Parametric whirl flutter study using different modelling approaches

This paper demonstrates the importance of assessing the whirl flutter stability of propeller configurations with a detailed aeroelastic model instead of local pylon models. Especially with the growing use of electric motors for propulsion in air taxis and commuter aircraft whirl flutter becomes an important mode of instability. These configurations often include propeller which are powered by lightweight electric motors and located at remote locations, e.g. the wing tip. This gives rise to an aeroelastic instability called whirl flutter, involving the gyroscopic whirl modes of the engine. The driving parameters for this instability are the dynamics of the mounting structure. Using a generic whirl flutter model of a propeller at the tip of a lifting surface, parameter studies on the flutter stability are carried out. The aeroelastic model consists of a dynamic MSC.Nastran beam model coupled with the unsteady ZAERO ZONA6 aerodynamic model and strip theory for the propeller aerodynamics. The parameter studies focus on the influence of different substructures (ranging from local engine mount stiffness to global aircraft dynamics) on the aeroelastic stability of the propeller. The results show a strong influence of the level of detail of the aeroelastic model on the flutter behaviour. The coupling with the lifting surface is of major importance, as it can stabilise the whirl flutter mode. Including wing unsteady aerodynamics into the analysis can also change the whirl flutter behaviour. This stresses the importance of including whirl flutter in the aeroelastic stability analysis on aircraft level.

Choice of Interceptor Aerodynamic Lifting Surface Location based on Autopilot Design Considerations

Interceptors operate at wide range of operating conditions in terms of Mach number, altitude and angle of attack. The aerodynamic design caters for such wide operating envelope by appropriate sizing of lifting and control surfaces for meeting the normal acceleration capability requirements. The wide range of operating conditions leads to an inevitable spread in center of pressure location and hence spread in static stability. The performance of control design is a strong function of the aerodynamic static stability. The total operating envelope can be bifurcated into statically stable and unstable zones and the aerodynamic lifting surface location can be used as a control parameter to identify the neutral stability point. During the homing phase lesser static stability is desirable for good speed of response, hence the lifting surface location needs to be chosen based on the capability of control to handle instability. This paper analyses the limitations of autopilot design for the control of an unstable interceptor and brings out a method to identify the optimum aerodynamic lifting surface location for efficiently managing static margin while satisfying the control limitations and homing phase performance. This provides an input on the most appropriate lifting surface location to the aerodynamic designer during the initial CFD based aerodynamic characterisation stage itself, before commencing the rigorous wind tunnel based characterisation.

Steady flow analysis of a slender wing by lifting surface method

In the paper hereby, steady flow around a thin-walled wing is analysed by means of the Lifting Surface Method. In order to carry out tests, the wing has been divided into a finite number of quadrilateral panels. All panel edges in turn are replaced by discrete straight vortex segments which induce velocities within the flow field. The problem boils down to working out velocity circulation distribution on the wing surface. For this purpose, numerical realization has been developed in C by Minimalist GNU for Windows compiler and Code::Blocks IDE. To work out a solution to the linear non-homogeneous algebraic system, the Gauss – Seidel stationary iterative method has been applied. The obtained results for various angle of attack values are depicted by means of ParaView.

Aerodynamic analysis of a generic wing featuring an elasto-flexible lifting surface

A three-dimensional-membrane-type wing is investigated applying fluid-structure-interaction computations and complementary experiments. An analysis for three Reynolds numbers is conducted at various angles of attack. The computations are performed by means of the TAU-Code and the FEM Carat++ solver. Wind-tunnel tests are carried out for performance analysis and to estimate the accuracy of the computations. In the results, the advantages of an elasto-flexible-lifting-surface concept are highlighted by comparing the formvariable surface to its rigid counterpart. The flexibility of the material and its adaptivity to the freestream allow the membrane to adjust its shape to the pressure distribution. For positive angles of attack, the airfoil’s camber increases resulting in an increase in the wing lifting capacity. Furthermore, the stall onset is postponed to higher angles of attack and the abrupt decrease in the lift is replaced by a gradual loss of it.