Global Sensitivity of Constraint Analysis method: Drag coefficient and Stall Speed selection for Small UAVs

Abstract

The Constraint analysis method is applied in the early stages of aircraft conceptual design for determining preliminary sizing parameters such as wing loading and power loading. However, its application to small UAVs is challenged by uncertainties in several key input parameters, particularly those that require detailed geometric or aerodynamic information that is typically unavailable during the early design stages. To help designers to identify which parameters demand precise specification and which have negligible impact, a global sensitivity analysis was conducted. This allowed us to quantify the influence of each design parameter on constraint analysis outputs and classify them based on their influence. The results identify several dominant parameters. While some are predefined, the zero-lift drag coefficient requires detailed aerodynamic data that is unavailable early in design. To address this, we propose using parasite drag estimation method, adapted for small UAVs through the use of UAV-specific wetted-area ratios. A parametric study of stall speed shows that both power loading and wing loading exhibit a local maximum, suggesting that stall speed can be treated as a trade off parameter rather than a fixed requirement. For UAV designers, this work provides: (1) a sensitivity framework for constraint analysis, and (2) practical methods for selecting appropriate values for drag coefficient and stall speed during the early design stages.