Designed for Speed.

Designing a Mach 0.8 Aircraft.

Employing a small, closely integrated team of students at the University of Bath has facilitated the rapid iteration of designs, with the team moving from concept to manufacture of its Kingfisher aircraft in under 8-months.

Each team member took ownership of a subsystem, first developing analytical and empirical models for initial sizing. Initial concept generation was performed using TRIZ, and the team navigated the design loop three times, evaluating and re-designing each time to meet the requirements of the world record. With each iteration, the fidelity of the design increased, and the team took advantage of computational methods like CFD and FEA to provide further optimisation of the design. To validate results these were combined with experimental techniques like wind-tunnel testing, low-speed flight prototypes and hardware in the loop setups.

Where Are We Now?

The current design called the Kingfisher, is a 1.3m long blended delta wing powered by a Jetcat P300 Pro micro-turbojet engine. The Kingfisher will launch from a pneumatic catapult system, performing a series of accelerated turns to reach 610 mph (Mach 0.8 at sea level) in the timing track. To keep the inlet on the bottom for propulsive performance, the Kingfisher performs most of its flight upside down, before rolling over for a belly landing to avoid the need for landing gear. The Kingfisher also carries a parachute flight termination system, in conjunction with redundant avionics and flight-control systems, using a safety hierarchy to ensure it never leaves the test range.

The team worked closely with Callen Lenz during the design, learning from their wealth of expertise in developing commercial UAVs, which included working with their pilots to ensure the aircraft is flyable. The fibreglass semi-monocoque structure, chosen for its radio transparency, was optimised using Finite Element Analysis, with internal ribs, bulkheads and spars also providing mounting points for off-the-shelf components and access hatches for maintainability. Meanwhile, four elevons control the aircraft in pitch and roll, with these sized to trim the aircraft even if one fails. These are controlled using a Pixhawk Cube autopilot which also supports autonomous GPS waypoint missions, along with an FPV camera and laser altimeter to maintain a 35m ground height in the timing track. The avionics and control systems were tested using a hardware in the loop setup, meanwhile, a series of low-speed sub-scale flight prototypes will provide further validation.

The delta-wing uses leading-edge vortices to maximise lift on landing in the same way Concorde did, meanwhile, its low thickness helps to minimise drag. The aircraft has a thrust-to-weight ratio of 3:1 on take-off, with the s-duct air intake shaped to deliver uniform, slow-moving airflow at the compressor face, to prevent stall. The aerodynamic analysis was accelerated using Simcenter Star-CCM+, the industry-leading CFD (Computational Fluid Dynamics) software provided by sponsor, Siemens. Star-CCM+ provides a fast and inexpensive method of characterising the 3-dimensional flow over the airframe, enabling the team to understand the vortex generation over the wing at low speeds, quantify the drag at Mach 0.8, and ensure the s-duct inlet can decelerate the flow entering the engine with minimal pressure losses and recirculation. These CFD results were validated during wind tunnel tests at the University of the West of England, with results matching predictions.

The Previous Supersonic Concept

Before converging on the Kingfisher’s high subsonic design. The team initially looked at what it would take to break the sound barrier, developing the Mach 1.2 capable supersonic concept shown above.

The shape was focused on managing the formation of shocks.  As an aircraft approaches the sound barrier, shock waves form, consuming large amounts of energy and causing a sharp increase in drag. The drag increase from Mach 0.8 to Mach 1 can be as great as 3-fold, and breaking the sound barrier requires sufficient thrust to exceed this drag, with small-scale turbojet engines ideal thanks to their wide range of operating speeds, high dynamic thrust, and relatively small form factor. After months of iteration, the resulting concept was a 2m long carbon fibre monocoque design, weighing 22kg when fuelled and gaining thrust from an afterburning Jetcat P400 Pro micro-turbojet engine. Its shape was optimised using the area rule to smooth out changes in cross-sectional area and minimise wave drag, while also providing sufficient volume for avionics, servo actuators, the fuel system and retractable landing gear.

The P400 Pro alone provides insufficient thrust to reach Mach 1, so the team added an afterburner: a lightweight modification that adds 50% more dynamic thrust without increasing the diameter of the fuselage (minimising drag). Even with these additions, the design would not be able to break the sound barrier at sea level, instead having to ascend to 11km altitude where the atmosphere is thinner and the speed of sound is lower, consuming half of its 6L fuel tank in the process. The aircraft would then enter a 30-degree dive where the effect of gravity provides additional “thrust”. In this configuration, a predicted top speed of Mach 1.2 could be momentarily reached.

Additionally, a normal shock inlet decelerates the flow entering the engine to subsonic speeds, ensuring the compressor doesn’t stall. Meanwhile, the highly swept delta-wing, with its 4% thickness, minimises drag at supersonic speeds, while providing sufficient lift at landing. Even so, the aircraft would have to land at 80 mph since if the wings were made any larger, the drag would be too high.

It is clear at this point that building and flying this aircraft would be beyond the scope of a student team, which is why the team instead opted for a subsonic Mach 0.8-capable design called “Kingfisher”. The Kingfisher has a larger wing area and a lower landing speed, making it more pilot-friendly. Moreover, it can reach its top speed at sea level without an afterburner, bypassing other logistical challenges.