Development of an Analytical Model to Investigate the Integrated Effects of Ground Effect and Rotor–Wing Interaction on Tiltrotor Download-to-Thrust Ratio

Mahdavi, Amir; Ghorbanian, Kaveh

doi:10.22034/joae.2026.590709.1333

Development of an Analytical Model to Investigate the Integrated Effects of Ground Effect and Rotor–Wing Interaction on Tiltrotor Download-to-Thrust Ratio

Articles in Press

Document Type : Original Article

Authors

amir mahdavi ¹

Kaveh Ghorbanian ²

¹ PhD Student, Department of Aerospace Engineering, Sharif University of Technology, Tehran, Iran

² Professor, Department of Aerospace Engineering, Sharif University of Technology, Tehran, Iran

10.22034/joae.2026.590709.1333

Abstract

This study presents a novel analytical method for deriving a formulation to calculate the download-to-thrust ratio for a tiltrotor aircraft. Using this relationship, the influence of height above ground—a key operational parameter—on download and thrust forces is investigated, enabling a comparison between out-of-ground-effect (OGE) and in-ground-effect (IGE) conditions. The results demonstrate that accounting for ground effects significantly alters predictions regarding download and thrust. Furthermore, the study analyzes the impact of the rotor-to-wing spacing (rotor position) and the extent of wing coverage by the rotor disk on the download force. To validate the proposed analytical model, the results are compared with previous studies, showing good agreement. The model also provides a computationally efficient framework for the rapid parametric assessment of download in tiltrotor aircraft. It is found that at a height of 1R (IGE condition), increasing the rotor-to-wing spacing by approximately 50% (from 0.45R to 0.65R, where R is the rotor radius) results in an approximately 8% reduction in the download-to-thrust ratio. Conversely, increasing the height by 50% (from 1R to 2R) with a rotor-to-wing spacing of 0.45R leads to an approximately 50% increase in the download-to-thrust ratio. On average, rotor-to-wing spacing and height above ground account for 14% and 86%, respectively, of the download-to-thrust ratio. This highlights the dominant role of ground effects regarding download in tiltrotor aircraft and underscores the necessity of incorporating these effects into analytical models.

Keywords

Tiltrotor aircraft

Rotor-wing interaction

Download-to-thrust ratio

Ground effect

Subjects