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24 Dossier | Phoenix UAS the aerodynamic design of the vehicle and its flight controls. Given the need – as with all aircraft – to keep weight to a minimum, the ideal shape for a pressurised fuselage is a sphere, as it provides the least surface area to contain a given volume. However, a sphere can generate up to an order of magnitude more drag than an aerofoil- based teardrop shape of the same frontal area. A properly designed teardrop shape removes the flow separation caused by a sphere, and then the only drag is that caused by the friction of the air moving over the surface. The region of air affected by friction is known as the boundary layer, and the wake of such a shape is formed only of these boundary layers; the associated, much lower, retarding force is known as friction drag. This is less important at low speeds because the drag of a vehicle is proportional to the square of its speed. However, with a predicted design operating speed of 15 m/s (~33 mph) the drag is a major factor for a vehicle with no conventional propulsion. The behaviour of the boundary layers can be very sensitive to Reynolds number. Consequently, the aerodynamics of a relatively small, slow-moving vehicle flying at low altitude can be significantly different from a larger vehicle of exactly the same shape flying faster at high altitude, where density, speed, size and viscosity are all different. All of these aerodynamics considerations point towards a requirement for a long, thin teardrop shape (like the fuselage of a conventional aeroplane), while there is a competing, weight-driven desire for a sphere. The optimum solution is thus a compromise between the two. The aerodynamic performance was predicted using CFD, which indicated the aerodynamic-induced pressure forces acting on the skin of the fuselage. At the same time, FEA was used to model the aircraft’s structure (including the material of the fuselage) and calculate the stresses and strains therein. By incorporating the results of the CFD analysis into the FEA calculations, the aerodynamic forces and those from the internal pressure of the lift gas are combined, and the maximum forces in the fuselage fabric can be predicted, as well as the locations at which they occur. These values were used to select fabrics capable of withstanding these loads without stretching too much. Stretching of the fabric, especially when inflating the internal air bladder, would increase the displacement and negate the effect of adding the mass of the compressed air. The lift gas – helium – is contained within the fuselage, so the volume of the fuselage determines the buoyancy and thus the lifting capability of the aircraft. However, that can lead to an aircraft designer’s worst nightmare – the divergent weight spiral. December/January 2020 | Unmanned Systems Technology In varying the craft’s buoyancy, the flight control system has more in common with that for a submarine than an aircraft The pumps and valve open the airway to the internal bladder