Athena is a long range cargo drone designed to rapidly transport payloads to a destination. It was designed with simplicity and safety in mind to ensure safe, reliable and easy operation from challenging environments.
The basic inspiration came from high performance gliders, from H frame quad copters and the Rutan Voyager aircraft. Placing the vertical lifting motors within two booms was a simple solution to allow the even spacing of the lift motors, and the booms could simply be extended to support the tailplane, as with SAAB 21, P-38 and numerous other aircraft.
The payload was inspired by various cargo concepts from the past, such as of the Sikorsky 'Skycrane' helicopter, with interchangable pods that could be attached under the aircraft.
I also wanted the aircraft to use commercial off the shelf components where possible to reduce costs and ease construction, so the motors, batteries and propeller combinations were specified to fit closely with common components.
8 Lifting motors, 1 forward flight
8 Aerodynamic Control surfaces; flaperons, ailerons, elevators and rudders
Wingspan 4.4m, AR 17
Wing loading 23kg/m^2
Disc loading 19kg/m^2
Cruising Speed 33m/s (120kph)
Stall Speed 18m/s
L/D Cruise = 15
The landing gear is kept simple, with two fixed tail wheels incorporated into the bottom of the vertical fins, and with the main body of the aircraft resting on small feet. I wanted to avoid long gear legs, as these would be very fragile and would need to be very stiff not to wobble, meaning extra mass, as well as aerodynamic drag. The payload concept does not require large amounts of ground clearance, as will be shown later.
While the motors are exposed to the elements, they are brushless DC motors, and as such can operate in wet conditions with no problems. Water proofed Electronic Speed Controllers (ESC) should be used on this design, as the heatsinks will require airflow to be cooled.
the servos are mounted on the underside of the aircraft surfaces and as such will be somewhat sheltered from rain; however the use of waterproof servos is required to reduce the risk of a failure.
A simple rubberised skirt, similar to a car door should be run around the edge of the opening to seal the gap between the payload container and the fuselage.
A similar technique to the payload bay is used with simple rubber gaskets preventing water from entering the fuselage
The Aircraft can be broken down into these component parts for transport:
Athena Folded into Cuboid Container
- Horizontal stabiliser/elevator
- 2 Booms with lift rotors
- 2 Tail sections, with rudders and tail wheels
- Main Fuselage and Cargo bay
- Removable Pitot tube/handle
Ease of Handling:
The aircraft is designed for easy ground handling operations. By lifting the nose with the forward handle the aircraft can be towed or maneuvered around similar to a wheel barrow. The mechanical advantage given by the level arm from the tail wheels lets a single person move the aircraft and handle the payload.
The cargo unloading/loading operation does not require a person to be kneeled down next to lifting propellers or reaching under wings and booms, the aircraft is simply pulled over the payload and lowered on top.
Batteries can be accessed easily though a large hatch in the top of the aircraft. The batteries are secured with velcro straps, so they can be replaced quickly, and the position adjusted to achieve the correct C.G.
The airframe mass breakdown is as follows;
- 7.2kg Structure
- .42kg Servo motor actuators & linkages
- 3.4kg Required Ignition items
- 2.3kg Lift Motors & Rotors
- 0.5kg Forward flight Motor & Prop
About 1kg of mass is unused from the 25kg limit, giving some room for error during construction and the possibility of using it for extra battery capacity, conventional landing gear etc.
All control surfaces have a level of redundancy.The 8 lift motors and lift propellers mean the aircraft can maintain adequate control and TWR with up to two power system failures provided they are not both on the same corner.A parachute is fitted to rescue the aircraft in the event of catastrophic failure of control and power systems.The switch panel on the nose of the aircraft can be used to override the flight control system, and disable lift and cruise motors during ground operations. All motors and actuators are set to a default ‘off’ or 0 state immediately when landed and can only be enabled manually when ready to take off again.
The aircraft is made up of relatively simple shapes, allowing the surfaces to be easily manufactured from a light weight foam, eg hotwire cut, molded or routed. While this foam does not provide much strength or stiffness, it is skinned with carbon or glass fibre and resin to create a very strong and lightweight surface skin around the foam.
To provide extra structure in key areas, large diameter carbon fibre tubes are interlocked to form the wing-boom-fuselage joins.
Structural Concept and Control Component Locations
Fuselage and Boom plate structure
Payload and Cargo Concept;
The payload is contained within a removable open top box. This box can be swapped with another pre-loaded container to improve turn around times. The aircraft can also be configured with alternate payloads, such as a high resolution or thermal imaging camera for aerial surveying, SAR operations and remote inspections. Another alternate payload would be a air data module, which could gather atmospheric and enviromental samples for weather research or after a natural or human disaster.
Payload Loading ConceptCamera Module with Retractable Landing Skid
Unusual shapes can be accommodated and secured by Velcro or similar straps;Securing straps in payload compartment
The payload compartment is larger than required to allow easy removal and loading of the cargo into the module. The dimentions are shown below;Payload container detail
Aerodynamic design was based on minimising induced and form drag at speed.
This meant Athena has a high aspect ratio wing, as well as a moderately high wing loading for good lift to drag ratio at a high cruise speed.
Using some simple optimisation code I could plot the thrust required to complete the cruise phase of the mission with respect to flight speed for a range of wingspans and configurations. The output for Athena is as follows;
Cruise Energy, No losses
It is obvious that the ideal configuration is found at around 25m/s, however I wanted the aircraft to be able to complete the 100km mission in under one hour, so a higher speed was selected.
The aerodynamics were also simulated in XFLR5, it was found the fuselage/wing join produced turbulent vortices that would disrupt flow over a conventionally placed horizontal stabiliser and elevator. So it was lowered out of the wake from the wing and cruise propeller.
For XFLR analysis simple NACA aerofoils were used, with NACA0010 on tail surfaces and NACA2414 root foils and NACA2210 tip foil. Specialised aerofoils should be used in the final design.
The wing has a 6 degree twist along its length to get as close to an elliptical (ideal) lift distribution and a high Oswald factor (e). Although, a positive side effect of flying at high speed is that the induced drag becomes irrelevant compared to the form drag, so form drag is to minimised in the rest of the design.
Minimising Form Drag;
- Lift rotor pairs are shrouded in streamlined booms
- Fuselage lengthened to minimise frontal area
- Tail wheels enclosed within fairings at bottom of tail
- No unnecessary legs/struts, minimised protrusions
The biggest problem with drag comes from the exposed lift propellers, unfortunately this is a problem with all designs, however if aligned with the free stream flow they should produce minimal drag.
Control Surface Concept Sketch
Battery: 6S (24V) LiPo
Required Performance: 5-6.5kg Thrust
Lifting motors: around 450kv driving 16"-20" prop
Example: TMotor MN5212 420kv & 18x6.1CF propeller
6kg Thrust @ 1500W @ 60A
Battery: 6S (24v) LiPo
Required Performance: 16N thrust at 33m/s
Using Propeller theory I found around 9000RPM and a 15"x8" prop was a able to provide adequate thrust at that flight speed.
Accounting for a voltage sag and drag on the motor, the target RPM was estimated at 10000RPM, this meant a ~420kv Motor was required.
Propeller data sheets
showed at low speeds a 15x8 propeller would draw over 2kw.
Motor: >=420kv, >=2000W.
Example: Scorpion SII-4025-440KV & APC 15x8E propeller
Thin wing sail plane servos are required due to the low thickness of flight surfaces. Many are commercially available in the standard 'thin' size of 30 x 10 x 35mm.
Typical Specifications are, 30g, 8kg/cm and operating voltages of 4.8-7v
The same type of servos are used to control all the control surfaces, and have more than adequate torque and speed for the size of control surfaces.
This took a lot longer than expected, good luck everyone,David.