## 2 UAV Platform Evolution
### UAV Platforms Based on Aerodynamic Features  
UAVs exhibit great diversity in their aerodynamic characteristics, which enables them to serve varied roles and applications. Based on the various attributes, such as their lift generation mechanism, flight speed, and range, UAVs can be categorized into quadcopters, fixed-wing, hybrid VTOL, lighter than air, and flapping wings. **Quadcopters** have rapidly become one of the most ubiquitous drone categories deployed across metropolitan areas due to their exceptional maneuverability and ability to take off, land, and hover in confined spaces. As rotorcrafts, quadcopters utilize four horizontally oriented lifting rotors to generate the vertical thrust needed for hovering and low-speed flights [55]. Unlike helicopters relying on a single main rotor for lift, quadcopters distribute thrust across four smaller rotors, providing a higher payload capacity for a given size and improved control redundancy and safety [3]. Adjusting the speed of each rotor enables omnidirectional stability and maneuvering in tight urban areas not accessible by fixed-wing UAVs. This makes quadcopters ideal for the proximate scanning and inspection of buildings, infrastructures, urban canyons, parks, and other environmental elements, which comprise complex cityscapes. Table 1 summarizes the differences between different UAV platforms based on their aerodynamic features, applications, and technical limitations. While quads were initially limited by short flight times, the advances in batteries, motors, and materials have improved their endurance of 30 min or longer, even for small models [56]. Larger octocopter or hexacopter UAVs can extend their flight duration further and lift heavier sensor payloads. Camera gimbals minimize image blurring during flight maneuvers and hovering. When choosing multirotors for climate change research in urban areas, factors, such as maximum sensor weight, wind resilience, noise profiles, safety features, and integration options, for data communications and processing warrant consideration. **Ducted-fan** UAVs utilize an enclosed rotor system combining the benefits of multirotor and fixed-wing drones. They employ enclosed fans mounted within circular ducts to generate a vertical lift, eliminating exposed blades that can pose safety risks [58]. Ducting also streamlines the airflow, increasing the efficiency over open rotors [59]. This provides greater endurance than quadcopters, with flight times of around 45-60 minutes, depending on the conditions [55]. Ducted fans typically have a fixed wing for a forward flight, such as conventional UAVs. Transitioning between hovering and high-speed cruise flight modes gives ducted fans flexibility for constrained takeoff and landing tasks and sustained transits [60]. Compared to quadcopters, they can cover larger areas rapidly while still allowing a close inspection through stable hovering, as illustrated in Figure 3. Ducting shields rotors from obstruction, enabling drone operations in confined spaces inaccessible to fixed wings [39]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Platform** & **Flight Speed** & **Flight Range** &
\begin{tabular}{c} **Applications in the** \\ **Built Environment** \\ \end{tabular} & **Limitations** \\ \hline Quadcopters & 0–35 mph & 1–3 km & Urban inspection & Limited payload capacity \\  & & & Urban microclimate & Short flight times \\ Ducted Fan & 0–60 mph & 2–7 km & Utility inspection & Limited payload \\  & & & Vertical infrastructure mapping & Complex maintenance \\ Fixed Wings & 50–90 mph & 10–40 km & Urban thermal mapping & Require assisted launch/landing \\  & & & Air pollution monitoring & Minimal maneuverability \\ Hybrid VTOL & 0–80 mph & 5–25 km & Large-scale mapping & Complex transition mechanism \\  & & & Environmental monitoring & Heavier than fixed wings \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Features of different UAV platforms and their applications in the built environment [57; 57; 15].