Position Detector

A position detector for detecting the position of a helicopter blade comprising a sensor arranged to generate a signal in dependence on the position of a helicopter blade with respect to the sensor and a data processing module arranged to receive the signal from the sensor and generate an output indicative of the blade position from the received signal, the sensor comprising;

• a mask having a plurality of angularly divergent slits formed therein;
• a lens assembly having a field of view and being arranged to direct incident light onto a focal plane of the lens assembly at which the mask is located;
• and
• a light detector arranged to detect the level of illumination passing through each of the plurality of slits in the mask and provide a signal indicative of the illumination level to the data processing module,
• wherein the data processing module is arranged to receive the signal for each slit and sum the signals to generate a combined signal, wherein the ratio of combined signal amplitude when a blade is in the field of view of the lens assembly to when a blade is not in the field of view is indicative of blade height in relation to the sensor position.