How stakeholder requirements constrain drone-based medical delivery: probabilistic sizing of eVTOL aircraft at high altitude.

Drone-based medical delivery can overcome geographic barriers to healthcare in highland regions, but designing aircraft for this mission requires navigating uncertainty that originates outside engineering: physicians define cold-chain requirements, regulators mandate safety systems, and health planners set delivery speed targets, each imposing design penalties whose magnitudes are uncertain. No existing aircraft sizing framework propagates this requirement-origin uncertainty alongside classical technological uncertainty. Here we introduce Requirement Impact Kits (RIKs)-composable modules with uncertain engineering coefficients-within a three-level probabilistic framework that distinguishes requirement-scenario, engineering-translation, and parameter-level uncertainty. Applied to eVTOL medical delivery at 2,800 m altitude in the Quito Metropolitan District, comparing Lift+Cruise, Tilt-Rotor, and Multirotor architectures, the framework reveals that stakeholder decisions dominate the design space: demanding urgent delivery reduces aircraft feasibility by 24 percentage points-four times the impact of adding cold-chain thermal management. Sobol sensitivity decomposition shows that 98% of parameter-level variance is epistemic and reducible through rotor testing, while battery energy density contributes less than 2%. The Lift+Cruise architecture achieves the highest robustness, and an altitude-cruise speed feasibility map provides health system planners with a quantitative tool for negotiating delivery time requirements against operational reliability.