concept

Power curve

A graph of how much power a flier must spend to fly at each speed — set against the most its muscles can supply. Flying too slowly costs power (you must thrash to keep from falling); flying too fast also costs power (you must beat the drag), so the "power required" line sags into a U-shape, with a cheapest speed at its bottom. As an animal grows, the required line rises faster than the power its muscles can deliver; where the two meet is the heaviest a flier can still flap. The physiologist Colin Pennycuick mapped these curves.