Fig. 2. Illustration of the technique for estimating the mechanical power output of the muscles of blue-breasted quail during take-off. To provide a vertical force, the wings must impart a downward momentum to the air to balance the weight and any vertical acceleration of the bird. The actuator disc represents the area over which the wings interact with the air to give it a downward impulse, and it generates a vertical force equal to the weight and acceleration force M_b(g+) by imparting an induced velocity kw to air entering the disc with vertical velocity . The rate at which work is done by the disc is the induced power, P_ind, and is equal to this force multiplied by the total air velocity. The second and third terms of the induced power equation are the rate of increase of potential energy of the body (M_bg) and the rate of increase of kinetic energy (M_b). Both these quantities were calculated from the movement of the centre of mass. The profile power (P_pro) required to overcome the pressure and friction drag acting on the wing was calculated from the resultant velocity and area of the wing using blade-element analysis. The parasite power (P_par) required to overcome drag on the body was calculated from the velocity and frontal area of the bird. The total aerodynamic power (P_aero) is equal to the sum of the induced power, profile power and parasite power. See Askew et al. (2001) for further details of the calculations. M_b is body mass, g is acceleration due to gravity, is vertical velocity, . is vertical acceleration, k is the induced power factor, w is induced velocity, v is the velocity of the bird, is air density, S_i is wing strip area, V_R,i is the velocity of the wing strip, S_b is body frontal area, C_D,pro is the profile drag coefficient and C_D,par is the parasite drag coefficient.

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