VTOL development journey: Case Study
Following the successful development of our flagship airframe, D3’s Cometa, Carbonix embarked on the journey in 2014 to develop a fixed-wing airframe with vertical take-off and landing (VTOL) capabilities to be offered ‘off-the-shelf’ to meet the commercial market’s need to dispense of bulky and expensive launch and retrieval equipment.
Carbonix undertook R&D with both short-term and long-term goals.
THE FUTURE OF VTOL
Long-term, we wanted to develop an all-new airframe at the upper range of the 25Kg weight category with vertical take-off and landing (VTOL) capability designed into the airframe from the outset. This airframe will be aimed at extended and beyond visual line of sight (BVLoS) operations. Thus it will have the carrying capacity to accommodate safety equipment as required to integrate and communicate with other airspace users. This airframe, The Visio, is due out in late 2018.
OUR LATEST VTOL SOLUTION
Short-term, and to satisfy customer demand in the 10Kg to 15Kg weight range, Carbonix worked extensively to create a variant of the D3 designed Cometa with added VTOL capability. In the process, we developed the first commercially available fixed-wing/VTOL hybrid UAV made in Australia.
Selecting a VTOL configuration for a fixed-wing aircraft is a wonderfully nuanced challenge. The basic trade-off comes down to the following: Do you use the same motors and rotate their orientation between horizontal and vertical flight? Or do you keep the two systems separate and carry dedicated motors for each flight mode?
SOLVING THE DESIGN ISSUES
On the face of it, using common motors seems attractive to the elegance-seeking designer because it does both jobs using fewer elements. However, on close analysis the compromises are subtler. If we look at part count carefully, we must include the actuators needed to rotate the motors, and some means of changing propeller pitch to accommodate the difference between the low-axial-airspeed/high-power vertical regime and the high-axial-airspeed/low-power mode needed in horizontal flight. Add the fact that a dedicated horizontal flight motor can be optimised for range/endurance (for example it can optionally be petrol-powered and have a fixed-pitch propeller matched for loiter or cruise), and the decision looks like it could go either way.
This is where expertise in evaluating design solutions to the specific brief, and quantifying such pros and cons really gives Carbonix an advantage. We advocate a disciplined, numbers-driven approach, informed by design intuition. Always guided by the values of wanting to create beautiful solutions.
One of many VTOL retrofit options explored by Carbonix as early as 2014: Tricopter with swivelling motors. Minimum number of rotors, but variable pitch required for the rear. Dedicated canard-like struts for forward rotors double as landing gear springs and keep airflow ahead of wing clean.
Initial prototype with dedicated VTOL system mounted to existing rear booms (obvious minimum-impact solution) and new forward boom extensions. This was unique until recently but has since been widely adopted by other twin-boom UAVs. The trick is in the execution: Subtle geometry of rotor positioning and inclination is key to effective transition between vertical and horizontal flight. Minimising drag in horizontal flight from motor pods and parked rotors is key to maintaining range and endurance.
Current model ‘off-the-shelf’ airframe pictured outside the Carbonix workshop on Cockatoo Island where series production is underway. In this version, the tail-booms plug in aft of the rear VTOL pods. Thus the airframe can be flown as an ordinary quadcopter with wings and empennage left on the ground.
As we can see from the examples above, not all applications call for the same solution. Variables such as overall weight and the percentage of mission time spent in vertical flight can tip the balance one way or the other. For example, our 2018 Visio airframe could well use tilting motors in one alternative configuration. The aim is always to offer the best solution to suit the mission and the customer requirements. Flexibility and modularity are vital, as one size will not always fit all.
Carbonix prides itself on offering bespoke solutions to its customers, so our customers will have a UAV that is perfect for their requirements. Like choosing the colour of your airframe, our customers can choose modifications like longer nose cones, VTOL capability, and tailored locations of RF-transparent patches and equipment housing shelves and hardpoints. We apply our unique experience to come up with the designs that will meet these requests.
Being able to ‘mix and match’ different airframe elements is vital to satisfying different specific mission requirements. The base structural component is what we call the ‘wing-box’, which is effectively the central fuselage, and inboard blended wing-roots including the main carry-through spar.
The wing-box contains all the motor mounts, a fuel tank, and a dedicated avionics area. The base wing-box can be combined with different nose shapes that are essentially fairings around the payload. These nose cones can be one of several off-the-shelf sizes we can make from existing tooling, or customised as needed. They accommodate different shapes and sizes of payload, from simple fixed cameras, to multispectral arrays and other specialised sensors.
Conventional take-off Cometa in the background, and VTOL version in the foreground. Airframe in the foreground has a removable nose fairing. The one in the background has a fixed nose with access hatch on top.
FROM PROTOTYPE TO PRODUCTION
All production tooling is either carbon or metal. Metal is used for small and shallow components where crisp/fine detailing is required. The hard and non-porous surface can be repeatedly polished and gives a clean release with virtually no mould deterioration.
Carbon tools are more suitable to larger parts with deep curvature, and where thermal expansion considerations start to matter. We source our tooling resins from various suppliers and have an extensive history of experimentation to adjust tooling properties to suit the application.
Good tooling with properly thought-out geometry and vital information such as cut-lines and witness marks machined-in is vital to accurate and repeatable manufacturing.
As always, the parts are almost exclusively made from prepreg carbon, with various moulding and curing techniques used, according to part characteristics.
Our in-house development serves to establish some building blocks and tools to help provide the best combination of off-the-shelf and customised components to suit our customers. We then offer the best airframe solution for each customer application, and can provide airframes in various stages of completion.
We have perfected an effective closed-mould technique to make tubular and conical parts in a single-cure process. The extra material visible on the left forms a slip-joint that ties the top and bottom halves of the part together as one during cure.
Carbon mould for lower skin of wing-box. The mould includes all information necessary for downstream processes. For example, outlines for cut-outs and screw centre witness-marks are machined into the master plug/pattern. And the mould flanges contain guides for aligning the booms and spars.
Read the beginning of this development journey here: CTOL Development Case Study