she became a true aviation history on November 27, 2014 at the Zeppelin Museum in Friedrichshafen and was officially handed over to the Museum of History as a gift.

Overall evaluation
On the occasion of the handover of the solar-powered airship "LOTTE" to the Zeppelin Museum in Friedrichshafen, an article was published in the "Sudkurier".

Prof. Bernd Helmut Kröplin recalls: "When we built the solar airship "LOTTE", there were no suitable lightweight solar cells for the hull. Therefore, we milled and processed the existing solar cells to make them lighter. These were the first unofficial "thin-film solar cells", so to speak. Today, one might be able to use foil solar cells in a subsequent model of the first solar airship; we were the real pioneers of development at that time".

The idea was to show on an unmanned airship that a photovoltaic energy supply could provide sufficient capacity for the airship's rational use. The 16-meter-long, 98-kilogram lightweight solar airship proved this during several scientific measurement missions (smoke detection, weather data logging) and during the Solar Race in Australia in 1993. With a maximum altitude of 1,000 meters and a top speed of 45 km/h, the solar airship was a star visible from a distance at the 1993 International Horticultural Exhibition in Stuttgart.