Elastic Aircraft for High Altitudes ELHASPA Electric High Altitude Solar Powered Aircraft HABLEG High Altitude Balloon Launched Experimental Glider HAP High Altitude Platform HAPS High Altitude Pseudo-Satellite IMU Inertial Measurement Unit UAV Unmanned Aerial Vehicle

The group Flying Robots at the DLR Institute of Robotics and Mechatronics in Oberpfaffenhofen conducts research on solar powered high altitude aircraft. Due to the high altitude and the almost infinite mission duration, these platforms are also denoted as High Altitude Pseudo-Satellites (HAPS) or High Altitude Platforms (HAP). After the successful flight of HABLEG [1], which was presented at ESA PAC 2015 [2], work continued with the goal to reach the stratosphere under own power with a reasonable sized platform. In order to achieve continuous flying, the overall goal with HAP designs is to obtain a very high battery-tostructure mass ratio. This leads to very fragile aircraft which greatly influences operational availability due to the dependence on calm weather conditions. In fact, thermals and wind have led to several catastrophic failures in the past. (a) Helios [NASA] (b) Helios break-up [NASA] (c) Zephyr [Airbus] (d) ELHASPA [DLR] (e) Solara 50 [Titan/Google] (f) Aquila [Facebook] Figure 1: Current HAP Designs This overview paper proposes a new way of building high altitude platforms. The idea behind ELAHA (Elastic Aircraft for High Altitudes) is to build a segmented airplane with an extremely elastic wing, only elevators as control surfaces and appropriate control algorithms that allow it to survive in turbulent weather conditions. To date, a proof-of-concept has already been flown successfully and the construction of a larger 10m wingspan version is ongoing. This paper discusses the concept, what has already been achieved and the current status of the development. 1. PROBLEMS OF CURRENT HAP DESIGNS Fig. 1 shows a few examples of HAP designs. Of those, only the upper row, Fig. 1 (a) – (c), have actually flown in the stratosphere whereas the others only did low level flight testing so far. Current HAP designs are built using modern fiber compound materials. The mandatory weight reduction as a key design driver currently leads to lightweight but also extremely fragile platforms. Carbon fiber, which is commonly used, has a very high tensile strength and stiffness, but is only capable of small amounts of compression before failing. This geometrically limits the achievable bending radius of a wing with a given thickness. In combination with high wingspans and low wing loadings, these fragile structures lead to tight flight envelopes in which an overspeed condition is reached rather easily. It can be stated that there were several incidents involving structural failure of existing HAP designs. The airplanes in the lower row of Fig. 1, ELHASPA, Solara 50 [3], and Aquila [4], all encountered structural failure in consequence of piloting or autopiloting shortcomings which caused a violation of flight envelopes. We assume that a more deformable and thus forgiving wing structure in combination with high local control authority to cope with reduced stiffness can help to extend the operation boundaries and avoid getting into undesired situations. A non-sufficient local control authority of a high aspectratio wing also played a role in the Helios breakup. Here, the aircraft morphed into a high dihedral state. The “procedure to reduce dihedral was to increase airspeed” [5]. This approach failed and the wing disintegrated after a successional occurrence of deficient longitudinal control stability. Furthermore, some of the existing designs don’t scale well in size. The increase in size comes with the need for an over-proportionally heavier structure diminishing the potential gain of a bigger platform. This can only be adverted by truly span-loading platforms. 2. THE CONCEPT We propose to approach these problems with a highly elastic wing and a segmented aircraft. By highly elastic, we mean a wing that is able to bend up to 90° from wingtip to wingtip. If, as shown in Fig. 2, a thermal updraft catches a part of the aircraft, the wing bends all the way up until the projected surface against the updraft is reduced to almost zero and thereby has no further harming influence. We refer to this, as passive safety. Figure 2: Thermal bending wing upwards The airplane is made up of segments that can be joined together as needed, which can be seen in Fig. 3 and following. We distinguish between payload and propulsion segments: