Landing Simulation of a High-Altitude Platform with Skid-Type Landing Gear - Flight Procedure, Controller, and Loads

The German Aerospace Center (DLR) is currently developing a solar-powered high-altitude platform. The underlying vehicle is a fixed-wing aircraft designated to be stationed in the stratosphere for several days and to carry payload used to perform Earth observation missions. The project HAP addresses the complete design process of the aircraft, from conceptual studies and detailed design up to its construction, flight test campaigns and its final operational clearance. The aircraft operates at very low airspeeds, in the order of 10 m/s equivalent airspeed, and is therefore very susceptible to atmospheric disturbances, particularly in ground proximity. In addition, due to the absence of air brakes and due to the the very high glide ratio, the aircraft only reaches low sink rates, which additionally prolongates the landing duration. Altogether, the landing is a highly critical flight phase that needs thorough consideration. This paper addresses three issues associated with the landing: first, it presents a landing procedure that has a potential of reducing the risk of the landing, second, it describes the positioning process of skids that are used as landing gear, and third, it includes the resulting landing loads in the aeroelastic design. A high number of landing simulations of the aircraft in atmospheric disturbances is performed using a 6 degrees-of-freedom flight dynamics model. For this purpose, a landing controller based on an outer energy management loop and an inner pitch loop is implemented. During the simulations, the best main skid position is found to be around 40 cm in front of the centre of gravity. A dynamic aeroelasticity simulation is performed to analyse the associated loads. The results show that the landing loads obtained with this landing procedure are not sizing and thus do not influence the structural weight