Research ArticleBIOMIMETICS

A bioinspired Separated Flow wing provides turbulence resilience and aerodynamic efficiency for miniature drones

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Science Robotics  29 Jan 2020:
Vol. 5, Issue 38, eaay8533
DOI: 10.1126/scirobotics.aay8533
  • Fig. 1 Endurance, Reynolds number for MAVs and natural flyers, and the effect of atmospheric turbulence on laminar separation.

    (A) The flight time of existing commercial and research fixed-wing MAVs and NAVs (blue and light blue circles) is below 40 min. Lines represent estimated maximum flight times obtained using the scaling analysis in section S2 for flight speeds of 5 m/s (light purple), 7.5 m/s (purple), and 10 m/s (dark purple). (B) Most of the existing outdoor MAVs have higher Re and smaller AR (reported in red) than birds and large insects of comparable mass. The dashed dark blue reference line represents how the Re of a vehicle equipped with an AR = 6 wing and flying at CL = 0.6 scales with mass (eq. S9). (C and D) Below Re = 50,000, external turbulence has a high impact on wings with thick airfoil sections (U is the wind speed). Comparison fixed-wing vehicles: [1] Black Widow (35), [2] Wasp (40), [3] University of Florida MAV (41), and [4] EPFL MC2 (42).

  • Fig. 2 Separated flow airfoils in natural flyers and for MAVS.

    (A) The surface roughness on the wing of the common swift facilitates flow separation promoting transition to turbulence for more resilient performance across different Reynolds regimes and angles of attack (23). Airfoil section reprinted from (23). (B) Photograph of the 104-g MAV demonstrator and a section of the bioinspired Separated Flow airfoil. The top surface of the wing is removed to highlight the integration of batteries and electronics inside the wing. (Bottom) A section of the bioinspired Separated Flow airfoil illustrates the flat plate (in blue) and the rear flap (in red). The flow separates at the sharp leading edge, transitions to turbulence, and reattaches over the flap. c is the airfoil chord, t is the bottom plate thickness, b1 and b2 are the lengths of the flap portions fore and aft of the bottom plate trailing edge, and β is the flap angle. (C) The time averaged horizontal velocity field, u¯/U, captured over the wing mid-span plane, using PIV in a water tunnel, illustrates the airfoil aerodynamic principle (Re = 40,000, α = 5, Tu = 2.5%). Near the leading edge, a region of negative speed (fluid moving toward the leading edge) marks the presence of flow separation, S. After transitioning to turbulence, T, the flow reattaches, R, over the plate as speed increases near the surface. The boundary layer travels over the flap leading edge and remains attached over the back. (D) Soon after separation, the Reynolds stress, u'v'¯/U2, grows, as shown by the evolution of the maximum values along y/c, indicating the transition to turbulence of the separated shear layer (23, 39).

  • Fig. 3 Lift production and performance resilience to turbulence.

    (A) A high increase in lift coefficient, CL, results from variations in external turbulence levels, Tu, for an Eppler E423 wing at Re = 40,000 (blue symbols). The variation is dependent on the level of external turbulence and on the angle of attack. The Separated Flow wing is relatively insensitive to external turbulence levels (red symbols). (B) High levels of external turbulence result in a great variation (improvement) of wing efficiency, CL/CD, for the Eppler E423 wing compared with only a minor change (degradation) for the Separated Flow wing.

  • Fig. 4 Atmospheric turbulence has only a minor effect on the flow field of the Separated Flow wing compared with the Eppler wing.

    The continuous lines and the black dashed lines represent the local velocity and the zero velocity at each chord-wise station. (A) Under conditions of low turbulence, Tu = 0.04%, flow separation occurs along more than half of the E423 wing as indicated by the zero speed contour line (gray dashed line). (B) Increased turbulence, Tu = 6.9%, eliminates separation, causes higher speeds in the leading edge region and a CL increase of 80% from 0.63 to 1.13 (α = 8; Fig. 3A). (C) Under the condition of wing maximum efficiency, α = 5, with low external turbulence, Tu = 0.04%, the separated flow transitions to turbulent, T, at about one-fifth of the chord (the criteria for determining the transition location is described in Materials and Methods). (D) With increased external turbulence, Tu = 6.9%, transition occurs very close to the leading edge, reattachment, R, occurs earlier, and the separation region is smaller. However, the change in the flow field and the speed distribution around the airfoil is only minor (fig. S5A) and results in a small lift variation (Fig. 3).

  • Fig. 5 MAV implementation and power required for cruise flight.

    (A) 3D model of the MAV and its main subsystems. (B) Detailed view of the wing structure. The top cover and a section of the flap are removed to show the structural and functional components. (C) The propulsion power required for flight as measured in the wind tunnel at two turbulence levels. In the speed region for minimum power, atmospheric turbulence tends to increase the power by less than 10%, and the increase is due to a reduction in wing efficiency with external turbulence (Fig. 3B). To estimate the flight time, we add receiver losses and servomotor consumption to obtain the total power required in cruise flight (see Materials and Methods).

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/5/38/eaay8533/DC1

    Section S1. Reynolds numbers for natural flyers

    Section S2. Flight time dependency

    Section S3. MAV and NAV aerodynamic efficiency: low or high aspect ratio?

    Section S4. Design principles for a Separated Flow airfoil

    Section S5. Separated Flow wing working principle, turbulence effects on lift

    Section S6. Battery capacity measurement

    Section S7. Wing-propeller interaction for lift production

    Section S8. Roll control

    Fig. S1. Lift coefficient versus angle of attack and maximum efficiency scaling with Reynolds number.

    Fig. S2. Subsystem scaling.

    Fig. S3. Separated Flow airfoil performance.

    Fig. S4. The angle of attack influences the reattachment position and not the distance required for laminar to turbulent transition (Re = 40,000 and Tu = 2.5%).

    Fig. S5. Reattachment position of the separated shear layer.

    Fig. S6. Velocity profiles around the Separated Flow wing at α = 7°, Re = 40,000 and at two turbulence levels.

    Fig. S7. Lift increase due to propeller action.

    Fig. S8. Ailerons characterization.

    Table S1. Airfoil geometries tested in the wind tunnel.

    Table S2. The capacity of two different batteries was measured at two different discharge rates.

    Movie S1. Video of flight.

    Data S1. Reynolds numbers of large insects and small birds.

    Data S2. Propellers efficiency and power.

    Data S3. Motors efficiency and power output.

    Data S4. Lithium polymer batteries–specific energy.

    Data S5. Data and scripts used to generate figures.

    References (4356)

  • Supplementary Materials

    The PDF file includes:

    • Section S1. Reynolds numbers for natural flyers
    • Section S2. Flight time dependency
    • Section S3. MAV and NAV aerodynamic efficiency: low or high aspect ratio?
    • Section S4. Design principles for a Separated Flow airfoil
    • Section S5. Separated Flow wing working principle, turbulence effects on lift
    • Section S6. Battery capacity measurement
    • Section S7. Wing-propeller interaction for lift production
    • Section S8. Roll control
    • Fig. S1. Lift coefficient versus angle of attack and maximum efficiency scaling with Reynolds number.
    • Fig. S2. Subsystem scaling.
    • Fig. S3. Separated Flow airfoil performance.
    • Fig. S4. The angle of attack influences the reattachment position and not the distance required for laminar to turbulent transition (Re = 40,000 and Tu = 2.5%).
    • Fig. S5. Reattachment position of the separated shear layer.
    • Fig. S6. Velocity profiles around the Separated Flow wing at α = 7°, Re = 40,000 and at two turbulence levels.
    • Fig. S7. Lift increase due to propeller action.
    • Fig. S8. Ailerons characterization.
    • Table S1. Airfoil geometries tested in the wind tunnel.
    • Table S2. The capacity of two different batteries was measured at two different discharge rates.
    • References (4356)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Video of flight.
    • Data S1 (Microsoft Excel format). Reynolds numbers of large insects and small birds.
    • Data S2 (Microsoft Excel format). Propellers efficiency and power.
    • Data S3 (Microsoft Excel format). Motors efficiency and power output.
    • Data S4 (Microsoft Excel format). Lithium polymer batteries–specific energy.
    • Data S5 (.7z format). Data and scripts used to generate figures.

    Files in this Data Supplement:

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