“We have much to learn by studying nature and taking the time to tease out its secrets.” (David Suzuki)
Following that quote, the AirBee concept is inspired by nature and by bees in particular. Bees are high performance cargo carriers related to their weight in terms of maneuverability, range, speed and carriage. They may act independently and autonomously, but are also capable of collective intelligence and cooperation supported by a highly sophisticated communication system. They also may adjust their brain chemistry depending on the task that is to be accomplished.
Our goal with the AirBee concept was to build upon these amazing characteristics and design and conceptualize a simple, modular and at the same time intelligent high performance cargo drone that shall be deployable in a wide range of mission settings, e.g. quick reaction and observation/emergency missions after crises and disasters, delivery of urgent goods such as spare parts, organs etc., surveillance missions, and mass parcel delivery. The Airbee concept shall exploit modern processes, technologies and materials for light weight and modularity and wherever possible harness simple mechanisms for ease of handling.
B. Brief Concept Description
Inspiration for Design
The design of the AirBee concept was inspired by the legendary Lockheed SR-71 as well as modern approaches like the Eurofighter, for example. To meet the dimensional requirements and to enable both VOTL and conventional fixed-wing long distance flight, we chose a blended wing configuration with a W-shaped tailplane and canard-like foreplane. The front wings which also entail the ailerons are mounted to the upper part of the fuselage. In combination with the W-shape, we can avoid disturbances in the air current ahead of the main wings and, thus, increase lift efficiency. The elevators are mounted on the main wing. Furthermore, AirBee has one single vertical tail plane with a rudder. Therefore, it is highly maneuverable for the fixed wing flight. Both wings are slightly swept and have winglets to reduce induced drag and improve aerodynamic efficiency.
We decided to go for a 4+1 direct driven prop and electric motor concept for the sake of high maneuverability, power redundancy as well as weight distribution. Four off-the-shelf 2-blade propellers that are mounted on top of the winglets are attached upon an electric motor for each. These props are responsible for VTOL and are not used during cruise flight. One push propeller together with another electric motor sits at the back of the fuselage and provides thrust for conventional cruise mode. VTOL mode is similar to a quadrotor. In transition from hover to forward flight, the nose will pitch down while the push motor starts operating similar to a helicopter. When critical forward speed is reached, the VTOL motors will be shut down. During final approach, the nose will pitch up to reduce forward speed. VTOL motors will be restarted until hover mode is reached.
Since the VTOL is realized with four external installed props, the drone is able to act like an ordinary quadrotor with its nature to roll, pitch and yaw. Even though the systems lack VTOL thrust reverse which leads to supposedly dead weight, the rotors are able to start and orientate very quickly. Later on, the mentioned configuration is able to stop abruptly right before landing by taking countermeasures which saves a significant amount of time to deliver urgent goods to the customer.
Since the four mounted VTOL motors have to carry the maximum weight of about 25 kg, its calculated electrical power results in 1.875 kW per motor by a power-weight ratio of about 300 W/kg. The given cruise height of 300 ft within 1 min results in a required velocity of 5 ft/s, or 1.52 m/s. To meet the requirements, the drone will be able to start and land just once which corresponds to a value of 2 min for both procedures. Multiplied with the calculated electrical power gives an electrical energy of about 62.5 Wh. The dimensioning of the rechargeable batteries depends on the electrical charge value which is given by the division of the electrical energy by the required voltage times four to get all motors into account. The calculation results into about 16.9 Ah which then leads to the weight of a common rechargeable battery.
Since the given frame sheet is not clear enough to get calculations related to cruise flight simply done, further calculations were based on the condition that mass and gravity do no longer appear since the wings fully compensate those multiplied forces. Therefore, just standard values like cw, air density, surface area and velocity were taken into account.
Fail Safe Components & Safety Provisions
Several systems and components are responsible for overall flight related characteristics, communication and safety purposes. Beside standardized components, e.g. IMU or common positioning and communication systems, AirBee follows a comprehensive and sophisticated safety concept to prevent cargo damage and injuries to people during operation. This concept includes sensors for dynamic flight path adjustments, quick landing procedure after early problem indication, e.g. lack of energy due to strengthened headwind or general bad weather condition, quick turn-off all rotors after landing, and a parachute implementation for unforeseen events of malfunction. A flight termination parachute is installed, too.
Furthermore, satellite cameras and related sensors ensure obstacle avoidance and merge position feedback control near ground level. The switchable navigation lights and common harassing fire devices encloses the concept of having a state of the art flying drone to transport medical cargo safely from hospital to hospital, or from hospital to operation site. An additional beeping signal while hovering near ground will warn bystanders.
For the sake of multi-purpose capability, we went for a conventional 3-point landing gear system to enable fixed-wing take-off and landing where VTOL is not feasible. By mounting the gears directly to and partly inside the fuselage we reduce weight and drag and add simplicity by avoiding a retraction mechanism. Due to the low MTOW the required braking power is negligible. The gears are located near COG to reduce the load on the wings while landing.
Modularity, Shipability & Ease of Handling
The body of AirBee consist of seven sections that are easily assembled and fastened by means of clips, tongue-and-groove systems and/or hook and loop fasteners (Velcro strips). No tools are needed for assembly. The parts may easily be carried by two adults and are easily shippable in standard boxes. The high tech textile skin and carbon fiber rod and spar system is easily serviceable as well as fixable with standard repair materials and tools.
To service or change components, only the skin has to be removed at that area. Once finished, the skin can be stretched on the bionic structure again within less than one minute. Connection between textile skin and bionic polyamide structure through studs, zippers or hook and loop fasteners.
The structural concepts entails a low weight grid structure based on bionic principles and engineering made of polyamide or polyamide derivatives, e.g. carbomide, by means of additive layer manufacturing. The bionic grid structure of the fuselage and wings is manufactured by selective laser sintering and made of an aeronautical polyamide alloy. The spars are made of off-the-shelf low specific weight and high strength CFRP profiles manufactured by braiding. The overall frame is covered with industrial high tech textiles widely adopted in the aeronautical and space industry, similar to the Solar Impulse 2.
The structural concept of AirBee is light and strong at the same time. It ensures operation in all realistic weather conditions according to the requirements. The overall structure is waterproof once the single components are mounted to the fuselage (rubber sealing). The design was CAD optimized regarding dead volumes and redundant structures. Low overall weight will be obtained by using bionic light weight ALM structures in combination with CFRP rods and spars. The structural frame is covered with high performance and ultralight industrial textiles. Consequently, the multi-material design obtains the best performance due to an optimal use of the right material, with the best properties at the right position.
Payload & Cargo
The cargo bay is located behind the foreplane at the lower side of the fuselage very close to the COG. It is enclosed by two gull-wing doors which hinge aside by means of telescopic rods for the sake of simplicity and reserved weight and space. The doors are unstressed all the time as the battery package as well as the cargo and sensor bay are fastened by means of a permanent and switchable magnet rail system inside the cargo bay. This system offers great modularity and a wide range of mission scenarios from dropping food packages or medicine during flight to automated loading procedures. The cargo will be placed in a in a standard box (455 x 355 x 205 mm) made of light sandwich structures with CFRP top layers and rigid foam cores manufactured by innovative compression molding processes. Inside the box, different sizes of load may be fixed by means of belts.
We use standard and off-the shelf TATTU Lipo battery packs with a capacity of 12000 mAh, a weight of about 1 kg and dimensions 190x35x70 mm.
Depending on the mission setting, we would need 4 (60 km, 5 kg) or 6 (100 km, 3 kg) battery packs, respectively. A small additional onboard battery ensures electric operation while battery swapping. This battery will be recharged by the main batteries afterwards.
For the sake of modular configuration and heat conduction, each battery pack is placed within a standard PA 3D printed battery housing with the dimensions 210x90x310 mm. These boxes connect the battery packs to the onboard electric system once they are inserted and locked at the right position on the magnetic railing system in the battery bay.
To change batteries, the cargo bay must be emptied. For graphical explanation of the exchange process, please see the images.
The cargo is mounted inside the drone via an intelligent mechatronic interface which enables the whole system to fully automatic couple and decouple, transport loads, identify them, and ensure power and/or signal supply, as required. The large set of electronic components (sensors, cameras, GPS etc.) and modular ports allow for a wide range of configuration with respect to the mission setting and range of tasks. Reserved weight, space and power is provided along to the requirements.