Low-drag bodies and the Kutta edge in the wing-body-tail configuration

Get Complete Project Material File(s) Now! »

Research background

In the last two decades, major research initiatives around the world have been working on new aircraft configuration development, under the priority heading of “The Green Aircraft”. The most well-known of these initiatives is the National Aeronautics and Space Administration (NASA)’s Environmentally Responsible Aviation (ERA) project, the European Commission’s New Aircraft Concepts Research (NACRE) and the Clean Sky project. Spurred by the growing consensus that the current dominant aircraft configuration will have to be substituted, the search is intensifying for a superior new configuration as the pressure of the growing aviation industry on the environment demands substantially better flight efficiency.
The current dominant configuration (CDC) in commercial air transport is that of the tube-and- wing. The Douglas DC-3 entered service in 1936, having all the elements now common to all commercial transport aircraft. It is composed of a long tubular body, (hard, load-bearing external shell) supporting a single main pair of wings and a tailplane at the aft end, the purpose of which is to ensure static longitudinal stability. In the 80 years since then, very little has changed (perhaps engines may be mounted on the fuselage, perhaps on the wings). However, in times of continued scarcity and cost increases in fossil fuel, we are ever more concerned about whether potentially fundamental changes are necessary.
One of the favoured alternative configurations is the blended wing body (BWB) (Liebeck et al., 1998; Liebeck, 2004). Although a review of the CDC is necessary, the general suitability of the CDC should not be lost (Huyssen et al., 2016). Some of the BWB’s most renowned qualities are the lift-producing centre-body, the elimination of the wing-body junction interference effects and the improvement of volumetric efficiency – all these qualities accumulate to a drag reduction of approximately 27% (Liebeck, 2004; Kuntawala, 2011). If general suitability of the configuration is to be maintained, a tube-and-wing configuration, with a similar fuel reduction, would be a meaningful avenue to explore. One such candidate already exists which could replace the CDC, known as the N+3 D8 configuration by Drela (2011). The reasons for apparent penalties of the tube-and-wing configuration are mostly associated with the fuselage, and its existence as volume-carrying structure. Multhopp (1942), Dodbele et al. (1987), Wickens (1990), Zedan et al. (1994), Coiro and Nicolosi (1994) and Reneaux (2004) have emphasised the importance of reducing drag over the fuselage surface by noting that all turbulent airplane fuselage drag comprises 50% of the total profile drag (friction and form drag).
The fuselage is not required to provide a portion of the lift, in fact, the necessary addition of the fuselage to the wing leads to a local loss of lift (Munk, 1923; Prandtl, 1932; Jones, 1980) and associated drag increase (Hoerner, 1965). Huyssen et al. (2012) proposed an alternative wing-body-tail (WBT) arrangement. The key argument is that, in principle, a tailplane is unnecessary, since static longitudinal stability can be achieved with variations in the main wing geometry alone (Agenbag et al., 2009). Without this requirement, the long tubular body is neither necessary, nor close to an optimum, for either drag reduction or for packing efficiency (minimum wetted area per unit volume). If a different, shorter body is employed, it is possible to modify the flow around that body so that the circulation distribution between the body and the wings is much more uniform than with a tailplane. If that is so, both the induced drag and the total viscous drag can be reduced, owing to an improved body shape with a reduced wetted area.
Therefore, two design objectives are proposed: a body with lower drag and a body that can provide some lift to the whole aircraft. In terms of drag reduction, the fuselage design can consider a low-drag body (LDB) shape, which is typically shorter than traditional fuselage bodies. Various studies have found (experimentally and computationally) that the optimum drag body would have a fineness ratio, λ, (length to maximum body diameter ratio, l/d) of between 4 and 6, which is significantly different from the current dominant passenger transport configuration (the so-called “tube-and- wing”) that uses 9 < λ < 13. These studies include Young (1939), Gertler (1950), Stoney (1961), Boltz et al. (1960), Carmichael (1964; 1966), Chevray (1968), Krauss (1968), Parsons and Goodson (1972), Patel et al. (1974), Myring (1972; 1981), Hess and James (1976), Patel and Lee (1977), Chappell (1978), Meier and Kreplin (1981), Pinebrook and Dalton (1983), Markatos (1984), Patel and Chen (1986), Cebeci (1989), Lutz and Wagner (1998), Hammache et al. (2002) and Li et al. (2011) and mostly relate to underwater bodies and airships. In terms of the second design objective, the proposed WBT configuration would modify the flow around the wing-body by using a deflector plate called the Kutta edge (KE), shown in Figure 1.2. This KE would control the downwash distribution in order to maintain the highest possible span efficiency for the wing during all flight conditions. This implies that lift can be supported on an LDB simply by adding camber to the body of revolution.
When combined with a trailing-edge tail/plate it could yield quite significant performance benefits by improving wing effectiveness and at some deflection positions, act as a high-lifting device. In principle, if a KE contributes to the lift of an aircraft, it allows flight at a lower lift coefficient, CL, of the main wing, further reducing the drag of the overall system, offsetting any additional wetted surface area required for the KE. The experimental investigations were conducted at Re = 5 x 104 (Huyssen et al., 2012) and Re = 105 (Davis and Spedding, 2015). The KE appeared to modify the global flow around the body so that the circulation distribution between the body and the wings was more uniform, which, in principle, leads to a reduction in induced (inviscid) drag. However, the argument was only indirectly supported by the findings of modified downwash profiles in the near wake, and the mechanism by which the global flow-field was changed could not be directly identified, either from particle image velocimetry (PIV) in the wake (Huyssen et al., 2012; Davis and Spedding, 2015) or with global force balance measurements (Davis and Spedding, 2015). It is also noted that no attempt was made to optimise the WBT, and both studies used a NACA0012 wing, a simple geometric form for the shorter body (not an LDB), and a discrete KE, which was not blended or integrated into the body shape. The KE itself was also fixed to the conical aftbody, so that KE deflection was always linked with deflection of the aftbody. The deflection was described by a single angle, δ.

CONTENTS :

• ABSTRACT
• JOURNAL AND CONFERENCE PAPERS
• ACKNOWLEDGMENTS
• LIST OF FIGURES
• LIST OF TABLES
• NOMENCLATURE
• 1. INTRODUCTION
  • 1.1 Research background
  • 1.2 Importance of the proposed work
    • 1.2.1 Low-drag bodies and the Kutta edge in the wing-body-tail configuration
    • 1.2.2 Agreement between experimental and computational results at a low Re
  • 1.3 Aim
  • 1.4 Research objectives
  • 1.5 Structure of the dissertation
• 2. LITERATURE SURVEY
  • 2.1 Introduction
  • 2.2 Non-dimensional parameters
    • 2.2.1 Reynolds number
    • 2.2.2 Lift coefficient
    • 2.2.3 Drag coefficient
    • 2.2.4 Body fineness ratio
  • 2.3 Description of drag sources for external fluid dynamics
    • 2.3.1 Drag on bodies of revolution
    • 2.3.2 Transition over the nose
    • 2.3.3 Separation over the aftbody
    • 2.3.4 Reynolds number sensitivity
    • 2.3.5 Low-drag bodies
      • 2.3.5.1 F-57 low-drag body
      • 2.3.5.2 Myring’s low-drag body
• 2.4 Lifting bodies and low aspect ratio tails
  • 2.4.1 The Kutta edge
  • 2.4.2 Low aspect ratio wings
  • 2.4.4 Lift from body-tail combinations
  • 2.4.5 Modification of the slender-body theory for the Kutta edge
• 2.5 Lack of agreement of experimental and computational fluid dynamics at low
• Reynolds numbers for the NACA
  • 2.5.1 Experimental difficulties
  • 2.5.2 Computational fluid dynamics difficulties
• 2.6 Proposed wing-body-tail configuration
  • 2.6.1 Downwash distribution of an ideal wing
  • 2.6.2 Initial experimental work of Huyssen et al. (2012) and Davis and Spedding (2015)
• 2.7 Numerical modelling at low Reynolds numbers
  • 2.7.1 Shear-stress transport k-ω turbulence model
  • 2.7.2 Transition modelling
  • 2.7.3 Modelling conditions
    • 2.7.3.1 Wall y+condition
    • 2.7.3.2 Transition model inputs

• 2.8 Summary and conclusions
• 3. EXPERIMENTAL AND NUMERICAL COMPARISON: A NACA0012 CASE
  • STUDY
  • 3.1 Introduction
  • 3.2 Dryden wind tunnel set-up
    • 3.2.1 NACA0012 wing model
    • 3.2.2 Force balance
  • 3.3 Computational modelling of the NACA
    • 3.3.1 Two-dimensional model geometry
    • 3.3.2 Pseudo two-dimensional model geometry
    • 3.3.3 Three-dimensional model geometry
    • 3.3.4 Solution domain and mesh generation
  • 3.4 Computational model verification and validation
    • 3.4.1 Uncertainty related to modelling assumptions and approximations
    • 3.4.2 Uncertainty due to input parameters
    • 3.4.3 Mesh convergence study
      • 3.4.3.1 Quantitative study: drag coefficients
      • 3.4.3.2 Qualitative study: wake structures
• 3.5 NACA0012 experimental and numerical comparison
  • 3.5.1 Two-dimensional results
  • 3.5.2 Three-dimensional results
• 3.6 Poor agreement between experimental results and computations
• 3.7 Summary, conclusions and recommendations
• 4. LOW-DRAG BODIES MODIFIED TO IMPROVE LIFT
  • 4.1 Introduction
  • 4.2 Numerical modelling
    • 4.2.1 Geometric model and mesh generation
    • 4.2.2 Boundary conditions, turbulence and transition models
    • 4.2.3 Comparison with reference bodies
  • 4.3 Numerical results for the experimental body-KE
  • 4.4 The aerodynamic effect of deflected tail plates on low-drag bodies
  • 4.5 Effectiveness of the Kutta edge to provide lift
  • 4.6 Summary, conclusions and recommendations on the function of the KE
• 5. WING-BODY-TAIL CONFIGURATION
  • 5.1 Introduction
  • 5.2 Numerical modelling
    • 5.2.1 Geometric model, solution domain and mesh generation
    • 5.2.2 Boundary Conditions, Turbulence and Transition Models
  • 5.3 Simulations of the experimental WBT configuration at Rec =
  • 5.4 Numerical investigation of LDBs in the WBT
  • 5.5 Discussion
  • 5.6 Summary, conclusions and recommendations
• 6. CONCLUSIONS AND PROPOSED FUTURE WORK
  • 6.1 Summary
  • 6.2 Conclusions
  • 6.3 Recommendations and proposed future work
  • REFERENCES

GET THE COMPLETE PROJECT