CTOL Airframe Development: Case Study

In 2013, D3 Applied Technologies approached Carbonix to build their Cometa design. D3 has world-leading expertise in designing and analysing efficient aerodynamic structures for applications as varied as racing yachts, fishing vessels, turbine blades, and even buildings.

OUR BRIEF

Our role was to design and build the tooling for manufacturing the airframe in advanced composites, to manufacture the airframe to the highest standards, as well as develop detailing and hardware solutions for prototyping and production of the new UAV.

Carbonix developed and optimised the tooling, construction method, structural arrangements, fittings, and detailing.

Throughout this development cycle significant gains were made in the build process and structure, with the main areas of focus being to achieve optimum aeroelastic behaviour of the loaded structure in flight and reduce labour content for production.

With our experience designing and building high-performance structures in advance composites, Carbonix was able to execute D3’s Cometa design providing structural enhancements to create a truly efficient and optimised airframe.

STRUCTURAL OPTIMISATION

Different arrangements were tested for the main spar construction and monocoque skin laminates. The main candidates were a shaped foam main spar with thin skins, a tubular spar, and variations on I, Z, and C beam section moulded spars. As well as deflection under load and overall weight, factors such as labour time required for assembly, tooling costs, and downstream impacts on resilience were considered.

Spars
Tubular spar (left) and moulded C spar comparison. A number of variations were designed, built, and tested by Carbonix to arrive at the current production solution.

Since the airframe is modular and comes apart for packing into a compact transport case, we identified some great opportunities for making gains through innovative solutions for the various interfaces and joints.

The outer wings unplug at roughly one-third span, near the root of the longitudinal booms. So an effective spar joining method had to be devised to transfer the bending moment and locate the outer wing securely. After testing various configurations of axial screws, transverse pins, and even magnets, we developed and optimised the T-Latch system™ (named for designer Taylor Thomas) that allows secure, tool-free assembly without needing access to the inside of the airframe, and leaving a flush surface on the wing (the design is such that an incomplete connection will be visually obvious to alert the user). This latch mechanism is an example of our in-house design and machining capability with a deliberate focus on elegant solutions that have ergonomic and tactile beauty to them.

Similarly, we developed quick-release systems for the tail-booms and various options for securing the payload bay and internal shelves. Such structural details and fittings enhance usability in the field and complete the look and feel of the airframe as a premium product for commercial use.

Latch
Quick release system designed in-house

Bending
Various methods were employed to quantify and prove behaviours under load, from simulations, to static tests, to in-flight measurements.

LAUNCH METHOD

The initial airframe brief was for applications where ground equipment would be available (flying out of dedicated bases in fixed locations). So, to minimise on-board weight and complexity, a catapult-launch and parachute-land system was selected for the Cometa RPAS. For initial testing, D3’s Cometa design also incorporated removable tricycle landing gear. Combined with flaps, this would allow the option of conventional landing at very low speed, minimising the chance of damage.

CTOL 1
Second prototype in 2014. Tricycle landing gear, dorsal parachute hatch, and flaps are visible. Entire airframe together with ground control station and extra payload modules packs into the box seen in this picture.

When Carbonix explored commercial applications of this airframe and upcoming new models, it became apparent that the ability to launch and retrieve from small, unprepared areas (often flanked by obstacles such as trees or cliffs) was very important to customers. Being able to slow to a hover mid-mission was also evidently desirable. And having the redundancy associated with a vertical flight system was also an attractive feature.

We identified a clear gap in the market for small and medium UAVs combining the long range/endurance of a sophisticated fixed-wing aircraft with the versatility and agility of a multi-rotor. And so, the R&D for our fixed-wing, VTOL hybrid airframe began in 2014.

Read the next installment of our journey: VTOL Development Case Study.