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The science
Power needed to fly
To hold level flight at speed U the animal must supply lift equal to its weight W = m g. The induced power is Pind = κ σ(h) W² / (2 ρ Ad U) with disc area Ad = π b²/4 (b = wingspan) and κ a form factor. The parasite power is Ppar = ½ ρ f U³, with f the equivalent flat-plate drag area. The total P(U) = A/U + B U³ has a minimum at Ump = (A / 3B)1/4 and the best distance per unit energy at Umr = (A / B)1/4.
Ground effect over water
Close to a surface the wing-tip vortices are squeezed, which cuts induced drag. A common empirical factor is σ(h) = (16 h/b)² / (1 + (16 h/b)²), which tends to 1 far from the surface and to 0 at h = 0. Skimming a few centimetres above the river therefore lowers the induced power.
Honest simplifications: a fixed-wing steady-state analogy for flapping flight, constant coefficients, no wing-beat unsteadiness, no wind, and the McCormick-style ground-effect formula holds best when h/b is below about 1. Real dragonflies are more complicated; this shows the trend.