Trevor M. Young

Adhesives bonding of a titanium alloy to a glass fibre reinforced composite material

Abstract Experimental results for 90° peel tests are presented for alternative titanium surface treatments. Based upon these results, sodium hydroxide anodisation (SHA) and surface cleaning by Excimer laser were selected for further study. The durability of a titanium/glass fibre composite lap joint in a hot/wet environment was investigated by single lap shear testing. Both thermosetting and thermoplastic composites were evaluated using two aerospace industry standard film adhesives. Surface treatment by Excimer laser resulted in a five-fold increase in peel strength (dry), but the environmental durability was shown to be poor. In comparison, SHA of the titanium resulted in relatively high peel strengths and provided a durable composite/metal bond.

Composite Repair in Wind Turbine Blades: An Overview

Renewable energy sources such as wind energy—together with energy-efficient technologies—are essential to meet global energy demands and address climate change. Fiber-reinforced polymer composites, with their superior structural properties (e.g., high stiffness-to-weight) that allow lightweight and robust designs, play a significant part in the design and manufacture of modern wind turbines, especially turbine blades, for demanding service conditions. However, with the current global growth in onshore/offshore wind farm installations (with total global capacity of ∼282 GW by the end of 2012) and trend in wind turbine design (∼7–8 MW turbine capacity with ∼70–80 m blade length for offshore installations), one of the challenges that the wind energy industry faces with composite turbine blades is the aspect of structural maintenance and repair. Although wind turbines are typically designed for a service life of about 20 years, robust structural maintenance and repair procedures are essential to ensure the structural integrity of wind turbines and prevent catastrophic failures. Wind blades are damaged due to demanding mechanical loads (e.g., static and fatigue), environmental conditions (e.g., temperature and humidity) and also manufacturing defects. If material damage is not extensive, structural repair is the only viable option to restore strength since replacing the entire blade is not cost-effective, especially for larger blades. Composite repairs (e.g., external and scarf patches) can be used to restore damaged laminate/sandwich regions in wind blades. With composite materials in the spar (∼30–80 mm thick glass/carbon fiber laminates) and aerodynamic shells (sandwich sections with thin glass fiber skins and thick foam/wood as core), it is important to have reliable and cost-effective structural repair procedures to restore damaged wind blades. However, compared to aerospace bonded repairs, structural repair procedures in wind blades are not as well developed and thus face several challenges. In this regard, the area of composite repair in wind blades is broadly reviewed to provide an overview as well as identify associated challenges.

Investigation of hybrid laminar flow control (HLFC) surfaces

Abstract Hybrid laminar flow control (HLFC) is an active drag reduction technique. A delay in transition of the boundary layer from laminar to turbulent flow is usually achieved by the application of suction over the first 10–20% of the chord. The design of the suction surface and the chambers underneath the perforated skin represents one of the most significant engineering challenges concerning HLFC. A review of design requirements, candidate materials and drilling methods for the production of the suction surface, is presented. Materials considered include titanium, aluminium and carbon fibre composite. Laser (Excimer or Nd–YAG) and electron beam drilling has been used to produce satisfactory suction panels.

Experimental Analysis of the Bondline Stress Concentrations to Characterize the Influence of Adhesive Ductility on the Composite Single Lap Joint Strength

Adhesively bonded composite single lap joints were experimentally investigated to analyze the bondline stress concentrations and characterize the influence of adhesive ductility on the joint strength. Two epoxy paste adhesives—one with high tensile strength and low ductility, and the other with relatively low tensile strength and high ductility—were used to manufacture composite single lap joints. Quasi-static tensile tests were conducted on the single lap joints to failure at room temperature. High magnification two-dimensional digital image correlation was used to analyze strain distributions near the adhesive fillet regions. The failure mechanisms were examined using scanning electron microscopy to understand the effect of adhesive ductility on the joint strength. For a given surface treatment and laminate type, the results show that adhesive ductility significantly increases the joint strength by positively influencing stress distribution and failure mechanism near the overlap edges. Moreover, it is shown that high magnification two-dimensional digital image correlation can successfully be used to study the damage initiation phase in composite bonded joints.

An investigation of surfactant and enzyme formulations for the alleviation of insect contamination on Hybrid Laminar Flow Control (HLFC) surfaces

Abstract Hybrid Laminar Flow Control (HLFC) is an active drag reduction technique that requires a small amount of air to be sucked through a porous skin surface, thus stabilising the boundary layer and permitting extended laminar flow along the wing surface. Contamination of the laminar flow surfaces by insects is a major concern for this technology. An overview of insect contamination and mitigating solutions is provided as background to research undertaken. The paper reports on an experimental investigation into the potential use of surfactants and enzymes as additives to contamination alleviation fluids. It was demonstrated that both approaches have merit. It is concluded that the use of fluorosurfactant containing solutions is preferable to enzyme borne solutions, partly due to the need to control the temperature of the enzyme solution.

Durability of hybrid laminar flow control (HLFC) surfaces

Abstract As a part of the European Commission sponsored HYLTEC (Hybrid Laminar Flow Technology) project, a SAAB 2000 aircraft – fitted with a number of small laser drilled panels on the wing leading edge – completed 20 months of routine service; the objective being to investigate contamination and durability aspects of Hybrid Laminar Flow Control (HLFC) suction surfaces. A post-flight test investigation of these panels, manufactured from Nd-YAG laser drilled titanium, aluminium and carbon fibre polyetheretherketone (PEEK) composite, has been conducted. Using Scanning Electron Microscopy (SEM), evidence of corrosion and damage was investigated. An optical inspection technique was used to measure hole geometries and the results were compared to pressure loss measurements through the panels. Titanium was found to be the most robust material, displaying no adverse affect from this exposure, whilst aluminium was found to be substantially less durable. The PEEK carbon fibre composite showed signs of surface degradation after only two months of flight trials.

Control allocation with actuator dynamics for aircraft flight controls

This paper addresses the control allocation to several aircraft flight controls to produce required body axis angular accelerations. Control law is designed to produce the virtual control effort signals, which are then distributed by solving a sequential least squares problem using active set method to the flight control surfaces to generate this effort. Two cases are described: in the first case the control law and allocation for the healthy aircraft is implemented, and in the second case, jamming of one control surface is introduced at time zero. In this case, it was shown how the controller and allocation compensate for this failure without changing the control law. To implement this system it was assumed that there is a good fault identification system onboard. Normally aircraft are over-actuated and in the case of a control failure this over actuation is more pronounced due to coupling of aircraft dynamics. Instead of using one-to-one mapping between control allocator and control surfaces, actuator dynamics was included in the system. The discrepancy in the optimal signal from control allocation due to this additional dynamics was compensated using the scheme mentioned in this paper. Each gain corresponding to the actuator is tuned using genetic algorithms (GA). The controller and allocation design are implemented on a nonlinear B747 model with actuator dynamics. Nomenclature aor δ = right outboard aileron (deg) air δ = right inboard aileron (deg) aol δ = left outboard aileron (deg) ail δ = left inboard aileron (deg) eor δ = right outboard elevator (deg) eir δ = right inboard elevator (deg) eol δ = left outboard elevator (deg) eil δ = left inboard elevator (deg) ih δ = stabilizer (deg) ur δ = upper rudder (deg) dr δ = down rudder (deg) p = roll rate about body x-axis (rad/s) q = pitch rate about body y-axis (rad/s) r = yaw rate about body z-axis (rad/s) T V = true airspeed (m/s)

Aircraft design education at universities: benefits and difficulties

Abstract The value of teaching aircraft design at university by means of student design projects is explored. It is argued that conceptual design is an essential part of engineering education and it provides a foundation for the development of engineering judgement, which is required to establish a balance between safety, economics and functionality of an engineering system. The design process is constituted by two elements – a creative process involving the postulation of design alternatives, and an analytical process, which evaluates the envisaged designs. Detail design teaches vocational skills and instils an awareness of the complex, multidisciplinary and integrated nature of the aeronautical engineering business. The factors that limit the quality of design education include: support staff, time, financial resources, teamwork and lecturing staff.

An investigation into potential fuel savings for 110–130 seat passenger transport aircraft due to the incorporation of natural laminar flow or hybrid laminar flow control on the engine nacelles

A computer program, capable of accurately determining the required fuel for a given mission profile, has been developed for two ‘project’ aircraft types. The program was initially validated against reference aircraft and then modified to study the potential impact on trip fuel due to the incorporation of natural laminar flow or hybrid laminar flow control on the engine nacelles, by implementing changes the aircraft’s drag coefficient (CD), specific fuel consumption and operating empty weight. Trade studies, with changes to CD, specific fuel consumption and operating empty weight, were conducted using trip fuel as the objective function. The results can be superimposed, permitting estimates of the trip fuel reduction due to the incorporation of alternative drag reduction designs. The impact of trip distance on the fuel-saving potential of laminar flow technologies has also been explored. The potential fuel savings for the aircraft fleet (with the incorporation of hybrid laminar flow control or natural laminar flow) has been estimated using an aircraft utilisation model (representing the probability of the aircraft flying a given stage length). It was concluded that there would be a net fuel saving based on the utilisation model. Finally, to obtain a realistic assessment of the potential benefit of these technologies, the probable time-in-cloud, which will result in a loss of laminar flow, has been considered.

Impact of Freely Suspended Particles on Laminar Boundary Layers

Following recent volatility in fuel markets, laminar flow technologies are again being considered as an option for increasing aircraft efficiencies in order to cope with the forecasted growth of air traffic and to simultaneously reduce emissions. However, some aspects on the operation of such aircraft are not fully understood, such as the impact of ice crystals in cirrus cloud. This leads to uncertainties in regard to fuel planning for laminar flow aircraft, and would prevent the optimal benefits accruing in flight operations. The latest published in-flight experimental research, from the 1980s, resulted in information that is too sparse and incomplete to allow for conclusive deductions. A computational analysis, which has been set up in accordance to Northrop X-21 flight tests, revealed major discrepancies regarding some of the basic assumptions made for the original analytical study concerning the impact of ice particles on laminar flow. Recent laboratory experiments succeeded to demonstrate the effect of freely suspended particles. These experiments validated both the double pitot approach, which was originally used, as well as hot films as eligible candidates for the detection of the phenomena; however, the former approach is believed to produce an incomplete picture. Even though sensitivity studies on the development of the critical limits could be undertaken, definite statements will require more detailed investigation allowing for a better controllability of the particles’ injection. Furthermore, it is believed that greater insight into the basic phenomenon of how freely suspended particles deteriorate an initially laminar boundary layer will only be gained from capturing the flow field around a single particle close to its surface impingement location. For this purpose, experiments are in progress involving smoke flow visualisation and a high speed camera. The difficulties connected with reproducing the effect in a wind tunnel with realistic parameters might require further support from numerical analyses.