*7.4. Airframe Design and Certification Considerations*

Considerable recent published work has considered the alleviation of gust loads on aircraft [72–76] and in some cases even harvesting it [77–79]. Severe gusts around buildings can pose a major challenge for flight of different vehicular scales and configurations. Smaller UAVs are more sensitive to the disturbances, however larger UAVs are still affected albeit to a lesser extent. The latter will depend on the relative magnitude and scale of a gust with respect to the aircraft's scale. Also, the UAV configuration (rotary vs fixed wing) will respond differently to the disturbances. Hybrid configurations which have a combination of lifting surfaces (i.e., fixed wing) and an array of thrusting disks (i.e., rotary wing) are well suited for close proximity flight to buildings. However, there is a spectrum of design possibilities which require careful design choices to truly alleviate the disadvantages of both fixed and rotary wing. Further research is required to identify the intrinsic aerodynamic deficiencies of these hybrid configurations and what are they particularly susceptible to. For example, fixed-wing craft will stall if flown too slow, while rotary wing craft are susceptible to the vortex ring state and weather cock stability. Some deficiencies may be resolved with hybrid configurations while others may persist or even give rise to new deficiencies especially during hover. Vehicles with large surface areas facing the wind direction (e.g., tilt wings) will experience significant attitude control and flight-path tracking challenges due to the relatively large forces generated by these surfaces. Such designs should be avoided where possible if a UAV is expected to fly at low speeds near buildings and gust-generating infrastructure. The frontal projection area of the UAV regardless of the configuration needs to be minimized most critically during proximity flight. This may be even achieved

through active wing area reduction, but the structural and mechanical challenges of an airframe capable of reducing area or changing its wing planform. This design challenge is complex but not impossible. There are also other means of mitigating turbulence and gusts through the control systems [80–82], aerodynamic configuration [68,83–86], and novel sensors [66,67,87]. Counteracting such flow disturbances comes at the cost of increased weight and power demands which will affect range and battery consumption. The question then becomes, how smooth of a flight will the passenger demand? How much control will we need to give to the pilot and/or the system?

As a hybrid UAV flies slow and in proximity to a building, any fixed wing control surface on the airframe become ineffective in controlling attitude due to low speed. The effectiveness or relative force of control surfaces reduces by the square of the flow velocity it is exposed to. In this case the UAV relies mainly on the rotary wings for lift and attitude control. There are opportunities for unconventional fixed wings designs to increase the control authority and rapidity [84,85], however the rotary wings will be required to achieve the majority of the control and lifting work in such scenarios, and therefore require the ability to rapidly adjust thrust to mitigate any gusts encountered. Variable-pitch propellers are effective in generating rapid actuation and more efficient thrust vectoring to enable the vehicle to approach a vertiport at low approach speeds with more control authority and stability.

From a certification standpoint, AAM airframes need to demonstrate the ability to counter attitude disturbances and flight path deviations for a reasonable range of wind speeds and gust conditions to make AAM operational for the majority of the year despite weather. Coping with high wind speeds, certification should include a demonstration of limits on the angular perturbations allowed in the vehicles' three axes during the highest operational wind and gust magnitudes. These angular limits should be selected to ensure that the physical extremities of the vehicle do not collide with the vertiport during touchdown or take-off. Limits should also be imposed on how much flight-path drift occurs for a range of wind and gust speeds to reduce risk of collision with infrastructure. Airframe manufacturers can either conduct physical experimentations to demonstrate compliance with the limitations imposed or utilize numerical-based modelling (with a form of validation) [61].

Helicopter certifications requirements rely on the presence of human pilots on board that can assess hazardous situations. Regulations for autonomous UAV operations in cities (especially large air taxis) will be different and rely on measurable numerical thresholds, which are used by the flight control system for automated decision making and planning, given there is no human-in-the-loop to make such rapid judgments:


Regulations for autonomous UAV operations in cities (especially large air taxis) will have to be more detailed and require the reliance on measurable numerical thresholds, which are used by the flight control system for automated decision making and planning, given there is no human-in-the-loop to make such rapid judgments.
