concept

Reynolds number

A dimensionless number measuring the ratio of inertial forces to viscous forces in a fluid - loosely, it tells you whether, for an object moving through air or water, the heft of the flow or its "stickiness" dominates. Tiny insects fly at low Reynolds numbers, where air is as clinging as honey; large flyers like the ikran operate at high Reynolds numbers, where lift comes from pressure differences across a streamlined airfoil, far more efficiently. The same atmosphere can be a viscous syrup to a midge and a clean airstream to a banshee.