Many highly efficient gliders have the following characteristics; high wing, T-tail glider and tadpole fuselage. Quadcopters are also among the most manoeuvrable aircraft. Therefore, it would seem that the most logical hybrid would be a combination of these two. However, it was found that given the nature of this competition that another configuration, the wide-body, canard with retractable quad arms, may be more suitable.
The first thing that was noted from the rules is that 25 kg is the target mass. Even though this is the maximum there is no need to aim for anything less. Additionally, a quick analysis of motors shows that four motors are likely to be the most efficient form of lift, since the rules stipulate you have at least four lifting motors. Rotors could be put within the wing, but this would make them less efficient and weaken the structural integrity of the component they are within (by necessitating giant 29” holes). Even prop guards significantly reduce the performance of rotors, so we decided it would be preferable to mount the motors external to the aircraft in clean air.
Given that a forward camera is also required, a tractor motor is unlikely to be a possible configuration. Multiple forward motors could be utilised, but this is likely to cause unnecessary additional mass. This called for an unconventional layout. The inspiration for this aircraft is drawn from Burt Rutan, known for his peculiar designs, and in particular his home-built, ‘Long-EZ’. This was a very high performance canard aircraft and was cause for much research into canard aircraft. The research found that the pusher configuration and shorter fuselage enabled significant drag savings.
A wide-body was chosen for ease of use. Given the dimensions of the payload, it was likely that an elliptical cross-section would be required. The decision was made to have the major axis lie horizontally so that only a single fuselage deck would be required (instead of stacking items vertically, we can stack horizontally). A removable top-hatch then enables easy access to all components. This was very important for this design as it is intended to be as modular as possible. Being able to access all areas of the aircraft with the removal of a few covers makes life much more convenient for the user. This prevents the need to remove multiple components to gain access for maintenance. This design allows for a very open-plan layout, that can be customised for various operations. The single deck also means that a complex truss structure is not required internally, saving weight.
Another key design feature of this aircraft is an attempt to have as ‘clean’ a body as possible. On this basis we decided to retract all external components into the fuselage. Obviously retraction mechanism adds weight and complexity to the design, but we felt that the drag savings and safety of this design provides the costs. This will be discussed in greater detail in a later section, but a preliminary result from wind tunnel testing that we carried out at our university showed that the drag cost of the quad-rotor system could be over . To put this in perspective, the wing in this design produces 6.6N of drag. The rotors are in effect doubling the drag of the aircraft, making the kind of mission described in this challenge very difficult. We felt that although the 60 km mission may be possible with this drag, the 100km mission would not be. By removing external components we were able to achieve significant performance gains.
It is noted that adding extra moving parts the risk of failure is increased, so we implemented one of the simplest designs we could find. The retraction system is similar to the servo landing gear systems of many heavy-lift quads. It encompasses a tension spring to aid the motor against drag when retracting and extending during flight and is rated to hold over 20kg. As such we feel any failure in this component is well mitigated and the risk is of comparable order to many other structural elements in the aircraft.
The last major consideration was the design speed of the vehicle. This vehicle is competing in an industry that has many transport options such as ‘traditional’ drones, cars, motorbikes and the like. We, therefore, felt that unless the vehicle could travel at a reasonably fast speed, there would be little uptake of the technology. This vehicle is also intended to be an emergency response vehicle, so it should be capable of travelling fast. As such we optimised for a cruise speed of 35m/s. However, the aircraft was designed to operate efficiently over a range of velocities for different mission profiles.
In order to land, usually, you would like to approach at a speed less than 15m/s. If this wasn’t the case you would find it hard to find runways long enough for you to land. This is less than half of our cruise speed, which would mean that we would need to increase our lift coefficient by more than four times. Given that this would require complex high-lift devices on the wing, this idea was abandoned. Furthermore, greater performance can be gained from the wing if landings are not a concern. For a canard, especially (where the canard limits Cl max), incorporating a stable landing into the design would result in significant performance losses. Given that the wing and tail configuration cannot support a fixed wing landing, wheels were not incorporated in the landing system. This enabled a slender landing gear design that retracts neatly into the fuselage.
For these reasons we would like to canvass our design the FD-7 (‘fat duck’). Further justification is provided in the ensuing documentation to complement the summary provided above. Please feel free to view all the relevant, drawings, figures and data to feel confident that our design is both viable and fully capable of surpassing the requirements of this challenge.