Unmanned Systems Technology 022 | XOcean XO-450 l Radar systems l Space vehicles insight l Small Robot l BMPower FCPS l Prismatic HALE UAV l InterDrone 2018 show report l UpVision l Navigation systems

69 have resulted in the PHASE-8 being made available to any company seeking to develop and mature their own unique HAPS systems and technologies. Fluid dynamics A critical factor in the development of the PHASE-8 was that it could accurately verify the capability of aero- structural designs and vital subsystems to be flown above 55,000 ft – but do so at sea level. “To enable this accurate verification, despite the difference in altitude, the PHASE-8 needs to operate at the same Reynolds number when flying at sea level as the HALE UAV operates at its altitude,” explains Paul Brooks, managing director at Prismatic. The Reynolds number is dimensionless, and is named after Osbourne Reynolds, who popularised the concept relating the inertial forces of a fluid to its viscous forces. “As such, two different-sized objects of the same shape in different fluids, but operating at the same Reynolds number, can be expected to behave in a similar manner, particularly considering the transition from laminar to turbulent flow,” Brooks says. The Reynolds number is defined by: where: ρ is the density of the fluid v is the velocity of the fluid relative to the object L is the characteristic length of the object (or the chord in the case of an aerofoil) and μ is the fluid’s dynamic viscosity When comparing these values for the PHASA-35 operating at 55,000 and 65,000 ft, as shown in the table above, Prismatic found that the PHASE-8 needed to be one-quarter scale to match the Reynolds numbers. To that end it has an 8.75 m wingspan compared with the 35 m of the PHASA-35. Flight configuration Following on from this, the aircraft has a 35:1 aspect ratio, weighs 7 kg excluding batteries, a 2 kg payload limit and a flight ceiling of 10,000 ft, for a 12-hour endurance without recharging. Cruising speed is between 32 and 43 kph. Aside from the complexity in the calculations behind the wingspan and aspect ratio, Prismatic has aimed for simplicity in most aspects of the craft, as HALE systems must fly for months at a time without maintenance. Flight control uses an IMU and pitot tube to provide the data for operating the elevator, rudder and two optional ailerons. “Our focus is on designing for reliability and then optimising the graceful degradation of the system in the event of failures, for example by enabling differential thrust to alleviate a problem with the rudder and by using separate elevator surfaces and servos to alleviate a loss of pitch control,” says Prismatic’s projects engineering director Jonathan Dixon. The perennial HAPS problem of high bending moment forces on the high aspect-ratio wings is alleviated by using two brushless DC motors with tractor propellers at the midpoint of each wing, rather than undertaking a complex span- loaded configuration. Also, while the use of a pitot tube might seem odd given the thinner air pressure that the PHASA-35 will experience at 55,000 ft, the greater speed of the larger platform compared with the PHASE-8 is anticipated to result in a similar dynamic pressure Prismatic HALE UAV | Digest Unmanned Systems Technology | October/November 2018 Parameter PHASA-35 PHASA-35 PHASE-8 Altitude (ft) @55,000 @65,000 @10,000 Air temperature (C) -70 -65 0 Density (kg/m 3 ) 0.16 0.1 1.2 Dynamic viscosity (Ns/m 2 ) 1.2 x 10 -5 1.5 x 10 -5 1.7 x 10 -5 Flight speed (m/s) 24 28 9-12 Critical length (chord, m) 1.0 1.0 0.25 Reynolds number 320,000 175,000 170,000-230,000 The wings are built from carbon fibre skinned-foam to minimise weight, and separate into five parts for ease of transport