Blade Root Region Research Articles

Experimental Study on Flow around Blades of Horizontal Axis Wind Turbine in Wind Tunnel

This paper describes the measurement of the flow field around rotor blade. Three-bladed upwind rotor was tested in an open jet type wind tunnel. The rotor has a diameter of 2.4 m. Flow field around rotor blade was measured with the use of two-dimensional LDV. The flow field was measured in x-y plane and z-y plane. The circulation around the blade sections were calculated by flow vectors around the rotor blade. The velocity vectors at optimum operation show a smooth flow around the blade and the bound vortex around blade cross-section seems to be persistent. On the other hand, the velocity vectors at stall condition demonstrate significant fluctuations in the near wake and separation on the blade suction side was observed. The circulation around blade span-wise section was calculated at the certain control volume. By the observation of flow field and calculated results of circulation, it seems that the flow is separated at the blade from middle-span region to tip region at stall condition. No separation was observed at the blade root region.

Helicopter Vibration Reduction Using Discrete Controllable-Stiffness Devices at the Rotor Hub

The present study demonstrates that optimal multi-cyclic stiffness variations of discrete controllable stiffness devices in the rotor blade root region can reduce the 4/rev vibratory hub loads of a four-bladed hingeless rotor helicopter. The controllable stiffness devices (flap, lag, and torsion devices) are modeled as discrete springs whose stiffness coefficients can be varied, and a gradient-based optimization scheme is used to determine optimal multi-cyclic device stiffness variations that minimize a composite index comprising of all six components of vibratory hub loads. Multi-cyclic stiffness variations of the flap and lag devices are most influential, and when optimal 2/rev and 3/rev stiffness variations of these devices were used in combination the vibratory hub drag force was practically eliminated and the vibratory hub side force was reduced by 55%. No significant detrimental effects were observed on the first through the fifth harmonics of the vibratory blade root loads. Multi-cyclic (3/rev and 4/rev) stiffness variations of the torsion device produced only small reductions in the 4/rev hub vertical force. Multi-cyclic stiffness variations of the flap and lag devices were seen to be effective in reducing hub vibration even when there were changes in fundamental rotor properties such as the flap, lag, and torsion stiffness of the root (flexure) element, and the cruise speed.

Optimal control of helicopter vibration through cyclic variations inblade root stiffness

This study demonstrates that optimal multi-cyclic variations of theblade root flap and lag stiffness can produce simultaneous reductions in allof the components of vibratory hub loads of a four-bladed hingeless rotorhelicopter. Both gradient- and non-gradient-based optimization schemes aresuccessful in reducing the hub vibrations. The required stiffness variationscan be reduced (without significantly compromising performance) by introducinga penalty on the input in the objective function used for minimization.Reductions in the vibration performance index of over 90% were seen withoptimal 2/rev and 3/rev flap and lag stiffnessvariations. The concept was effective in reducing vibrations over a range ofvariations in configuration (fundamental flap, lag, and torsion frequencies)and operational parameters (forward speed). Furthermore, it was shown thatstiffness variations of discrete flap and lag springs introduced tothe blade root region are effective in reducing vibratory hub loads. Thus, theintroduction of discrete controllable stiffness elements (devices) is a viablemethod for varying the stiffness of the blade root region.

Helicopter Vibration Reduction through Cyclic Variations in Rotor Blade Root Stiffness

This study analytically examines the influence of cyclic variations in flap-, lag-, and torsion-stiffness of the blade root region (at harmonics of the rotational speed), for reduction of vibratory hub loads of a helicopter in forward flight. The results indicate that considerable reduction in hub vibrations is possible using small-to-moderate amplitude cyclic variations in stiffness (no greater than 15% of the baseline stiffness value). Torsion stiffness variations produced moderate reductions in vertical hub force, lag stiffness variations produced substantial reductions in all hub forces and the hub yaw moment, and flap stiffness variations produced very significant reductions in all hub forces and the hub roll and pitch moments. The amplitude of the cyclic stiffness variations required generally increase with increasing forward speed, for comparable reductions in vibration. At any given forward speed, if the amplitudes of cyclic stiffness variation are too large, the hub vibrations can actually increase. The stiffness variations that reduce the vibratory hub loads could produce increases in certain vibratory blade root load harmonics. Vibration reductions are achieved due to a decrease in the inertial contribution to the hub loads, or a change in relative phase of various contributions.