Therefore the blades encounter an apparent frontal wind when moving downwind, which is the resultant of {HEAD ~ the blade speed} minus {TAIL ~ the wind speed} 
minus
Therefore the blades encounter an apparent frontal wind when moving downwind , which is the resultant of { HEAD ~ the blade speed } minus { TAIL ~ the wind speed }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL