Gas turbine engine with short inlet and mistuned fan blades

A gas turbine engine comprises a fan rotor having fan blades received within an outer nacelle. The fan blades are provided with at least a first type having a first natural frequency, and a second type having a second natural frequency. The fan rotor has a first mount structure intended for the first type and a distinct second mount structure intended for the second type. The first type of fan blade fits into the first mount structure intended for the first type, but there is a first obstruction preventing the first type of fan blade from being placed into the second mount structure intended for the second type. The second type of fan blade fits into the second mount structure intended for the second type, but there is a second obstruction preventing the second type of fan blade from being placed into the first mount structure intended for the first type.

BACKGROUND OF THE INVENTION

This application relates to a gas turbine engine having mistuned fan blades.

Gas turbine engines are known and typically include a fan delivering air into a bypass duct as propulsion air and into a compressor as core flow. The air is compressed in the compressor and delivered into a combustor where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors, driving them to rotate.

Recently, a gear reduction has been incorporated between a fan drive turbine and the fan rotor. This has increased the design freedom for the gas turbine engine designer. In particular, the fan can now be made to rotate slower than the turbine. With this change, the diameter of the fan has increased.

It has recently been proposed to provide a gas turbine engine, where the inlet or area of a surrounding housing or nacelle forward of the fan rotor, is shorter than in the past. Providing a shorter inlet reduces the weight of the engine and also reduces external drag. Other benefits include reducing a bending moment and corresponding load on an engine structure during flight conditions such as takeoff. Further, by making the inlet shorter, the overall envelope of the engine is reduced.

However, the shorter inlets raise various challenges.

One challenge has to do with fan blade resonant stress. As the fan blades rotate, they are subject to stresses and, particularly, have challenges when the engine is operating such that flow non-uniformities, often termed inlet distortions, result in periodic excitation at or near the natural frequency of the fan blade. For an assembly of fan blades with similar vibrational characteristics, i.e., a tuned fan blade assembly, adjacent fan blades can contribute to the challenges on each fan blade, as their operation is interrelated. These challenges can become particularly acute in a short inlet fan which can have increased levels of inlet distortion compared to longer inlets.

It has been proposed in other engine types to mistune the fan blades such that adjacent fan blades have different natural frequencies. This decouples the interaction between the adjacent blades and can result in a significant reduction in resonant stress compared to a tuned fan blade assembly. Such intentionally mistuned fans have primarily been utilized in integrally bladed rotors such as for military engines.

SUMMARY OF THE INVENTION

In a featured embodiment, a gas turbine engine comprises a fan rotor having fan blades received within an outer nacelle. The fan blades are provided with at least a first type having a first natural frequency, and a second type having a second natural frequency. The fan rotor has a first mount structure intended for the first type and a distinct second mount structure intended for the second type. The first type of fan blade fits into the first mount structure intended for the first type, but there is a first obstruction preventing the first type of fan blade from being placed into the second mount structure intended for the second type. The second type of fan blade fits into the second mount structure intended for the second type, but there is a second obstruction preventing the second type of fan blade from being placed into the first mount structure intended for the first type.

In another embodiment according to the previous embodiment, a distance is defined from a plane defined by leading edges of the fan blades to an axial location of a forwardmost part of the nacelle, and an outer diameter of the fan blades being defined, and a ratio of the distance to the outer diameter is between about 0.2 and about 0.5.

In another embodiment according to any of the previous embodiments, the nacelle is formed with droop such that one portion extends axially further from the fan blades than does another portion, and wherein a distance measured to the one portion of the nacelle will still result in a ratio less than about 0.45, and the distance being measured to the another portion of the nacelle still results in the ratio being greater than about 0.20.

In another embodiment according to any of the previous embodiments, the ratio is greater than or equal about 0.25.

In another embodiment according to any of the previous embodiments, the ratio is greater than or equal to about 0.30.

In another embodiment according to any of the previous embodiments, the ratio is less than or equal to about 0.40.

In another embodiment according to any of the previous embodiments, a fan drive turbine drives the fan rotor through a gear reduction.

In another embodiment according to any of the previous embodiments, a gear ratio of the gear reduction is greater than about 2.3.

In another embodiment according to any of the previous embodiments, the mount structure is the rotor, and a portion of the first type of fan blade is received in a first type of slot in the rotor, and a portion of the second type of fan blade is received in a second type of slot in the rotor which is distinct from the slot of the first type of slot.

In another embodiment according to any of the previous embodiments, the portion of the first type of fan blade is a first flange. The portion of the second type of fan blade is a second flange which is positioned at a distinct location as compared to the first flange. The slots of the first type have an opening to receive the first flange. The slots of the second type have an opening to receive the second flange at the distinct location.

In another embodiment according to any of the previous embodiments, the first flange on the first type of fan blade extends in a first circumferential direction and the second flange on the second type of fan blade extends in an opposed circumferential direction.

In another embodiment according to any of the previous embodiments, the first flange on the first type of fan blade is at a radial position that is different from the second flange on the second type of fan blade.

In another embodiment according to any of the previous embodiments, a first flange is received in first slots formed in one of the rotor and the first type of fan blades, and the other of the rotor and the first type of blades is formed with a first groove to receive the first flange.

In another embodiment according to any of the previous embodiments, a second flange is received in second slots formed in one of the rotor and the second type of fan blades, and the other of the rotor and the second type of blades is formed with a second groove to receive the second flange,

In another embodiment according to any of the previous embodiments, the first grooves and the first flange for the first type of fan blade, and the second grooves and the second flange for the second type of fan blade are at distinct locations.

In another embodiment according to any of the previous embodiments, the first flanges and the second flanges are formed in the rotor.

In another embodiment according to any of the previous embodiments, the first flanges and the second flanges are formed, respectively, on the first type of fan blades and the second type of fan blades.

In another embodiment according to any of the previous embodiments, the first and second types of fan blades each has a dovetail with circumferential sides, and the circumferential sides are formed differently for each of the first and second type of fan blades.

In another embodiment according to any of the previous embodiments, an angle of at least one of the circumferential sides along which at least one of the circumferential side extends is different for the first type of fan blade and second type of fan blade.

In another embodiment according to any of the previous embodiments, the circumferential sides of the first type of fan blades and the second type of fan blades have a radially higher circumferential side and a radially lower circumferential side, and the higher and lower circumferential sides are reversed between the first type of fan blades and second type of fan blades.

DETAILED DESCRIPTION

FIG. 2shows an engine known as a short inlet engine. As shown, a nacelle94has forwardmost ends96and97. As can be seen, the forwardmost ends do not necessarily lie in a common plane perpendicular to a center axis of the engine. Rather, point96is further forward than point97. Fan blades98have an outer diameter99. The nacelle94is shown to have a radially inwardly extending innermost point104. Point104is inward of the outer diameter99of the fan blade98. As shown schematically, the fan blades have a root section or dovetail101received in a hub103of the fan rotor.

The short inlet may be defined by a distance L measured from: (a) a plane X perpendicular to a central axis C, which plane also being tangent to a leading edge or forward most point102of the fan blade9to (b) a plane defined by the forwardmost points (including ends96,97) of the nacelle948. A ratio is defined of L:D with D being the outer diameter of the fan blades98.

In one embodiment L:D is between about 0.2 and about 0.50, or even 0.45. Alternatively, the ratio may be greater than about 0.25 and in alternative embodiments greater than about 0.30. In embodiments, the ratio of L:D may be less than about 0.40.

As can be appreciated, the L:D quantity would be different if measured to the forwardmost point96than to the forwardmost point97. However, in embodiments the ratio at the forwardmost point96would still be less than about 0.45, and the ratio at the shortest forwardmost point97would still be greater than about 0.2.

Stated another way, the forwardmost end of said nacelle extends outwardly for varying extents across the circumference of the nacelle, and the ratio of the L:D for all portions of the varying distance of the nacelle being between about 0.2 and about 0.45.

The engine shown inFIG. 2, wherein the end96extends outwardly for a greater distance than the end97is said to have droop. In such an engine, the inlet leading to the fan blades98is not only short at the longer end96, but especially short at the shorter end97. There is a challenge from air distortion in the air reaching the fan blade98, and this increases the stresses on the blades.

Thus, as shown inFIG. 3A, it is proposed to have a fan rotor170provided with at least two distinct designs of fan blades172(A) and176(B). In an embodiment, fan blades172are designed to have a first natural frequency, while fan blades176are designed to have a second natural frequency. In embodiments, nominal natural frequency differences between the two types would be at least 2% for the first two lowest frequency blade-only structural modes. Generally, the two types are alternated across the circumference of rotor150. While the rotor170is shown with two types of fan blades, rotors incorporating additional numbers of fan blades are also envisioned within the scope of this disclosure.

There would be challenges for a fan rotor such as fan rotor170. In particular, it would not be desirable to have a fan blade172inadvertently inserted into the hub170at a location intended for a fan blade176, or vice versa.

FIG. 3Aschematically shows the dovetail174associated with the blade172being distinct from the dovetail178associated with the blade176. Of course, the hubs170would have a broach or slot which corresponds to the shapes of dovetails174and178. It should be understood that the illustrations are not actual dovetail shapes, but rather merely serve to show that the overall shape of the dovetail may be made differently.

As shown inFIG. 3B, the dovetail174would not be able to fit into the slot associated with the dovetail178, and vice versa. In this manner, the potential for inadvertently putting a blade into the wrong slot or broach is addressed.

FIG. 4shows an embodiment150wherein a first type blade152(A) and a second type blade154(B) are shown. Here, the dovetail156of blade152includes a flange or ear158extending in a first circumferential direction from the blade152, and fitting into a slot160in the hub150. A second flange162extends from the second type blade154in an opposed circumferential direction and fits into a slot164. Again, the rotor150would be provided with appropriate slots to receive the dovetails156and the flanges158/162. Again, this would eliminate the possibility of inadvertently putting the wrong blade at an improper location on the hub150.

FIG. 5shows another embodiment wherein a hub150receives two types of fan blades182and190. A flange188extends from the dovetail186at a radially inner location. On the blade type190, the dovetail192has a flange194extending at a distinct radial location into a slot in the hub150. Again, this would prevent the fan blades from being inadvertently placed into the wrong slot or broach.

FIG. 6shows a hub196wherein a dovetail198receives a flange199from the hub196into an opening197in the dovetail198. This embodiment is intended to show that while the projections in all other Figures are shown extending outwardly of the blade, the projections could also extend from the hubs into the blades.

FIG. 7shows an embodiment wherein the blades220have side surfaces wherein a first blade type has a radially outer surface222and a radially inner side surface224at opposed circumferential sides. The second type blade has the radially inner side surface226on the opposed circumferential side and the radially outer surface228on the circumferential side associated with the radially inner side wall224of the first blade type. Again, this will prevent inadvertent positioning of the wrong type blade into a particular slot.

FIG. 8shows yet another embodiment230wherein the side walls on a first blade type232and the side walls on a second blade type234extend along distinct angles. Again, the distinct angles are such that there was be an obstruction to placing an improper blade type into a slot.

FIG. 9is a cross-sectional view along an axial length of a blade240. That is, it shows a radially inner surface between the blade and hub. The hub242has an extending portion246extending into a groove244in the bottom of the dovetail for the blade240. An extending portion250from the first type blade extends outwardly into a groove248in the hub242.

For the second blade type, the position of the extension and slot are reversed as shown at252and254. Again, there would be an obstruction to placing the wrong blade type in the wrong slot.

FIG. 9also shows a feature wherein retention rings260are placed at the axial ends of both the blade240and hub242. This supports an understanding of the embodiment shown inFIG. 10. A ring270and a ring272are shown at ends of a rotor274securing a first blade type276. An extension278extends into a groove280in ring270. This extension is at one circumferential extent on the blade type276. The second blade type282has its extension284extending into a groove286on the ring270at the opposed circumferential end. It should be understood that with this embodiment, the ring270would require some indexing feature such that its screws280and286are proper aligned. As shown in phantom at290, the extension can also extend into the rear locking ring272. With this embodiment, the wrong blade type could perhaps be placed in a slot, but cannot be secured.

Again, with regard to all of the embodiments, the suggestion shown inFIG. 6wherein the extension could be in the lock ring, and extend into grooves in the blades would also hold true.

What is common for the embodiments ofFIGS. 3-6is mount structure is provided with structure and the two fan blade types are provided with structure that would prevent one fan blade type from being inadvertently placed into a slot intended for the other fan blade type. There is a mechanical obstruction to such inadvertent insertion. The mount structure could be the hub slot or a retention ring.

It should also be understood that the shapes of the obstructions may look different from those illustrated.