Blade pitch

About: Blade pitch is a research topic. Over the lifetime, 5321 publications have been published within this topic receiving 63134 citations.

Rotor and aircraft passively stable in hover

Abstract: The present invention discloses a rotor that enables an aircraft to be passively stable in hover The rotor, having a generally fixed geometry is tiltably connected to its rotor shaft The outer part of the rotor blades have a pitch fixed relative to the rotational plane, whereas the inner part of the rotor blades have a pitch fixed relative to a reference plane perpendicular to the rotor shaft

Vortex-Shedding Noise and Potential-Interaction Noise Modeling by a Reversed Sears' Problem

Abstract: The classical Sears’ problem of calculating the lift fluctuations, due to the impingement of upstream disturbances on the leading edge of a flat-plate airfoil, is recast in the present paper, essentially by reversing the mean flow carrying the velocity disturbances. The latter are considered to interact with the trailing edge when being evacuated downstream, instead of impinging on the leading edge from upstream, and the Kutta condition is ignored. A compressible solution is proposed, which is a straightforward adaptation of Amiet’s formulation, dedicated to the calculation of the far-field noise. The first application is the narrow-band vortex-shedding noise associated with the von Karman street shed in the wake of a thick plate. For this, a convection speed lower than the surrounding flow speed is accounted for. Once the PSD and the spanwise correlation length of the transverse velocity fluctuations in the near wake are known as input data, the model is shown to predict sound levels, which agree well with the measurements. The second application is the modeling of the potential-interaction noise generated as rotor blades operate in the flow distortion caused by a radial cylinder placed at a short downstream distance. The distortion and the blade loading harmonics are derived analytically. The shrouded tail rotor of an helicopter is investigated as a special case. The effect of a variable blade pitch angle, related to a variable thrust, is assessed. A noise increase by 0.4 dB per degree is found, compatible with previously reported measurements. Both test cases suggest that the proposed model is a reliable pre-design tool for configurations in which the unsteady aerodynamics of blades clearly results from downstream disturbances.

Speed and collective pitch bias of helicopter longitudinal cyclic pitch

Abstract: A bias actuator, such as an extensible link, in the longitudinal cyclic pitch channel of a helicopter is provided with inputs as a function of airspeed multiplied inversely with collective pitch, and as a function of the rate of change of collective pitch stick position, so as to enforce positive angle of attack and speed stability and positive static pitch trim gradient and to decouple collective pitch from the longitudinal cyclic pitch channel at cruise airspeeds, the invention compensates, inter alia, adverse control effects of tail stabilizer surfaces at cruise speeds. A pair of indicators display bias commands and actual bias responses.

Variable diameter rotor having an offset twist

Abstract: A variable diameter rotor for a tilt rotor aircraft has an inboard section having a linear twist from a root end to an outboard end thereof, and an outboard section telescoped over the inboard section which additionally has a linear twist from a root end to an outboard end thereof. The outboard section is disposed at an offset of about 4° to 15° relative to the inboard section to provide a step wise change in twist angle when in the extended condition, such that the rotor when operated in the helicopter mode, can be operated at high blade pitch angles without premature stalling along the inboard airfoil surfaces This allows efficient operation at high thrust coefficient/solidarity ratios. Optionally, the trailing edge of the outboard section is rotated along the aft 30% of chord of the airfoil section to provide an equivalent twist increase in the offset at the transition between the blade sections. When in the forward flight cruise mode, the inboard equivalent twist increment allows operation of the inboard airfoil sections at higher pitch angles to reduce negative lift regions and increase cruise propulsion efficiency.

Apparatus and method for reducing asymmetric rotor loads in wind turbines during shutdown

Abstract: An apparatus and method for reducing asymmetric rotor load in a wind turbine includes calculating a time delay for pitching each blade toward feather upon initiation of a shutdown condition. The blades with the larger blade angle begin moving toward feather with an initial pitch rate, while the blade with the smallest blade angle begins moving toward feather with a final pitch rate. Once all the blades have reached approximately an identical blade angle, the blades move simultaneously together to feather at the final pitch rate. By introducing the time delay for pitching the blades having higher blade angles at the final pitch rate, a simple, time-based correction of initial conditions during shutdown reduces the extreme loads on turbine components.