Lift offset: an analytical insight

Abstract

Purpose This paper aims to present an analytical insight into the effect known as lift offset. This effect may be of advantage in terms of power consumption when configurations that consists of two main rotors (coaxial, tandem, etc.) are operated at high speed forward flight. Fundamentally, each of the main rotors is producing a roll moment in opposite direction that is overall canceled to allow trim. The first-order phenomena of lift offset are therefore related to the load distribution over each rotor and may be analyzed independently of the interaction between the rotors. Design/methodology/approach Using simple analytical model, this paper analytically and explicitly evaluates the lift offset effect. It also demonstrates in a closed analytical form the various phenomena involved, including the optimal values obtained. Findings The paper shows that the main effect of lift offset is its influence on the lift coefficient distribution over the disk. It is also demonstrated that lift offset substantially reduces both the average and the azimuthal variation of the lift coefficient. Originality/value Comparison with numerical predictions shows adequate correlation. Overall, the insight provided may be use as a guideline for advance design of rotorcraft configurations of coaxial/tandem rotor systems.