As a helicopter translates to cruise, the induced power decreases, and the profile power increases, with the profile power dominating the tail rotor. The main rotor speed corresponding to the minimum main rotor power increases, if the change of tail rotor power in hover is considered. In hover, the tail rotor may not be able to provide enough thrust to counter the main rotor torque, if it is slowed to follow the main rotor speed. However, it increases the main rotor torque and the corresponding required tail rotor thrust to trim, which then decreases the yaw control margin of the tail rotor. Reducing the main rotor speed can result in lower main rotor power at certain flight conditions. The predictions of the main and tail rotor powers are generally in good agreement with flight tests, which justifies the use of the present method in analysing main and tail rotors. A helicopter model able to predict the main rotor and tail rotor powers is presented, and the flight test data of the UH-60A helicopter is used for validation. The correlation of vibratory hub loads is generally poor by both methods, although the coupled analysis somewhat captures general trends.ĪBSTRACTVariable tail rotor speed is investigated as a method for reducing tail rotor power, and improving helicopter performance. Maximum reduction of IBC actuator force is better predicted with CAMRAD II, but general trends are better captured with the coupled analysis. Coupled CAMRAD II/OVERFLOW 2 shows excellent correlation with the measured rotor power variations with the IBC phase at both μ = 0.35 and μ = 0.4. However, the correlation degrades at μ = 0.4. CAMRAD II predicts the rotor power variations with the IBC phase reasonably well at μ = 0.35. At the optimum IBC phase for rotor performance, IBC actuator force (pitch link force) decreased, and neither flap nor chord bending moments changed significantly. Measured data show a 5.1% rotor power reduction (8.6% rotor lift to effective-drag ratio increase) using 2/rev IBC actuation with 2.0° amplitude at μ = 0.4. Wind tunnel measurements of performance, loads, and vibration of a full-scale UH-60A Black Hawk main rotor with an individual blade control (IBC) system are compared with calculations obtained using the comprehensive helicopter analysis CAMRAD II and a coupled CAMRAD II/OVERFLOW 2 analysis. The study produced eleven options that can improve and/or enhance the next generation Black Hawk's FOV if incorporated into the new design. However, under dynamic conditions the UH-60A cockpit design and normal flight characteristics substantially reduce the FOV in critical areas. The only exception is the obstructed view that the door and windshield vertical structures create. The study revealed the current UH-60A design meets the requirements of MIL-STD-850B under static conditions. Also, the study team collected technical data related to military rotary wing design, administered a survey to pilots and interviewed users and other technical experts. Close attention was given to dynamic flight characteristics that affect FOV. The study involved a comprehensive review of Army requirement documents, existing FOV studies, and accident data. Army tasked an independent contractor to investigate the problem and propose alternatives. To improve the FOV in the next generation Black Hawk, the U.S. Changing flight tactics and increased use of Night Vision Goggles (NVGs) has focused attention on the limited Field of View (FOV) of the Army UH-60A Black Hawk Helicopter.
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