A continuous frequency domain method with roots on the classic Hill’s determinant analysis is presented to approximate the time-varying characteristics of a linear periodic system. The method is particularly useful to derive a time-invariant equivalent form of the time-varying aeroelastic problem of a rotor blade in forward flight. The proposed technique allows methodology usually employed in fixed wing aircraft to obtain closed-loop control laws be extended to rotary wings. The method is first validated solving Mathieu’s equation. Next, the two-degree-of-freedom (flap bending and torsion) problem of rotating beam subject to unsteady and incompressible aerodynamics in forward flight is solved in the laplace domain. As a demonstration of the proposed method, the transfer functions in the ‘s’ plane between a sudden and uniformly distributed input pressure perturbation applied along the beam and the output response of the two elastic degrees of freedom considered are obtained at a set of local sections.

The past has been interesting, but what of the future? As Fig. 1 shows, extrapolation is an attractive tool, with pitfalls for the incautious - most often exemplified in aerospace by the prediction that by 2015 the entire US defence budget would fund one aeroplane, or more recently by the belief that avionics will expand into infallibility and reduce costs to invisibility. The first extrapolation is premature, and the latter two as near as makes no difference wrong.

In the case of the single affordable US defence aircraft the extrapolation probably correctly estimated the 2015 US defence budget, but erred in expressing the US Department of Defence requirement in terms of aircraft rather than damage-effects on target. The subsequent step change in avionics miniaturisation undermined the extrapolation and now looks likely to enable the DoD’s damage-effect requirements to be attained using products that lie on a different cost-effectiveness trend line.

The objective of the work described in this paper was to identify differences in the flow fields immediately downstream of air-jet and vane vortex generators. The flows were assumed to be incompressible and fully turbulent and were solved using the finite volume, Navier-Stokes code CFX 4 (CFDS, AEA Technology, Harwell) on a non-orthogonal, body-fitted grid using the k-ε turbulence model and standard wall functions. The behaviour of the longitudinal vortices produced by the vanes and air jets is presented in terms of stream-wise and cross-stream velocity profiles, circulation and peak vorticity decay, peak vorticity paths in the cross-stream and streamwise direction, cross-stream vorticity profiles, and cross-stream distribution of shear stress. The predicted results show that the vanes and air jets considered produce vortices with significantly different circulation strengths, but that the enhancement of skin friction was of similar magnitude in both cases.

An experimental investigation has been carried out to study the effect of notches on the flow and noise characteristics of 2:1 elliptic-slot jets. The effect of variation in notch geometry is also investigated. The presence of the notch is found to provide low-velocity regions in the notched-minor-axis plane thereby intensifying the jet growth along that plane. The interaction of these low-velocity regions with the mean flow results in shorter core-lengths. At under-expansion, the shock-cell lengths are weakened considerably in notched jets resulting in significant reductions in the overall jet noise level. Azimuthal directivity of elliptic jets indicate higher noise intensity radiated along major-axis ends relative to minor-axis sides. Notches seem to circumferentially modify the noise emission characteristics and significantly bring down the noise levels along major-axis-unnotched plane. Polar directivity indicates reduction in both shock noise and jet mixing noise in the forward quadrant.

In the present investigation, the results obtained during the ground vibration tests of a closed-loop control system conducted on a full-scale fighter to attenuate vertical fin buffeting response using strain actuation are presented. The experimental results are supported by numerical analyses using a finite element aeroelastic model of the structure. Two groups of actuators consisting of piezoelectric elements distributed over the structure were designed to achieve authority over the first and second modes of the vertical fin. The control laws were synthesised using the linear quadratic Gaussian (LQG) method for a time-invariant two-input, two-output (2x2 MIMO) control system. Three different pairs of sensors including strain gauges and accelerometers at different locations were used to close the feedback loop. The results demonstrated that actual reductions of up to 18% in the root-mean-square (RMS) values of the fin dynamic response measured by the strain transducer at the critical point for fatigue at the root were achieved for the second mode under the most severe buffet condition.

This paper provides a concise overview of the aeroengine industry serving the large civil aircraft market together with a discussion of the key market drivers and the strategic issues within this industry. With the airlines feeling increased pressure to improve their market performance and their associated strategies for focusing more on their core businesses, the outsourcing of aeroengine overhaul and maintenance has thus provided the original equipment manufacturers (OEMs) with the opportunity to strategically align with their customers. As the OEMs therefore become service providers, effectively providing thrust for 20 years, the new core competences that must be attained are discussed. They must extend their deep product knowledge in design and manufacture to those of negotiation of the long-term service agreements for their tailored service offerings and in the lifecycle engineering of their products to ensure successful fulfilment of these contracts. In the eyes of their customers the aeroengine OEMs will be seen as risk mitigators and critical to their success will the development and utilisation of knowledge-based tools for the provision of informed decision-making. The future need for such tools and the challenges are identified within this paper so that both the airlines and OEMs can minimise their risks and cost base.