Table of Contents
22.214.171.124 Measures Against Bird Strike Damage
- Sufficiently early discussion with authorities and operators to determine the certification requirements for bird strike resistance before beginning construction. Otherwise there is a danger of the alternatives being limited considerably. This includes the interpretation of allowable damages and influences on the engine.
- Sufficiently early, realistic verification of the engine behavior required during bird strikes.
- Optimization of the fan area (spinner geometry, fan blades, splitter, Fig. "Geometry optimization", Ref. 126.96.36.199-4, also see Fig. "Spinner geometry")
- Constructive design of “robust” blading
- Impact resistant materials (high impact- and notch impact strength) for especially highly stressed engine blading. If this is not sufficiently safely possible (for example, in fan stators), one may consider materials such as fiber-reinforced synthetics, which may fracture, but will only cause minor consequential damages.
- Use of thickest possible blade profiles with the maximum thickness located towards the leading edge, rounded leading and trailing edges, and short aspect ratio; these demands do not correspond to the geometry of modern compressor blades with wide chords and supersonic profiles. Despite this, the demands posed by bird strikes should be given as much consideration as possible.
- Wide spacing between stages, tightly spaced blades.
- Bracing long, slender (fan) blades with clappers, if possible set on alternating sides to further improve the (slicing effect) bird strike resistance
- Resilient, heavily damped rotor blade fastening
- Stator blades/vanes that are as stiff as possible and fastened together in rings or ring segments
- Hollow (e.g. for de-icing air) guide vanes should be designed to prevent buckling, through use of stiffening spars and a non-bursting trailing edge.
- Guide vanes in housings made from weaker materials such as light metals (Al and Mg alloys)should be fastened with more robust hammerheads rather than dovetails (Fig. "Damage mechanisms in the compressor").
- Carefully designing inner bearings and bearing connections of adjustable guide vanes.
- Predetermined break points should prevent closing and blocking of the blades in case of extreme overloading of the guide blade adjustment levers.
- Burst protection rings above fan rotor blades.
- Sufficient ring-reinforcements on the compressor casings to prevent bursting.
- Preventing large amounts of flammable dusts from being created due to imbalances cause by bird strikes; no flammable rub-tolerant coatings and blade materials in potential contact areas.
- If there is a risk of high-speed direct hits, there should be no inlet stator annulus or housing/casing struts ahead of the first rotor stage.
- Avoiding swan-neck air ducts, including in the rear compressor area. If a swan-neck duct is unavoidable, at the least the first rotor blading immediately following the duct must be especially robust in order to prevent overstressing due to collected bird pieces.
- Air extraction bores and slits (vents, for cabin air, de-icing air, cooling air, etc.) should be designed in such a way that they will not become blocked.
- Rotor disk connections and rotor shaft teeth should be designed to be shear-proof in case of polar weight moments of inertia.
- With bladings made from titanium alloys, the ignition of titanium fires as consequential damage after a bird strike should be minimized through suitable pairing of materials (rub coatings) on potential rub surfaces.
- Rotor disks should be sufficiently solid near the annulus to prevent the annulus breaking off due to the high loads caused by a bird strike.
- Design of the nose cone; there are two “philosophies”:
- Sufficiently resistant rotating (spinner) or fixed nose cone (Figs. "Low speed impact I" and "Influence of inlet guide vanes", Example "Spinner fragments"). If the cone is thin and “soft”, then the impact force of a bird strike is decreased and the energy is absorbed by deformation. This may have the drawback of resulting in unallowable imbalances, damaging bearing stresses, and in extreme cases, failure of the bearings or casings.
In this case bearings should be selected that are suited for high imbalances.
There are potential advantages to cones made from fiber-reinforced synthetics. When parts made from these materials fail, the resulting fragments are far less dangerous than fragments of metallic parts.
- Use of a stiff cone; while this construction is robust with regard to the creation and toleration of imbalances, it is relatively heavy and its stiffness results in high impact forces that are transmitted into the bearings and the supporting structure, making these more likely to become overloaded and cause extensive consequential damages. In this case it must especially be ensured that the rotor bearings are sufficiently strong against axial impact forces resulting from bird strikes in the hub area.
- Sufficient strength of the fan area in case of blade failure and/or large imbalances. This includes continuous housing/casing flanges with additional external tracks, oversized screwed flange joints possibly with increased expandability (tension screws), and a strong housing/casing against radial deformation.
- Including the inlet duct in the design concept for the bird strike resistance of the engine. For example, the shape of the inlet duct should ensure that no direct hits of the bird on core engine are possible.
- Making use of devices such as shutters or fences in the inlet duct (Fig. "Inlet screen as safety precaution" and Fig. "Various FOD grill designs") which stop and/or slice up the bird in a sufficiently safe manner, but do not endanger the engine in case of a failure/fracture.
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