Necessary conditions to bird's flight

　In order that a bird flies, the lift and thrust that are necessary in the
flight are not only produced, but also various mechanical conditions must
be satisfied.

　Balance of forces
　　The sum of the force that the bird receives from the surrounding air, the gravity
　　and the inertia force must be zero.

　Balance of moments
　　The moment of force that the bird receives from the surrounding air with respect
　　to the center of gravity must be zero.

　Stability
　　The flying posture must be kept stably.

　Control ability
　　The posture must be restored in response to a disturbance of air and so on.

　Steerage performance
　　The turning must be free and sharp.

　Curtailment of drag
　　The bird should make decrease the induce drag, should make protect the boundary
　　layer separation and occurrence of large turbulence, and should minimize
　　the frictional loss with air.

　Burden of force
　　The power must be minimized and should be able to fly within the muscular power.

　Consumption of energy
　　The consumption energy should be minimized and it should be within the physical
　　strength.

　Structural strength
　　The wing and feathers have to be sufficient strength so as to support the bird's
　　weight and to flapping, and also to be able to bear unexpected external forces.

　The pose that is synthetically excellent over all these conditions, in conformity
with the flight purpose, the environment and bird's ability, that is the mechanically
ideal pose, is felt most naturally and beautifully.

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