Publications:

ALLEVIATION OF DYNAMIC STALL INDUCED
VIBRATIONS USING ACTIVELY CONTROLLED FLAPS

ABSTRACT

This paper presents a successful treatment of the helicopter vibration reduction problem at high advance ratios, taking into account the effects of dynamic stall. The ONERA model is used to describe the loads during stall, in conjunction with a rational function approximation for unsteady loads for attached flow. Single and dual actively controlled flaps are used to reduce vibrations. Successful vibration reduction is demonstrated over the entire range of advance ratios considered (0.3 = µ = 0.45). This study represents the first successful implementation of vibration reduction in presence of dynamic stall, and physical explanation for the vibration reduction process is also provided. A methodology for accounting for the increased drag and power penalty associated with flap deflection is also described. Finally, saturation limits on the control deflections are imposed, which keep flap deflections in a practical range. Effective vibration reduction is achieved even when imposing practical saturation limits on the controller. Download the entire paper (~261KB)

 

MODELING APPROACHES TO HYPERSONIC AEROELASTICITY

ABSTRACT

The hypersonic aeroelastic problem of a double wedge airfoil typical cross-section is studied using three different unsteady aerodynamic loads: (1) third order piston theory, (2) Euler solution, and (3) unsteady Navier Stokes aerodynamics. Computational aeroelastic response results are obtained, and compared with piston theory solutions for a variety of flight conditions. Aeroelastic behavior is studied for 7 < M < 15 at an altitude of 70,000 feet. A parametric study of offsets and wedge angles is conducted. Piston theory and Euler solutions are fairly close below the flutter boundary, and differences increase with increase in Mach number, close to the flutter boundary. Differences between viscous and inviscid aeroelastic behavior can be substantial. The results presented serve as a partial validation of the CFL3D code for the hypersonic flight regime. Download the entire paper (~687KB)

 

 

ALLEVIATION OF ROTOR VIBRATIONS INDUCED BY
DYNAMIC STALL USING ACTIVELY CONTROLLED FLAPS
WITH FREEPLAY

ABSTRACT

This paper presents a successful treatment of the helicopter vibration reduction problem at high advance ratios, taking into account the effects of dynamic stall. The ONERA model is used to describe the loads during stall, in conjunction with a rational function approximation for unsteady loads for attached flow. This study represents the first successful implementation of vibration reduction in presence of dynamic stall, using single and dual trailing edge flap configurations. A physical explanation for the vibration reduction process is also provided. Saturation limits on the control deflections are imposed, which limit flap deflections to a practical range. Effective vibration reduction is achieved even when imposing practical saturation limits on the controller. Finally, the robustness of the vibration reduction process in the presence of a freeplay type of nonlinearity is also demonstrated. Download the entire paper (~896KB)

 

ACTUATOR SATURATION AND ITS INFLUENCE ON VIBRATION
REDUCTION BY ACTIVELY CONTROLLED FLAPS

ABSTRACT

The influence of actuator saturation on the vibration reduction abilities of an actively controlled flap is investigated. An aeroelastic model of a four bladed hingeless rotor with a free wake is used for the analyses. Three methods for constraining flap deflections are studied at two limiting values, two and four degrees, of maximum flap deflection. Results indicate that neither scaling nor clipping of the optimal control flap deflection to the maximum flap deflection provides acceptable vibration reduction. A newly developed control method with saturation constraints shows exceptional reduction of vibrations. This new control method reduces vibrations to similar levels as the unconstrained optimal control while constraining maximum flap deflections to the limiting values. Download the entire paper (~215KB)

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