We read in the opening chapter of Early Developments of Modern Aerodynamics by Ackroyd, Axcell and Ruban:
- Our stress on viscosity will no doubt surprise newcomers to the subject. However, it must be emphasized at the outset that, without viscosity, all flight, even in nature, would be impossible since wings would produce no lift. From this it follows that both marine and aircraft propellers – essentially rotating wings – would be useless, while ships driven since antiquity by sails – flexible wings – would have suffered an extremely short history. Moreover, without viscosity we would all be in severe danger of asphyxiating in our own exhalations. Thus an understanding of the role of viscosity emerges as not only vital to any explanation of flight but also vital, quite literally, to life itself.
This is the central postulate of modern fluid mechanics attributed to the Prandtl as its Father: Lift and drag of a body moving through air or water, such as the wing of an airplane or sail/keel of a sailing boat, are effects of viscosity originating from a thin boundary layer.
We show that this is incorrect by computing solutions of the Navier-Stokes equations with a slip boundary condition, which do not exhibit any boundary layers yet have lift and drag in close accordance with observation. The central postulate of modern fluid dynamics is thus incorrect.