CFD-Derived Parasite-Drag Law of an Octocopter Airframe with Endurance Implications

Octocopters are increasingly utilized, but designers lack a straightforward and reliable method for calculating airframe drag, critical for accurate power and endurance estimation. This study isolates airframe-induced drag, excluding propellers, using computational simulations of a 7.2-kg octocopter operating at speeds from 1 to 15 m/s. Standard simulation techniques resolve near-wall flow phenomena, and the computational setup is verified for accuracy. Comparative analysis of airflow at 4 and 15 m/s shows that increased drag at higher speeds is mainly due to the expansion of disturbed flow regions, especially at arm–hub and landing-gear junctions, where flow separation and wake formation intensify. The measured drag values closely follow a single airspeed-based formula, allowing straightforward drag prediction within this velocity range. Expressed in terms of electrical power, this law enables the determination of optimal cruising speeds for maximum range without further simulations. The results also show that geometric modifications, such as rounding sharp corners at critical junctions and refining landing-gear profiles, can significantly reduce drag.