ICAS 2000 CONGRESS THREE-SURFACE AIRCRAFT - A CONCEPT FOR FUTURE LARGE AIRCRAFT G. Wichmann, D. Strohmeyer, Th. Streit German Aerospace Center, DLR Lilienthalplatz 7, 38108 Braunschweig, Germany Keywords: three-surface aircraft / transonic canard design / preliminary aircraft design / canard optimisation / fuselage-canard configuration Abstract ratio wing with transonic performances, stabiliser In this paper the activities of the Institute of De- and control surfaces and under wing mounted en- sign Aerodynamics within the DLR project gines. Improvements only can be reached by size “Dreiflächen-Flugzeug (3FF)” - Three-Surface effects or by employing more sophisticated tech- Aircraft, TSA - concerning the investigation of nologies, which, without proper guidance, is three-surface aircraft configurations are pre- counterproductive to competitive and economic sented. Results of aircraft predesign calculations targets or even operational requirements. In a of the retrofitted wind-tunnel model DLR-F11 long run only new unconventional civil transport show higher performances for the optimised TSA aircraft configurations show major improvements configuration compared to the conventional de- and promise a considerable progress in produc- sign. However, the performance increase is cou- tivity even and in particular under future eco- pled with a loss of static stability. Under the nomic and environmental demands. The key driv- boundary condition of equal static stability, the ers of aircraft efficiency are weight and aerody- same performance gain is achieved with a con- namics. By means of their configurational fea- ventional design. The possible performance po- tures unconventional configurations can offer a tential of TSA configurations can only be realised much greater potential to improve these charac- if the concept of a “free-floating canard”, which teristics than might be achieved by further im- does not influence the static stability, is taken into proving conventional aircraft configurations with account. Corresponding transonic canard designs the same technology level. Studies carried out demonstrate the aerodynamic feasibility of the during the last decades have shown a large im- canard concept for transport aircraft configura- provement potential which can be exploited if one tions at high cruise Mach numbers for a back- is ready to accept unconventional solutions. ward swept canard at a low aft position and a The three-surface aircraft with an additional forward swept canard at a low front position. third wing in the forebody region of the aircraft, the “canard”, represents such a concept for future large transport aircraft, Fig. 1. The objective of 1 Introduction the canard in addition to the TSA’s horizontal Analyses of the International Air Traffic Asso- tailplane (HTP) is to achieve additional lift in the ciation (IATA) predict an annual increase of the forebody region and therefore to world-wide air traffic between 5% and 7% for the • reduce negative lift required from the HTP to next decades. Based on these predictions the large trim the aircraft at cruise and high lift con- aircraft companies and national aeronautical re- ditions reducing the induced drag of the total search institutes work on alternative concepts configuration, such as the Megaliner (A3XX), which go beyond • allow a larger aircraft with higher maximum the conventional stretching of the existing wide take-off weight at fixed wing size bodies. Conventional aircraft configurations are - or - allow a smaller wing at fixed aircraft defined by a fuselage for payload, a high aspect size and take-off weight, 115.1 G. Wichmann, D. Strohmeyer, Th. Streit • improve rotation capability at take-off. • general: determination of the perform- ance potential of a three-surface aircraft An installation of a canard has a significant versus conventional configurations, influence on the overall design of the configura- • aerodynamics: transonic canard design, tion. The potential of the TSA concept depends determination of interference effects be- strongly on the flight mechanics concept. That is, tween canard, main wing and tailplane, the benefits of using a canard are most promising • flight mechanics: clarification of the lon- for a neutrally stable or even unstable aircraft. gitudinal and lateral stability of a three- Stability augmentation could then be performed surface configuration, solution of the by automatic scheduling of the canard to keep it problem of reduced longitudinal stability, at a fixed local incidence that is needed for trim. • aeroelasticity: investigation of the influ- However, by the canard installation also addi- ence of the flexible fuselage and the ca- tional problems will occur, e.g. diminishing of nard on the oscillation behaviour of the fuselage’s accessibility, or introduction of new three-surface aircraft, detection of new aerodynamic interference effects, like canard flutter forms due to the presence of the downwash effects on the wing, particularly at canard, yawing conditions. • predesign: design/optimisation of a three- In order to investigate the influence of the surface configuration, analysis concern- canard on the overall performance of the aircraft ing effectivity and economy and final and to contribute in solving fundamental prob- evaluation of the complete aircraft system. lems combined with the canard installation, DLR started a project “Three-Surface Aircraft”. The research methods reach from prelimi- In this paper the contribution of the Institute nary aircraft design tools over numerical flow of Design Aerodynamics in the project is consid- simulation and wind-tunnel experiments up to ered. After a short description of the project, a flight-simulation tests with the DLR experimental basic field of investigation, the further develop- aircraft ATTAS. ment and application of the predesign code A wind-tunnel half model with the name PrADO for the prediction of the overall perform- DLR-F11 has been planned and constructed at ance of three-surface configurations in compari- DLR, within a separate investment, for KKK son to conventional aircraft designs is presented, (Cryogenic Wind Tunnel Cologne) and ETW including results of preliminary three-surface air- (European Transonic Wind Tunnel) cryogenic craft designs. Additionally some characteristic investigations in close co-operation with Daim- features of the transonic aerodynamic canard de- lerChrysler Aerospace. The wind-tunnel tests will sign in the complex front fuselage flow of a three- be performed for clean and high lift configura- surface configuration will be shown and first de- tions with and without canard covering the com- signs presented. plete Reynolds number regime up to the high Reynolds numbers of ETW. A sketch of the 2 The DLR Project “Three-Surface Aircraft” model also showing the basic equipment for high Within the DLR project “Three-Surface Aircraft”, lift configuration is presented in Fig. 2. The ge- running 4 years from 1977 to 2000, basic ques- ometry consists of a scaled A340-200 fuselage tions concerning the application of canard wings with fairing and a transonic wing that has been for future and today’s transport aircraft are inves- designed based on the variable camber concept tigated. All basic disciplines like aerodynamics, (VC concept). For smaller total lift coefficients flight mechanics, aeroelastics and structure me- the wing works at high cruise Mach numbers in chanics are involved in several working packages clean configuration, for higher lift coefficients including predesign studies for the evaluation of additional flap deflections are necessary to yield the complete aircraft system. reasonable pressure distributions without shock- The main objectives of the project are: induced separation. The model will be equipped 115.2 THREE-SURFACE AIRCRAFT - A CONCEPT FOR FUTURE LARGE AIRCRAFT by a canard wing at the position resulting from modelling the retrofit of an existing aircraft with predesign optimisation runs completed by a a canard. scaled A340 horizontal tailplane in order to in- The integration of a canard in the nose re- vestigate the three-surface configuration. Due to gion of the aircraft reduces the static stability and the fact that the model is designed for the A3XX influences the controllability of the aircraft. For cruise Mach number M=0.85 its basic configura- this reason, static stability (a static margin of at tion without canard was selected for first wall least 10 % of the reference chord length) and correction measurements in ETW. Thus the tests controllability (a maximum trim angle of ±10º will represent an essential contribution to the in- for the canard and ±15º for the horizontal tail- stallation of the half model technique in ETW. plane) as well as the location of the main landing gear (to guarantee a minimum load of 5% of the 3 Preliminary Aircraft Design maximum take-off weight on the nose landing gear during taxi on the runway) are checked at the The task of the preliminary aircraft design within end of the design process for each configuration. the framework of the project is twofold: on the For the optimisation of the 30º-swept canard one hand the basic geometry of an optimised ca- four basic geometry parameters were chosen as nard for the DLR-F11 wind-tunnel model had to optimisation variables and varied in a reasonable be defined, on the other hand, the performance of range: span (9m≤b ≤18m), aspect ratio a three-surface aircraft had to be analysed and C (3≤A ≤8), taper ratio (0.3≤λ ≤0.8) and twist (- evaluated in comparison with a conventional air- C C 6º≤ε ≤6º), assuming a linear twist distribution. craft design. C Further optimisation variables were the canard x- position of the swept back canard (10m≤xC≤13m) 3.1. Design Code and Boundary Conditions and the cruise Mach number (0.82≤M≤0.87). Ac- The integrated aircraft design and optimisation cording to the aim of the study and following studies are based on the design code PrADO, Bréguet’s range equation, the product of aerody- Fig. 3. A detailed description of the design pro- namic efficiency and cruise Mach number L/D * cedure and the implemented extensions for TSA M was chosen as objective function.