This article has been published as part of Ch.1 of my PhD thesis
UAVs Then
What the Wright brothers did on the 17th of December 1903 was a stepping- stone for humanity in its quest to conquer the skies by flying machines of their own creation. A quest that started hundreds of years before with many attempts of human-powered flight mimicking the flapping-behavior of birds. After years full of trial, errors, and crashing bodies, humans converged to the solution of fixed-wing aircrafts for sustained flight and not flapping-wing ones. Ever since the Wright Flyer (Fig. 1.1.a) flew in 1903, fixed-wing aircrafts have evolved at a tremendous rate motivated, unfortunately, by its hostile capabilities. Only 11 years were enough for powered aircrafts to become an important branch of the armed forces in World War I. Although, aircrafts were used initially for reconnaissance only, by the end of the war armed air-forces included fighter and bomber aircrafts as well.
The capabilities of aerial vehicles have triggered a massive amount of military funds to be allocated for the development of the technology which in turn accelerated the evolution of fixed-wing aircrafts. Such funds have allowed the increase of the maximum speed of these vehicles from 80 km/h in the beginning of the war in 1914 to 240 km/h at the end of the war in 1918 [Munson, 1969]. It is also remarkable that during this early development stage, the first autopilot controlled UAV (Fig. 1.1.b), designed by P.C. Hewitt and E. Sperry, made its first successful flight on the 12th of September 1917 [Valavanis and Vachtsevanos, 2015].
Most development efforts were concentrated initially on fixed-wing aerial vehicles, which were the most suited for military purposes due to their high endurance and long-range capabilities. However, there were many appealing features in such vehicles such as vertical take-off and landing (VTOL), flying at low altitudes, and hovering. This led to the advent of rotary-wing or multirotor aerial vehicles. The first successful flight of a quad-rotor aircraft took place in the early 1920s by the de Bothezat helicopter (Fig. 1.2.a). Thirty nine years later, the first unmanned helicopter developed was the QH-50 DASH (Fig. 1.2.b), which demonstrated its first flight in 1959 [Valavanis and Vachtsevanos, 2015].
The military development of fixed-wing and rotary-wing aerial vehicles, both manned and unmanned, continued for decades. Such development yielded a vast variety of designs at many scales. Moreover, the mathematical under- standing of the flight dynamics and control of such vehicles evolved greatly in the process.
On the other hand, the development of flapping-wing aerial vehicles (aka ornithopters) was overshadowed by the success of fixed-wing and multirotor vehicles. Thus, flapping-wing aerial vehicles were abandoned for many years and considered a futureless technology due to its high complexity [Goodheart, 2011]. However, the old dream of “flying like the birds” motivated a few individual visionaries to unlock the secrets of the flight dynamics of birds and build flapping-wing aerial vehicles, but at a much slower pace compared to fixed-wing and multirotor vehicles. The first human-powered flapping-wing vehicle successfully flew in 1929 as a result of the work of A. Lippish. Seventeen years later, in 1946, the first engine-powered flapping-flight was achieved by A. Schmid (Fig. 1.3.a). Then after another fourty-five years, the first successful engine-powered small-scale flapping-wing UAV, known as “Mr. Bill” (Fig. 1.3.b), was designed in September 1991 [Goodheart, 2011].
UAVs Now
Nowadays UAVs have come a very long way in becoming smaller, cheaper, smarter, and closer to people. The advancements in the fields of miniaturized sensors, microprocessors, electric drives, and batteries have allowed the down- scaling of UAVs to a great extent and simultaneously with lower costs. Moreover, the advancement in robotic technologies have allowed the development of advanced UAVs with perception, planning, and decision making capabilities, that allowed them to accomplish complicated tasks autonomously. The combination of UAVs and robotic technologies gave rise to the field of aerial robotics.
The small scale and low cost of current aerial robots have been primary factors that enabled aerial robots to gain unprecedented notability in the civilian market. Historically military applications have been the main driving force for the development of UAV technologies. However, nowadays the market share of civil applications is increasing at a very high pace. According to Finnegan [2019], the commercial unmanned aerial systems (which includes UAVs and their auxiliary equipment) market is expected to increase from $5 billion in 2019 to $14.5 billion in 2028. In addition, UAVs are allowing for commercial services and solutions that are estimated to potentially have a market value of over $127 billion [Mazur and Wi´sniewski, 2016]. These market predictions, and others, imply that civilian applications are becoming an equally important driving force, if not more, for the development of UAV technologies.
Multi-rotor aerial robots have gained more popularity in civilian applications as well as academic research compared to fixed-wing ones. This can be contributed to their lower cost, versatility, portability and most importantly its hovering and VTOL capabilities. With the wide variety of sensors that could be mounted onboard, aerial robots enabled many companies to offer cost effective solutions for various civilian applications such as surveillance, visual inspection, aerial photography, search and rescue, mapping, entertainment, and law enforcement, just to mention a few.
In recent years, there has been also an increasing interest in small-scale flapping-wing UAVs in academic research groups and civilian companies. One factor that helped revive interest in this class of aerial vehicles was their aerodynamic properties at very small scales. The efficient operation of these vehicles at low Reynolds numbers has motivated the development of insect-like micro UAVs such as the RobobBee [Jafferis et al., 2019] as well as hummingbird-like UAVs such as the AeroVironment Nano Hummingbird [Keennon et al., 2012], shown in Fig. 1.3. The natural appearance of bird-like aerial robots, such as the Robird [Folkertsma et al., 2017], has also found a niche market in the civilian sector to be used as a deterrent for real birds at airports, harbors, and waste management facilities. Successful companies that utilized such unique feature of flapping-wing robots include The Drone Bird Company (formerly Clear Flight Solutions) and AERIUM Analytics.
References:
Finnegan, P. (2019). World civil unmanned aerial systems market prole & forecast 2019. Technical report, Teal Group Corporation, Fairfax, VA, USA.
Folkertsma, G. A., Straatman, W., Nijenhuis, N., Venner, C. H., and Stramigioli, S. (2017). Robird: a robotic bird of prey. IEEE Robotics and Automation Magazine, 24(3):22{29.
Goodheart, B. J. (2011). Tracing the history of the ornithopter: Past, present, and future. Journal of Aviation/Aerospace Education & Research, 21(1):31-44.
Jafferis, N. T., Helbling, E. F., Karpelson, M., and Wood, R. J. (2019). Untethered flight of an insect-sized flapping-wing microscale aerial vehicle. Nature, 570(7762):491-495.
Keennon, M., Klingebiel, K., and Won, H. (2012). Development of the nano hummingbird: A tailless flapping wing micro air vehicle. In 50th AIAA aerospace sciences meeting including the new horizons forum and aerospace exposition, page 588.
Mazur, M. and Wisniewski, A. (2016). Clarity from above-pwc global report on the commercial applications of drone technology. Technical report, PwC Polska, Warszawa, Poland.
Munson, K. G. (1969). Aircraft of World War I. Ian Allan Ltd.
Valavanis, K. P. and Vachtsevanos, G. J. (2015). Handbook of unmanned aerial vehicles, volume 1. Springer.