During the academic year of 2017-2018, Lambach Aircraft participated in the British Model Flying Association (BMFA) Payload challenge.
The annual payload challenge consists in designing, building, and flying a UAV able to carry a payload of water. The winning team would be the one able to carry the highest amount of payload weight compared to the weight of the UAV, for a given propulsion system.
Student teams came from all over the world, bringing their design and passion to a fierce competition of skill as well as nerves! Try after try, all participants had set back: some teams crashed their landing gear while trying to take off from the grass, some others broke their propellers while testing.
We also had our challenges: during our first trial runs, right after we had gained the attention of all other competitors, who didn’t believe we would be able to take off with such a heavy looking UAV, after about two minutes of flight, our engine quit on us!
Our motto since then has been:
we make heads turn, and engines BURN!
Of course we got right back on track. Spirit of Delft was designed with an high aspect ratio, and as such, was able to get to the ground intact even without propeller. Thanks to the support of fellow teams, we were able to find a spare engine that would fit our engine mount, and get back to the runway to successfully complete the first round of the competition!
The Design
It was designed to carry up to 3 bottles of 1 liter each, 1 in the fuselage and 2 under the wings. Special pods and ring holders where developed to accommodate these. The pods under the wing also served as the main landing gear attachment points, connected the twin tail booms and where streamlined to minimize drag.
With a long slender wings it was able to glide long distances to land softly even with the heavy payload in grass. The wings where made by placing long carbon fiber rods to carry compression and tensile loads from the wing bending moments. The shear between these rods where carried by a balsa wood web.
The ribs consisted of glass fiber re-enforced balsa wood to maintain the shape of the airfoil. These included lightening holes to save weight as well as allowing the wings to be taken apart during transport through the inclusion of connecting rods. This made the design light, portable and strong enough to endure some impacts. The winglets at the edge allowed for better wing vortex performance.
The twin booms consisted of carbon fiber tubes which where connected to the edges of the horizonal tail plane. The fuselage consisted of a light weight carbon rod truss structure which could also house additional batteries if needed. The loads where introduced into the wing by large glass re-enforced balsa back bones.
The whole structure was covered in shrink foil to improve drag. This allowed the whole aircraft to lift off the ground with more than 5 kilograms with a relatively small engine. Lastly, the nose landing gear was actuated to be able to steer, improving ground roll.
Manufacturing
Part of the engineering experience is building with your own hands the designs you have been working on. At Lambach, we make use of our workshop and tools, together with the Aircraft Hall of the TU Delft to manufacture our UAVs.
The Spirit of Delft (BMFA 2018 project) was one such project which was build completely by our members.
Wing
The build started with the wings. The wing consisted from ribs which would make the shrink foil take the shape of the airfoils. Were composed of a closed cell foam core sandwiched between of 2 layers of glass fibre on each side. These layers where angled in 4 different directions chosen for optimal load distribution. The panel was produced in the Composite Laboratory of TU Delft, with the airfoil shapes cut out by a water jet cutter at an external facility.
The wingbox design used carbon fibre as part of its assembly and served as spars to carry the large bending loads. Balsa wood panels where cut to shape using our own CNC machine to exacting precision to save mass. This served to maintain good torsion performance and holding the spars in place.
Fuselage
The body was connected to the wing as an extension of the two central glass fibre ribs. These parts contained large lighting holes to save weight. The shape was such as to allow for load transmission from the nose gear to the wings. The central cavity created also could contain payload which subsequently rested on the spars.
The fuselage was constructed through a carbon fibre truss structure with connecting elements made of 3D printed PLA to crossing members for stiffness in all directions.
All the 3D printed elements, joints for the fuselage as well as connections for the payload, were build with Lambach’s very own 3D printer machine, in our workshop.
At the front of the fuselage is the nose of the aircraft. This contained the battery, engine, engine controller, fuses, receivers, and the overall brains of the system. The nose gear steering was given a mechanism to allow for ground roll control.
The skin of the wing was made of foil. This made the aerodynamic profile adhere to the designed airfoil thanks to the rubs. Foil also provided a very smooth surface with little drag and some crucial tension on the wing structure to stiffen it.
Empennage
The empennage airfoils were built almost fully from extra thin balsa wood, cut with our CNC machine. The support spars are made with carbon fibre rods in a similar way as the wings to add strength and rigidity. Similarly to the wings, these too where covered with foil for smooth aerodynamic areas. The booms composed of carbon tubes which also contained the electric wiring and power to the control surfaces. The rods were connected to the outsides and then directly to the re-enforced landing gear mounts.
Control Surfaces
The control surfaces were build like the main structure of their wings. However their skin was reinforced with extra thin balsa wood panels reinforced with triangular inserts. These made them stiffer while still remaining hollow and containing the trailing edge shape of the airfoil for good aerodynamic performance.
The surfaces where actuated through small servos connected by control rods. These rods where connected to large arms glued to the surfaces of the control surface. This gave them a torque advantage and hence quick response. This made it much more responsive while the servo size and power consumption could be made much lower.
