High Altitude Pseudo Satellites (HAPS) are establishing themselves as a new class of unmanned aerial vehicles. Mostly solar powered, these UAVs operate in the stratosphere as a substitute for research and communication satellites. A few examples of the current projects are AIRBUS Zephyr, Aurora/Boeing Odysseus and ELHASPS. The market for HAPS was estimated at $2.7 billion in 2018 and is expected to grow at 20% compound annual growth rate to over $7 billion in 2024.
Due to their energy-efficient propulsion, HAPS operate at very low speeds of 50 to 100 kph, leading to lengthy ascents and descents and, hence, low response capabilities to weather conditions with icing potential. Ice accumulation on the aircraft surface increases the mass and reduces the aerodynamic efficiency of the affected areas such as airfoils, horizontal and vertical stabilizers, engine intakes, propeller surfaces and even flight measurement probes. Occurrence of ice can, therefore, lead to increase in drag and loss of lift, longitudinal and lateral instability of the aircraft, and incorrect sensor measurements that can lead to catastrophic accidents. The possibility of critical ice buildup due to the unique boundary conditions makes it imperative to understand the effects of icing on the aerodynamic and flight mechanical characteristics of these low-speed high altitude aircrafts.
The main objective of the AVALLON research project is to investigate the aerodynamics of low-speed aircraft and effects of icing in detail. The research approach includes experimental analysis on an airfoil model with flap at the Technische Universität Braunschweig’s Icing Wind Tunnel, as well as numerical analysis by improving numerical icing codes for low-speed aircrafts. The validation and optimization of the calculation methods is carried out by comparison with measurements from the ice wind tunnel tests. The outcome of the analysis will help in identifying the critical flight conditions and mission parts, allow inclusion of this information in the weather data evaluation during flight guidance, and assist in deriving strategies for safe and efficient operation. The project results will also be used to develop a suitable human-machine-interface and assistance system that can support flight guidance, especially for HAPS, under icing conditions during operation.
The project objectives are summarized as follows: