Vasileios Pastrikakis

Method for Calculating Rotors with Active Gurney Flaps

This paper builds on the Helicopter Multi-Block version 2 computational-fluid-dynamics solver of the University of Liverpool and demonstrates the implementation and use of Gurney flaps on wings and rotors. The idea is to flag any cell face within the computational mesh with a solid, no-slip boundary condition. Hence, the infinitely thin Gurney can be approximated by “blocking cells” in the mesh. Comparison between thick Gurney flaps and infinitely thin Gurneys showed no difference on the integrated loads; the same flow structure was captured and the same vortices were identified ahead and behind the Gurney. The results presented for various test cases suggest that the method is simple and efficient, and it can therefore be used for routine analysis of rotors with Gurney flaps. Moreover, the current method adds to the flexibility of the solver because no special grids are required, and Gurney flaps can be easily implemented. Simple two-dimensional aerofoil, three-dimensional wing, and rotors in hover and f...

Rotor Computations with Active Gurney Flaps

This paper builds on the Helicopter Multi-Block CFD solver of the University of Liverpool and demonstrates the implementation and use of Gurney flaps on wings, and rotors. The idea is to flag any cell face within the computational mesh with a solid, no slip boundary condition. Hence the infinitely thin Gurney can be approximated by “blocking cells” in the mesh. Comparison between thick Gurney flaps and infinitely thin Gurneys showed no difference on the integrated loads, the same flow structure was captured and the same vortices were identified ahead and behind the Gurney. The results presented for various test cases suggest that the method is simple and efficient and it can therefore be used for routine analysis of rotors with Gurney flaps. Moreover, the current method adds to the flexibility of the solver since no special grids are required and Gurney flaps can be easily implemented. Simple aerofoils, wings, and rotors in hover and forward flight were tested with fixed, linearly actuated, and swinging Gurneys, and the ability of the code to deploy a Gurney flap within the multiblock mesh is highlighted. The need for experimental data suitable for validation of CFD methods for cases of rotors with Gurney flaps is also highlighted.

Effect of Gurney Flaps on Overall Helicopter Flight Envelope

This chapter presents a study of the W3-Sokol main rotor equipped with Gurney flaps. The effect of an active Gurney is tested at low and high forward flight speeds to draw conclusions about the potential enhancement of the rotorcraft performance for the whole flight envelope. The effect of the flap on the trimming and handling of a full helicopter is also investigated. In the previous chapter, the Gurney proved to be efficient at medium to high advance ratios, where the power requirements of the rotor were decreased by up to 3.3%. However, the 1/rev actuation of the flap might be an issue for the trimming and handling of the helicopter. The current study builds on the idea that any active mechanism operating on a rotor could alter the dynamics and the handling of the helicopter. A closed loop actuation of the Gurney flap was put forward based on a pressure divergence criterion, and it led to further enhancement of the aerodynamic performance. Next, a generic light utility helicopter was built using 2D aerodynamics of the main aerofoil section of the W3-Sokol blade along with a robust controller, and the response of the rotorcraft to control inputs was tested. This analysis proved that the 1/rev actuation of the Gurney did not alter the handling qualities of the helicopter, and as a result, it can be implemented as a flow control mechanism for aerodynamic enhancement and retreating blade stall alleviation.

Alleviation of Retreating Side Stall Using Active Gurney Flaps

This chapter presents CFD results for the performance of the W3 Sokol rotor in forward flight and an investigation of the potential effect of the implementation of an active Gurney. Rigid and elastic blade models were considered and calculations were guided using flight test data. The Gurney flap was extended from 40 %R to 65 %R and was located at the trailing edge of the blade. The size of the Gurney was selected to be 2 % of the chord based on previous chapter for the same rotor in hover. All results were trimmed to the same thrust as flight tests. The harmonic analysis of the flight test data proved to be a useful tool for identifying vibrations on the rotor caused by stall at the retreating side, and a carefully designed Gurney flap and actuation schedule were essential to alleviate the effects of flow separation.

Performance Enhancement of Rotors in Hover Using Fixed Gurney Flaps

This chapter demonstrates the potential effect of a Gurney flap on the performance of the W3-Sokol rotor blade in hover. A rigid blade was first considered and the calculations were conducted at several thrust settings. The Gurney flap was extended from 46%R to 66%R and it was located at the trailing edge of the main rotor blade. Four different sizes of Gurney flaps were studied: 2%, 1%, 0.5%, and 0.3% of the chord. The biggest flap proved to be the most effective. A second study considered elastic blades with and without the Gurney flap. The results were trimmed at the same thrust values as the rigid blade and indicate an increase of aerodynamic performance when the Gurney flap is used, especially for high thrust cases.

Introduction and Literature Survey

The advent of modern simulation and experimental methods has so far contributed great insights in the flow physics of rotorcraft flows. Starting from this solid base, flow control methods are now emerging that offer extended range of operation to lifting surfaces. This chapter compares several flow control ideas with the aim to identify one or more candidate methods for deployment on a rotor. For rotor aerofoils, any method offering higher lift to drag ratio without severe penalties in pitching moment is going to affect the performance of the rotor system in a positive way. The chapter highlights the merits of the Gurney flap due to its effectiveness and potential for practical implementation.

CFD Method for Modelling Gurney Flaps

A CFD method for dealing with Gurney flaps is presented in this chapter. The flap is implemented as a thin solid surface without the need to generate a detailed CFD mesh around it. The method is compared with results for fully resolved Gurney flaps and is also demonstrated for complete rotor configurations. The advantage of the method is its efficiency and flexibility and can be made compatible with any CFD solver.