How Does a Windmill Work? Sails That Have to Be Aimed

A windmill sail works like a wing rather than a sheet being pushed. Air flowing past an inclined surface is deflected, and the reaction produces a force with a component along the direction of rotation, which is what drives the shaft. The sail is therefore set at an angle to the plane of rotation, and because the tip travels much faster than the root, the ideal angle changes along the length, which is why traditional sails are twisted and why the twist is described as weather. Too flat an angle and the wind pushes without turning, too steep and the airflow separates and the force collapses. Power available from wind rises with the cube of wind speed, so doubling the speed makes eight times the power available, which is why windmills are useless in light airs and must be shut down in gales, and why controlling the amount of sail exposed is a continuous operating task rather than a setting.

A mill that cannot turn into the wind works only when the wind happens to be right, and the solutions define the main types:

• •The post mill, in which the entire body of the mill, machinery and all, is built on a massive vertical post and is turned bodily by a long tail pole pushed by the miller
• •The tower mill, in which a fixed masonry tower carries a rotating cap holding the sails and the drive shaft, so only the cap turns
• •The smock mill, the same principle with a timber-framed tower, lighter and buildable on soft ground
• •The fantail, invented in England in 1745, a small wind vane mounted at right angles behind the cap and geared to the turning mechanism, which turns the cap automatically whenever the wind shifts, making the mill self-orienting with no attention at all
• •Sail control, from removable cloth spread over lattice frames to spring sails with shutters held by springs and finally the patent sail of 1807, which adjusted itself to wind strength while running
• •A brake acting on the great spur wheel, since a runaway mill in a storm could destroy itself through friction and fire

The mechanism inside a corn mill is a gear train converting a slow horizontal shaft into a faster vertical one. The windshaft carries a large brake wheel, which drives a smaller gear turning the vertical main shaft, which in turn drives the stone nuts turning the millstones. The gearing ratios matter because millstones need a specific speed. The grinding itself depends on a detail that is easy to miss: the stones do not touch, and the gap between them, called the tentering, is adjusted continuously to suit the grain and the wind strength, since too close scorches the flour and too far leaves it coarse. Both stones carry a pattern of grooves cut into the face, which draw grain from the centre outwards, cut it progressively and allow the ground meal to escape, and those grooves wear and must be recut regularly by a specialist, which is where the phrase about showing your mettle comes from, since the steel chips embedded in a dresser's hands proved they had done the work.

Grinding grain is the familiar use and far from the only one. Dutch mills drained land, driving scoop wheels and later Archimedes screws to lift water out of polders, which is how a substantial part of the Netherlands exists at all, and at one point thousands of mills worked at it. Others sawed timber, crushed oilseed, ground pigments, made paper, pumped brine and powered forges. Steam engines displaced them from the nineteenth century, because a steam mill works regardless of weather and can be built where it is needed rather than where the wind is, which is the decisive advantage. The principles returned in modern wind turbines, which differ in using few slender blades optimised as aerofoils, running at much higher tip speeds, generating electricity rather than driving machinery directly, and yawing under computer control rather than by fantail, while facing the same underlying constraints of cubic power, aiming and shutdown in high winds.

Windmill sails act as wings rather than sheets, set at an angle that varies along their length because the tip moves faster than the root. Available power rises with the cube of wind speed, so sail area must be adjustable and the mill shut down in gales. Post mills turn their whole body into the wind while tower and smock mills turn only a cap, and the fantail of 1745 made that automatic. Inside, gearing drives millstones whose gap is continuously adjusted.