Figure "Inlet duct design": Slightly curved inlets, annulus collector outer contours (“swan-necks”), and inlets and intermediate casings with many struts can have either a damage-decreasing or a damage-increasing influence.
In accordance with the fairly realistic image of a bird striking an engine at high impact speeds corresponding to the flight speed “spraying” in a manner similar to a jet of water, the smaller “droplets” (bird pieces) are easier for the blading to process due to their distribution and low weight, resulting in considerably less damage than when a bird enters the engine whole.
These conditions can be optimized through specific constructive measures, such as inlet duct design, making direct hits on the engine impossible. Naturally, this process also includes suitable design of the wall strength and stiffness. This type of strategy can, for example, make possible the use of light fiber materials in the blading of fighter aircraft engines.
Even if the impact angle and/or the yielding of the impacted surface are sufficient to “spray” the bird, it must be assumed that the bird may be decelerated by friction with the wall and/or collect in areas such as on struts or in a swan-neck duct (as in Fig. "Sensitivity caused by S-shaped ducts"), etc. This creates conditions for a low speed impact (Fig. "Influence of flight speed") with a larger impact angle on the rotor blades. This shifts the conditions for the “critical flight speed” towards the normal-impulse-maximum (Fig. "Low flight speeds").
Internals in the inlet duct such as air guide vanes and flaps can be broken off by an ingested bird and cause additional damages.
In certain Russian fighter aircraft types, grill-like structures in the engine inlet duct are used to catch ingested birds (Fig. "Inlet screen as safety precaution").
This certainly requires relatively solid and heavy constructions in order to avoid the risk of parts of the grill being broken off by a large bird. In addition, all internals in front of the engine present an increased danger of FOD through loose or fractured parts (Fig. "Risk of complex inlets"). Influences on the inlet flow to the engine must also be considered.
An important, virtually decisive influence on inlet design is its radar signature. Because the rotating fan blades could create a strong radar echo, a grill or specially designed inlet guide vanes are placed in front of it (Ref. 5.2.2.3-7).
This type of internal (called radar blockers or) would most likely have a negative effect on the bird strike danger. They could hardly be made solidly enough to withstand a dangerous high speed impact without damage or fracturing. For this reason, curved inlets which cover the engines are advantageous.

Figure "Multiple engine failure by single bird": There have been several reported cases in which multiple engines on an aircraft suffered virtually simultaneous bird strikes (see Example "Twin engine failure" and Example "The seagull split"). Experience has shown that this is entirely possible through several birds of a single flock. It seems practically impossible for a single bird to strike the fuselage or landing gear so symmetrically, that the pieces damage both engines and cause them to fail (top left diagram). However, experience has shown that this type of incident can and does occur, with both civilian and military aircraft.
The affected aircraft are twin-jet types in which the engine inlets are located on the fuselage.
It is especially remarkable that when the bird strikes the tip of the fuselage, it does not spray apart and be deflected outward by the fuselage as expected, but rather splits into relatively large pieces that travel along the fuselage and enter the engine inlets (top left diagram).
The two bottom diagrams show different engine configurations in modern twin-jet fighter aircraft that should have considerably different bird strike risks (danger of a single bird damaging both engines). The probability of a larger bird damaging both engines simultaneously should be considerably greater for the configuration with both engine inlets close together under the fuselage (in order to minimize radar echo?) than for the configuration in the bottom diagram.

Illustration 5.2.2-3.1 (Ref. 5.2.2.3-5): This modern Russian fighter aircraft has angled grills (Ref. 5.2.2.3-3.2) in the engine inlets that are designed to protect against FOD. According to Ref. 5.2.2.3-6, the grills are retractable. It can be assumed, that these grills are only extended into the intake duct in especially FOD-susceptible operating conditions. If foreign objects include birds, then this is one of the rare cases in which this type of protective measure is used (compare with Fig. "Vortex prevention"). A very early use of an “inlet screen” shows the sketch of an old fighter type at the bottom (Ref. 5.2.2.3-8).

Figure "Various FOD grill designs" (Ref. 5.2.2.3-9): This two variations of FOD protection grills are from early Russian specialist literature. They serve not only the protection against bird impact but also against other foreign objects like bigger stones and hail. Obviously these have been realized in several types of fighters (Fig. "Inlet screen as safety precaution"). As drawback are termed:

• Increased intake drag equivalent 3-5 % thrust loss respectively increase of the SFC.
  
• De-icing of the grill (only the static type).
• Increased weight.

The left sketch shows a static grill made from profiled metal sheet with slots of 2×2 mm up to 6,5×6,5 mm. It is located in the region of low flow velocity.
At the right a pivoting grill from several segments is displayed. It is mounted at the casing on the outside with pneumatic or hydraulic piston rods. If the main landing gear is actuated also the activation of the grill takes place.

Figure "Bird strike-proof inlet design": The shape and location of the inlet to the engine core can considerably reduce the probability of a bird, or at least large pieces of a bird, from entering the sensitive compressor.
In propeller engines, the rotating nose cone (spinner) and the propeller act to deflect foreign objects. A similar effect is consciously used in fan engines (middle diagram).
With regard to FOD, the configuration in the right diagram should be the safest (also see Fig. "Optimal design of the inlet duct"). However, it is necessary that the flow conditions at the engine inlet are acceptable for the operating behavior of the compressor and the longer design. This concept has been successfully used for years in an engine type for business jets. These engines use three blisk stages in the high-pressure compressor, and these have never had any FOD problems.

Figure "Concept of additional safety device" (Ref. 5.2.2.3-10): Also at turboprop engines the danger of a bird impact exists (Fig. "Turboprop I"). This picture shows the concept of a deflection (sketch in the middle) of the bird out of the S-shaped intake duct of an aeroengine with separate positioned prop gear (upper sketch). For this, obviously a flap is provided. Additionally there exists a heated grill against icing to minimize failures by FOD. The deflection of the bird should especially prevent the danger of a torsion fracture of the gas producer shaft (sketch below) caused by braking of the rotor from this relatively small aeroengine. The exact layout of the whole design could not be seen from the available literature and was completed by the author correspondent to his understanding.

Figure "Protection grills for turboprop engines" (Ref. 5.2.2.3-11): Already at the beginning of the seventies last century, protection grills in the intake duct against bird impact have been tested. Also activities from the middle of the eighties deal with this problem (Fig. "Concept of additional safety device"). Firstly the question arises: Why just for turboprop engines which yet seem to be protected by the rotation propeller? However as emerged cases show, this is obviously not true (Fig. "Turboprop I" and Fig. "Turboprop II"). A special problem is the curved intake duct in which the bird mass, also after a splashing at the wall, can again accumulate in front of the aeroengine (Fig. "Sensitivity caused by S-shaped ducts", Fig. "Axial stiffening rib" and Fig. "Influence of flight speed"). Because of the braking by friction with the wall, this leads to a „low speed impact“. From this the relatively filigree compressor rotor blades of the first stage from this relatively small aeroengine are especially endangered.
As remedy there are concepts of grills in the intake duct.

The left version concerns a slidable system. In the probable impact area of the wall of the duct, pre-stressed elastic exit flaps are mounted. During impact of big or small birds a spring triggers the opening. Then the birds can exit from the aeroengine nacelle through a channel.
The sketched case at the right shows a pivoting mechanism. It is driven by a motor. In the point of impact the duct wall has a `retention chamber' which obviously shall gather remains of birds. The energy absorber placed behind is thereby obviously deformed by the retention chamber. An energy absorbing filling shall prevent the overloading of the nacelle.
In both cases to minimize the impact forces, the grill is oriented angular (30°) to the bird trajectory. Anyway, in an extreme case during the direct hit of a 2 kg bird with about 320 km/h a force of about 50 tons, must be absorbed without unacceptable deterioration (Fig. "Influence of proximity to ground"). This force will be doubled at 400 km/h.
In layout the steel lamella of the grit have been 2.25 mm thick, its distance was 15 mm.

Figure "Protection grill concepts for airliners" (Ref. 5.2.2.3-11): Obviously concerned are not realized studies for the protection against bird impact. Anyway these are interesting as basis for a discussion and stimulation.
This arrangement of the left is as well designed for the mounting at the wing with a pylon as also for aeroengines at the rear aircraft fuselage. In this case the orientation of the device, which bears the pylon, is horizontal. The sketches show an elder aeroengine type with a lower bypass ratio. The bird is directed around the aeroengine. For modern aeroengines with high bypass ratio the tilted grill merely deviates outward into the bypass region of the fan. Main problem seems to be the additional weight of the construction. As problematic also the icing of the system is mentioned, which may require a de-icing device. Further also possibly efficiency loss and vibration excitations of the aeroengine by deteriorations of the flow (vortex formation) must be considered.
The arrangement at the right shows an into the intake duct hinged protection grit, similar Fig. "Protection grills for turboprop engines". An additional weight in the range of 100 kg will be expected for aeroengines with lower bypass ratio. A version for aeroengines with high bypass ratio is not evident.

Figure "Influence of blade material": The reaction of a blade to a to impact momentum is largely dependent on the blade material. This usually results in elastic and plastic deformations and is followed by vibrations at various resonance frequencies due to the blade springback (see Fig. "Phases of deformation"). The impact time is an increment of 10^-3 seconds. Experience has shown that steel blades perform considerably better under bending loads than do titanium ones under the same condition, and titanium blades perform better than blades made from Al alloys or fiber composites (for example, boron fiber-reinforced aluminum or carbon fiber-reinforced synthetics). While steel and titanium blades primarily show plastic deformation under bending loads, the same amount of energy makes the fiber material fracture brittle after purely elastic deformation (primarily due to the brittle fracture behavior of the reinforcing fibers; bottom diagram).
This behavior at first seems rather implausible, since the strength of currently used fiber-reinforced materials and titanium alloys is certainly comparable to that of steel. However, as the top diagrams show, the blade tip deflection after absorbing a certain amount of energy is greater, the lower the density of the blade material and the greater its elasticity (the smaller the E-modulus). This is understandable, since less deflective movement would be expected from a heavy, stiff steel blade than from a light blade under the same impact energy (Ref. 5.2.2.3-2). Due to this behavior, the same amount of energy deflects the light blade more and therefore subjects it to correspondingly higher bending loads, depending on the elasticity modulus. This makes the light blade more sensitive to bird strikes.
A further drawback of large deflection is the possibility of contact with neighboring blade rows, which is combined with extensive consequential damages.
This behavior has also led to blades in smaller engines not being made from light, fiber-reinforced materials, especially fiber-reinforced synthetics. Only recently have fan rotor blades in large engines, which weigh more than 10 kg, started being made from fiber-reinforced synthetics. Blades this solid are sufficiently robust to handle impacts even from large birds.
In order to use the advantages of fiber-reinforced synthetic blading in fighter aircraft engines, a material-specific safety philosophy should be considered, but it would not necessarily have to worsen the failure rate of the entire engine. On the contrary, when using fiber-reinforced synthetics in rotor blades, this can be accomplished by shaping the inlet duct so that birds cannot strike the engine directly, but first hit the duct wall and spray into small, harmless pieces which then enter the engine.
The use of fiber-reinforced synthetics for guide vanes in the fan is realistic if one considers, for example, twin-jet fighter aircraft and must only achieve the same reliability as with titanium blades. With the relatively thin and narrow guide vanes used in these engines, high speed impact of a somewhat larger bird (Fig. "Influence of flight speed") will in all likelihood result in failure of the blade which was struck. In this case, even the fracture of a fiber-reinforced synthetic is acceptable, especially since fragments of this material will cause considerably less damage downflow than ductile, metallic fragments would.

Figure "Notch impact strength": The different compressor blade materials (alloys based on Al, Ni, Ti, heat treated steels, and fiber-reinforced materials) can all behave very differently during foreign object strikes, depending on the material characteristics. The modulus of elasticity and yield strength are of primary importance for elastic deformations. However, impact damage also depends on the notch impact strength and the density of the blade material (Fig. "Influence of blade material"). The diagram shows that high-strength titanium alloys have a considerably lower notch impact strength than steels of similar strength. Al alloys have rather poor strength as well as poor notch impact strength, which was undoubtedly a factor (along with low dynamic strength and sensitivity to erosion and corrosion) in the decision no longer to use these materials for compressor blading in modern engines.
Low notch impact strength can drastically increase the extent of damage if, for example, during a bird strike notches in the blading are created through contact with other engine parts or fragments, and this is followed by heavy stressing of the blading (through heavy rubbing, etc.). This type of notch can be the initiation point for an unexpected blade fracture. Blades made from high-strength titanium alloys, especially, may show this undesirable behavior and thus deviate from their pre-calculated behavior.
Fundamentally, all blades and disks should be made from material with the most possible impact resistance. This is not only so that the directly struck blades do not fracture, but also in order to limit the consequential damages after a blade failure and prevent haircuts, etc. (a haircut is the fracturing of all blades on a rotor stage).
Interestingly, Ti-alloys in general have a considerably lower notch impact strength than steels or forged super alloys. The plastic deformation and possible stable and unstable crack initiation or fracture are influenced by the plastic deformability and the hardening behavior. The ultimate strain is a characteristic value for the undamaged material under static force. The lower the notch sensitivity of the material, the less likely it is that dynamic stress during vibrations will cause crack growth. At the high stress speeds relevant here, the plastic strain behavior, the impact bending strength, and the notch impact bending strength all decrease. The material tends to fracture more brittly with increasing deformation speed. At the same time, the ultimate strength and the yield strength usually increase, the latter increasing more quickly before meeting with the ultimate strength. In this case the material is completely brittle (Fig. "Material behavior"). Because of this material behavior, for fractions of a second the stress may be greater than the static yield strength (deformation speed effect). In structural steels yield strength may be exceeded by 2 or 3 times without any noticeable permanent deformation. With total damage to bladings this embrittlement can sometimes be observed on fracture surfaces.

Figure "Influence of inlet guide vanes": In engines equipped with them, inlet guide vanes (front guide annulus, top diagram) are especially susceptible to damage. Unlike rotor blades, which experience a decrease in the normal impulse beyond the critical flight speed (Fig. "Low flight speeds"), with guide blades the normal impulse continues to increase due to the fact that the impact angle does not approach zero. Therefore, with increasing intake speeds, the damage to be expected also increases. Other high-risk areas are adjustable guide vanes, especially inlet guide vanes, during landing, because the blade annulus is closed and may be struck by a foreign object at a large angle.
Inlet guide blades or bearing struts at the compressor inlet can create low-speed impact conditions for the following rotor blades during high-speed flight by decelerating the bird mass (compare with Fig. "Influence of flight speed")
The high impact speeds can bend (Fig. "Variable vanes") rotor blades or crack and/or fracture them at the transition to the housing (Fig. "Influence of flight speed"). Adjustable guide vanes can fracture at the adjustment bearings and/or all guide vanes may close due to the coupling of the adjustment mechanism, causing a sudden thrust loss in the engine. Even if only single adjustable guide blades suffer permanent angle changes, it can lead to serious operating problems in the compressor (stalls, blade vibrations).
Additionally, guide vanes are fundamentally subject to higher stresses during bird strikes than rotor blades:
While, in normal cases, multiple rotor blades of a stage slice up birds and distribute the stress, all of these bird pieces strike the same stator blade position. For this reason, the design and fastening of stator blades on the housing must be undertaken with sufficient care.
Experience has shown that,

• in light metal housings,/casings especially, fastening blades with a T-root is preferable to using dovetail systems.
• fastening the guide vane tips with an inner ring (shroud), or even grouping them into guide vane sections, provides favorable results. These designs stiffen the construction, distribute loads, and prevent the blades from bending over and rubbing on the following rotor stage. If contact occurs, there is an additional danger of self-increasing deformation caused by the stator annulus being pulled into the rotor due to the characteristic blade angles (Fig. "Contact of rotor with stator vanes"). It is also important that the inner shroud is sufficiently strong to prevent fractures, since fractures of the stiffening ring will usually result in catastrophic damages.
  

Elastic and plastic deformations of the V-guide vane annuluses (Fig. "Influence of flight speed") in the area of the bird strike, combined with decelerating effects cause foreign object movement in the compressor that varies greatly from the movement of the normal flow, increasing the impact angle and therefore also the stress on the following rotor blades (middle diagram; Ref. 5.2.2.3-2). On the other hand, a sufficiently strong and stiff guide annulus in front of the first rotor stage can considerably reduce rotor damage during bird strikes if it causes the bird to strike the rotor blades at a more favorable angle of impact.

Illustrations 5.2.2.3-8 and 5.2.2.3-9 (Ref. 5.2.2.3-3): The stator and rotor blade zones most sensitive to FOD and bird strike damage are:

• the leading edges of the blade, which are extremely thin for reasons of weight and aerodynamics (supersonic flow speed at the blade tips). The edges fail by bending or fracturing (Fig. "Low speed impact I").

• the area of the blade near the root (most stressed area is usually a little above the root platform, in Fig. "Modern blade shapes II" a little above 20% of blade length). If the blade is already fairly wide at this point, then the most sensitive zone is not directly at the blade root. The blades are overstressed by bending loads, plastically deformed, and may fail due to dynamic fatigue in case of further operation.
  As would be expected, blades with thick profiles with a large inlet edge radius and the maximum thickness located far towards the leading edge sustain less damage during bird strikes than modern blades with slim, sharp-edged high speed profiles. With regard to the blade bending stress (Fig. "Modern blade shapes II"), it decreases with wider chords and shorter blade lengths.
  

However, the leading edge stress may be different (Fig. "Modern blade shapes I" top left and right). This stress is directed by the torsion loads when the foreign object strikes the leading edge area. In this case a wide chord can be a disadvantage, as practical experience has shown (Fig. "Damage symptoms on rotor blades" bottom).

Figure "Influence of clappers": Practical experience (see Fig. "Low speed impact II") has shown that the relatively narrow fan blades with clappers that are used in older engine types have considerable advantages over wide-chord fan blades without clappers. If a large bird strikes the blade tip area, then the clappers act as braces and distribute the bending loads across several blades. In addition, the lever arm to the braced area is relatively small, which means that overloads result in the creation of relatively small fragments, which can pass through the bypass without threatening the core engine.

Due to the small resulting fragments (and also the thinner and narrower blades), the resulting imbalances are easier to control, making overstress of the main bearings less likely. In order to safely control the large imbalances caused by fractured wide-chord fan blades and prevent unallowable damages (such as overstressing of the engine suspension), it may be necessary to take elaborate measures to construct the bearings to behave elastically, at least in case of overstress.
The weight of the casing in the fan area is largely determined by the containment, which must prevent the escape of blade fragments. The relatively small and low-energy fragments of blades with clappers require correspondingly lighter containments, which is an advantage with regard to the total engine weight, especially in big fan engines with large bypass ratios.
However, despite this advantage, modern fan engines exclusively use wide-chord blades for aerodynamic reasons.

Figure "Sensitivity caused by S-shaped ducts": Experience has shown that so-called swan-neck ducts in intermediate compressor casings can behave poorly in case of bird strikes (also see Fig. "Axial stiffening rib"). These casings are used in multi-shaft engines and direct the compressor air flow of the core engine into the high-pressure compressor zone. Although birds are sliced into small pieces by the blades of the fan and/or low-pressure compressor before reaching the swan-neck duct, these small pieces can collect on the struts to the outer wall of the s-duct and enter the high-pressure compressor as a larger mass. Also, the mass travels slowly and is concentrated at the tip region of the rotor blades. This creates conditions for a low-speed impact, resulting in maximum bending loads (Fig. "Influence of flight speed"). This process can easily overstress filigreed rotor bladings. For this reason, it is recommended that at least the first rotor stage of the high-pressure compressor be made especially robust.

Figure "Axial stiffening rib": If pieces of a bird collect in a s-duct, on casing struts, and/or on stator blades and are decelerated, it can cause extensive damage to the blades of the rotor stage immediately downflow (also see Fig. "Sensitivity caused by S-shaped ducts"). In this case in a modern fan engine for use in passenger aircraft, the leading blade edges were so deformed near the tips that an axial stiffening fin became necessary. The seriousness of the problems can be seen in the acceptance of the flow disturbances caused by the poor aerodynamics of the fin.