Deefer: I''ve found a handy article published by the Flight Safety Foundation:
http://flightsafety.org/hs/hs_nov-dec90.pdf
The rotor blade icing process and its subsequent effect on helicopter performance cannot be analyzed in the straight-forward manner used to explain ice accretion on the leading edges of fixed-wing aircraft. Spanwise elements of a rotor blade, unlike the leading-edges of an airplane's wing, move through the air at various speeds.
Rotor blade icing is made even more complex by the constantly changing angle of attack experienced by the helicopter's main rotor blades in normal forward flight. These obvious and unique characteristics of the helicopter;s lifting system, combined with differing surfaces temperatures along the blades' spanwise sections and smaller airfoil thickness, make helicopter rotor blade icing complex and extremely hazardous.
A while back I recalled my encounter with the late John 'Chalky' White recounting his days test flying the Wessex in known icing conditions and he made no bones about the fact that he thought he may have embarked upon his last flight!
In relation to the above PPRuNer Cornish Jack wrote:
Re: the Wessex and icing trials - not Norway but Canada in the mid 70s the A&AEE winter trials included a quadruple prismatic camera filming the rotor blades in flight. The movements recorded caused not a little 'interest' and one comment to the effect that "had I known that was what was going on I'd never have set foot in the
thing!" Remarkably robust bit of kit!
You may also find the following of interest:
http://www.pprune.org/rotorheads/405...ditions-3.html
http://www.pprune.org/rotorheads/240...mitations.html
http://www.pprune.org/rotorheads/605...elicopter.html