Relevance of flying qualities deterioration boundary for a helicopter

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Tajar, A.R.
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Flying Qualities is one of the very important aspects of helicopter design and operational use. The flight envelopes associated with flying qualities are to be clearly understood and established. The regulations and operational requirements are in many cases, no more than guidelines and essentially to be converted into engineering parameters. The establishment of helicopter limitations majorly depends on the theoretical analysis and component testing. These limitations are generally demonstrated during development and certification flights. The establishment of limitations also ensures the smooth extension of the flight envelope for the growth potential. The conversion of these limitations to engineering parameters is very important from the operation and safety point of view. During some flight regimes, it becomes vital to display these engineering parameters as guidance to indicate helicopter limitations. These indications also ensure safety of the helicopter by respecting the operational boundaries associated with environment. This paper examines the various levels of limitations associated with rotor stall. These flight limitations come from Engine/Transmission limits, control margins and rotor aerofoil characteristics. These limitations appear during the flight depending on the helicopter configuration and the environment. The maximum speed at low altitude is generally limited by the Transmission limit however it is limited by Engine limits at high altitudes. The Flying Qualities boundary comes into picture when the maximum speed and steady bank turn (without loss of altitude) is limited by the control margin and not by the Engine power. The flying qualities limitations are above the Engine limits at low altitude however it moves below the engine limits during high altitude operations. To achieve maximum level flight speed, pilot gets driven by engine limits. When the engines are new, the engine limits may be far from the flying qualities because of availability of higher power available from new engines. The pilot may get misguided by the power available and can still push for higher speeds. Pushing for speeds beyond flying qualities deterioration boundary results in loss of control margin and pitch, roll or yaw oscillations. The several level of limitations associated with rotor stall boundary are function of Blade loading and advance ratio. In turn, blade loading and advance ratio reflect the properties such as maximum lift coefficient, Mach number, drag divergence number and blade geometry. The prediction of flying qualities deterioration was based on the blade loading and advance ratio. The prediction of Flying Qualities (FQ) deterioration limit was also validated by availability of control margins. It was observed that the FQ deterioration limit was also related to loss of control margin. Flight tests were carried out with different All up weights and altitudes condition from low speed to maximum level flight speed. Flight tests data was gathered and stall area of the rotor disk was derived with the help of constant inflow downwash model and aerofoil data. The rotor capability was also compared for two different helicopters with same rotors. It was proved that the reduction in fuselage drag and employment of auxiliary lift source of lift such as wing improves the overall performance of the helicopter. Furthermore, a warning logic was developed to predict the flying qualities deterioration boundary for a helicopter.