Ahmed A. Hassan

Smart Material-Actuated Rotor Technology – SMART

Vibration, noise, and aerodynamic design compromises are primary barriers to further improvements in effectiveness of the helicopter. The MD900 light utility helicopter main rotor system is modified to include in-blade smart material actuation for active control. A piezoelectric (PE)-driven trailing edge flap is used for vibration, noise, and aerodynamic performance improvements. A shape memory alloy (SMA)-driven trailing edge trim tab is used for in-flight blade tracking. Sizing and conceptual design of an active flap and trim tab system for the MD900 helicopter were completed. Several two-dimensional airfoil and flap/tab models were wind tunnel tested and an aerodynamic database was established. Structural samples of the flap actuator mounts, flap section, tab, and full-span flap were fabricated and successfully tested. Several prototype actuators were developed and extensively tested to establish their performance and robustness in the dynamic operating environment. The flap actuator uses two biaxial PE stack columns, operating in a push pull mode, a column mid-support, and a stroke amplification mechanism. The tab actuator uses two biaxial SMA tubes for actuation/bias, an SMA-activated lock for power-off operation, and integrated microprocessor control electronics. Results to date confirm that smart material in-blade active control of a rotor is feasible and offers significant performance and cost benefits. Projected payoffs from reduced vibrations and noise as well as in-flight tracking include improved component lives, reduced maintenance and improved crew, passenger, and community acceptance.

Oscillating air jets for implementing blade variable twist, enhancing engine and blade efficiency, and reducing drag, vibration, download and ir signature

A porous surface on an aircraft structure driven with oscillating positive and negative pressures is used as an active control device for attenuating negative aerodynamic interactions. The porous surfaces can be driven with positive and negative pressures either continuously or when predetermined flight conditions are present. The porous surfaces can be used on rotor blades to reduce BVI noise in descent flight conditions. The porous surfaces can be configured on rotor blades for affecting blade variable twist in accordance with various flight conditions, and can further be incorporated for reducing rotor hub vibrations as well. Porous surfaces placed on aerodynamic surfaces below the rotor blades of a tiltrotor aircraft can attenuate or eliminate download and fountain flow conditions. When placed on the trailing edges of a tip jet-exhaust driven rotor blade, the porous surfaces of the present invention can supplement the tip jet momentum of the exhaust to thereby reduce an amount of exhaust needed to drive the rotor blade.

A Study of Rotor Tip Vortex Structure Alteration Techniques

Numerical studies of the tip-vortex structure from a hovering rotor with blowing are presented, and compared with the tip-vortex structure from a clean rotor, and from a rotorusing a passive trailing-edgetip device. A hybrid, Navier‐Stokes potential e ow method is used to model the e owe eld. A scheme that is e fth-order accurate in space is used to accurately capture the tip vortices. Velocity and vorticity data in the core of the vortex are studied at various planes behind the blade trailing edge. These data for the clean rotor are e rst compared with experimental results obtained for the same rotor. Previously published results for the tip-vortex structure from the same rotor employing a passive tip device are discussed next. Finally, results for a rotor blade with upper and lower surface blowing are presented. It is concluded that the tip-vortex strength may be modie ed through blowing. Blowing is found to be just as effective as a spoiler in altering the tip-vortex strength, but does not have the high drag and torque penalty associated with spoilers.

Prediction of blade-vortex interaction noise using airloads generated by a finite-difference technique

The present numerical finite-difference scheme for helicopter blade-load prediction during realistic, self-generated three-dimensional blade-vortex interactions (BVI) derives the velocity field through a nonlinear superposition of the rotor flow-field yielded by the full potential rotor flow solver RFS2 for BVI, on the one hand, over the rotational vortex flow field computed with the Biot-Savart law. Despite the accurate prediction of the acoustic waveforms, peak amplitudes are found to have been persistently underpredicted. The inclusion of BVI noise source in the acoustic analysis significantly improved the perceived noise level-corrected tone prediction.

Airfoil Design for Helicopter Rotor Blades— A Three-Dimensional Approach

A finite difference procedure has been developed for the design of airfoil sections for helicopter rotor blades. The procedure is based on the coupled three-dimensional direct solutions to the full potential equation inherent in the rotor flow solver (RFS2) and the two-dimensional inverse solutions to an auxiliary equation. Here, the evolution of the airfoil geometries, at a number of a priori defined radial control stations is driven by the user-prescribed pressure distributions and the flowfield requirements imposed by the RFS2 flow solver. In this respect, the influence of the finite aspect ratio blade, sweep, taper and, more importantly, the tip vortex wake, are reflected in the final airfoil designs. The lifting-line CAMRAD/ JA trim code was incorporated into the design procedure to allow for the simulation of the tip vortex wake effects. Results are presented for the redesign of a number of airfoil sections for a generic hovering rotor (with rectangular blades) with and without allowance for the tip vortex wake effects. Aerodynamic performance characteristics of the original blade and the redesigned blade in hover are assessed using the three-dimensional TURNS Navier-Stokes rotor flow solver.

Applications of zero-net-mass jets for enhanced rotorcraft aerodynamic performance

Numerical studies were conducted to investigate the benee cial effects of using arrays of zero-net-mass (ZNM) “ synthetic” jets on the aerodynamic characteristics of the NACA-0012 airfoil. Flowe eld predictions were made using modie ed versions of the NASA Ames ARC2D, U.S. Army 2DBVI unsteady, two-dimensional, compressible thin-layer Navier ‐Stokes e ow solvers. An unsteady surface transpiration boundary condition was enforced over a user-specie ed portion of the airfoil’ s upper, or lower, surface to emulate the time variation of the mass e ux out from and into the airfoil’ s surface. Special emphasis is placed on two-dimensional model problems that are representativeofthemorecomplexthree-dimensionalhelicopterrotore owe eldenvironment.Thenumericalresults have indicated that ZNM jets can be used to enhance the lift characteristics of airfoils (helicopter rotor blades ) and alleviate the impulsive aerodynamic response of a helicopter blade during encounters with the tip vortex wake. The effectiveness of ZNM jets for aerodynamic control is shown to increase with the increase in freestream Mach number and, more importantly, with the decrease in the ratio between the peak jet Mach number to the freestream Mach number. The striking similarities with the aerodynamics of an airfoil having an array of surface protuberances are presented.

Simulation of realistic rotor blade-vortex interactions using a finite-difference technique

A numerical finite-difference code has been used to predict helicopter blade loads during realistic self-generated three-dimensional blade-vortex interactions. The velocity field is determined via a nonlinear superposition of the rotor flowfield. Data obtained from a lifting-line helicopter/rotor trim code are used to determine the instantaneous position of the interaction vortex elements with respect to the blade. Data obtained for three rotor advance ratios show a reasonable correlation with wind tunnel data.

Effects of leading and trailing edge flaps on the aerodynamics of airfoil/vortex interactions

A numerical procedure has been developed for predicting the two-dimensional parallel interaction between a free convecting vortex and a NACA 0012 airfoil having leading and trailing edge integral-type flaps. Special emphasis is placed on the unsteady flap motion effects which result in alleviating the interaction at subcritical and supercritical onset flows. The numerical procedure described here is based on the implicit finite-difference solutions to the unsteady two-dimensional full potential equation. Vortex-induced effects are computed using the Biot-Savart Law with allowance for a finite core radius. The vortex-induced velocities at the surface of the airfoil are incorporated into the potential flow model via the use of the velocity transpiration approach. Flap motion effects are also modeled using the transpiration approach. For subcritical interactions, our results indicate that trailing edge flaps can be used to alleviate the impulsive loads experienced by the airfoil. For supercritical interactions, our results demonstrate the necessity of using a leading edge flap, rather than a trailing edge flap, to alleviate the interaction. Results for various time-dependent flap motions and their effect on the predicted temporal sectional loads, differential pressures, and the free vortex trajectories are presented