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Tailoring wing structures for reduced drag penalty in off-design flight conditions Melih Papila Raphael T. Haftka Sabanci University, Turkey University of Florida SPONSOR: NASA Langley Research Center William H. Mason Virgina Tech. Rafael Alves Instituto Tecnologico de Aeronautica, Brasil

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2 Motivation Airplanes are prone to frequent deviations from cruise design condition during service life. The wing of an airplane is not optimal with respect to induced drag at all flight conditions as the structural deformation and the effective twist change The drag penalty for flight at off-design conditions can translate to thousands of gallons of jet fuel over the lifetime of an airplane.

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3 Motivation-Analytical proof Given a rigid wing planform Assume minimum induced drag for two different lift coefficients (elliptic circulation or spanload) associated with two different angles of attack. Constant angle of incidence (no twist) Elliptic circulation Constant angle of incidence, along the span (no twist) and elliptical circulation is only possible if the wing also has an elliptic chord distribution Elliptic circulation Difference

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4 Objectives Demonstrate the effect of fixed geometric wing twist on induced drag due to change in flight conditions Compare with a wing of variable twist as the flight condition changes

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5 Outline Motivation and objectives Example wing problem Approach Analysis models Results for near elliptic distribution for straight-line wrapped surfaces Concluding remarks and future work

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6 Example: Airbus A380-like swept wing Flies at Mach jpg y x crcr ctct b/2 Λ 1/4

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7 Approach Two cruise conditions at Mach 0.85 distinguished by two lift coefficients Two optimal wings (minimum induced drag) associated with the cruise conditions Design condition – Optimal wing (1) Off-design condition – Optimal wing (2) Compare to Optimal wing (1) flying at off-design condition at different angle of attack

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8 Approach Induced drag penalty for the design condition optimal wing operating at the off-design condition Compare drag coefficients and span efficiencies flight path Optimal wing (1) incidence (design) flight path Non- optimal incidence (off- design) Adjust angle of attack flight path Optimal wing (2) incidence (off- design)

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9 Cruise scenarios Two Cruise conditions at Mach 0.85 (1) Design condition (2) Off-design condition Aircraft on short hops for which the lower flight altitudes may be required Lower payload (fewer passengers)

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10 Simplifications Weight change during flight due to fuel consumption ignored Assume level flight (ascent and descent ignored)

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11 Analysis for induced drag penalty Effect of fixed geometric wing twist on induced drag at different flight conditions reflected by span efficiency factor e.

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12 Analysis for induced drag penalty Aerodynamic model MSC.NASTRAN static aeroelasticity solver for spanload (Doublet-Lattice subsonic lifting surface) 8x50 model, aerodynamic pressure coefficients at the center of each box Given spanload, LIDRAG (FORTRAN code from Virginia Tech) computes span efficiency e and total lift coefficient C L Local lift-coefficient Spanload

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13 Rib Shear web Spar Shear web Skin panel Rib caps Spar caps Analysis for induced drag penalty Structural model MSC.NASTRAN structural model Leading and trailing edge spars Fifteen equally spaced ribs Upper and lower skins are assumed identical Each bay has uniform thickness for structural optimization: 14 skin panel thicknesses variables. Structural optimization: minimize weight under stress constraints (a load factor of 2.5 on the dynamic pressure and a safety factor of 1.5 on the stress allowable) Aluminum

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14 Near elliptic distribution Recall the goal: assess performance of optimal geometric wing twist at off- design condition. Optimal total angle of incidence via Aerodynamic Analyses, LAMDES. Total angle of incidence flight path )( tot

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15 Induced drag penalty- Rigid wing Near elliptic distribution Attributed to LAMDES not finding true optimum or true elliptic spanload, but near optimal. How the structural deformation will affect results?

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16 Induced drag penalty- Elastic wing Near elliptic distribution ~2 drag count (0.0002)

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17 Induced drag penalty- Elastic wing Near elliptic distribution Spanload Total angle of incidence

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18 Induced drag penalty- Rigid wing Straight-line wrapped surfaces

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19 Induced drag penalty- Elastic wing Straight-line wrapped surfaces Penalty was 0.8 for rigid wing, Insensitive to structural deformation

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20 Induced Drag Penalty- Elastic Wing Straight-line wrapped surfaces Spanload Total angle of incidence

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21 Cost penalty – Scenario I Near elliptic distribution Consider induced drag about 25% of total drag, then 0.25x0.01= 0.25% penalty on total drag liter/km, then loss 16.65x0.0025= 0.04 liter/km Max range 15000, cruise about 5000 km loss 200 liter/flight Assume 300 flights/year, half flown at a lower altitude than the design altitude liter/year fuel loss

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22 Two types of total angle of incidence distributions investigated at off- design condition at a lower lift coefficient than design lift coefficient. For near elliptic spanload (near optimal wing with respect to induced drag) such changes resulted in about a two drag-count increase which may be sufficient to suggest tailoring the structure as the flight condition changes The straight-line wrapped surfaces was found more effective at the off-design conditions if the wing was not tailored because it was insensitive to the structural deformation and the penalty level was lower. Concluding Remarks

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23 Work in progress Study complete flight envelope, ascend, cruise and descend Optimize the wing structure so that the deformation provides near optimal wing twist associated with the changing flight condition. Composite wing skin and treat fiber orientation(s) as design variable

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