The propeller is the engine of the windmill, taking the power of the wind and converting it into rotary force to drive the alternator. It gets its power from the wind by effectively changing the wind direction and slowing the wind down as it passes through the propeller.

The air behind the windmill has lost most of its forward direction and is instead "swirling" in a spiral, until it regains its forward direction some distance downstream.

There is a lot of science and maths involved in this process and I’ll only touch on the basics here – you could write a book on the subject and still not cover everything.

The blades

This set of timber blades were carved by Dennis Latham. Length is 1150mm and they are performing well on the F&P windmill. They're painted blue to reduce visual impact.

The propeller blades need to extract as much energy from the wind as possible and provide as must rotational speed as possible. Propellers used in power generation are designed to rotate faster than the wind speed. This is called the TSR, or Tip Speed Ratio.

A propeller with a TSR of five means the tips of the propeller are travelling at five times the wind speed, so if the wind speed is 25km/h, the tips are travelling at 125km/h.

A good TSR for power generation is between four and seven. A TSR of over eight is achievable but at these speeds the tip velocity is so high that blade wear and noise become a serious problem.

Modern blades are designed like aerofoils and need to factor in angle of attack, lift, drag and stalling.