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The purpose of this study is to use the individual blade pitch control (IBC), reduce actively both the rotor hub vibratory loads and airframe vibration responses for the lift-offset compound helicopter at a high-speed flight condition.

The Sikorsky X2 technology demonstrator (X2TD) is used as the lift-offset compound helicopter. The X2TD lift-offset rotor is modelled and its rotor hub vibratory loads at a flight speed of 250 knots are predicted using a rotorcraft comprehensive analysis code, CAMRAD II, and the airframe structural dynamics is represented with a finite element analysis code, MSC.NASTRAN. When the propulsive trim methodology is applied for rotor trim, the best input condition for IBC using multiple harmonic inputs is searched to reduce the rotor vibration, while the rotor aerodynamic performance (the rotor effective lift-to-drag ratio) is improved or maintained at least. Finally, the reduction in airframe vibration responses is investigated when the best input condition for IBC with multiple harmonics is applied to the lift-offset rotor.

When the IBC with the single harmonic input using the 2/rev actuation frequency, amplitude of 2° and control phase angle of 120° (2P/2°/120°) is considered for X2TD rotor, the rotor vibration is reduced by about 26.37% only and the rotor effective lift-to-drag ratio increases slightly by 0.98%. When X2TD rotor uses the IBC with multiple harmonic inputs (2P/2°/45° + 5P/1°/90°), the rotor hub vibratory loads and airframe vibration responses are reduced by 44.69% and from 0.48 to 79.10%, respectively, while rotor effective lift-to-drag ratio is improved by 0.77%, as compared to the baseline without IBC.

This study is the first study to use the 2/rev actuation for IBC to the four-bladed lift-offset coaxial rotor and to investigate to obtain simultaneously the rotor vibration reduction, rotor performance improvement and airframe vibration reduction, using IBC with multiple harmonic inputs.

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.

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.

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.

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.