Performance improvements

Citation

(2003), "Performance improvements", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 3. https://doi.org/10.1108/aeat.2003.12775cac.001

Performance improvements

Performance improvements

Further papers from the 2002 CEAS Aerodynamics Conference dealing with fixed wing applications are detailed. One paper featured the recent achievements of the European High Lift Programme EUROLIFT. The overall objective of this programme is to reduce the time and cost of the design process of high lift systems, to improve the design accuracy at flight conditions and thus to reduce the economic risk for the aircraft manufacturer. At present the high lift design is verified using sub-scale aircraft models, and the low-speed flight performance is extrapolated from sub-scale wind tunnel results, both posing a high risk in time and money. In order to improve this situation, more reliable computational and experimental tools for an efficient design of advanced high lift systems will be developed and verified in the EUROLIFT project.

The work programme is divided into three main packages: WP1: generation of a detailed experimental 3D database for WP2 high lift code validation. WP2: assessment and improvement of industrial high lift codes using the WP1 database. WP3: verification of the new European Transonic Wind tunnel (ETW) for high Reynolds number low-speed half model testing and test of an advanced high lift system up to flight conditions as well as a project co-ordination workpackage. The EUROLIFT consortium consists of 14 partners from 7 countries. The ongoing activities and main achievements are as follows: WP1 dominant high flow phenomena are under investigation and a detailed experimental database is under generation to be used in WP2 for the validation of high lift CFD codes. WP2 results cover grid generation for selected 3D high lift configuration and pre-test computations; implementation of improvements concerning turbulence modelling, solver accuracy and efficiency as well as grid generation into partner codes. WP3 after verification of the ETW for low speed testing by using an existing A321 cryogenic half model, will prepare the high lift validation experiments in the ETW, and feasibility study for future high lift flight tests on an A321 including a flight test instrumentation proposal. This paper was given by Airbus Deutschland.

Presented by Airbus UK was HELIX: Innovative Aerodynamic High Lift Concepts – which looks to explore 23 concepts for generating sufficient low speed aerodynamics performance for conventional transport aircraft. This is through the application of technological developments in the physical disciplines and design tools, to a range of new (and some old) high lift concepts. Conventional high lift systems (slat and flap) are approaching high maturity levels making it difficult to improve the technology further. Novel high lift concepts may offer significant total aircraft benefits.

This paper presented a selection of the latest experimental and theoretical results on the high lift concepts under evaluation in the first year of the project from June 2001. Consideration will be given to the multi-disciplinary ''trade'' tools that have been developed within the HELIX project to assess all the high lift concepts. These tools have been developed by Airbus, Alenia and Israel Industries and form the technical framework by which HELIX is managed. About 7-10 projects will be in the second year. These include: multi-surface concepts; vortex lift systems; blown systems; leading edge systems; short chord systems; variable camber systems; and simpler slotted flaps.

Circulation control for high lift applications was given by Manchester University and referred to the known concept of circulation control, which historically has concentrated on unswept wings with fully attached flow, whereas this work will concentrate on the performance benefits for a delta wing with massive regions of separated flow. The application of delta wings is driven by the constraint of limited pitch trim available to support conventional trailing edge devices.

Contributions came from various authors, the first concerned with the HYLTEC project. About 50 per cent of the drag, an aircraft experiences occurs due to the friction between the aircraft skin and the air. Laminar flow technologies reduce this surface friction significantly. The main benefit for the customer is fuel reduction which results in an improved ecological friendliness and major cost reduction. HYLTEC is a joint European programme with the following objectives.

Improvement of knowledge on operational and manufacturing procedures; investigations of realistic solutions for the combination of the air suction system with other systems for anti-icing; anti-contamination; study of retrofit capabilities of modern aircraft; and, further clarification of practical topics like surface roughness, suction inhomogenity, etc., with the aid of a wind tunnel test and application of CFD tools.

The technical approach has three tasks: two flight tests by SAAB 2000 and Do228 address operational issues and icing and contamination, respectively. Lab test and manufacturing problems are main tasks. Task 2 is devoted to retrofit requirements for the application of HLFC to aircraft in service. An airbus medium range aircraft has been selected as a baseline vehicle for the study.

Task 3 is concerned with the generation of new experimental data needed for the validation of numerical predictions and to validate design strategies. The measurements are especially devoted to problems on icing, surface contamination and constraints on retrofit issues. Wind tunnel tests and flight test data also form part of this exercise.

From Qinetic came Predicting; the mission performance of a Retrofit HLFC System which presented a detailed mission assessment technique for an aircraft employing an HLFC system of the type described as a new aerodynamic approach to suction system design. The properties of realistic porous surface and the modelling of discrete suction system components allow practical aerodynamic design for HLFC. A case study is presented in the paper for the A310 aircraft used in the EU HYLTEC project.

There are three sections of the paper: the first presents the analysis used to convert two-dimensional design process into a design and analysis tool for the complete aircraft, taking into account local variations in lift coefficient and Reynolds number. The second section focuses on the reduction of HLFC penalties – pump power and system weight – by the judicious positioning of suction control and by perturbations to the wing section which could feasibly be implemented as part of a retrofit programme. The third section concerns the impact on the HLFC design of using either classical or PSE stability methods to control leading edge crossflow instability, and, whether the existing criteria for over-suction (suction rates which precipitate rather than delay transition) are accurate at high Reynolds numbers. The results of the performance study for the A310 are summarised and design modifications suggested can reduce the system specification reviewed, leading to recommendations for further work in this area.

Airbus and AOA presented a simplified suction system which form an HLFC Leading Edge Box of an A320 Fin. One central objective of the EC funded programme ALTTA is the simplification of the suction system for an Airbus A320 Fin. An efficient structural solution overcoming undesirable effects is a second sheet between stringers and ribs. The outer porous surface, the supporting stringers and the inner sheet form a double sheet surface with a relatively high number of chambers, as illustrated. This structure allows for a very efficient control of the suction distribution. Assuming that the porosity of the suction surface is constant in chord and span direction, the local mass flow between two stringers can be adjusted by metering orifices in the inner sheet, making the whole leading edge box a single suction duct. The extent of the laminar flow varies between nearly 40 per cent on the outer half of the fin and about 30 per cent on the lower part. The simplified layout of the suction system fulfils the design objectives agreed by the ALTTA.

Unconventional wing-tip devices

Another contribution, from ONERA, France, noted that lift induced drag is a very important part of the total drag of large subsonic transport aircraft such as the Airbus A380 for which the span is limited by structural considerations at the wing root and also at the airport. The classical way to decrease the lift induced drag is to increase the aspect ratio and the span of the wing but if the span is limited, the alternative is to look for some wing tip devices acting on the tip vortex which is at the origin of the lift induced drag. The status of work at ONERA is shown with the first results obtained on a A3XX wing with a blended winglet and with a blended spiroid tip. The optimised design is obtained and the final shape analysed in a fine grid with an Euler code and drag component extracted from the computed flow field. The original wing tip goes to a blended winglet shape through continuous deformation and an airfoil is then chosen for the winglet tip and the local airfoil along the winglet span is interpolated between the win airfoil at the origin of the winglet and the tip airfoil. A fine analysis of the two final shapes derived with the Euler code confirms the potential of blended winglets for drag reduction.

For the first analysis of a wing with a spiroid tip, a concept tested in flight on the Gulfstream II is quoted, with the vertical part ahead of the horizontal part. The local airfoil is assumed to be constant except in a prescribed area where its camber is inverted. The location where the camber of the spiroid is inverted is a design variable and it should be almost at the location where the local lift is inverted. The spiroid tip was assumed to be almost circular with its height and its width almost identical to the width of the blended winglet, and the sweep angle of its leading edge has been chosen to match the mean sweep angle of the wing. With some improvements, a performing code is possible for a wing with a blended winglet and a blended spiroid tip. For the latter, however, its optimisation in transonic flow conditions involves many design variables, with particular attention to the junction of the spiroid with the wing.