The Box Wing is a Great platform for innovation with a unique layout for safety and efficiency with a human touch.
Safe and approachableDrones have a problem with reputation, most drones looks like predators or military hardwareEven if there's no pilot, humans are an integral part of the system
I tried my best to stay away from the aggressive predator cliché, insteard favoring neonatale features to design something approachable and likeable that still looks professional and trustworthy.
The interface is a central part of that with its central screen. From a practical standpoint it’s used for communication, to avoid confusion and uncertainties. It can be used for unlocking, planning and field analysis of flight data. This makes the system self contained so it does not rely on any ground infrastructure. But it’s also a natural center of focus, the “face” of the drone.
Propellers are clearly guarded from prop strikes, sensors in the wing turn of the rotor if something is inside of the guards/wings.
The small footprint allows for easy ground handling.
The 3 point landing gear extends in a wide equilateral triangle around the center of gravity for stability. Their length allows operating in rough terrain and allows a long travel for dampening a rough landing. They are also designed to work as landing skids in the event of a conventional wing borne landing. The rear landing gear has a scale to check that weight and CG is within tolerance. This data could possibly be relayed to the flight controller to adjust control laws for optimum performance. It is mechanically simple, self locking design with only two moving parts and can be 3D printed in one piece in case of failure.
In case of landing gear failure the it can belly land without the rotors contacting the ground.Rotor clearance is 30 cm with landing gear and 6cm without.
The hexacopter layout also provides redundancy in case of a motor, controller or propeller failure. It also means that the individual rotor RPM can be modulated without a net control force. This could be used for modulating sound frequencies or avoid vortex ring state during fast vertical decent. The cargo is housed in the center section, right in the center of gravity and is detached by removing the whole section of the fuselage. This gives easy access from above or below, both manual or automated.
Being able to remove the whole cargo section allows for great flexibility when designing alternative modules.
The battery compartment is located in a smaller module behind the cargo and is detachable in the same manner.
When the cargo and battery modules are removed, hinges in the wing attachment points allows for simple folding without tools or disassembly and without connectors that can fail or parts to loose. Cargo- and battery module can be carried together as a backpack, just like the folded plane. This splits up the weight evenly for easy transportation by two persons.
In case of catastrophic failure the suggested parachute system is used.
High wing loadingLess gust sensitivity, small footprint, ease of transportation, efficiency higher speed. Maximise the strength of the configuration.
By loading the wing to 50kg/m2 the stall speed is 19m/s which should leave a comfortable margin for the transition corridor. Overpowered cruise motors to push thru transition drag.
Propulsion/YawThe efficiency of controlling yaw by differential RPM on a multicopter platform diminishes with increasing aircraft size. This is further complicated by the larger aerodynamic area and therefore crosswind sensitivity of a quadplane, requiring a larger control force in gusty conditions.
The greater structural integrity provided by the box wing allows putting the horizontal flight propulsors on each wing tip. They provide strong yaw authority from hover, all the way to cruise speed without taxing capacity from the lift rotors. Because the propellers are operating in clean air this offsets the disadvanage compared to a pusher propeller operating in the slip stream. VLM analysis of the propeller configuration in VSPAERO shows favorable interaction with wing tip vortices, increasing L/D from 29 to 31 for the top wing.
Structural efficiencyThe box wings main feature is providing a barrier for all rotors without the usual drag penalty of protective shrouds. The box wings structural integrity is a perfect platform for attaching the lift rotors, providing better bending and torsional stiffness. It also protects the central cargo section in the event of a crash.
The fuselage shape is driven by the payload dimensions and uses a NLF airfoil cross section.
The lower reynolds number compared to a single wing of the same area is partly offset by the box wings higher spanwise efficiency.
Lead-lag is allowed on blades to dampen vibration for radial flow, The rear lift propellers are mounted inverted so that gyroscopic precession don't push the propeller closer to the arm.
The whole construction is watertight with a total volume over 120 liter which means that the vehicle floats like a cork in case of a ditching. The internal honeycomb forms compartments, ensuring the vehicle stays afloat even if the skin is punctured.
All pieces are monocoque composite constructions. Spread tow carbon fiber over a 3D printed core of polycarbonate or ULTEM.
Lift systemThe disc loading for the lift system is a reasonably low 10kg/m2.The hexacopter configuration gives a wider range of COTS choices for lift propellers.I propose the use of KDE direct motors and propellers specified by the manufacturer to provide 58kgf lift and an efficiency of 9g/watt at 25kgf thrust.Motor weight is 305g, controller 86g and propeller 65g.
Batteries are LG HG2 18650 3000mah cells rated at 20 amphere. Cell weight is 47g and the operational temperature ranges from -20c to +70c. LG Chem is one of the biggest battery manufacturers in the world and performance numbers are validated by third partie.The use of 18650 standard cells gives great flexibility in adapting for new battery technologies which are usually introduced in 18650 cells before prismatic cells.
The battery module is a 3d printed polycarbonate core carbon monocoque like the other modules.Simple cell level fuses as used by Tesla provides safety against cell failures with very low weight penalty. A electric heater supplies heat when battery pack temperatures fall below -20.The pack contains 140 cells and delivers 210wh under hover load (5A/cell). Total battery pack weight is 6.9kg. Battery management is handled by the charger on the ground.
3 rudders with direct drive actuators minimize complexity and allows for redundance in case one actuator fails. High speed actuators allows for future possibility for relaxed stability for increased range by simple firmware upgrade (all hardware for that functionality is already there since multicopter part is actively stabilized, just use air speed meter for variable PID in forward flight).
The modular construction makes it easy to replace a broken component in the field. All electronics are located in the nose making replacement simple.
The location of the cargo and batteries means they can be prepared beforehand and simply switched out as one package for a turn around time measured in seconds.
When folded, the width is small enough to fit a standard pallet, the short wingspan gives a max length of 1.3 meters when folded
Lift and Cdi was calculated with two different VLM solvers (VSPAERO and Athena Vortex Lattice)
Form drag with Navier Stokes/LES (Autodesk Flow Design)Total drag agreeing with frame sheet (22 N)
- Cruise speed 40ms/140kmh
- Transision speed >19ms/68kmh
- MTOW 25kg (with 6kg payload)
- Surface Area 3.34m2
- AR: 25
- Vertical wing spacing: 500mm (20% of wingspan)