Each class of UAVs is driven by aerodynamic considerations that are either unique or very important for the future development of UAVs. This section describes some of these issues.
HALE UAVs developed in the past 30 years represent a wide range of flow conditions. From the low-speed Predator (Ernst, 1996) and Condor (Johnstone and Arntz, 1990) to Global Hawk (Heber, 1996) and Darkstar (Berman, 1997), these aircraft share several aerodynamic challenges, but also illustrate the differences among UAVs in this class. This section deals with some of the common aerodynamic challenges.
Although HALE UAVs may be required to operate at speeds higher than those for maximum aerodynamic efficiency for reasons of cost or mission effectiveness, the requirement for long endurance leads to lower speed operation, with a subsequent increase in vortex drag. Low-speed, high-altitude operations could also require that dynamic pressure be less than ideal. The standard approach to reducing induced drag is to increase wingspan (e.g., the wingspan of the 26,000-pound, jet-powered Global Hawk is 116 feet, the propeller-driven Boeing Condor of the 1980s 210 feet, and the solar-powered AeroVironment Centurion 240 feet). Large span, high-aspect-ratio wings pose difficulties, ranging from storage and transport to aeroelastic control, in addition to the performance penalties associated with the high unit-weights of the wings. Vortex drag can also be reduced by nonplanar lifting systems, including winglets, joined wings, C-wings, and other geometries (Kroo et al., 1996). Although these configurations reduce induced drag, their overall advantages over larger-span planar wings are small and mission specific. More radical approaches to drag reduction, such as tip turbines, may be more practical for UAVs than for commercial aircraft, but the potential for savings is uncertain at best.
Boundary-layer characteristics are among the most important issues for future UAV research and development. These issues are related to low Reynolds number, predicting and modifying boundary-layer transition, boundary-layer sensing and control, and airfoil section design.
Because HALE UAVs have high-aspect-ratio wings and fly in low-density conditions, often at low speeds, airflow is characterized by low Reynolds numbers (see Figure 3-1). Typical Reynolds numbers for the wings of HALE UAVs are