The construction of the LZ120 Bodensee 1:50 semiscale By the Windreiter Team Introduction: The LZ120 Bodensee was the first airship of the Zeppelin Reederei after World War I. In many ways the Bodensee was an outstanding Zeppelin. After lot s of war airships, the Bodensee was a Zeppelin fully aimed for civil purpose. It carried a lounge-like passenger bay and performed a regular service for several years. Further it was the first Zeppelin that was built with a beautiful streamlined shape and equipped with 3 engine gondolas which propelled the Bodensee to relatively high speeds. But the most fascinating feature of the Bodensee is the relatively small size for a rigid airship. Just 120 m long it is one of the shortest rigid airships ever built and still had about half of its weight as usable payload. The Bodensee was the first of a couple of outstanding Zeppelins like the Graf Zeppelin and the Hindenburg that where built in the following decade until the Hindenburg disaster and the second World War stopped the marvellous age of the Zeppelins. This lead to the wish to dive into the secrets of this Zeppelin and to build a model of the Bodensee, able to fly. This documentation is about the huge problems we faced during the construction and how to make the Bodensee finally flying. The whole story was covered by DMAX Die Modellbauer Luftschiffe and is available as a movie (german language). Calculations: The bigger an airship is, the better. However we had a few constraints in size, since the filigree final product had to be carried a couple of hundreds kilometre to the place where the virgin flight should take place. Since a rigid airship can not easily be deflated like a blimp, it has to be carried in a box. This box was available from another project and has roughly 2.5 m x 0.5 m x 0.5 m. So our model of the Bodensee must fit in that box. To calculate that, we employed an excel calculation tool based on the
work of Johannes Eissing. With a total length of 2.5 m and a maximum diameter of 0.5 m an internal volume of about 300 l can be approximated. This means the total airship model including all electronics and battery must weigh less than 300 g. Additional the city of Friedrichshafen, where the presentation should take place is about 300 m above our location (Düsseldorf) which further lowers the lift. We calculated this by the Windreiter Lift Calculator and in Friedrichshafen the remaining lift will be about 287 g. The total weight of the Bodensee can be splitted in different parts: - Internal Structure - Outer Cover - Inner Gas Envelope - Electronics - Dead Weight (Gondola, Paint, etc.) - Trim Ballast Construction: We tested various different ways of building the airships main body and in the end decided to use a balsa wood framework. This frame consists of 8 gores, each covered with thin paper. These 8 gores connect to each other via central rings. After all 8 gores are connected, the inner rings are removed to free the space for the big single gas envelope. This is of cause not perfectly how the original Bodensee was built, but it is a way of construction which allows a nice shape, close to the shape of the original, while using a minimum of weight. Each gore was made after a template that was painted to our working table. This way each gore was perfectly identical. The material for the gores is 1 mm thick balsa wood cut to 5 mm wide bars of 1 m length.
On the same table each gore was covered with ultra thin paper. The gores were then glued to the internal support rings. We used clips for hair and clips for gardening to glue everything together under pressure.
Since on the table nasty tension will develop in the framework, we decided to hang the airship from the ceiling while working on the frame. This way each of the gores was glued to the internal support rings. However the trick is to connect the edges of each gore to the neighbouring once. We did this by a small strip of paper that was glued over both edges. This glue-reinforced layer of paper and balsa wood provided extraordinary stability with almost no weight. Building this framework is an extremely tedious work and the more gores are connected to the airship the harder it is to work inside, since the inside is very hard to reach. We decided to keep the last gore without paper cover and leave it open till the very end to enable us to still work in the inside. One of the gores was then painted with the original Bodensee style letters. We did this by projecting the letters visible on an old picture of the Zeppelin to our gores, using a digital projector. Since the weight is extremely critical in our model, we decided that this is the only piece of colour we would use on this model.
After all gores are glued to the frame, the frame became rather rigid and was able to carry its own weight when placed on the table. In the next step the inner support rings where removed by just cutting them with knife and pliers. The inner envelope was made from thin mylar rescue blankets as found in the emergency kit of a car. It was roughly cut to follow the shape of the airship after inflation and then glued using double sided sticky tape. Before adding electronics and dead weight like the gondola, we filled the airship with lifting gas to test the total lift. By adding some ballast, we were able to assess how much weight we could spend on electronics like motors and batteries. Even though we did our best to save weight where possible, only about 60 g were left for all additional parts of the airship.
This unfortunately did not allow to include moving elevators and the two left and right motor gondulas. We just added one servo that moves the lover rudder and the main engine in the center of the ship for forward thrust. To simulate the trim of the airship a small motor was placed in the center to push downwards and lift the airship on the point. All electronics were connected by thin coated cupper lines to save weight. The electronic components were: Microservo 2.2 g; 2x Microbrushless Motor 3.2 g; 2x MicroESC 0.5 g; 4 Channel Receiver 6g; 300 mah 1S Lipo 8g. To enable the transportation of the Bodensee, the fins were folded to the side.
On the day before we left to the final presentation, we made another inflation test with all the components attached and found that the Bodensee has become too heavy in the tail section. This means we were unable to balance the airship to level, which is essential for the flight operation. The following became a nightmare for us. We had to cut of the tail of the airship and rebuilt it shorter. At the same time we saved several grams of weight in the tail section be removing pieces of balsa wood that are not absolutely necessary for strength. It was a tough piece of work, but we stripped about 13g of weight of the airship by removing everything that could be removed without destroying the whole thing. So the Bodensee was ready to travel to Friedrichshafen for its virgin flight.
Final Setup and Virgin Flight Setting up a rigid airship is rather easy, just get it out of the box and put helium inside. To remove any remaining air in the gas envelope we use a vacuum cleaner. This ensures only pure lifting gas inside. In the picture below the new tail section can be seen, which is a bit short but still does it job. For comparison, a silent_runner airship is in front of the Bodensee. In the nose of the Bodensee, a very small magnet was built in. Like the big original Zeppelins, the model could anchor this way on a metal mast, like a tripod. j Tethered to the tripod, the Bodensee gently moves with the wind and looks awesome. Then, the virgin flight was scheduled and it was pretty tough since several problems appeared. If both engines are
driven more than half of the speed, the battery voltage would drop significantly because of the very long thin wires. This means, the ESC will go off and restart once the voltage is going up again. This made the control of the Bodensee extremely hard. Further the power of the forward engine was so low, that the maximum speed of the airship was less then walking speed. The same applies to the curve radius which was about 8 meters. So everything is like in the original and we have a magnificent extremely hard to handle piece of craftsmanship floating in the air. Fortunately just in the beginning of the virgin flight, the air conditioning of the big hall was shut off and the airspace was perfectly calm. Using the downward facing engine, the Bodensee slowly raised above our heads. Then the forward thruster was activated and accelerated the Zeppelin. It was an incredibly feeling to finally see our almost impossible contraption flying above our heads. And it was quite a tough job to keep the airship away from the walls. It was like flying a huge ocean cruiser that has no brakes.
The model of the LZ 120 Bodensee was built by Andreas Burkart and André Sobotta. The built was covered by DMAX Die Modellbauer das Duell in 2015, thanks to Jo, who did the movie work! Special thanks go to Anke, who made the airship finally flying by cutting the tail. www.windreiter.de