Flying in conditions conducive to ice formation is problematic for virtually all helicopters. Moreover, many twin engine IFR helicopters are not certified for flight in known icing conditions. As such, helicopter pilots should understand the problems an encounter with icing can create for the rotor system.
Ice buildup on rotor blades will change the shape of the airfoil and consequentially, its ability to produce lift while increasing drag. The increased drag will slow the main rotor requiring the pilot to add power – which in some cases might not be available. Ice accumulation on the airframe can increase the helicopter’s gross weight requiring more power as well. Ice buildup is rarely, if ever, symmetrical causing an imbalance that produces vibrations in the rotor system. These vibrations can cause shedding of the ice and if all the ice comes off, vibration levels, lift and drag will return to normal. Asymmetrical shedding, however, can make the vibrations worse. Hopefully, the increased vibration will shed the remaining ice before any damage can occur. Ice accumulation is less on the outboard section of the rotor blade which is helpful because this area produces a larger amount of lift. However, an autorotation could be more difficult as the driving region is closer to the blade’s center.
Deicing refers to removing ice that has accumulated, while anti-icing is the prevention of ice formation. The few helicopters that having ice protection on the main rotor system use a de-icing system as the power required to anti-ice a main rotor system is extremely high. One of these is the Sikorsky S92 and it uses heater mats in the rotor blades to melt a thin layer of ice in contact with the blade surface causing the remaining ice to shed from the blade. According to Sikorsky, heat is applied to the mats to melt the ice in specific zones at precisely the right time for controlled shedding. Opposite main rotor blades are deiced simultaneously in order to prevent rotor imbalance and small sections of the rotor blades are deiced alternately to reducing the amount of electrical power required at any given time. The tail rotor ice protection system can be set to de-icing mode, which applies power in a scheduled manner or anti-icing mode in which heat is continuously applied to tail rotor heating mats.