I Introducing Fixed-Wing UAVs
Fixed-wing aircraft have now been successfully designed and flown for over 100 years. Aero-modelers have been flying quite large aircraft at low subsonic speeds for decades, sometimes at scales as large as one-third the full size. Given the accumulated experience, it is therefore a relatively straightforward task to design, build, and fly a workable fixed-wing unmanned air vehicle (UAV) platform, armed with one of the many textbooks available on aircraft design (perhaps the most famous of these being that by Torenbeek , though there are many others). Even a cursory search of the Web will reveal hundreds of UAVs, many of them fixed-wing, and a number being offered for sale commercially. What is much less simple is to quickly make robust and reliable airframes in a repeatable manner at low cost, tailored to specific missions and suitable for commercial-grade operations.
If one has to rely on the craft skills of a highly gifted model-maker to construct an aircraft, costs rapidly rise, timescales lengthen, and repeatability becomes difficult to ensure. The use of bespoke molds and various forms of composites allows a much higher standard of airframe, but the initial production costs become then high and the ability to alter designs becomes very limited. Conversely, by using commodity off-the-shelf components combined with computer-aided design (CAD)-based digital manufacture, craft skills can be eliminated, costs lowered, and repeatability guaranteed. Clearly, if one has always to manually adapt an existing design to come up with a specification for a new aircraft, much design flexibility is lost; if, instead, lightweight decision support tools are linked to sophisticated parametric CAD models, high-quality design concepts can be rapidly developed to specific needs.
This is the fundamental design philosophy adopted by the UAV team at the University of Southampton (Figure 1.1) and forms the guiding approach of this book. The basic idea is to work in a digital, online world, buying parts where possible and manufacturing custom items only where absolutely necessary - essentially the aim is to source a kit of components either from part suppliers or companies offering online CAD-based manufacture, which then simply requires assembly to produce the finished aircraft. This means that the resulting UAVs are of a high and repeatable quality with as much emphasis on smart design as possible. This philosophy has become possible largely because of a revolution in bespoke digital manufacturing capabilities afforded by advanced CAD, Internet-based sourcing, low-cost computer numerical controlled (CNC) machining, and the widespread availability of 3D printing of functional components. In particular, the use of SLS nylon and metal has transformed the way in which main
Small Unmanned Fixed-wing Aircraft Design: A Practical Approach, First Edition. Andrew J. Keane, Andras Sobester and James P. Scanlan.
©2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.
Figure 1.1 The University of Southampton UAV team with eight of our aircraft, March 2015. See also https://www.youtube.com/c/SotonUAV and https://www.sotonuav.uk/.
fuselage components and bespoke aircraft fittings can now be made. The core aims throughout our work have been to seek
- 1. low costs with highly repeatable and robust products,
- 2. rapid conversion of design changes into flying aircraft to meet new requirements, and
- 3. flexible payload systems
- 4. duplication of all flight critical systems,
- 5. sufficiently sophisticated avionics to allow fully autonomous takeoff, flight, and landing,
- 6. large and strong fixtures and joints to provide tolerance of uneven landing sites and day-to-day ground handling, and
- 7. low take-off and landing speeds to minimize risks of damage during operations.
These aims ensure long-lived and robust commercial-grade aircraft, which can survive hundreds of flight cycles and thousands of flight hours - something that model aircraft never see.