Hybrid Laminar Flow Control Research Articles

Evaluation of insect residue resistant coatings – Correlation of a screening method with a conventional assessment technique

Laminar flow technologies such as laminar flow control (LFC) and hybrid laminar flow control (HLFC) are drag reduction techniques that allow the stabilization of the boundary layer and therefore permit an extended region of laminar flow to be achieved. The laminar flow region is, however, easily disturbed by surface contamination caused by insect debris, ice or dust particles. This causes a transition of the flow, and results in a turbulent boundary layer. Mitigation of surface contamination through the use of low surface energy coatings is thus of great interest. In this paper, the use of a screening method (utilizing a wet abrasion scrub test) to determine the effectiveness of candidate coatings against insect contamination has been employed. The method was compared to a conventional assessment technique, namely an insect delivery device. It was demonstrated that both approaches have merit; both methods provide useful means of directly quantifying the adhesion of insect debris to aircraft wing surfaces. An analogue, which can be used as a substitute to live insect testing, was also evaluated. The effect of humidity and water on the adhesive properties of insect haemolymph has been investigated.

Conceptual aircraft design with hybrid laminar flow control

This paper describes a methodology for conceptual design and optimization of aircraft with hybrid laminar flow control (HLFC) systems integrated into wing and tails. An existing conceptual aircraft design platform is enhanced by the necessary methods for sizing of HLFC system architecture and prediction of aerodynamic drag polars. These include transonic drag characteristics as well as transition prediction by analysing boundary layer instability mechanisms. The implemented methods are described and its sensitivities against relevant aircraft design parameters are discussed. The integrated sizing methodology allows to assess the net benefit of HLFC system integration on overall aircraft level and to minimize aircraft fuel consumption by variation of aircraft design parameters, cruise conditions and HLFC system parameters. To demonstrate the applicability of the developed methodology in conceptual aircraft design it is used for design and assessment of an HLFC long-range passenger aircraft. The influence of the HLFC main drivers Mach number and wing sweep angle on aerodynamics, systems and aircraft design characteristics is investigated. Further, aircraft component resizing is analysed to further exploit the fuel reduction potential of the HLFC technology.

High Speed Laser Micro Drilling for Aerospace Applications

To realize a reduced fuel consumption of civil jet transport aircrafts, hybrid laminar flow control (HLFC) is a key technology in aerospace industry. For drag reduction caused by boundary layer suction, small holes at the leading edges of the aircraft wings and tail planes are needed. Much effort has been spend in drilling of small holes into different materials. Nevertheless, the economically drilling of small holes in the range of 50 to 100μm in diameter on large areas is up to date crucial. Apart from processing time this is also due to the demand of close tolerances in case of suction holes. Additionally, minimization of the thermal distortion when drilling large areas gets more challenging. This paper deals with the transfer of the laser drilling process to an adequate system technology for the fabrication of large suction inserts using a short pulsed fiber laser with a power of 200W at a pulse repetition rate of 200kHz. The laser combined with a galvanic scanner and a plane field optic leads to a precise and fast drilling over an area of 100mm x 100mm. Using a precise stage several of these drilling fields can be placed side by side to machine the complete panel area. The amount of drilled through-going holes and their roundness as well as the distortion of the panel is influenced by the applied drilling sequence and energy input. As a result, round micro holes down to 30μm in diameter can be produced with a drilling rate of more than 400 holes per second in 0.8mm thick titanium sheets.

Numerical study of laminar nacelles: natural and hybrid laminar flow designs

This paper presents the numerical design of laminar nacelle engine. Both natural laminar flow and hybrid laminar flow control strategies that have been used to achieve laminar nacelle will be described. Numerical process is based on Euler computations coupled with boundary layer stability analyses. Influence of parameters acting on transition such as mean flow distortion (due to the pylon or wall suction), surface imperfections and wall heating will be analysed.

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.

Automatic Transition Predictions Using Simplified Methods

Laminar-turbulent transition remains a critical issue in a number of cases, including drag reduction, performance prediction of high-lift systems, improved accuracy in general computational fluid dynamics, and reduction of computation cycles for development of optimization tools. Transition delay remains one of the most promising technologies for reducing air transport energy consumption, through natural or hybrid laminar flow control. The use of linear stability theory, either local or nonlocal, remains rather demanding in terms of knowledge and user interaction. Hence, a demand exists for simplified, robust, and accurate transition prediction tools to be inserted into general flow solvers, of boundary-layer or Reynolds-averaged Navier―Stokes types. The problem can be solved by developing transition criteria or database methods. In this last case, characteristics of an actual flow are derived from known solutions of model flows. ONERA, the French Aerospace Laboratory, has long been involved in the development of such methods, and the present paper aims at providing a comprehensive view of the tools developed in the second category, applicable from low-speed two-dimensional to transonic three-dimensional flows, and even to three-dimensional supersonic flows.

Fuel-Sensitivity Analyses for Jet and Piston-Propeller Airplanes

Two mathematical relationships have been derived to estimate the impact on fuel consumption during cruise – one for jet airplanes and one for piston-propeller airplanes – arising from design changes that alter the airplane’s weight (W), lift-to-drag ratio (L/D) and Specific Fuel Consumption (SFC). Using cruise fuel burn as the figure of merit, the expressions may be used to rapidly assess proposed airplane design changes (or modifications) where simultaneous changes to these three fundamental design parameters occur. Two case studies are presented. The first considers the incorporation of an active drag reduction system, such as Hybrid Laminar Flow Control (HLFC), which reduces the skin friction drag, but increases the airplane’s weight, and – as such systems require power – increases the power off-take from the engines, leading to an SFC penalty. Using a commercial jet airplane as a reference vehicle, the results of the numerical expression are compared to those obtained using a computer program, which is capable of accurately determining the cruise fuel. The underlying assumption of linear independence of the three parameters is explored – the results indicate that the parameters are only weakly linked, the nature of which depends on the airplane’s performance characteristics. The second case study concerns a proposed design change to piston-propeller airplane, where the incorporation of a retractable landing gear reduces drag, but a weight penalty is incurred. A target for the weight change is rapidly computed for the associated drag change.

Civil-Transport Wing Design Concept Exploiting New Technologies

Current trends in the design of transport aircraft have shown that in order to be economically viable and competitive it is necessary to investigate technologies that can give an improvement in performance and operational flexibility goal, but must be shown to be cost effective. The current competitive environment forces the airlines to buy advanced technology aircraft and requires manufacturers to provide more operational flexibility, without drastic performance penalties. This is a challenging task, which might be solved by the use of new technologies. It is believed that the application of a hybrid laminar-flow control (HLFC) and variable camber (VC) to a wing would assist in achieving such a task. This paper describes an investigation aimed to examine the suitability of an aerodynamic wing design, allowing for the use of a combined HLFC-VC concept for transonic transport aircraft. The paper describes the phenomenon of laminar flow and outlines the wing design process. It then discusses the benefits and penalties of the variable-camber wing concept. Description is then given of the aerodynamic design of a wing that incorporated both laminar-flow and variable-camber technologies. It concludes with a discussion of the results and recommendations for future work.

Status and perspectives of laminar flow

AbstractAfter identifying the ecological and economic drivers for use of laminar flow technology, we outline the mechanisms of laminarturbulent boundary layer transition and review the status of natural laminar flow (NLF) and hybrid laminar flow control (HLFC). New ways to reduce the complexity of HLFC systems are presented, and the remaining steps to achieve technology readiness are discussed.

Liquid anti-contamination systems for hybrid laminar flow control aircraft—a review of the critical issues and important experimental results

Contamination of the aircraft leading edges by insects and other airborne particles is a major operational concern for the implementation of hybrid laminar flow control (HLFC)-an active drag reduction technique, which has the potential to produce significant fuel savings for airliners. A review of the critical issues is provided, encompassing the transition mechanism, insect distributions and concepts for contamination mitigation. The use of liquid/foam-based systems has proved to be very successful, as is evident in the review of experimental work.

Fuel sensitivity analyses for active drag reduction systems

Active drag reduction systems, such as hybrid laminar flow control (HLFC), have the potential for significant fuel savings; however, this is at the expense of an increase in aircraft weight and engine power off-take. A computer program – capable of accurately determining the trip fuel for a given mission profile – has been developed. The program was validated against manufacturer’s payload-range data, and then modified to emulate the installation of an active drag reduction system, by incorporating changes to the drag polars, specific fuel consumption (SFC) data and operating empty weight (OEW). Results of sensitivity studies are presented that enable the reduction in trip fuel to be determined for given changes in CD, SFC and OEW. The underlying assumption of linear independence of the three parameters is explored. The linearised method was observed to underestimate the fuel savings of the HLFC aircraft by approximately 0·6% compared to an analysis which took into account the coupling between the parameters. A mathematical relationship has been established to estimate the impact on cruise fuel burn arising from relative changes to the aircraft’s mass, lift-to-drag ratio and SFC.

Durability of hybrid laminar flow control (HLFC) surfaces

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.

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

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.

Potential fuel savings due to hybrid laminar flow control under operational conditions

Hybrid laminar flow control (HLFC) is an active drag reduction technique that permits extended laminar flow on an aircraft surface and thus offers the potential for significant fuel savings. This is at the expense of an increase in system weight and specific fuel consumption. An overview of HLFC system failure types and consequences is presented as an introduction to this study, which investigated the impact of a potential loss of laminar flow due to flight in cirrus clouds. At typical cruising altitudes, the ice crystals are of a sufficient size and may result in sufficient particle flux to cause a temporary transition of the boundary layer. A computer performance model of a twin engine aircraft in the class of the Boeing 757 has been used to study the impact of alternative fuel planning scenarios on the fuel consumed by a HLFC aircraft, taking into account a model of probable cloud encounter. Based on the models, the study showed that if the fuel planning assumed 25% time-in-cloud (TIC) during the cruise, then in the extreme case of 55% TIC during the cruise, the contingency fuel (taken as 3% of the trip fuel), would be sufficient for the aircraft to complete the mission (including the alternate leg and hold).

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.

Predictions and observations of the flow field induced by laminar flow control microperforations

Hybrid laminar flow control (HLFC) aims to reduce aircraft skin friction drag by controlling the boundary-layer characteristics through a combination of surface suction and surface profile shaping. Suction is applied through an array of microperforations in the surface; and, to enable HLFC design criteria to be established with confidence, a full understanding of how these suction perforations affect the boundary layer is required. The objective of this paper is to predict the flow field induced by surface suction through single and multiple rows of microperforations, at transonic cruise conditions. A broad range of cases are studied for a variety of geometric and flow configurations by solving the compressible, laminar, Navier-Stokes equations. The geometric parameters considered are perforation diameter, inclination to the surface, and perforation duct profile. The flow parameters consist of the boundary-layer displacement thickness and suction mass flow rate through the hole. From the predictions and analyses of the results, a wide variety of flow field patterns and features are observed; including longitudinal vortices, streamline curvature, large cross-flow velocities, inherently unstable velocity profiles, and a recirculation region at the perforation entrance. The perforation inlet shape is found to have a minimal effect on the induced flow field, but the level of streamwise vorticity is increased for inclined perforations. The size and shape of the sucked stream tube, which is currently used to predict the critical suction velocity, also is determined. For multiple rows of perforations, the flow field characteristics are shown to be influenced by significant interhole effects. The mass flow rate characteristics of microperforations are found to be insensitive to the ratio of hole diameter to boundary-layer displacement thickness. Also, conical bore holes are shown to provide substantial static pressure recovery due to diffusion effects.