The Cargo Plus
The Cargo Plus is a purely functional and simplistic airframe design, with a focus on modularity, low structural requirements and stability. Many structural components are used twice in the airframe, are interchangeable and of extremely simple construction. The overall design is inspired, and built around the common "plus" configuration octocopter.
Most SLT drone airframes out there attempt to combine an “X” configuration multi-rotor, with a conventional fixed wing airframe, which I believe is a mistake since a conventional fixed wing airframe naturally lends itself to being combined with a “+” configuration. In addition to requiring excessive additional structure to accommodate an “X” configuration multi-rotor layout, it also results in needless flexing and torsional stress issues which then have to be dealt with in a variety of ways which reduce performance. One way most designs cope with structural demands is by keeping vertical flight rotors on short booms which are located close to the root of the wings. While this may be helpful in reducing flexing, it greatly reduces stability of the aircraft during hover which must then be compensated for by using overpowered vertical flight rotors.
The Cargo Plus has many structural benefits. Instead of lifting the entire weight of the aircraft by the wings, half of the lifting forces generated by the vertical flight rotors lift the aircraft directly by the fuselage. The remaining vertical flight rotors are located on the wings which only support only half of the weight of the aircraft during hover. To control yaw, the lateral vertical flight rotors power up or down together while front and rear vertical flight rotors also power up or down together just like any other plus configuration multi-rotor. This means that there are absolutely no twisting forces on the airframe.
Vertical flight rotors are located at the front, rear, left and right extremities of the airframe, resulting in the maximum amount of stability possible with the given geometry.
The Cargo Plus places as much weight as possible toward the center of the aircraft, and as little as possible toward the extremities. This is essential for stability, during both vertical and horizontal flight modes.
Redundancy and Safety
The Cargo Plus utilizes two vertical flight rotors at every point of vertical lift. Redundant rotors are placed in a more efficient linear arrangement as opposed to the more common coaxial arrangement, avoiding the associated losses in efficiency. This means that if one ESC, motor or prop should fail, the Cargo Plus will be able to make a safe landing. If a malfunction happens during horizontal flight maneuvers, the aircraft can go into vertical flight mode. When all else fails, the specified flight termination system was included in the design. Redundant battery systems are also used.
The Cargo Plus is little more than a few carbon tubes connected to a wing, and does have a nice clean profile. On the downside, there is not much that can be done as far as optimizing aerodynamics without departing from some of the advantages of modularity and dead simplicity.
The Cargo Plus is big on modularity and portability. Components easily break down into sections well under 2000mm.
The image above demonstrates the use of simple, duplicate components used in the Cargo Plus. The components on the left hand side are both duplicate and interchangeable. Only the three main assemblies on the right are unique.The Head Assembly
The head assembly is one simple unit that slides onto the end of a carbon tube. It houses the nose skid and both the camera and the communication system, balancing out the weight from the rear horizontal flight motor and the tail assembly.
The Tail Assembly
The tail assembly is one simple unit that slides onto the end of a carbon tube. It houses the horizontal flight rotor, tail skid and two servos which drive the rear stabilizers directly, eliminating linkages and smaller fragile control surfaces.
The Main Assembly
The main assembly is a square fuselage section, and houses the batteries, payload bay and most of the electronics. The flat bottomed central wing section, and the square fuselage section are easy to connect, resulting in a strong and lightweight structure.
Wings and Ailerons
The outer wing and aileron sections are identical in construction and so are interchangeable. The wing tube holes run the length of the sections, and are capped off with wing tip plates.
Both the payload bay and the dual, front and rear batteries are accessible from the bottom. Worm gear driven electromechanical actuators are used in place of locking mechanisms. This increases ease of use for users who are manually loading/unloading cargo or batteries while still accommodating automated loading/unloading solutions. By using multiple electromechanical latches, failure of any single mechanism will not cause the payload or batteries to be jettisoned. As you can see in the image above, the payload/battery tray has an open port, exposing the red payload bay. This is designed to accommodate payloads which may require protruding instruments, cameras or sensors.
All wiring and electronics are located inside of the carbon tubes, main assembly and underneath the front and rear fuselage cones. Brushless motors do alright in damp conditions, but will need attention toward waterproofing the connections. Weather seals protect the payload/battery tray contents from the elements.
Payload Bay Dimensions
450 x 350 x 200mm
Dual removable wing tubes.
Helicopter skids, tail and nose skid.
Electronic Components and Flight Termination
Red - Flight Control Computer
Orange - Internal Measurement Unit
Yellow - ADS-B Transponder
Blue - Flight Termination Parachute
Purple - Flight Termination System/Launcher
Brown - Camera System
Black - Communication System
White - Air Data System
Lateral Vertical Flight Rotors
Managing a wing during windy conditions is one of the challenges of VTOL aircraft. Wings catch a lot of wind by design and their length gives them a lot of leverage which the wind uses against us. The Cargo Plus utilizes lateral vertical flight rotors located toward the wingtips of the aircraft, turning the length of the wing into an advantage, using the leverage to stabilize the aircraft. The lateral vertical flight rotors stabilize roll only during hover, and so can be mounted on very short and lightweight booms while causing absolutely no twisting forces on the wing.
Yaw Control During Hover
While all the vertical flight motors and rotors are the same, with the exception of rotors which turn in opposing directions, they are not all located at equal distances from the center of gravity of the aircraft (they are close, but not perfect) and so will have unequal effects on yaw from the torque they generate. This does not prevent a multi-rotor from flying, but does prevent the aircraft from taking full advantage of available power. Luckily this is actually easy to offset by mounting some of the rotors at very slight angles, a common practice among those who build multi-rotors. I would opt to angle the fuselage mounted rotors, since they are mounted to a more solid base compared to the lateral, wing mounted rotors and so can be more easily and precisely mounted.
Each battery pack contains 24 Ultra High Power NMC cells. Each cell measures 82 x 183 x 7mm, and each battery pack measures 50 x 200 x 350mm. Below is a link to the web page for these cells.
Vertical Flight Propulsion
Vertical flight will be powered by the MN5212-420KV by T-Motor, spinning 18x6.1 T-Motor props, which should give us 25.720KG of static thrust at 65% throttle on a 24 Volt system. This is convenient since forward propulsion was based on 25 Volts.
Ecalc and Forward Propulsion
While there is always room for further exploration, I settled on the Hacker, Q80-13S-20p F3A (215) for forward propulsion for now. There are many larger motors to choose from if more power is needed, but for only 610 grams, this looks pretty good.
Under different dimensional constraints, the fuselage could be elongated, allowing the front and rear rotors to be placed farther from the center of gravity which would be beneficial for yaw control. In another embodiment, the front and rear rotors could be flipped over so that they are facing downward. In some cases, it could be beneficial to move the lateral vertical flight rotors to the extreme wingtips, or even move them closer inboard. Finally, dual horizontal flight rotors could be used which could be helpful in assisting with yaw control during vertical and horizontal flight, providing forward flight propulsion redundancy and in avoiding having the horizontal flight propulsion in line with vertical flight propulsion, allowing smoother transitions.