Fig. 1 . Propeller body (A) and plan and side views (B) of the complete `see-saw' propeller rig. Roman numerals identify parts of the propeller body. i, sting for attachment to wings; ii, propeller head; iii, smoke chamber (smoke in this chamber feeds into the hollow shaft and up to the propeller head); iv, cut-away section showing torque strain gauges (electrical connections run down the hollow axle); v, strain gauge bridge supply and first-stage amplification by electronics rotating with axle; vi, electrical contacts on multi-wiper slip-rings carrying power and strain gauge signal; vii, gearbox; viii, motor; ix, tachometer.