The goal of life extending control (LEC) is to enhance the service life of complex mechanical systems, such as aircraft, spacecraft, and energy conversion devices, without any significant loss of performance, and can be achieved by making a trade-off between dynamic performance and structural durability. This paper presents the concept and a design methodology for robust life extending control of aircraft structures that are typically subjected to cyclic mechanical stresses. The controller design procedure relies on the specifications of flight performance and allowable fatigue crack damage at critical points of aircraft structures that serve as indicators of the effective service life. As an example, an aeroelastic model of the aircraft wings has been formulated and is incorporated into a nonlinear rigid-body model of the flight-dynamics. The H∞-based structured singular value (μ) synthesis method has been used to design robust life extending controllers based on a linearised model of the aircraft and a (nonlinear) state-space model of fatigue crack growth. The results of simulation experiments show significant savings in fatigue life of the wings while retaining the dynamic performance of the aircraft.

The stresses around the passenger door, emergency door and window of an aircraft, Airbus A300B, are considered in this paper. By idealising these opening as rectangular holes with rounded corners in an infinite plate subjected to inplane loads and considering the presence of adjacent holes the stresses are determined using the complex variable method along with Schwarz Alternating Method. The results of the analytical solution are found to be in good agreement with those of the finite element analysis using ANSYS 5.0 software within 5%.

Reduction of aircraft manufacturing cost benefits aircraft direct operating cost (DOC). The degree of stringency in specifying aircraft smoothness influences cost, i.e. the tighter the tolerance, the higher is the manufacturing cost. Discrete surface roughness arising from manufacturing tolerance at the wetted surface may be seen as an ‘aerodynamic’ defect. Features such as steps, gaps, waviness and fastener flushness (termed excrescence), seen as defects, contribute to aircraft parasitic drag.

The study is conducted on an isolated nacelle which is considered to be representative of an entire aircraft. Eleven key manufacturing features at the wetted surface of a generic long duct nacelle are identified, each associated with surface roughness. The influence of tolerance allocation at each of the key features is investigated to establish a relationship between aircraft aerodynamics and associated costs. The initial results offer considerable insight to a relatively complex problem in a multi-disciplinary environment.

Excrescence drag arising out of these ‘aerodynamic’ defects is assessed by using CFD and semi-empirical methods. Cost versus tolerance relationships are established through in-house methods using industrial data.

The aircraft unit price typically contributes from two to four times more than the fuel burn to aircraft direct operating costs. A trade-off study between manufacturing cost and aircraft drag indicates that, in general, there is scope for some relaxation of present-day tolerance allocation, to reduce aircraft acquisition cost, which would in turn reduce direct operating costs.

The paper reviews present practices in the prediction of scale effect at transonic speeds and considers how these may have to change in the future in the light of possible changes in wing design standards. Particular issues highlighted in the paper include the research that is needed to obtain the potential benefits that should come from the ability to test models at near-full-scale Reynolds numbers in cryogenic tunnels such as the ETW and second, the different experimental techniques that may have to be introduced if aerodynamic designers attempt to realise their long-held ambition of obtaining extensive laminar flow in flight.

If an aerofoil of chord c has a parabolic nose with radius of curvature r, and is placed at angle-of-attack α to a stream, the laminar boundary layer on its upper surface remains unseparated for α<0.8l8. In the present paper we consider some smooth local modifications to the leading edge. Symmetric modifications of the nature of local sharpening of the nose can improve this result to at least α<0.897. Further improvements are possible for unsymmetrical (e.g. drooped) noses, and an example of a ‘drooped’ nose with α<0.912 is shown.

Using an exact solution of the Euler equations, the pressure distributions over some axisymmetric bodies with a negative pressure gradient over the majority of the surface (from 99.7% to 88% of the total body length) have been calculated. The results of wind tunnel tests for these bodies are presented.