Icing. Aircraft icing intensity and its dependence on various factors The role of experimental research and numerical modeling in icing problems

Aircraft icing intensity in flight(I mm / min) is estimated by the rate of ice growth on the leading edge of the wing - by the thickness of ice deposition per unit time. In terms of intensity, they are distinguished:

A) slight icing - I less than 0.5 mm / min;

B) moderate icing - I from 0.5 to 1.0 mm / min;

B) severe icing - I more than 1.0 mm / min;

When assessing the danger of icing, the concept of the degree of icing can be used. Icing degree total ice deposition for the entire time of the aircraft stay in the icing zone. The longer the flight of the aircraft under icing conditions, the greater the degree of icing.

For a theoretical assessment of the factors affecting the intensity of icing, the following formula is used:

Icing intensity; - aircraft airspeed; - water content of the cloud; - integral coefficient of capture; - freezing coefficient; - the density of growing ice, which ranges from 0.6 g / cm 3 (white ice); up to 1.0 g / cm 3 (transparent ice);

The intensity of airborne icing increases with an increase in the water content of the clouds. The values ​​of the water content of the clouds vary over a wide range - from thousandths to several grams per cubic meter of air. The water content of the clouds at the AD is not measured, but it can be indirectly judged by the temperature and shape of the clouds. When the water content of the cloud is 1 g / cm 3, the most severe icing is observed.

A prerequisite for aircraft icing in flight is a negative temperature of their surfaces (from 5 to -50 degrees C). Icing of an aircraft with gas turbine engines can occur at positive air temperatures. (0 to 5 degrees C)

With an increase in the airspeed of the aircraft, the intensity of icing increases. However, at high air speeds, a kinetic heating of the aircraft occurs, which prevents icing.

The intensity of aircraft icing is different for different forms.

In cumulonimbus and power cumulus clouds, at subzero air temperatures, severe aircraft icing is almost always possible. These clouds contain large droplets with a diameter of 100 microns or more.

In the massif of stratus and altostratus clouds, with increasing altitude, a decrease in the size of drops and their number is observed. Heavy icing is possible when flying in the lower part of the cloud mass. Intra-mass stratus and stratocumulus clouds are most often watery and are characterized by an increase in water content with height. At temperatures from -0 to -20, weak icing is usually observed in these clouds; in some cases, icing can be strong.

When flying in altocumulus clouds, slight icing is observed. If the thickness of these clouds is more than 600 meters, the icing in them can be severe.

Flights in areas of heavy icing are flights in special conditions... Heavy icing is a dangerous meteorological phenomenon for flights.

Signs of severe aircraft icing are: rapid ice build-up on windshield wipers, windshield; a decrease in the indicated speed 5-10 minutes after entering the clouds by 5-10 km / h.

(There are 5 types of icing in flight: transparent ice, matt ice, white ice, frost and frost. The most dangerous types of icing are transparent and matt ice, which are observed at air temperatures from -0 to -10 degrees.

Transparent ice is the densest of all types of icing.

matte ice has a rough, bumpy surface. Strongly distorts the profile of the wing and aircraft.

white ice coarse ice, porous deposits, loosely adheres to the aircraft, and easily falls off when vibrated.)

Aircraft icing is a dangerous meteorological phenomenon for flights.
Although modern aircraft and helicopters are equipped with anti-icing systems, while ensuring flight safety, one constantly has to reckon with the possibility of ice deposition on aircraft in flight.
For the correct use of anti-icing means and rational operation of anti-icing systems, it is necessary to know the features of the aircraft icing process in different meteorological conditions and under different flight modes, as well as to have reliable predictive information about the possibility of icing. Of particular importance is the forecast of this dangerous meteorological phenomenon has for light aircraft and helicopters, which are less protected from icing than large aircraft.

Aircraft icing conditions

Icing occurs when supercooled water droplets of a cloud, rain, drizzle, and sometimes a mixture of supercooled droplets and wet snow, ice crystals collide with the surface of an aircraft (AC), which has a negative temperature. The aircraft icing process occurs under the influence of various factors associated, on the one hand, with a negative air temperature at the flight level, the presence of supercooled drops or ice crystals and with the possibility of their settling on the aircraft surface. On the other hand, the process of ice deposition is due to the dynamics of the heat balance on the icing surface. Thus, when analyzing and predicting the conditions of aircraft icing, not only the state of the atmosphere, but also the design features of the aircraft, its speed and flight duration should be taken into account.
The degree of icing hazard can be estimated by the rate of ice build-up. The characteristic of the rate of rise is the intensity of icing (mm / min), that is, the thickness of the ice deposited on the surface per unit of time. Weak icing is distinguished by intensity (1.0 mm / min).
For a theoretical assessment of the intensity of aircraft icing, the following formula is used:
where V is the aircraft flight speed, km / h; b - water content of the cloud, g / m3; E is the total capture coefficient; β - coefficient of freezing; Rl - ice density, g / cm3.
With an increase in water content, the intensity of icing increases. But since not all of the water settling in the drops has time to freeze (some of it is blown away by the air flow and evaporates), the freezing coefficient is introduced, which characterizes the ratio of the mass of accumulated ice to the mass of water that has settled over the same time on the same surface.
The rate of ice growth on different parts of the aircraft surface is different. In this regard, the total particle capture coefficient is introduced into the formula, which reflects the influence of many factors: wing profile and size, flight speed, droplet size and their distribution in the cloud.
When approaching the streamlined airfoil, the droplet is exposed to the force of inertia, which tends to keep it on the straight line of the undisturbed flow, and the drag force of the air medium, which prevents the droplet from deviating from the trajectory of air particles enveloping the wing airfoil. The larger the drop, the greater the force of its inertia and the more drops are deposited on the surface. The presence of large droplets and high flow velocities lead to an increase in the icing intensity. Obviously, a thinner profile causes less curvature of the trajectories of air particles than a larger profile. As a result, on thin profiles, more favorable conditions are created for the deposition of drops and more intense icing; the ends of the wings, struts, air pressure receiver, etc. freeze up faster.
The droplet size and the polydispersity of their distribution in the cloud are important for assessing the thermal conditions of icing. The smaller the radius of the droplet, the lower the temperature it can be in the liquid state. This factor turns out to be significant if we take into account the effect of flight speed on the aircraft surface temperature.
At a flight speed not exceeding the values ​​corresponding to the number M = 0.5, the intensity of icing is the greater, the higher the speed. However, with an increase in flight speed, a decrease in droplet settling is observed due to the influence of air compressibility. The freezing conditions for droplets also change under the influence of the kinetic heating of the surface due to the deceleration and compression of the air flow.
To calculate the kinetic heating of the aircraft surface (in dry air) ΔTkin.s, the following formulas are used:
In these formulas, T is the absolute temperature of the surrounding dry air, K; V - aircraft flight speed, m / s.
However, these formulas do not make it possible to correctly estimate the icing conditions during flight in clouds and atmospheric precipitation, when the temperature rise in the compressing air occurs according to the wet-adiabatic law. In this case, part of the heat is spent on evaporation. When flying in clouds and atmospheric precipitation, the kinetic heating is less than when flying at the same speed in dry air.
To calculate kinetic heating in any conditions, the formula should be used:
where V is the flight speed, km / h; Yа is the dry adiabatic gradient in the case of flight outside the clouds and the wet adiabatic temperature gradient when flying in the clouds.
Since the dependence of the moisture-adiabatic gradient on temperature and pressure is complex, it is advisable to use graphical constructions on an aerological diagram for calculations or use the data in the table sufficient for rough estimates. The data in this table refer to the critical point of the profile, where all kinetic energy is converted into heat.

The kinetic heating of different parts of the wing surface is not the same. The greatest heating is at the leading edge (at the critical point), as it approaches the rear of the wing, the heating decreases. The calculation of the kinetic heating of individual parts of the wing and side parts of the aircraft can be carried out by multiplying the obtained value of ΔTkin by the coefficient of recovery Rv. This coefficient takes values ​​of 0.7, 0.8 or 0.9, depending on the considered area of ​​the aircraft surface. Due to the uneven heating of the wing, conditions can be created under which the temperature is positive on the leading edge of the wing, and the temperature is negative on the rest of the wing. Under these conditions, there will be no icing on the leading edge of the wing, and icing will occur on the rest of the wing. In this case, the conditions for the air flow around the wing significantly deteriorate, its aerodynamics are disrupted, which can lead to the loss of stability of the aircraft and create a prerequisite for an accident. Therefore, when assessing the icing conditions in the case of high-speed flight, kinetic heating must be taken into account.
For these purposes, you can use the following chart.
Here, the abscissa represents the aircraft's flight speed, the ordinate represents the ambient air temperature, and the isolines in the figure field correspond to the temperature of the frontal parts of the aircraft. The order of calculations is shown by arrows. In addition, the dashed line of zero values ​​of the temperature of the side surfaces of the aircraft is shown with an average recovery coefficient kb = 0.8. This line can be used to assess the possibility of side surfaces icing when the temperature of the leading edge of the wing rises above 0 ° C.
To determine the conditions of icing in the clouds at the flight level of the aircraft, according to the schedule, the surface temperature of the aircraft is estimated from the air temperature at this altitude and the flight speed. Negative values airplane surface temperatures indicate the possibility of its icing in the clouds; positive temperatures exclude icing.
The minimum flight speed at which icing cannot occur is also determined from this graph by moving from the value of the ambient air temperature T horizontally to the isoline of the zero surface temperature of the aircraft and then down to the abscissa axis.
Thus, the analysis of the factors influencing the intensity of icing shows that the possibility of ice deposition on an aircraft is primarily determined by meteorological conditions and flight speed. Icing of piston aircraft depends mainly on meteorological conditions, since the kinetic heating of such aircraft is negligible. At a flight speed above 600 km / h, icing is rarely observed, this is prevented by the kinetic heating of the aircraft surface. Supersonic aircraft are most prone to icing during takeoff, climb, descent and approach.
When assessing the danger of a flight in icing zones, it is necessary to take into account the length of the zones and, consequently, the duration of the flight in them. In about 70% of cases, the flight in the icing zone lasts no more than 10 minutes, however, there are some cases when the flight duration in the icing zone is 50-60 minutes. Without the use of anti-icing agents, flight, even in the case of light icing, would be impossible.
Icing is especially dangerous for helicopters, since ice builds up on the blades of their propellers faster than on the surface of an aircraft. Icing of helicopters is observed both in the clouds and in precipitation (in freezing rain, drizzle, sleet). The most intensive is the icing of the helicopter propellers. The intensity of their icing depends on the speed of rotation of the blades, the thickness of their profile, on the water content of the clouds, the size of the droplets and on the air temperature. Ice build-up on the propellers is most likely in the temperature range of 0 to -10 ° C.

Aircraft icing forecast

Aircraft icing forecast includes the determination of synoptic conditions and the use of calculation methods.
Synoptic conditions favorable for icing are primarily associated with the development of frontal cloudiness. In frontal clouds, the probability of moderate and severe icing is several times higher than in intramass clouds (respectively 51% in the front zone and 18% in a homogeneous air mass). The probability of severe icing in the front zones is 18% on average. Heavy icing is usually observed in a relatively narrow strip, 150-200 km wide, near the front line at the earth's surface. In the zone of active warm fronts, strong icing is observed 300-350 km from the front line, its recurrence is 19%.
The intramass clouds are characterized by more frequent cases of weak icing (82%). However, in the intramass clouds of vertical development, both moderate and severe icing can be noted.
Studies have shown that the frequency of icing in the autumn-winter period is higher, and at different heights it is different. So, in winter, when flying at altitudes up to 3000 m, icing was observed in more than half of all cases, and at altitudes over 6000 m it was only 20%. In summer, up to heights of 3000 m, icing is very rare, and at flights above 6000 m, the recurrence of icing exceeded 60%. Such statistics can be taken into account when analyzing the possibility of this dangerous atmospheric phenomenon for aviation.
In addition to the difference in cloud formation conditions (frontal, intramass), when predicting icing, it is necessary to take into account the state and evolution of cloudiness, as well as the characteristics of the air mass.
The possibility of icing in clouds is primarily associated with the ambient temperature T - one of the factors that determine the water content of the cloud. Additional information about the possibility of icing is provided by the data on the dew point T-Ta deficit and the nature of advection in the clouds. The probability of no icing depending on various combinations of air temperature T and dew point deficit Td can be estimated from the following data:

If the values ​​of T are within the specified limits, and the value of T - Ta is less than the corresponding critical values, then weak icing can be predicted in zones of neutral advection or weak advection of cold (75% probability) moderate icing - in cold advection zones (80% probability) and in zones of developing cumulus clouds.
The water content of a cloud depends not only on temperature, but also on the nature of vertical movements in the clouds, which makes it possible to clarify the position of the icing zones in the clouds and its intensity.
To predict icing, after the presence of cloudiness is established, an analysis of the location of the 0, -10 and -20 ° C isotherms should be performed. Analysis of the maps showed that icing occurs most frequently in cloud layers (or precipitation) between these isotherms. The probability of icing at air temperatures below -20 ° C is small and is no more than 10%. Icing of modern aircraft is most likely at temperatures below -12 ° C. However, it should be noted that icing is not excluded even at lower temperatures. The frequency of icing in the cold period is twice as high as in the warm one. When predicting icing of aircraft with jet engines, the kinetic heating of their surface is also taken into account according to the graph presented above. To predict icing, it is necessary to determine the ambient temperature T, which corresponds to the surface temperature of the aircraft 0 ° C when flying at a given speed V. The possibility of icing an aircraft flying at a speed V is predicted in the layers above the isotherm T.
The presence of upper-air data makes it possible in operational practice to use the ratio proposed by Godske to predict icing and linking the dew point deficit with the saturation temperature above the ice Tn.l: Tn.l = -8 (T-Td).
The curve of T „values ​​is plotted on the aerological diagram. l, determined with an accuracy of tenths of a degree, and layers are distinguished in which Γ ^ Γ, l. The possibility of aircraft icing is predicted in these layers.
The intensity of icing is estimated using the following rules:
1) at T - Ta = 0 ° C, icing in the AB clouds (in the form of frost) will be from weak to moderate;
in St, Sc and Cu (as pure ice), moderate to strong;
2) at T-Ta> 0 ° C, icing is unlikely in pure water clouds, in mixed clouds it is predominantly weak, in the form of frost.
The application of this method is expedient when assessing icing conditions in the lower two-kilometer layer of the atmosphere in cases of well-developed cloud systems with a low dew point deficit.
The intensity of airplane icing in the presence of aerological data can be determined from the nomogram.


It reflects the dependence of the icing conditions of two easily determined in practice parameters - the height of the lower boundary of the clouds Hngo and the temperature Tngo on it. For high-speed airplanes at a positive airplane surface temperature, a correction for kinetic heating is introduced (see the table above), the negative ambient air temperature that corresponds to zero surface temperature is determined; then the height of the location of this isotherm is found. The obtained data are used instead of the Tngo and Nngo values.
It is advisable to use the graph for forecasting icing only in the presence of fronts or intramass clouds of high vertical thickness (about 1000 m for St, Sc and more than 600 m for Ac).
Moderate and severe icing is indicated in a cloud zone up to 400 km wide ahead of a warm and cold front near the earth's surface and up to 200 km wide behind a warm and cold front. The justification for the calculations according to this graph is 80% and can be increased by taking into account the signs of cloud evolution set out below.
The front is sharpened if it is located in a well-formed baric trough of the surface pressure; temperature contrast in the front zone at AT850 is more than 7 ° С per 600 km (repeatability is more than 65% of cases); the propagation of the pressure drop to the frontal area or the excess of the absolute values ​​of the prefrontal pressure drop over the pressure increase behind the front is observed.
The front (and frontal cloudiness) is blurred if the baric trough in the surface pressure field is weakly expressed, the isobars approach rectilinear ones; temperature contrast in the front zone at AT850 is less than 7 ° С per 600 km (repeatability 70% of cases); the increase in pressure extends to the prefrontal area, or the absolute values ​​of the prefrontal increase in pressure exceed the values ​​of the pressure drop ahead of the front; there is a fall of continuous long-term precipitation of moderate intensity in the front zone.
The evolution of cloudiness can also be judged by the values ​​of Т-Тd at a given level or in the probed layer: a decrease in the deficit to 0-1 ° С indicates the development of clouds, an increase in deficit to 4 ° С and more indicates erosion.
To objectify the signs of the evolution of clouds, K. G. Abramovich and I. A. Gorlach investigated the possibility of using aerological data and information on diagnostic vertical currents. The results of the statistical analysis showed that the local development or erosion of clouds is well characterized by the previous 12-hour changes in the area of ​​the forecast point of the following three parameters: vertical currents at AT700, BT7OO, sums of dew point deficits at AT850 and AT700, and total atmospheric moisture content δW *. The last parameter is the amount of water vapor in an air column with a cross section of 1 cm2. The calculation of W * is carried out taking into account the data on the mass fraction of water vapor q obtained from the results of radio sounding of the atmosphere or taken from the dew point curve plotted on the aerological diagram.
Having determined the 12-hour changes in the sum of dew point deficits, total moisture content and vertical currents, local changes in cloudiness are specified using a nomogram.

The order of calculations is shown by arrows.
It should be borne in mind that the local forecast of the evolution of clouds makes it possible to estimate only the changes in the intensity of icing. The use of these data should be preceded by the prediction of icing in stratus frontal clouds using the following refinements:
1. With the development of clouds (keeping them unchanged) - in case of falling into region I, moderate to severe icing should be predicted, when entering region II - weak to moderate icing.
2. In case of cloud erosion - if it enters area I, weak to moderate icing is predicted, if it enters area II - no icing or weak ice deposition on the aircraft.
To assess the evolution of frontal clouds, it is also advisable to use sequential satellite images, which can serve to refine the frontal analysis on the synoptic map and to determine the horizontal extent of the frontal cloud system and its change over time.
A conclusion can be drawn about the possibility of moderate or severe icing for intramass positions based on the forecast of the shape of clouds and taking into account the water content and the intensity of icing during flight in them.
It is also useful to take into account information on the intensity of icing obtained from scheduled aircraft.
The presence of upper-air data makes it possible to determine the lower boundary of the icing zone using a special ruler (or nomogram) (a).
The horizontal axis in the scale of the upper-air diagram shows the temperature, and the vertical axis in the scale of the pressure indicates the flight speed of the aircraft (km / h). A curve of values ​​-ΔTkin is plotted, reflecting the change in the kinetic heating of the aircraft surface in humid air with a change in flight speed. To determine the lower boundary of the icing zone, it is necessary to combine the right edge of the ruler with the 0 ° С isotherm on the aerological diagram, on which the T (b) stratification curve is plotted. Then, along the isobar corresponding to the given flight speed, they are shifted to the left up to the -ΔTkin curve drawn on the ruler (point A1). From point A1, they are displaced along the isotherm to the intersection with the stratification curve. The obtained point A2 will indicate the level (on the pressure scale) from which icing is observed.
Figure (b) also shows an example of determining the minimum flight speed that excludes the possibility of icing. To do this, at a given flight altitude, point B1 is determined on the stratification curve T, then they are shifted along the isotherm to point B2. The minimum flight speed at which icing will not be observed is numerically equal to the pressure value at point B2.
To assess the intensity of icing taking into account the stratification of the air mass, you can use the nomogram:
On the horizontal axis (to the left) on the nomogram the temperature Tngo is plotted, on the vertical axis (downward) - the intensity of icing / (mm / min). Curves in the upper left square are isolines of the vertical temperature gradient, radial straight lines in the upper right square are lines of equal vertical thickness of the cloud layer (in hundreds of meters), oblique lines in the lower square - lines of equal flight speeds (km / h). (Since it is rarely read to the end, let's assume that Pi = 5) The order of calculations is shown by arrows. To determine the maximum intensity of icing, the thickness of the clouds is assessed using the upper scale indicated by the numbers in the circles. The justification for calculations on the nomogram is 85-90%.

Icing is the deposition of ice on the streamlined parts of aircraft and helicopters, as well as on power plants and external parts of special equipment when flying in clouds, fog or sleet. Icing occurs when there are supercooled droplets in the air at the flight altitude, and the surface of the aircraft has a negative temperature.

The following processes can lead to icing of aircraft: - direct settling of ice, snow or hail on the surface of the aircraft; - freezing of drops of a cloud or rain in contact with the surface of the aircraft; - sublimation of water vapor on the surface of the aircraft. To predict icing in practice, several fairly simple and effective methods are used. The main ones are as follows:

Synoptic forecasting method. This method consists in the fact that according to the materials available to the forecaster, the layers in which cloudiness and negative air temperatures are observed are determined.

Layers with possible icing are determined by the aerological diagram, and you, dear reader, are quite familiar with the procedure for processing the diagram. In addition, it can be said once again that the most dangerous icing is observed in the layer where the air temperature ranges from 0 to -20 ° C, and for the occurrence of severe or moderate icing, the most dangerous is the temperature drop from 0 to -12 ° C. This method is quite simple, does not require a significant amount of time to perform calculations, and gives good results. It is impractical to give other explanations on its use. Godske method.

This Czech physicist proposed to determine the value of Tn.l. from the sounding data. - saturation temperature above ice according to the formula: Тн.л. = -8D = -8 (T - Td), (2) where: D - dew point temperature deficit at any level. If it turns out that the saturation temperature above the ice is higher than the ambient temperature, then icing should be expected at this level. Icing forecast using this method is also given using an upper-air diagram. If, according to the sounding data, it turns out that the Godske curve in some layer lies to the right of the stratification curve, then icing should be predicted in this layer. Godske recommends using his method for forecasting aircraft icing only up to an altitude of 2000 m.

As additional information when predicting icing, the following established relationship can be used. If in the temperature range from 0 to - 12 ° С the dew point deficit is more than 2 ° С, in the temperature range from -8 to - 15 ° С the dew point deficit is more than 3 ° С, and at temperatures below - 16 ° С the dew point deficit is greater 4 ° C, then with a probability of more than 80% icing will not be observed under such conditions. And, of course, an important help for the forecaster in predicting icing (and not only it) is the information transmitted to the ground by flying crews, or crews taking off and landing.

Air element…. Boundless space, resilient air, deep blue and snow-white cotton wool of the clouds. Great:-). All this is present there, above, in fact. However, there is something else, which, perhaps, cannot be classified as enthusiastic ...

The clouds, it turns out, are far from always snow-white, and the sky has enough grayness and often all kinds of slush and wet rubbish, besides cold (even very :-)) and therefore unpleasant.

Unpleasant, however, not for a person (everything is clear with him :-)), but for his aircraft. The beauty of the sky, I think, is indifferent to this machine, but the cold and, so to speak, excess heat, the speed and influence of atmospheric currents and, in the end, moisture in its various manifestations is what the plane has to work in, and what it , like any machine, makes work not always comfortable.

Take, for example, the first and last of this list. Water and cold. The derivative of this combination is ordinary, well-known ice. I think any person, including those who are not versed in aviation matters, will immediately say that ice for an airplane is bad. Both on the ground and in the air.

On earth is icing taxiways and runways. Rubber wheels are not friendly with ice, it is clear to everyone. And although the take-off run on an icy runway (or taxiway) is not the most pleasant activity (and whole topic for discussion :-)), but in this case the aircraft is at least on solid ground.

And in the air, everything is somewhat more complicated. Here in the zone special attention there are two things that are very important for any aircraft: aerodynamic performance(both the airframe and the turbojet engine compressor, and for the propeller-driven aircraft and the helicopter also the characteristics of the propeller blades) and, of course, the weight.

Where does the ice in the air come from? In general, everything is quite simple :-). There is moisture in the atmosphere, as well as negative temperatures.

However, depending on external conditions, ice can have a different structure (and hence, accordingly, strength and adhesion to the aircraft skin), as well as the shape that it takes when settling on the surface of structural elements.

During flight, ice can appear on the glider surface in three ways. Starting from the end :-), let's name two of them as less dangerous and, so to speak, unproductive (in practice).

First type is the so-called sublimation icing ... In this case, water vapor is sublimated on the surface of the aircraft skin, that is, it is transformed into ice, bypassing the liquid phase (water phase). This usually happens when air masses saturated with moisture in contact with strongly cooled surfaces (in the absence of clouds).

This is possible, for example, if there is already ice on the surface (that is, the surface temperature is low), or if the aircraft rapidly loses altitude, moving from the colder upper atmosphere to the warmer lower layers, thereby maintaining a low skin temperature. The ice crystals formed in this case do not firmly adhere to the surface and are quickly blown away by the oncoming flow.

Second type- the so-called dry icing ... This, simply speaking, is the settling of ready-made ice, snow or hail during the flight of an aircraft through crystalline clouds, which are cooled so much that moisture is contained in them in a frozen form (that is, already formed crystals 🙂).

Such ice is usually not held on the surface (it is immediately blown off) and does not cause harm (unless, of course, it clogs up any functional holes of a complex configuration). It can remain on the casing if it has a sufficiently high temperature, as a result of which the ice crystal has time to melt and then freeze again upon contact with the ice already present there.

However, this is already, perhaps, a special case of another, third type possible icing... This type is the most common, and, in itself, the most dangerous for exploitation. aircraft... Its essence is the freezing on the surface of the casing of moisture drops contained in a cloud or in rain, and the water that makes up these drops is in hypothermic condition.

As you know, ice is one of the aggregate states of matter, in this case water. It turns out through the transition of water to a solid state, that is, its crystallization. Everyone knows the freezing point of water - 0 ° C. However, this is not quite "that temperature". This is the so-called equilibrium crystallization temperature(differently theoretical).

At this temperature, liquid water and solid ice exist in equilibrium and can exist as long as you like.

In order for the water to freeze, that is, to crystallize, additional energy is needed to form crystallization centers(otherwise they are also called embryos). Indeed, in order for them to turn out (spontaneously, without external influence), it is necessary to bring the molecules of the substance closer to a certain distance, that is, to overcome the elastic forces.

This energy is taken due to additional cooling of the liquid (in our case, water), in other words, its hypothermia. That is, the water is already becoming supercooled with a temperature significantly below zero.

Now the formation of crystallization centers and, ultimately, its transformation into ice, can occur either spontaneously (at a certain temperature, the molecules will enter into interaction), or in the presence of impurities in the water (some speck of dust, interacting with molecules, can itself become a crystallization center ), or under some external influence, for example, a shock (molecules also interact).

Thus, water cooled to a certain temperature is in a kind of unstable state, otherwise called metastable. In this state, it can be for a sufficiently long period until the temperature changes or there is no outside influence.

For example. You can store a container of purified water (free of impurities) in an unfrozen state in the freezer compartment of the refrigerator for quite a long time, but as soon as this water is shaken, it immediately begins to crystallize. The video shows this well.

Now let's get back from the theoretical digression to our practice. Supercooled water- this is exactly the substance that can be in the cloud. After all, a cloud is essentially a water aerosol. Water droplets contained in it can have sizes from several microns to tens and even hundreds of microns (if the cloud is rain). Supercooled droplets are typically 5 μm to 75 μm in size.

The smaller the volume of supercooled water in size, the more difficult the spontaneous formation of crystallization centers in it. This directly applies to small water droplets in the cloud. It is for this reason that in the so-called droplet-liquid clouds, even at a sufficiently low temperature, it is water, not ice, that is found.

It is these supercooled water droplets that collide with the structural elements of the aircraft (that is, experiencing an external effect), quickly crystallize and turn into ice. Further, new ones are layered on top of these frozen drops, and as a result we have icing in its purest form :-).

Most often, supercooled water droplets are contained in two types of clouds: layered ( stratus cloud or ST) and cumulus ( Cumulus clouds or Cu), as well as their varieties.

On average, the probability of icing exists at an air temperature of 0 ° C to -20 ° C, and the highest intensity is achieved in the range from 0 ° C to -10 ° C. Although there are known cases of icing even at -67 ° C.

Icing(at the inlet) can occur even at a temperature of + 5 ° C .. + 10 ° C, that is, the engines are more vulnerable here. This is facilitated by the expansion of the air (due to the acceleration of the flow) in the air intake duct, as a result of which there is a decrease in temperature, condensation of moisture, followed by its freezing.

Light icing of the turbojet engine compressor.

Compressor icing.

As a result, a decrease in the efficiency and stability of the compressor and the entire engine as a whole is likely. In addition, if pieces of ice hit the rotating blades, they may be damaged.

Heavy icing of the compressor (SAM146 engine).

For known such a phenomenon as carburetor icing , which is facilitated by the evaporation of the fuel in its channels, accompanied by general cooling. At the same time, the outside air temperature can be positive, up to + 10 ° C. This is fraught with freezing (and therefore narrowing) of the fuel-air channels, freezing of the throttle valve with a loss of its mobility, which ultimately affects the performance of the entire aircraft engine.

Carburetor icing.

The speed (intensity) of ice formation, depending on external conditions, can be different. It depends on the flight speed, air temperature, on the size of the droplets and on such a parameter as the water content of the cloud. This is the amount of water in grams per unit volume of the cloud (usually a cubic meter).

In hydrometeorology icing intensity it is customary to measure in millimeters per minute (mm / min). The gradation here is as follows: slight icing - up to 0.5 mm / min; from 0.5 to 1.0 mm / min - moderate; from 1.0 to 1.5 mm / min - strong and over 1.5 mm / min - very strong icing.

It is clear that with an increase in flight speed, the intensity of icing will increase, but there is a limit to this, because at a sufficiently high speed, such a factor as kinetic heating ... By interacting with air molecules, the skin of the aircraft can heat up to quite noticeable values.

You can give some approximate (average) calculated data on kinetic heating (though for dry air :-)). At a flight speed of about 360 km / h, the heating will be 5 ° С, at 720 km / h - 20 ° С, at 900 km / h - about 31 ° С, at 1200 km / h - 61 ° С, at 2400 km / h - about 240 ° C.

However, one must understand that this is data for dry air (more precisely, for flight outside the clouds). When wet, the heating is reduced by about half. In addition, the heating value of the side surfaces is only two-thirds of the heating value of the frontal surfaces.

That is, kinetic heating at certain flight speeds must be taken into account to assess the possibility of icing, but in reality it is more relevant for high-speed aircraft (somewhere from 500 km / h). It is clear that when the skin is warmed up, about no icing it is not necessary to speak.

But supersonic aircraft do not always fly at high speeds. At certain stages of the flight, they may well be susceptible to the phenomenon of ice formation, and the most interesting thing is that they are more vulnerable in this regard.

And that's why:-). To study the issue of icing of a single profile, such a concept as "capture zone" is introduced. When flowing around such a profile with a flow that contains supercooled drops, this flow bends around it, following the curvature of the profile. However, in this case, droplets with a greater mass, as a result of inertia, cannot sharply change their trajectory and follow the flow. They crash into the profile and freeze on it.

Capture zone L1 and protection zone L. S - spreading zones.

That is, some of the droplets that are at a sufficient distance from the profile will be able to round it, and some will not. This zone, which the supercooled drops fall on, is called the capture zone. In this case, the drops, depending on their size, have the ability to spread after impact. Therefore, more drip zones.

As a result, we get the zone L, the so-called "protection zone". This is the area of ​​the wing profile that needs to be protected from icing in one way or another. The size of the capture zone depends on the flight speed. The higher it is, the larger the zone. In addition, its size increases with an increase in the size of the droplets.

And most importantly, which is relevant for high-speed aircraft, the capture zone is the larger, the thinner the profile. Indeed, on such a profile, the droplet does not need to greatly change the flight trajectory and struggle with inertia. She can fly further, thereby increasing the capture area.

Increased grip area for thin wing.

As a result, for a thin wing with a sharp edge (and this is a high-speed aircraft 🙂), up to 90% of the droplets contained in the incoming flow can be captured. And for a relatively thick profile, and even at low flight speeds, this figure drops to 15%. It turns out that an aircraft designed for supersonic flight at low speeds is in a much worse position than a subsonic aircraft.

In practice, the size of the protection zone usually does not exceed 15% of the profile chord length. However, there are times when the aircraft is exposed to particularly large supercooled droplets (more than 200 microns) or falls under the influence of the so-called freezing rain(the drops are even larger in it).

In this case, the protection zone can significantly increase (mainly due to the spreading of drops along the wing profile), up to 80% of the surface. Here, moreover, a lot depends on the profile itself (an example of this is severe flight accidents with an airplane ATR -72- more on that below).

Ice deposits appearing on the structural elements of an aircraft may differ in appearance and nature, depending on the conditions and mode of flight, the composition of clouds, and the air temperature. There are three types of possible deposits: frost, rime and ice.

Frost- the result of sublimation of water vapor, it is a plaque of a fine-crystalline structure. It is poorly adhered to the surface, easily separated and blown away by the flow.

Rime... Formed when flying through clouds with temperatures much lower - 10 ° C. It is a coarse-grained formation. Here, small droplets freeze almost immediately after colliding with the surface. It can be easily blown away by the oncoming stream.

Ice itself... It is of three types. First is transparent ice. It is formed when flying through clouds with supercooled droplets or under supercooled rain in the most dangerous temperature range from 0 ° C to -10 ° C. This ice adheres firmly to the surface, repeating its curvature and not greatly distorting it until its thickness small. With increasing thickness, it becomes dangerous.

Second - matt(or mixed) ice. Most dangerous species icing. Temperature conditions from -6 ° C to -10 ° C. Formed when flying through mixed clouds. At the same time, large spread and small non-spread drops, crystals, snowflakes freeze into a single mass. All this mass has a rough, bumpy structure, which greatly impairs the aerodynamics of the bearing surfaces.

Third - porous white, croupy ice. Formed at temperatures below -10 ° C as a result of freezing of small drops. Due to its porosity, it does not adhere tightly to the surface. As the thickness increases, it becomes dangerous.

From the point of view of aerodynamics, the most sensitive, probably, is still icing leading edge of the wing and tail... The above-described protection zone becomes vulnerable here. In this zone, growing ice can form several characteristic forms.

The first- this is profile shape (or wedge-shaped)... Ice, when deposited, repeats the shape of the part of the aircraft structure on which it is located. Formed at temperatures below -20 ° C in clouds with low water content and small droplets. It adheres firmly to the surface, but is usually of little danger due to the fact that it does not greatly distort its shape.

Second form – grooved... It can be formed for two reasons. First: if the temperature on the leading edge of the wing toe is above zero (for example, due to kinetic heating), and on the other surfaces it is negative. This version of the form is also called horn-shaped.

Forms of ice formation on the toe of the profile. a - profile; b - grooved; c - horn-shaped; d - intermediate.

That is, due to the relatively high temperature of the profile toe, not all of the water freezes, and ice formations that look like horns grow along the edges of the toe at the top and bottom. The ice is rough and bumpy here. It strongly changes the curvature of the airfoil and thus affects its aerodynamics.

The second reason is the interaction of the profile with large supercooled droplets (size> 20 µm) in clouds with high water content at a relatively high temperature (-5 ° C ... -8 ° C). In this case, the drops, colliding with the leading edge of the profile toe, because of their size, do not have time to freeze immediately, but spread along the toe higher and lower and freeze there, layering on top of each other.

The result is something like a gutter with high edges. Such ice adheres firmly to the surface, has a rough structure and, due to its shape, also greatly changes the aerodynamics of the profile.

There are also intermediate (mixed or chaotic) forms. icing... Formed in the protection zone when flying through mixed clouds or precipitation. In this case, the ice surface can be of the most varied curvature and roughness, which has an extremely negative effect on the flow around the airfoil. However, this type of ice is poorly adhered to the wing surface and is easily blown away by the oncoming air flow.

The most dangerous from the point of view of changes in aerodynamic characteristics and the most common types of icing in the current practice are chute and horn-like.

In general, during flight through an area where there are conditions for icing, ice usually forms at all aircraft frontal surfaces... The proportion of the wing and tail unit in this regard is about 75%, and this is the reason for most of the severe flight accidents that occurred due to icing that took place in the practice of world aviation.

The main reason here is a significant deterioration in the bearing properties of aerodynamic surfaces, an increase in the profile resistance.

Changes in profile characteristics as a result of icing (quality and lift coefficient).

Ice build-ups in the form of the aforementioned horns, grooves or any other ice deposits can completely change the flow pattern around the wing profile or tail. The profile resistance grows, the flow becomes turbulent, in many places it stalls, the value of the lifting force decreases significantly, the value critical angle of attack, the weight of the aircraft is growing. Stall and stall can occur even at very low angles of attack.

An example of such a development of events is the well-known disaster of the aircraft ATR-72-212 (registration number N401AM, flight 4184) of American Eagle Airlines, which occurred in the United States (Roselawn, Indiana) October 31, 1994.

In this case, two things coincided completely unsuccessfully: a rather long stay of the aircraft in the waiting area in the clouds with the presence of especially large supercooled water droplets and features (or rather, disadvantages) aerodynamics and structures of this type of aircraft, contributing to the accumulation of ice on the upper surface of the wing in a special shape (roller or horn), and in places that, in principle (on other aircraft) are not very susceptible to this (this is exactly the case of a significant increase in the protection zone mentioned above) ...

American Eagle Airlines ATR-72-212 aircraft (Florida, USA, February 2011). Analogue of the disaster of 10/31/94, Roselawn, Indiana.

The crew used the onboard anti-icing system, however, its design capabilities did not correspond to the conditions of the resulting icing. A roll of ice formed behind the wing area served by this system. The pilots had no information about this, just as they did not have special instructions for actions on this type of aircraft with such icing. These instructions (specific enough) have simply not been developed yet.

Eventually icing prepared the conditions for the incident, and the crew's actions (incorrect in this case - retraction of the flaps with an increase in the angle of attack, plus a low speed)) were the impetus for its start.

Turbulence and flow stall occurred, the aircraft fell on the right wing, while entering rotation around the longitudinal axis due to the fact that the right aileron was "sucked" upward by a vortex formed as a result of flow separation and turbulence in the area of ​​the trailing edge of the wing and the aileron itself.

At the same time, the loads on the controls were very high, the crew could not cope with the car, more precisely, they did not have enough height. As a result of the disaster, all people on board were killed - 64 people.

You can watch a video about this incident (I haven't posted it on the site yet :-)) in the Russian version of National Geographic. Interesting!

A flight accident with an airplane developed approximately according to the same scenario. ATR -72-201(registration number VP-BYZ) of the company Utair who crashed on April 2, 2012 immediately after takeoff from Roshchino airport (Tyumen).

Retract flaps with autopilot engaged + low speed = aircraft stall... The reason for this was icing the upper surface of the wing, and in this case it was formed on the ground. This is the so-called ground icing.

Before departure, the plane stood at night in the open air in the parking lot at low negative temperatures(0 ° C ... - 6 ° C). During this time, precipitation in the form of rain and sleet was repeatedly observed. In such conditions, the formation of ice on the wing surfaces was almost inevitable. However, before departure, no special treatment was carried out to eliminate ground icing and prevent further ice formation (in flight).

Aircraft ATR-72-201 (reg.VP-BYZ). This board crashed on 02.04.2012 near Tyumen.

The result is sad. The aircraft, in accordance with its aerodynamic characteristics, reacted to the change in the flow around the wing immediately after the flaps were retracted. There was a stall, first on one wing, then on the other, abrupt loss heights and collision with the ground. Moreover, the crew probably did not even understand what was happening with the plane.

Ground icing It is often very intense (depending on weather conditions) and can cover not only the leading edges and frontal surfaces, as in flight, but the entire upper surface of the wing, empennage and fuselage. At the same time, due to the long-term presence of a strong wind in one direction, it can be asymmetrical.

There are known cases of ice freezing during parking in the slotted spaces of the controls on the wing and tail. This can lead to incorrect operation of the control system, which is very dangerous, especially on takeoff.

An interesting type of ground icing is “fuel ice”. An airplane making long flights to high altitudes long time is in the area of ​​low temperatures (down to -65 ° C). In this case, large volumes of fuel in the fuel tanks are strongly cooled (down to -20 ° C).

After landing, the fuel does not have time to heat up quickly (especially since it is isolated from the atmosphere), therefore, moisture condenses on the skin surface in the area of ​​the fuel tanks (and this is very often the wing surface), which then freezes due to the low surface temperature. This phenomenon can occur at a positive air temperature in the parking lot. And the ice that forms at the same time is very transparent, and often it can only be detected by touch.

Departure without removing traces of ground icing is prohibited in accordance with all guidelines in the aviation of any state. Although sometimes I just want to say that "laws are created in order to break them." Video…..

WITH icing the plane is connected and such unpleasant phenomenon, how aerodynamic "peck" ... Its essence is that the aircraft during the flight is quite sharp and almost always unexpectedly for the crew, lowers its nose and goes into a dive. Moreover, it is quite difficult for the crew to cope with this phenomenon and to transfer the aircraft to horizontal flight, sometimes it is impossible. The plane does not obey the rudders. Accidents of this kind were not without disasters.

This phenomenon occurs mainly during the landing approach, when the aircraft descends and the wing mechanization is in landing configuration, that is, the flaps are extended (most often to the maximum angle). And the reason for it is stabilizer icing.

The stabilizer, performing its function of providing longitudinal stability and controllability, usually works at negative angles of attack. At the same time, it creates, so to speak, a negative lift :-), that is, an aerodynamic force similar to the lift of a wing, only directed downward.

If it is present, a moment for pitching is created. It works in opposition to dive moment(compensates for it) created by the lift of the wing, which, moreover, after the release of the flaps is shifted in their direction, further increasing the diving moment. Moments are compensated - the plane is stable.

TU-154M. Diagram of forces and moments with the released mechanization. The plane is in balance. (Practical aerodynamics of TU-154M).

However, it should be understood that as a result of flaps extension, the slope of the flow behind the wing (downward) increases, and, accordingly, the slope of the flow around the stabilizer increases, that is, the negative angle of attack increases.

If, at the same time, ice build-ups appear on the surface of the stabilizer (bottom) (something like the horns or grooves discussed above, for example), then due to a change in the curvature of the profile, the critical angle of attack of the stabilizer can become very small.

Change (deterioration) of the stabilizer characteristics during its icing (TU-154M).

Therefore, the angle of attack of the incident flow (even more beveled by the flaps, moreover) can easily exceed the critical values ​​for the icy stabilizer. As a result, a flow stall occurs (bottom surface), the aerodynamic force of the stabilizer is greatly reduced and, accordingly, the pitching moment decreases.

As a result, the aircraft abruptly lowers its nose and goes into a dive. The phenomenon is very unpleasant ... However, it is well-known, and usually in the Flight Operations Manual for each given type of aircraft, it is described with a listing of the necessary actions of the crew in this case. Nevertheless, it still cannot do without serious flight accidents.

Thus icing- a thing, to put it mildly, very unpleasant and by itself it is assumed that there are ways to deal with it, or at least the search for opportunities to overcome it painlessly. One of the most common ways is (PIC). All modern aircraft, to one degree or another, cannot do without it.

The action of such technical systems is aimed at preventing ice formation on the surfaces of the aircraft structure or eliminating the consequences of icing that has already begun (which is more common), that is, removing ice in one way or another.

In principle, an airplane can freeze up anywhere on its surface, and the ice that forms there is completely out of place :-), regardless of what degree of danger it creates for the aircraft. Therefore, it would be nice to remove all this ice. However, it would still be unwise to make a solid PIC instead of the aircraft skin (and at the same time the engine input device) :-), impractical, and technically impossible (at least for now :-)).

Therefore, the areas of the most probable and most intense ice formation, as well as those requiring special attention from the point of view of flight safety, become the places of possible location of the actuating elements of the POS.

Layout of anti-icing equipment on an IL-76 aircraft. 1 - electric heating of angle of attack sensors; 2 - sensors of the icing signaling device; 3 - headlamp for lighting the socks of the air intakes; 4 - heating of air pressure receivers; 5 - POS of lantern glasses (electric, liquid-mechanical and air-thermal); 6.7 - POS engines (cooker and VNA); 8 - POS of socks of air intakes; 9 - POS leading edge of the wing (slats); 10 - POS plumage; 11 - headlamp to illuminate the toes of the plumage.

These are the frontal surfaces of the wing and tail assembly (leading edges), the shells of the air intakes of the engines, the inlet guide vanes of the engines, as well as some sensors (for example, angle of attack and slip sensors, temperature (air) sensors), antennas and air pressure receivers.

Anti-icing systems are divided into mechanical, physicochemical and thermal ... In addition, according to the principle of action, they are continuous and cyclic ... After switching on, POS of continuous operation work without stopping and do not allow the formation of ice on the protected surfaces. Cyclic POSs exert their protective effect in separate cycles, while freeing the surface from the ice formed during the break.

Mechanical anti-icing systems- these are just systems of cyclical action. The cycle of their work is divided into three parts: the formation of a layer of ice of a certain thickness (about 4 mm), then the destruction of the integrity of this layer (or a decrease in its adhesion to the skin) and, finally, the removal of ice under the action of a high-speed pressure.

The principle of operation of the pneumo-mechanical system.

Structurally, they are made in the form of a special protector made of thin materials (something like rubber) with cameras built into it and divided into several sections. This protector is placed on the surfaces to be protected. Usually these are wing and tail socks. Cameras can be located both along the wingspan and across it.

When the system is turned on, air is supplied under pressure to the chambers of certain sections at different times, taken from the engine (turbojet engine, or from a compressor driven by the engine). The pressure is about 120-130 kPa. The surface "swells", deforms, while the ice loses its integral structure and is blown away by the oncoming stream. After switching off, the air is sucked into the atmosphere by a special injector.

POS of this principle of operation is one of the first to find application in aviation. However, it cannot be installed on modern high-speed aircraft (max.V up to 600 km / h), because under the action of high-speed pressure at high speeds deformation of the tread and, as a consequence, a change in the shape of the profile, which, of course, is unacceptable.

B-17 bomber with a mechanical anti-icing system. Rubber protectors (dark in color) are visible on the wing and tail.

The leading edge of a Bombardier Dash 8 Q400 aircraft wing is equipped with a pneumatic de-icing toe cap. Longitudinal pneumatic chambers are visible.

Airplane Bombardier Dash 8 Q400.

In this case, the transverse chambers in terms of the aerodynamic resistance they create are in a more advantageous position than the longitudinal ones (this is understandable 🙂). In general, an increase in the profile resistance (in working condition up to 110%, in non-working condition up to 10%) is one of the main disadvantages of such a system.

In addition, the protectors are short-lived and are subject to the destructive effects of the environment (moisture, temperature changes, sunlight) and various types of dynamic loads. And the main advantage is simplicity and low weight, plus a relatively low air consumption.

Cyclic mechanical systems can also include electric pulse POS ... The basis of this system is special electric solenoid coils without cores called eddy current inductors. They are located near the skin in the icing zone.

Scheme of an electric pulse POS on the example of the IL-86 aircraft.

An electric current is supplied to them with powerful pulses (at intervals of 1-2 seconds). The duration of the pulses is several microseconds. As a result, eddy currents are induced in the skin. The interaction of the fields of currents of the skin and the inductor causes elastic deformations of the skin and, accordingly, the ice layer located on it, which is destroyed.

Thermal de-icing systems ... Hot air taken from the compressor (for turbojet engines) or passing through a heat exchanger heated by the exhaust gases can be used as a source of thermal energy.

Scheme of air-thermal heating of the profile nose. 1 - aircraft skin; 2 - wall; 3 - corrugated surface; 4 - spar; 5 - distribution pipe (collector).

Diagram of the air-thermal POS of the Cessna Citation Sovereign CE680 aircraft.

Airplane Cessna Citation Sovereign CE680.

POS control panel of the Cessna Citation Sovereign CE680 aircraft.

This kind of system is the most widespread now, because of its simplicity and reliability. They, too, are both cyclical and continuous. For heating large areas, cyclic systems are most often used for reasons of energy savings.

Continuous heating systems are used mainly to prevent ice formation in places where ice discharge (in the case of a cyclic system) could have dangerous consequences... For example, ice dropping from the center section of aircraft with engines located in the tail section. This could damage the compressor vanes if blown ice gets into the engine inlet.

Hot air is supplied to the area of ​​the protected areas through special pneumatic systems (pipes) separately from each engine (to ensure the reliability and operation of the system in the event of a failure of one of the engines). Moreover, air can be distributed over the heated areas, passing both along and across them (such have a higher efficiency). After fulfilling its functions, the air is released into the atmosphere.

The main disadvantage of this scheme is a noticeable drop in engine power when using compressor air. It can drop by up to 15% depending on the type of aircraft and engine.

This disadvantage is not possessed by a thermal system that uses for heating electric current... In it, the directly operating unit is a special conductive layer containing heating elements in the form of a wire (most often) and located between the insulating layers near the heated surface (under the wing skin, for example). It converts electrical energy into heat in a well-known way :-).

Airplane wing toe with heating elements of electrothermal POS.

These systems are usually pulsed to save energy. They are very compact and lightweight. Compared to air-thermal systems, they practically do not depend on the operating mode of the engine (in terms of power consumption) and have a significantly higher efficiency: for the air system, the maximum efficiency is 0.4, for the electric system - 0.95.

However, they are structurally more complex, laborious to maintain, and have a fairly high probability of failure. In addition, they require a sufficiently large generated power for their work.

As some exotic among thermal systems (or maybe their further development 🙂), it is worth mentioning the project initiated in 1998 by the research center NASA (NASA John H. Glenn Research Center)... It is called ThermаWing(thermal wing). Its essence is to use it to cover the wing profile with a special flexible conductive foil based on graphite. That is, not individual elements are heated, but the entire toe of the wing (this, however, is also true for the entire wing).

This coating can be used to both remove ice and prevent ice formation. It has a very high performance, great efficiency, compactness and durability. Pre-certified and Columbia Aircraft Manufacturing Corporation is trying this technology in the production of a glider using composite materials for the new Columbia 300/350/400 (Cessna 300350/400) aircraft. The same technology is used on the Cirrus SR-22 aircraft manufactured by the Cirrus Aircraft Corporation.

Columbia 400 airplane.

Airplane Ciruss SR22.

Video about the operation of such a system on the Ciruss SR22 aircraft.

Electrothermal POS are also used to heat various sensors and receivers of air pressure, as well as to de-icing the windscreen glazing of aircraft cabins. In this case, the heating elements are inserted into the sensor housings or between the layers of the laminated windshield. The fight against fogging (and icing) of the cabin glass from the inside is carried out by blowing warm air ( air-thermal software WITH ).

The less commonly used (in total) currently deicing method is physico-chemical... There are also two directions here. The first is a decrease in the coefficient of adhesion of ice to the protected surface, and the second is a decrease (decrease) in the freezing point of water.

In order to reduce the adhesion of ice to the surface, either various coatings such as special varnishes or separately applied substances (for example, based on fats or paraffins) can be used. This method has many technical inconveniences and is practically not used.

Decreasing the freezing point can be achieved by wetting the surface with liquids that have a lower freezing point than water. Moreover, such a liquid should be easy to use, wet the surface well and not be aggressive towards the materials of the aircraft structure.

In practice, in this case, most often used is suitable for all the required parameters. alcohol and its mixtures with glycerin... Such systems are not very simple and require a large supply special liquids... Moreover, they do not dissolve already formed ice. Alcohol also has one parameter that is not very convenient in everyday use 🙂. This is its indirect, so to speak internal use. I don't know if it's worth joking about this topic or not 🙂 ...

In addition, antifreezes are used for these purposes, that is, mixtures based on ethylene glycol (or propylene glycol, as less toxic). Aircraft using such systems have panels with rows of very small diameter holes on the leading edges of the wing and tail.

Through these holes, during the flight, when icing conditions occur, a reagent is supplied by a special pump and inflated along the wing in a counter flow. Such systems are used mainly in general-purpose piston aircraft, and also partially in business and military aviation. In the same place, a liquid system with antifreeze is used for anti-icing treatment of light aircraft propellers.

Liquids containing alcohol are often used for processing windshields, complete with devices that are, in fact, ordinary "wipers". It turns out the so-called fluid-mechanical system. Its action is rather prophylactic, since it does not dissolve the already formed ice.

Control panel for cockpit glass wipers ("wipers").

No less airplanes freeze up. They are exposed to this phenomenon not only the case with all the sensors installed on it, but also both screws - carrier and tail... Icing of the propellers is the greatest danger.

Main rotor... Its blade, representing in a certain sense a model of the wing, has, nevertheless, a much more complex picture of aerodynamic flow. As is known, the flow velocities around it, depending on the evolution of the helicopter, can vary from approaching sonic (at the tip of the blade) to negative in the backflow zone.

Hence, the formation of ice under conditions of possible icing can take on a peculiar character. In principle, the leading edge of the blade always freezes up. At sufficiently low air temperatures (from -10 ° and below), it freezes over the entire length, and the intensity icing increases with increasing radius (flow velocity is higher), although at the tip of the blade it can decrease due to kinetic heating.

V backflow area the trailing edge may freeze up. The leading edge in this zone is covered with ice less due to low peripheral speeds and incomplete turnover of the direct flow. With a high water content of the cloud and large supercooled drops in the area of ​​the butt of the blade, both the trailing edge and the upper surface of the blade can be covered with ice.

An approximate diagram of icing of a helicopter main rotor blade.

As a result, as on the wing, the aerodynamic characteristics of the blades are significantly deteriorated. The profile resistance greatly increases, the lifting force decreases. As a result, the lifting force of the entire propeller decreases, which cannot always be compensated for by an increase in power.

In addition, at a certain thickness of ice, its strength and adhesion turn out to be unable to withstand centrifugal force and the so-called self-release of ice... This happens quite chaotically and therefore, naturally, a certain asymmetry arises, that is, the blades receive different masses and different flow. As a result - strong vibration and quite probable loss of stability of the helicopter flight. All of this can end badly enough.

As for the tail rotor, it is even more susceptible to icing due to its small size. The centrifugal forces on it significantly exceed those on the main rotor (up to five times), therefore self-release of ice occurs more often and vibration loads are significant. In addition, the ice being dropped can damage the rotor blades and helicopter structural elements.

Due to the special sensitivity of the blades of helicopters to icing and the considerable danger for them of this phenomenon, when the possibility of moderate or severe icing is indicated in the weather forecast, helicopter flights are most often not performed.

An approximate diagram of an electrothermal heating system for a helicopter tail rotor. Here 5 and 6 are electric heating elements.

As for the POS used for helicopter blades, the most widespread are electrothermal... Air-heating systems are not used due to the complexity of air distribution along the blades. But they are used to heat the air intakes of helicopter gas turbine engines. To combat ice on windshields, alcohol is often used (at least on our helicopters 🙂 ).

In general, due to the complexity of the aerodynamics of the main rotor, determining the size and location of the protected area on its blade is sufficient difficult process... However, usually the blades along the leading edge protect the entire length (sometimes starting from 1/3 of the length). On the top, it is about 8-12% of the chord, on the bottom, 25-28% of the chord. The tail rotor protects the leading edge by about 15% along the chord length.

The trailing edge near the butt (which has a tendency to icing) is not fully protected in the electrothermal method due to the difficulty of placing the heating element in it. In this regard, with the danger of icing, the speed of the horizontal flight of the helicopter is limited.

Similarly, it happens icing propellers aircraft. Here, however, the process proceeds more evenly, since there are no reverse flow zones, no retreating and advancing blades, as on the main rotor of a helicopter 🙂. Icing starts from the leading edge and then goes along the chord up to about 25% of its length. The tips of the blades during cruising may not freeze due to kinetic heating. A large accumulation of ice occurs on the propeller coque, which greatly increases the resistance.

Self-release of ice occurs, so to speak, regularly 🙂. All these delights lead to a drop in thrust, efficiency of the propeller, its imbalance, significant vibration, which ultimately leads to engine damage. In addition, chunks of ice can damage the fuselage. This is especially dangerous in the area of ​​the sealed cabin.

As a PIC for aircraft propellers, most often electro-thermal ones are used, most often of cyclic action. Systems of this nature are easiest to use in this case. Moreover, their effectiveness is high. It is enough to slightly reduce the adhesion of ice to the surface and then centrifugal force 🙂 comes into play. In this method, the heating elements are embedded in the blade body (usually along the leading edge), repeating its shape, and along the surface of the rotor crank.

Of all the above types anti-icing systems some are used in combination. For example, air-thermal with electro-thermal or electro-pulse with electro-thermal.

Many modern anti-icing systems work in conjunction with icing sensors (or alarms)... They help to control the meteorological conditions of the flight and timely detect the process that has begun. icing... Anti-icing systems can be activated either manually or by a signal from these alarms.

An example of the location of the icing sensors. Airplane A320.

POS control panel on A320. The control panel for the air-thermal system is circled in yellow. The smaller console turns on electric heating.

Such sensors are installed on aircraft in places where the incoming air flow undergoes the least distortion. In addition, they are installed in the air intake ducts of the engines and can be of two types: indirect and direct.

The first detect the presence of water droplets in the air. However, they cannot distinguish supercooled water from ordinary water, therefore they have temperature correctors that turn them on only at negative air temperatures. Such alarms are highly sensitive. The action of their sensors is based on measurements of electrical resistance and heat transfer.

Second react directly to the formation and thickness of ice on the sensor itself. Sensitivity to conditions icing they are lower, because they only react to ice, and it takes time for it to form. The sensor of such a signaling device is made in the form of a pin exposed to the stream. Ice forms on it when the appropriate conditions occur.

There are several principles of operation of icing alarms. But the most common are two of them. First- radioisotope, based on the attenuation of β-radiation of a radioactive isotope ( strontium - 90, yttrium - 90) a layer of ice formed on the sensor. This annunciator responds to both the beginning and the end of icing, as well as its speed.

Radioisotope ice detector sensor (type RIO-3). Here 1 - profiled windows; 2 - radiation receiver; 3 - ice layer; 4 - radiation source.

Second–Vibrating. In this case, the indicator reacts to a change in the natural frequency sensing element(membrane) of the sensor, on which the newly formed ice settles. Thus, the intensity of icing is recorded.

Air intakes of engines can be equipped with CO type icing alarms, which work on the principle of a differential pressure gauge. The sensor has an L-shape, with its end face installed upstream and parallel to it. There are two chambers inside the signaling device: dynamic (5) and static (9) pressure. A sensitive membrane (7) with electrical contacts (6) is installed between the chambers.

Icing indicator sensor, CO type.

When the engine is not running, the pressure in the dynamic chamber is equal to static pressure (through nozzle 3) and the contacts are closed. During the flight, they are open (there is pressure). But as soon as ice appears at the input (1) of the sensor, which clogs the input, the dynamic pressure drops again and the contacts close. Signal about icing... It enters the engine anti-icing system control unit as well as the cockpit. Number 4 is a heater to prevent icing of the internal cavities of the signaling device.

In addition, indicators can be installed icing visual type... They usually stand within sight (near the windshield), have illumination and the pilot has the ability to visually control the build-up of ice on them, thereby obtaining necessary information about possible icing.

Layout of anti-icing equipment on a passenger aircraft. Here 1 - cockpit glass; 2,3 - sensors of angles of attack and pressure; 4 - leading edge of the wing (slats); 5 - socks of air intakes; 6 - socks of the tail unit; 7.8 - lighting headlights; 9 - the entrance to the engines; 10 - icing alarm.

On some types of aircraft, special headlights are installed to visually inspect the leading edges of the wing and tail, as well as engine air intakes at night from the cockpit and passenger compartment. This increases the possibilities of visual control.

Alarm sensors icing, as already mentioned, in addition to a certain place on the aircraft fuselage, they must be installed at the inlet to the air intake of each engine. The reason for this is understandable. The engine is a vital unit and special requirements are imposed on the control of its condition (including with regard to icing).

TO anti-icing systems, ensuring the operation of the engines, the requirements are no less stringent. These systems operate on almost every flight and their total operating time is 3-5 times longer than that of the general aircraft system.

An approximate diagram of an air-thermal POS for a turbojet engine (input).

The temperature range of their protective action is wider (up to - 45 ° C) and they operate on a continuous basis. The cyclical option is not suitable here. The types of systems used are - air-thermal and electro-thermal as well as their combinations.

In the fight against icing in addition to on-board systems, ground handling of aircraft is also used. It is quite effective, however, this efficiency, so to speak, is short-lived. The processing itself, in fact, is divided into two types.

First Is the removal of ice and snow already formed during parking (in English de —icing ). It is carried out in various ways, from simple mechanical, that is, removing ice and snow manually, using special devices or compressed air, to treating surfaces with special fluids.

Aircraft handling ATR-72-500.

These fluids must have a freezing point at least 10 º below the current air temperature. They remove or "melt" existing ice. If during processing there is no precipitation and the air temperature is near-zero or higher, you can process surfaces to remove ice with just hot water.

Second view Is the treatment of the surfaces of the aircraft in order to prevent the formation of ice and reduce its adhesion to the skin (in English anti -icing). Such processing is carried out under conditions for possible icing. Application is carried out in a certain way with special mechanical devices - sprayers of various types, most often on the basis of automotive technology.

De-icing treatment.

A special reagent liquid used for such treatments is made on the basis of water and glycol (propylene glycol or ethylene glycol) with the addition of a number of other ingredients such as thickeners, colorants, surfactants (wetting agents), corrosion inhibitors, etc. The amount and composition of these additives is usually trade secret of the manufacturer. The freezing point of such a liquid is quite low (up to -60 ° C).

Processing is carried out immediately before takeoff. The liquid forms a special film on the surface of the airframe that prevents the precipitation from freezing. After processing, the aircraft has a reserve of time for take-off (about half an hour) and climb the altitude, the flight conditions at which exclude the possibility of icing. When a certain speed is reached, the protective film is blown off by the oncoming air flow.

KS-135. Anti-Icing.

Treatment of the Boeing-777 aircraft (anti-icing).

Anti-icing of Boeing-777 aircraft.

For various weather conditions, there are four types of these fluids according to SAE standards (SAE AMS 1428 & AMS 1424). Type I- a liquid of a fairly low viscosity (most often without a thickener). Mainly used for surgery de -icing... In this case, it can heat up to a temperature of 55 ° - 80 ° C. After use, it easily flows from the surface along with the remnants of dissolved ice. It can be colored orange for easier recognition.

Type II... It is a liquid sometimes called "pseudoplastic". It contains a polymer thickener and therefore has a fairly high viscosity. This allows it to stay on the surface of the aircraft until it reaches a speed close to 200 km / h, after which it is blown away by the oncoming flow. It is light yellow in color and is suitable for large commercial aircraft.

Type I V ... This liquid is close in parameters to type II, but has a longer waiting time. That is, an aircraft treated with such a reagent has a greater margin of time before takeoff and in more severe weather conditions. The color of the liquid is green.

Special fluids for anti-icing treatment. Type IV and type I.

Type III... This liquid is in its parameters between I and II types. It has a lower viscosity than type II and is washed off by the oncoming flow at speeds over 120 km / h. Designed primarily for regional and general aviation. The color is most often light yellow.

Thus for anti -icing reagents of II, III and IV types are used. In this case, they are used in accordance with the weather conditions. Type I can only be used in lung conditions icing (such as frost, but without precipitation).

For the application (dilution) of special fluids, depending on the weather, air temperature and the forecast for possible icing, there are certain calculation methods that are used by technical personnel. On average, it can take up to 3800 liters of concentrate solution to process one large liner.

Something like this is the case on the front of the struggle against universal icing🙂. Unfortunately, no matter how perfect modern POS or ground anti-icing systems are, they have capabilities that are limited to a certain framework, constructive, technical or otherwise, objective or not very.

Nature, as always, takes its toll, and technical tricks alone are not always enough to overcome the emerging problems with icing aircraft. Much depends on the person, both on the flight and ground personnel, on the creators of aviation technology and those who put it into daily operation.

Always in the foreground. At least that's how it should be 🙂. If this is equally clear to everyone who is somehow involved in such a responsible area of ​​human activity as aviation, then all of us will have a great and interesting future 🙂.

This concludes. Thanks for reading to the end. Until next time.

Finally, a little video. A video about the effect of icing on the TU-154 (a good film, albeit an old one :-)), the next one about de-icing treatment and then the work of the PIC in the air.

Photos are clickable.

In regions with difficult climatic conditions, when building engineering structures, it is necessary to take into account a number of criteria that are responsible for the reliability and safety of construction sites. These criteria, in particular, should take into account atmospheric and climatic factors that can negatively affect the state of structures and the process of operating structures. One of these factors is atmospheric icing.

Icing is the process of formation, deposition and accumulation of ice on the surfaces of various objects. Icing can occur as a result of freezing of supercooled droplets or wet snow, as well as by direct crystallization of water vapor contained in the air. The danger of this phenomenon for construction sites is that ice build-ups formed on its surfaces lead to a change in the design characteristics of structures (weight, aerodynamic characteristics, margin of safety, etc.), which affects the durability and safety of engineering structures.

Particular attention should be paid to the issue of icing in the design and construction of power lines (PTL) and communication lines. Icing of power transmission lines disrupts their normal operation, and often leads to serious accidents and disasters (Fig. 1).

Fig. 1. Consequences of icing power lines

Note that the problem of icing power transmission lines has been known for a long time and there are various methods of dealing with ice build-ups. Such methods include coating with special anti-icing compounds, melting by heating with electric current, mechanical deicing, sheathing, and preventive heating of wires. But, not always and not all of these methods are effective, they are accompanied high costs, losses of electricity.

Knowledge of the physics of the icing process is required to identify and develop more effective control methods. In the early stages of the development of a new object, it is necessary to study and analyze the factors influencing the process, the nature and intensity of ice deposition, heat transfer of the icing surface, and the identification of potentially weak and most susceptible to icing places in the structure of the object. Therefore, the ability to simulate the icing process under various conditions and evaluate possible consequences this phenomenon is an urgent task, both for Russia and for the world community.

The role of experimental research and numerical modeling in icing problems

Modeling the icing of power transmission lines is a large-scale task, in solving which, in a complete formulation, it is necessary to take into account many global and local characteristics of the object and the environment. These characteristics include: the length of the area under consideration, the relief of the surrounding area, air flow velocity profiles, the value of humidity and temperature depending on the distance above the earth's surface, the thermal conductivity of cables, the temperature of individual surfaces, etc.

The creation of a complete mathematical model capable of describing the processes of icing and the aerodynamics of an iced body is important and extremely difficult. engineering challenge... Today, many of the existing mathematical models are built on the basis of simplified methods, where certain restrictions are deliberately introduced or some of the influencing parameters are not taken into account. In most cases, such models are based on statistical and experimental data (including SNIP standards) obtained in the course of laboratory studies and long-term field observations.

Setting up and carrying out numerous and multivariate experimental studies of the icing process requires significant financial and time costs. In addition, in some cases it is simply not possible to obtain experimental data on the behavior of an object, for example, under extreme conditions. Therefore, there is a tendency to supplement a full-scale experiment with numerical modeling more and more often.

Analysis of various climatic phenomena using modern methods engineering analysis became possible both with the development of the numerical methods themselves, and with the rapid development of HPC - technologies (high performance computing technology High Performance Computing), realizing the ability to solve new models and large-scale problems in an adequate time frame. Engineering analysis, carried out using supercomputer simulation, provides the most accurate solution. Numerical modeling allows solving the problem in a complete setting, conducting virtual experiments with varying various parameters, investigating the influence of many factors on the process under study, simulating the behavior of an object under extreme loads, etc.

Modern high-performance computing systems, with the competent use of calculation tools for engineering analysis, allow you to get a solution in an adequate time frame and track the progress of solving the problem in real time. This significantly reduces the cost of carrying out multivariate experiments taking into account multicriteria formulations. A natural experiment, in this case, can be used only at the final stages of research and development, as a verification of a numerically obtained solution and confirmation of individual hypotheses.

Computer simulation of the icing process

A two-stage approach is used to simulate the icing process. Initially, the parameters of the flow of the carrier phase (speed, pressure, temperature) are calculated. After that, the icing process is calculated directly: modeling the deposition of liquid droplets on the surface, calculating the thickness and shape of the ice layer. As the thickness of the ice layer grows, the shape and size of the streamlined body change, and the flow parameters are recalculated using the new geometry of the streamlined body.

The calculation of the parameters of the flow of the working medium is carried out by the numerical solution of a system of nonlinear differential equations describing the basic laws of conservation. Such a system includes the equation of continuity, the equation of momentum (Navier-Stokes) and energy. To describe turbulent flows, the package uses Reynolds-averaged Navier-Stokes equations (RANS) and the LES large eddy method. The coefficient in front of the diffusion term in the momentum equation is found as the sum of molecular and turbulent viscosity. To calculate the latter, in this work, the Spallart-Allmaras one-parameter differential turbulence model is used, which is widely used in external flow problems.

The icing process is simulated on the basis of two embedded models. The first is the melting and solidification model. It does not explicitly describe the evolution of the liquid-ice interface. Instead, the enthalpy formulation is used to define the portion of the liquid in which the solid phase (ice) forms. In this case, the flow should be described by a two-phase flow model.

The second model that makes it possible to predict the formation of ice is the thin film model, which describes the process of droplet deposition on the walls of a streamlined body, thereby making it possible to obtain a wetting surface. According to this approach, the consideration includes a set of Lagrangian liquid particles that have mass, temperature and velocity. Interacting with the wall, the particles, depending on the balance of heat fluxes, can either increase the ice layer or decrease it. In other words, both surface icing and melting of the ice layer are simulated.

As an example, illustrating the capabilities of the package for modeling the icing of bodies, the problem of air flow around a cylinder with a speed of U = 5 m / s and a temperature of T = -15 0C was considered. The cylinder diameter is 19.5 mm. To divide the computational domain into control volumes, a polyhedral type of cells with a prismatic layer at the surface of the cylinder was used. In this case, for a better resolution of the wake after the cylinder, a local thickening of the mesh was used. The problem was solved in two stages. At the first stage, using the model of a single-phase liquid, the fields of velocities, pressures and temperatures were calculated for "dry" air. The results obtained are in qualitative agreement with numerous experimental and numerical works on single-phase flow around a cylinder.

At the second stage, Lagrangian particles were injected into the flow, simulating the presence of fine water droplets in the air flow, the trajectories of which, as well as the absolute air velocity field, are shown in Fig. 2. The distribution of ice thickness over the cylinder surface for different points in time is shown in Fig. 3. The maximum thickness of the ice layer is observed near the stagnation point of the flow.

Fig. 2. Droplet trajectories and the scalar field of absolute air velocity

Fig. 3. Ice layer thickness at different points in time

Time spent on calculating a two-dimensional problem ( physical time t = 3600c), amounted to 2800 core hours, using 16 computational cores. The same number of core hours is required to calculate only t = 600 s in the three-dimensional case. Analyzing the time spent on calculating test models, we can say that for the calculation in a complete setting, where the computational domain will already consist of several tens of millions of cells, where a larger number of particles and complex object geometry will be taken into account, a significant increase in the required hardware computing power will be required. In this regard, to carry out a complete modeling of the tasks of three-dimensional icing of bodies, it is necessary to use modern HPC technologies.