EFFECTS OF ICING ON HELICOPTERS

General

Through practical experience, a wealth of knowledge has been accumulated operating fixed-wing aircraft in icing conditions. There are some other considerations, however, with rotary-wing aircraft. The conditions in which ice formation is possible are given below:

Categories. For convenience, helicopter icing is considered under four headings, in the following order:

Rotor System Icing

Icing Effects on Main Rotor System

The primary effect of ice on the rotor system is drag; the secondary effect is loss of lift due to the change in aerodynamic efficiency of the blade. The way in which ice forms on the blade is affected by five main factors.

Some blade forms produce more kinetic heating than others and this can be related to the design of the blade and its speed of rotation.

Continuous operation in rain ice/freezing rain is impossible; this is because the water content is so high that ice will form all over the blade surface giving maximum drag and change of aerodynamic shape at the same time. Ice shedding (see beneath) will tend to worsen this condition.

Each time a blade rotates in continuous icing conditions, a thin layer of ice is deposited on 20% of the leading edge, spanwise from the tip. If a section of this ice, which has been formed in temperatures below -10°C, is examined, it will be seen to have bands of slightly differing colour tone which can be seen by the naked eye. These bands are, in fact, growth bands and the greater the number of rotations, the greater the growth of ice.

Ice Formation on Different Blade Types

High Performance Blade. On a blade with a characteristically high performance profile and a high rotational speed, ice forms readily on the leading edge because the radius is small and the boundary layer shallow (see Fig 1); super cooled droplets can easily penetrate this layer allowing the formation of ice.

High Lift Blade. A blade having typical high lift characteristics, is deep in section, has a large tip radius and a slow rotational speed. Because the tip radius is greater than that of the high performance blade, the boundary layer which surrounds it is deeper and most of the super-cooled droplets that penetrate this layer are centrifuged off again and only a small proportion form ice on the leading edge (see Fig 2). This is a better blade configuration in icing conditions than the high performance blade.

Fig 1 High Performance Blade	Fig 2 High Lift Blade

Tail Rotor Blades. So few problems have been encountered with icing of the tail rotor blades that it is unnecessary to go into great detail; ice is picked up on only 20% of the blade from the root end towards the tip. Although ice does build on the pitch change mechanism, this can be kept clear by regularly cycling the controls.

Ice Formation at Different Temperatures

Ice Formation at, or Just Below, Freezing Point. Between 0°C and -3°C ice will form in natural icing conditions on the leading edge of the blades from the blade root towards the tip covering about 70% of the span and 20% of the chord from the tip of the leading edge, the remaining 30% of the span at the tip being free of ice due to kinetic heating.  If the blade ice is allowed to build up, the maximum accretion point will be the mid-point of this area, with another area of high accretion around the blade root caused by turbulence (see Fig 3). The ice formed on the leading edge at these relatively high temperatures will have the classical mushroom shape. At the blade root there may also be a degree of run-back which, in itself, is not important as little lift is produced in this area.

Fig 3 Blade Ice Coverage at Temperatures Just Below Freezing Point

Ice Formation at Temperatures Between -3°C and -15°C. It has been shown that at -3°C about 70% of the leading edge span will be covered by ice. As the temperature decreases, ice is deposited further along the blade until 100% coverage from root to tip takes place (see Fig 4) the lower temperature having overcome the kinetic heating. With 100% coverage of the leading edge, drag becomes very high and, if this ice cannot be shed, the drag will increase to a point where power is limited.

Fig 4 Blade Ice Coverage at Temperatures Between -3 deg C and -15 º C

Leading Edge Ice Formation at Temperatures Above -10°C. Fig 5 shows the ice formation on the leading edge at a temperature above -10°C with a definite depression at the stagnation point (point A). The ice build-up at point B is heavier than at A because only the freezing fraction, which is the smallest part of the supercooled droplet, freezes on impact, the remainder runs back towards point B and freezes between B and C. The drag factor produced by this type of ice accretion is high.

Fig 5 Leading Edge Ice Formation at Temperatures Above -10ºC

Leading Edge Ice Formation at Temperatures Below -10°C. At temperatures below -10°C, ice forms on the leading edge in a different way; there is no longer a concave depression at the stagnation point and the formation is more symmetrical (see Fig 6). This is because the freezing fraction of the supercooled droplet is much larger with very little run-back; consequently, the drag factor is not so high but the problem of asymmetric shedding is now posed. The rate of accretion is much slower because the air is drier.

Fig 6 Leading Edge Ice Formation at Temperatures Below -10 º C

Icing Effects on Rotor Head Control Rods

Although icing of the rotor head control rods will occur in flight, the control rod ends are always in a condition of movement and this keeps the vital area clear and does not normally restrict control movement. However, it is highly desirable to keep these areas as clear as possible from ice accretion and this is done by fitting an airflow deflector plate forward of the control rod area; a secondary reason for keeping the control rods free of ice is that in some designs they are adjacent to the engine intake and any shedding can result in engine ice ingestion.

Natural Ice Shedding

All main rotor blades have some degree of self-shedding and this always starts at a point 30% outboard from the blade root and continues to the tip. The reason for this is that, at this point, the blade is subject to mechanical forces and flexion and vibration are at their maximum here. The characteristics of the high lift blade are much better for natural shedding than those of the stiffer, high performance blade with its weak boundary layer.

Before any shedding can take place in the natural shedding range, sufficient ice must have been built up; this varies with different types of helicopters and blade design.

Flight in continuous icing conditions is not dangerous provided that the helicopter is not flown in temperatures at which natural shedding cannot be guaranteed; this temperature limit is known as the critical shedding temperature.

Determination of Critical Shedding Temperature. The critical shedding temperature is determined by test flying, at the hover, in an icing rig over a wide range of temperatures, water content and droplet size. The temperatures at which shedding is no longer reliable are carefully bracketed, but have to be exceeded under carefully controlled test conditions. These temperature limits are clear-cut and the icing rig test flying is followed by free flight over a wide time and condition range in icing cloud, freezing fog and wet and dry snow. There is a need to repeat many of these conditions in free flight with varying quantities of ice on the blades. This is because, whilst it may appear that conditions are satisfactory in the hover and low speed manoeuvres where the ice has been retained, in forward flight (eg climbing, descending, steep turns and autorotation), asymmetrical shedding may take place.

Asymmetric Shedding. Below critical shedding temperature, ice may be retained on all blades for some time; however, one or more blades can suddenly shed its ice, giving an asymmetric condition. If asymmetric shedding occurs in flight it can cause violent vibration, possibly leading to destruction. In such conditions, the only course is to land immediately and shut down as soon as possible, even if this means using the rotor brake harshly.

Damage to the Tail Rotor by Shed Ice. The incident rate of damage to the tail rotor from ice shed from the main rotors is very low and may amount only to slight denting of the leading edge, not sufficient in itself to cause vibration or balance problems.

Blade Anti-icing

The equipment for blade anti-icing consists of an electrical matrix which covers 20% of the leading edge chordwise from the tip along the length of the blade. Heat is phased into this matrix in different sectors, timed to coincide with the natural shedding cycle, ie when sufficient ice has built up.

This works well until the heat application and the natural shedding cycle get out of phase; heat may then be applied at the wrong time.This causes run-back, the ice reforming further back along the chord line, causing the blade CG to move backwards which, in turn, causes imbalance and flutter; it can also cause a residual build-up of ice. The extreme case is the failure of heating to one blade causing asymmetric problems.

The power supply for the matrix equipment is a drain on the electrical resources and, since the only satisfactory solution would be to heat the whole blade, a generator large enough to do this would impose weight installation problems.

Much research is going into solving this problem, but no clear solution is imminent. The only free, untapped source of heat that exists is from the engine efflux, but, until this can be harnessed to provide an efficient de-icing system, natural shedding and its restrictions must be accepted.

Engine Icing

Turbine Engine Icing

The only ice produced on a turbine engine is at the throat near the first compressor stage. This is not an insurmountable problem as there is sufficient heat available from hot air bleeds and hot oil, to heat this area, and the inlet guide vanes (where fitted).

Because of their delicate construction however, there is a problem of ice ingestion by high performance turbines. A sudden slug of slush, even as low as 350cc water equivalent, can put out the engine flame. Momentum separators are effective in preventing the ingestion of ice and slush and the multi-purpose air intake system, when in the anti-icing mode, separates out any ice particles which may be present and deposits them in an evacuation compartment.

Airframe Icing

Problem Areas. The main airframe icing problems are:

Appearance of Airframe Ice

At temperatures between -5°C and -10°C, ice usually appears clear; between 0°C and -5°C it may appear granulated because it will have been formed from fairly large droplets. At lower temperatures, ie at -15°C and below, ice appears whitish and opaque.  At the higher temperatures (0°C to +3°C) the ice, because of its appearance, may appear much more dangerous than it is; it is certain that at these temperatures the weight of fuel being burnt will be greater than the weight of ice deposited but this is not the case with rain ice/frozen rain which will deposit clear ice faster than fuel is being used and will not shed naturally at temperatures normally safe to fly in.

Operating Considerations

Indications of Main Rotor Blade Icing and Natural Shedding by Instrument Interpretation

Before a pilot contemplates flying in cloud in natural icing conditions it is essential that he can interpret these conditions by reference to his instruments; it is equally important that he is aware of the aircraft temperature limits in these conditions and at no time is it wise that he should attempt to exceed them - except in an emergency and then he must be aware of the consequences.

Depending on the temperature and water liquid content of the cloud, ice will start to form on the main rotor blades. This ice will produce increased drag which, in turn, will demand more power from the engine to maintain the rotor rpm. When this extra power is demanded, it is shown by an increase in torque for a set collective angle, ie the torque will be seen to increase although no alteration has been made to the position of the collective lever. Furthermore, a stage in the deterioration in the aerodynamic section may be reached such that maintaining Rrpm in autorotation is not possible; this being at a time when the engine(s) are susceptible to damage from ice ingestion.

As the ice builds up on the leading edge of the blades, the torque will show a steady rise up to 20% of its original value and at the same time a slight increase in the general vibration level will be apparent. At the point where sufficient ice has been built up to shed, natural shedding takes place and the engine torque returns to its original value, as will the vibration level. A steady cycling of this nature will continue as long as the helicopter remains in icing conditions.

Aircraft Limitations

Limitations on flying in icing conditions are defined in the relevant Aircrew Manual and are mandatory; flight in icing conditions is only permitted if the aircraft is suitably equipped or is modified to the necessary standard (eg intake door configuration, OAT gauge, lighting etc).

The Aircrew Manual or Release to Service for the particular helicopter may also need to state the following:

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