Aeroengine Safety

Figure "Inlet air filter devices": The diagrams show filter devices in the inlet flow of helicopter engines. The intake openings and sockets for the particle-enriched exhaust air.

Figure "Particle separators": Filters that rely on the filtering effect of porous materials are widely used in industrial gas turbines because there is usually a large amount of available space. These filters require sufficiently large surface areas in order to minimize the flow resistance.
In aircraft engines, especially in helicopters, particle separators that operate on basis of the principle of inertia have found widespread and successful use. While they do not reach the high separation rates achieved by filter materials, they have sufficiently low flow resistance at a compact size. However, it must be remembered that integrated particle separators in the inlet air flow will always increase the engine weight and worsen performance, i.e. increase the specific fuel consumption.
The top diagrams show separator types in which the air is diverted tangentially (e.g. through guide vanes, left diagram) and/or radially (right diagram). The heavy particles (diameter greater than 0.010 mm, see Fig. "Particle size") are less likely to be diverted or are sufficiently centrifuged outward, resulting in a satisfactory separating effect. It must be remembered, however, that the principle of these devices means that they can not completely remove all particles from the gas flow, and a certain amount of erosion stress in the compressor is to be expected.
However, in this case it is easier to realize erosion protection through wear-resistant, hard coatings (TiN, etc.), since the small, low-energy particles will not smash through the coating.

The bottom diagram depicts a modularly constructed system with filter cartridges in a parallel configuration. In these cartridges, guide vanes make the air stream rotate, and the particles which are centrifuged outward are carried overboard through a bypass by an exhaust air flow which is supported by air injected from the compressor. This configuration can be used as an optional retro-fitting, since it must not be integrated into the engine and the modular construction allows it to be adjusted to fit the individual air throughput of different engine types.

Figure "Precipitator efficiency" (Ref. 5.3.3-1): The in Fig. "Particle separators" described and in use existing systems of filters/separation systems in the intake air of helicopter aeroengines, naturally do not guarantee a totally particle/dust separation. The so called precipitator efficiency is understandably depending from the particle size. It is markedly lower, that means the systems are more penetrable as plate filters („A“). We find them in industrial applications like for industrial gas turbines, compressors respectively power stations. These systems are too large and too heavy helicopter engines to get a sufficient low intake drag.
Sketch „E” shows an exception. Here a low intake drag is achieved by a wire mesh/sieve with relatively wide mesh size. Naturally, with this the precipitator efficiency is corresponding bad.
For the application in helicopters most effective are parallel applied filter cartridges (Fig. "Inlet air filter devices"), which are based on the inertia principle („B“).
Then follow integrated separator systems („C”, „D“) at which the whole intake flow is brought into rotation and/or is radial deflected.

Figure "Spinner geometry" (Ref. 5.3.1-12): In order to protect the core engine from erosion and FOD, and therefore also to reduce maintenance costs and service interruptions, the fan area must be optimally designed. Important factors include:

• Proper selection of the cone angle of the spinner can control the trajectories of ricocheting particles so that they pass through the bypass duct.

• The distance of the fan blade leading edges from the splitter must be coordinated with the cone angle of the spinner in order to direct the largest possible percentage of particles into the bypass duct.

Figure "Erosion protection": In practice, the application of hard armor coatings/hard facing to compressor blades has proven effective not only in older engine types. However, with the narrow and sharp-edged blade profiles in modern compressors with high pressure ratios and high dynamic loads, this type of erosion protection can only find very limited use due to the lower tolerances for roughness increases, dynamic strength losses, and profile changes. Furthermore, no cell action must be allowed to occur between the armor and the blade material, which could cause dangerous corrosion in the transition zone (also see Fig. "Corrosion due to cell action").

Figure "Multilayer coatings" (Ref. 5.3.3-8): Just in helicopters the compressors of the aeroengines are especially affected by sand erosion (Fig. "Extreme erosion stress during sand storms"). This leads to intense efforts to develop suitable protection coatings against sand erosion (erosion resistant coating = ER-coating). This is applied especially for the forward stages, usually made from high strength titanium alloy (Ti6Al4V = TiAl64). The coatings must be hard, to resist the abrasive wear of the dust at high and low impact angle (Fig. "Erosion mechanisms"). Its thickness is for modern compressors with thin sharp blading profiles markedly limited (range of 0,01 mm). A further requirement is, that the coating does not trigger corrosion (no developing of a corrosion cell, Fig. "Corrosion due to cell action"). All properties shall cover as far as possible the typical operation temperatures in modern compressors (-60 up to + 600 °C).
It was already early tried to deposit titanium nitride (TiN, identifiable at the characteristic golden colour) because of its extremely high hardness (2800-3200 Vickers). However for single layer coatings it turned out during long time operation in desert environments, that these on rotor blades of the front stages are penetrated and split out by the relatively large and energy-rich sand particles (Fig. "Erosion mechanisms"). This leads not only to abrasion/change of profiles, especially at the leading edge and an unwanted roughness. Critical is, that the fine cracks at the impact of the sand particle let markedly drop the fatigue strength. This also limits heavily the use. For the rear stages the single layer TiN coating has proved well. These are merely impinged from fine dust of the splintered sand particles (Fig. "Erosion mechanisms"). Here the excellent wear behaviour at flat impact angle can be used.
Not before a coating development in Russia, obviously has proved excellent in a high number of shaft engines and fan engines in military and civil use. The coating consists of several layers. Thereby hard TiN alternates with tough, deformable metallic layers (of titan, sketch middle right). A special adhesion coating guarantees the binding at the substrate. The advantage of this multi layer coating is due to the high tolerance against energy-rich sand particles. A sintering of the coating does not occur. The roughening of the surface is obviously minimum. Even if it comes to incipient cracks in the heavily deformed zones of a hard coating layer, these will be defused from the following tough/soft layer. So dangerous fatigue cracks are prevented. This effect increases with the percentage of metal in the layer combination.
The near-operation conditions testing in a typical shaft engine (sketch of the aeroengine) from a big transport helicopter (sketch above), confirmed the expectations. In contrast to an uncoated blading, the erosion caused portion of demounting aeroengines for overhaul up to 45 % could be lowered to 0. Thereby the separation rate of rotor and stator blades dropped from 70-80 % to 2-3% . The most of these rest cases because of unacceptable FODs. The long time efficiency drop (deterioration) decreased from up to 30 % to below 3 %.
Unfortunately from the literature on hand the behavior of the coating in the forward compressor stages at the testing during development and operation is not evident. These are exposed to especially energy-rich particles (size). Comparative pictures of coated and uncoated blades (sketches middle left and below) obviously derive from the middle and rear part of the compressor. Here also the single layer coating behaved during former testings promising.

Figure "Prevention of erosion in housings": Proper designing of the inner side of the housing/casing can cause abrasive dust (see Fig. "Damage mechanism in labyrinths"), such as that from crumbling labyrinth tip armor/hard facing, hard rub-tolerant coatings, or abrasive particles from brush seals, to be carried safely out of the engine by a rotating flow. However, it must be ensured that this dust does not cause damage in other areas (wear, blocking of cooling air bores, etc.).