DEFINING CONSISTENT ADS-33-METRICS FOR AGILITY ENHANCEMENT AND STRUCTURAL LOADS ALLEVIATION

The aim of the present paper is to develop a new set of consistent, complementary metrics for agility and structural loads pertaining to vertical manoeuvres in forward flight. In this context, the ADS-33 low-speed attitude quickness parameter is converted into two new metrics: 1) agility quickness (gamma quickness) defined as the ratio of peak quasi-steady normal acceleration to flight path angle change during vertical manoeuvres, and 2) vibratory load quickness defined as the ratio of peak amplitude of vibratory load to flight path angle change. While the agility quickness characterises the helicopter performance during manoeuvring flight, the load quickness is used to quantify the build up of loads in the rotor. The paper presents how the new metrics can be used to optimise the rotor loading without compromising the aircraft manoeuvrability. In this context, open-loop second harmonic cyclic control inputs, superimposed on the primary longitudinal cyclic pitch controls are analysed, resulting in both helicopter performance improvement and vibratory loads reduction. The case of a helicopter flying at high speed with stall on the retreating blade is investigated, this case being particularly sensitive to higher harmonic blade pitch control. We show how the stalled area can be redistributed over the rotor disk by use of second harmonic cyclic pitch and how this transposes into changes in agility and load quickness parameters. While the use of higher harmonic control through the non-rotating swashplate has been the subject of considerable previous research, the current approach is intended to demonstrate how consistent metrics allow for more efficient and insightful multi-disciplinary optimisation. NOTATIONS