Preliminary conceptual design for a Box-Wing High Altitude Pseudo Satellite

The Italian Aerospace Research Centre (CIRA) works on the design of an unmanned stratospheric platform for Earth observation and telecommunications, commonly referred to as a High Altitude Pseudo-Satellite (HAPS). The stratospheric airship offers a unique and promising platform for broadband telecommunication relay missions, combining the advantages of both terrestrial and satellite communication. This paper presents the conceptual design of a Box-wing HAPS configuration that generates both aerodynamic and aerostatic forces to balance the weight during the different phases of a mission. With the current flight endurance record, the platform proves suitable for long-term missions that continuously monitor ground activities from the stratosphere. The conceptual design process divides into two steps. The first stage involves analyzing a reference wing: starting with input parameters such as cruise velocity, altitude, airfoil profile, and maximum weight for the platform, and using semi-empirical formulas to determine sub-system mass, aerodynamic coefficients, and structural dimensions. Some of the data obtained from this initial examination serve as input for the second phase of the analysis. Specifically, the next step involves folding the reference wing to obtain the Box-wing configuration, also known as the Prandtlplane, as first described by Ludwig Prandtl. This specific configuration is selected for its advantages, including minimum induced drag for a given lift value and high aerodynamic efficiency. Following the initial design phase, which targets an altitude of 20 km, a speed of 16 m/s, and a payload capacity of 10 kg, a suitable configuration using a E186 airfoil is selected. The aircraft lift-drag curve is evaluated using a stationary, incompressible Reynolds-Averaged Navier-Stokes (RANS) analysis with k-omega SST turbulence model in OpenFoam. A detailed longitudinal and lateral-directional stability analysis is also conducted using OpenFOam, in particular, the effect of buoyancy on longitudinal stability is analysed. The results indicate that the presence of a moment due to buoyancy shifts the moment curve downward, achieving trim conditions at lower angles of attack. Additionally, the quasi-elliptical shape of the upper wing leads to lateral instability of the platform.