DOI: 10.5061/dryad.tmpg4f59f
title: The stability implications of drag minimization by tail action modelled in the gliding barn owl (<em>Tyto alba</em>)

description: Tail posture influences lift, drag, trim, and stability for birds, yet the
 interaction between them as the tail spreads and pitches remains unclear,
 even during steady gliding. In this study, we investigated the aerodynamic
 consequences of tail morphing, exploring the interactions between weight
 support, drag, longitudinal trim, and stability using data obtained from
 computational fluid dynamics (CFD) simulations of high-fidelity,
 photogrammetry-derived geometry of a free-gliding barn owl. Assuming drag
 to be minimized over a range of speeds, the tail should be more spread and
 pitched at low speeds, and less so at high speeds. This influences the
 proportion of weight supported by the tail; in order to prevent net
 aerodynamic pitching moment and maintain longitudinal moment equilibrium,
 the relative position of the center of gravity must shift. These effects
 shorten the negative static margin at higher speeds, making the model bird
 less unstable, limiting the reduction in pitch divergence doubling time
 that would otherwise have been coupled with the increase in speed. The
 drag-minimizing model owl is aerodynamically unstable at all speeds, but
 the feedback and control challenges of maintaining steady glides at high
 speeds are partially ameliorated and lower than would be predicted without
 a morphing airframe. In Dryad, the original data used to generate the barn
 owl geometry is provided, as well as the raw geometrical and aerodynamic
 data obtained from CFD simulations (ANSYS Fluent V19.1). 
description: # The stability implications of drag minimization by tail action modelled
 in the gliding barn owl (*Tyto alba*) Dataset DOI:
 [10.5061/dryad.tmpg4f59f](10.5061/dryad.tmpg4f59f) ## Description of the
 data This data includes the original points cloud used to reconstruct the
 high-fidelity geometry of the CFD simulation, as well as the data we
 obtained from CFD simulation for the analysis. With these data, readers
 can reproduce the trim states (tail spread, pitch and center of gravity
 shift) of the barn owl, and validate the conclusions of this paper.  ###
 Files and variables **File: Geometrical_variables.zip** Description: This
 zip file includes four csv data files. (1)
 "data_spanwise_leading_AR_t.csv" indicates the aspect ratio of
 the BARN OWL variation with tail spread and pitch, the data structure is 5
 X 3, row number indicates the pitching angle changing from 46 degrees to
 -46 degrees, column number indicates the spread angle changing from 0 to
 35 degrees. (2) "data_spanwise_leading_AR_t.csv" indicates the
 planform area variation with tail spread and pitch. (3)
 "data_spanwise_leading_e_is.csv" indicates the span efficiency
 with tail spread and pitch. (4)
 "data_spanwise_leading_e_v.csv" indicates the viscous efficiency
 with tail spread and pitch. (1)-(4) share the same data structure, with
 row and column number indicating the same morphology.  **File:
 aerodynamic_parameters_variation_with_aoa.zip** Description: This zip file
 includes four csv files. All the data are obtained from CFD simulations.
 (1)"lift_coefficient_variation_with_tail_span_pitch_aoa.csv"
 shows the lift coefficient variation with angle of attack（aoa） for
 different tail postures. Row 1-5 indicates the value for the spread angle
 equalling zero but the pitching angle ranging from 46 degrees to -46
 degrees; Row 6-10 indicates the value for the spread angle equalling 17.5
 degrees but the pitching angle ranging from 46 degrees to -46 degrees; Row
 11-15 indicates the value for the spread angle equalling 35 degrees but
 the pitching angle ranging from 46 degrees to -46 degrees. The angle of
 attack ranges from -6 to 6 degrees with an interval of 3 degrees. (2)
 "moment_coefficient_variation_with_tail_span_pitch_aoa.csv" shows the moment coefficient variation with angle of attack（aoa） for different tail postures. The data structure is same as (1). (3)"moment_zero_lift_variation_tail_span_pitch.csv" indicates the zero-lift moment, i.e. the moment at aerodynamic center. The data structure is 5 X 3, row number indicates the pitching angle changing from 46 degrees to -46 degrees, column number indicates the spread angle changing from 0 to 35 degrees. (4)"static_margin_variation_tail_span_pitch.csv" indicates the static margin, i.e. the distance between center of gravity and aerodynamic center. The data structure is 5 X 3, row number indicates the pitching angle changing from 46 degrees to -46 degrees, column number indicates the spread angle changing from 0 to 35 degrees. **File: CG_effect_at_trim_state.zip** Description: The zip file includes six files, showing the gliding speed, u, v at trim state for three center of gravity positions. (1) "CG_shift_origin_u_eq.csv" indicates the horizontal speed u at original center of gravity. The data structure is 256 X 9, row number indicates the pitching angle changing from 46 degree to -46 degree, column number indicate the spread angle changing from 0 to 35 degree. For the values "**NaN**", that indicates the trim **CANNOT** be achieved at such tail positions, and float values indicate the trim **CAN** be achieved at such tail position with the speed of the float values. For example, at Row 201 of the file, the values are  "[NaN  NaN  NaN  NaN 11.538 NaN  NaN  NaN  NaN]" This indicates that the pitching angle is (201-1)/(256-1)*92-46 = 26.3 degree, and the spread angle is (5-1)/(9-1)*35 = 17.5 degree, and the horizontal speed is 11.538 m/s.  （2） "CG_shift_origin_v_eq.csv" indicates the vertical speed v at original center of gravity. The data structure is the same. At Row 201 of the file, the values is  "[NaN  NaN  NaN  NaN 1.263 NaN  NaN  NaN  NaN]" This indicates that the pitching angle is (201-1)/(256-1)*92-46 = 26.3 degree, and the spread angle is (5-1)/(9-1)**35 = 17.5 degree, and the vertical speed is 1.263 m/s.  (3) "CG_shift_backward_u_eq.csv" and "CG_shift_backward_v_eq.csv" indicate the results when the center of gravity shifts backward by 3% of the chord length (more unstable).  (4) "CG_shift_forward_u_eq.csv" and "CG_shift_forward_v_eq.csv" indicate the results when the center of gravity shifts forward by 3% of the chord length (less unstable).  **Software: bird_flight_model.zip** Description: This file includes two point cloud files, showing the upper and lower surface of the barn owl surface at free gliding. These files can be opened by open source softwares Meshlab, CloudCompare (v2.14alpha), and can also be directly loaded by MATLAB and Python for model reconstruction.  **Access information** Other publicly accessible locations of the data: \* NA Data was derived from the following sources: \* NA