Fan Case Reinforcement in a Gas Turbine Jet Engine

[0001] This application is a continuation-in-part application of application Ser. No. 10/947,923, filed Sep. 23, 2004 and entitled “Method and Apparatus for Improving Fan Case Containment and Heat Resistance in a Gas Turbine Jet Engine.”

[0002] In a full test of a gas turbine jet engine, a fan blade is deliberately released from the hub at a maximum engine rotation speed by an explosive bolt positioned at the base of the fan blade. This test is used to demonstrate the engine carcass's ability to contain the impact of the fan blade and handle the resulting out-of-balance forces. This impact is absorbed as vibration through the fan case containment system which surrounds the engine. The fan case is an element in a fan case containment system and is typically the heaviest component of a gas turbine jet engine due to its size, and due to the strength requirements the fan case may possess for containment purposes. In gas turbine jet engines susceptible to backfiring, heated air travels backward from the combustor to the fan area, increasing the temperature within the fan case and causing a rise in fan case temperature. These higher temperatures may be a factor in determining from what material the fan case is to be constructed. Maintaining or reducing the weight of the fan case, while at the same time maintaining or improving fan case containment strength and utilizing fan case materials that can withstand the fan case temperatures is a demonstrated need in the art.

[0021] Referring now to the Figures, in which like reference numerals and names refer to structurally and/or functionally similar elements thereof, FIG. 1 shows a schematic diagram of the overall structure of a typical gas turbine jet engine with a fan casing typical of the prior art. Referring now to FIG. 1, Gas Turbine Jet Engine 100 has Fan 102 having a plurality of Fan Blades 104 housed within Fan Case 106. The Fan 104 rotates on an axis of rotation along a centerline 107 to provide air intake and thrust Booster 108 is a low pressure compressor which feeds inlet air to High Pressure Compressor Rotor 110 and its attached blades and stators, which force air into Combustor 112, increasing the pressure and temperature of the inlet air. High Pressure Turbine Rotor 114 and its accompanying blades and stators are housed within High Pressure Turbine Case 116. Low Pressure Turbine Rotor 118 and its accompanying blades and stators are housed within Low Pressure Turbine Case 120. Low Pressure Turbine Rotor 118 and its accompanying blades and stators extracts energy from the high-pressure, high-velocity gas flowing from Combustor 112 and transfers energy to Low Pressure Turbine Shaft 122, which in turn drives Fan 102, providing most of the thrust for Gas Turbine Jet Engine 100.

[0022] FIG. 2 shows a cross section of a forging for a fan casing typical of the prior art. Referring now to FIG. 2, Fan Case Forging 200 after machining yields Fan Case 202, shown in dotted line outline. In this example, Fan Case Forging 200 is forged from a titanium cylinder in one piece. The operating temperatures and load characteristics of the particular gas turbine jet engine that Fan Case 202 is designed for may require that Fan Case 202 be made of titanium. The forging weight for this particular Fan Case Forging 200 is approximately 3,347 pounds. After machining, Fan Case 202 has a flight weight of approximately 975.2 pounds. On gas turbine jet engines, the fan case may also be made of aluminum, steel, or manufactured from composite materials. Composite materials typically include a core material, a reinforcing material, and a resin binder. Core materials are typically wood, foam, and honeycomb. Reinforcing materials include fiberglass, carbon fiber, and Kevlar®. The resin component typically includes polyesters, vinyl esters, and epoxies. As technology improves, and temperatures go up in the gas turbine jet engines, aluminum cases are often wrapped with Kevlar® to give added strength for fan case containment purposes. For even higher operating temperatures not appropriate for aluminum or steel, titanium is used, which may also by wrapped with Kevlar® if so needed for additional strength.

[0023] Structural features of the machined Fan Case 202 include First Stiffening Ring 204 and Second Stiffening Ring 206. These two stiffening rings help prevent Fan Case 202 from going oval under the load and temperature conditions experienced during engine operation. Accessory Flange 208 will have holes drilled through it and various engine components hung from it, such as gear boxes, tubes, wiring, etc. First Containment Ring 210 encircles the outside of Fan Case 202 and provides additional strength for fan case containment. Second Containment Ring 212 circles the inside of Fan Case 202 and also provides additional strength for fan case containment. The section of Fan Case 202 between First Containment Ring 210 and Second Containment Ring 212 is the region where a fan blade, such as Fan Blade 104 (FIG. 1) will likely strike should it break loose from its hub. Due to the size of the Fan Blades 104, which typically are the largest fan blades in a gas turbine jet engine, this section of Fan Case 202 is often designed to be exceptionally strong. Thus, First and Second Containment Rings 210, 212 provide additional strength.

[0024] FIG. 3 shows a cross section of a forging for a fan casing in an embodiment of the present description. Referring now to FIG. 3, a fan casing of the present description can be substituted for Fan Case 202 for use in the same gas turbine jet engine for which Fan Case 202 was designed either in a retrofit of an existing engine or in a newly manufactured engine. Features of the present description are applicable to a Fan case of a gas turbine jet engine used in a variety of applications in which a fan intakes air and produces thrust. Such applications include aviation, amphibious and other applications. Fan Case Forging 300 after machining yields Fan Case 302, shown in dotted line outline. In this example, Fan Case Forging 300 is also forged from a titanium cylinder in one piece. In this example, the forging 300 has a simpler shape which simplifies the forging process, but it is appreciated that the shape may vary, depending upon the particular application. The forging weight for Fan Case Forging 300 is, in this embodiment, approximately 2,595 pounds, 752 pounds lighter than Fan Case Forging 200. After machining, Fan Case 302 has a flight weight of approximately 751.3 pounds, 223.9 pounds lighter than Fan Case 202. It is appreciated that the materials, weight and features of the forging 300 may vary, depending upon the particular application.

[0025] Structural features of the machined Fan Case 302 include First Stiffening Ring Notch 304 and Second Stiffening Ring Notch 306 located in a middle portion of Fan Case 302. Two stiffening rings from two additional forgings (see FIGS. 5A, 5B, 6A, and 6B) will be seated in First Stiffening Ring Notch 304 and Second Stiffening Ring Notch 306 (see FIG. 4) to help prevent Fan Case 302 from going oval under the load and temperature conditions experienced during engine operation. Depending upon the design of a particular fan case, more or less stiffening ring notches may be utilized, and they may be located in various positions on a surface of the fan case. Although the stiffener rings 502, 602 are depicted as being seated on an outer surface of the fan case 302, it is appreciated that one or more of the stiffener rings may be installed on an inner surface of the fan case, depending upon the particular application. Accessory Flange 308 towards the back end of Fan Case 302 may have holes drilled through it or other attachment surfaces formed on it, and various engine components supported by it, such as gear boxes, tubes, wiring, etc.

[0026] Containment Ring Notch 310 circles the inside of Fan Case 302 circumferentially at the front end. A containment ring from an additional forging (see FIGS. 7A and 7B) will be seated in Containment Ring Notch 310. FIG. 12 shows a cross-sectional schematic diagram of the fan case 302 fitted with a containment ring 702 seated in the inner containment ring notch 310. The section of Fan Case 302 spanning Containment Ring Notch 310 is the region where a fan blade, such as Fan Blade 104 (FIG. 1) will likely strike should it break loose from its hub. This section of Fan Case 302 is in this embodiment, relatively strong, and the containment ring from the additional forging, machined to a predetermined shape to match with Containment Ring Notch 310, provides additional strength and containment function. Fan Case 302 has no structure comparable to First Containment Ring 210 which may be omitted from the Fan Case 302 of the present description due to the additional strength provided by the containment ring 702. It is appreciated that in other embodiments, a second or additional containment rings may be added, depending upon the particular application.

[0027] FIG. 5A shows a cross section of a forging for a first stiffener ring for the fan casing of FIG. 3 in an embodiment of the present description, and FIG. 5B shows the first stiffener ring of FIG. 5A about to be shrink interference fit to the fan casing of FIG. 3 in an embodiment of the present description. Referring now to FIGS. 5A and 5B, First Stiffening Ring Forging 500, after machining to a predetermined shape to match with First Stiffening Ring Notch 304, yields First Stiffening Ring 502, shown in dotted line outline in FIG. 5A. In this example, First Stiffening Ring Forging 500 is forged from an aluminum ring in one piece. The forging weight for First Stiffening Ring Forging 500 is approximately 154 pounds. After machining, First Stiffening Ring 502 has a flight weight of approximately 41 pounds.

[0028] In this example, First Stiffening Ring 502, fabricated separately from the fan case 302, is shrink interference fit into First Stiffening Ring Notch 304. At ambient temperature, the inside diameter of First Stiffening Ring 502 will be slightly smaller than the outside diameter of First Stiffening Ring Notch 304. First Stiffening Ring 502 is heated, which causes First Stiffening Ring 502 to expand, increasing the inside diameter to a diameter that is greater than the outside diameter of First Stiffening Ring Notch 304, and giving rise to First Ring Clearance 504, enabling First Stiffening Ring 502 to be positioned as shown in First Stiffening Ring Notch 304. In this position, First Stiffening Ring 502 is allowed to cool, which shrinks in diameter and seats itself circumferentially into First Stiffening Ring Notch 304. At ambient temperature, due to First Stiffening Ring 502 attempting to return to its smaller inside diameter, but being prevented from doing so due to the larger outside diameter of First Stiffening Ring Notch 304, a shrink with an interference fit results, with radially compressive circumferential force being applied to Fan Case 302 by First Stiffening Ring 502, and tensile circumferential force is applied to First Stiffening Ring 502 by Fan Case 302. In one embodiment, the radially compressive forces may be centered on the axis of rotation defined by center line 107. Also, the radially compressive forces are applied continuously around the entire circumference of the First Stiffening Ring 502 without interruption.

[0029] FIG. 6A shows a cross section of a forging for a second stiffener ring for the fan casing of FIG. 3 in an embodiment of the present description, and FIG. 6B shows the second stiffener ring of FIG. 6A about to be shrink interference fit to the fan casing of FIG. 3 in an embodiment of the present description. Referring now to FIGS. 6A and 6B, Second Stiffening Ring Forging 600, after machining to a predetermined shape to match with Second Stiffening Ring Notch 306, yields Second Stiffening Ring 602, shown in dotted line outline in FIG. 6A. In this example, Second Stiffening Ring Forging 600 is forged from an aluminum ring in one piece. The forging weight for Second Stiffening Ring Forging 600 is approximately 148 pounds. After machining, Second Stiffening Ring 602 has a flight weight of approximately 40.6 pounds.

[0030] In this example, Second Stiffening Ring 602, fabricated separately from the fan case 302, is shrink interference fit into Second Stiffening Ring Notch 306. At ambient temperature, the inside diameter of Second Stiffening Ring 602 will be slightly less than the outside diameter of Second Stiffening Ring Notch 306. Second Stiffening Ring 602 is heated, which causes Second Stiffening Ring 602 to expand, increasing the inside diameter to a diameter that is greater than the outside diameter of Second Stiffening Ring Notch 306, and giving rise to Second Ring Clearance 604, enabling Second Stiffening Ring 602 to be positioned as shown in Second Stiffening Ring Notch 306. In this position, Second Stiffening Ring 602 is allowed to cool, which shrinks in diameter and seats itself circumferentially into Second Stiffening Ring Notch 306. At ambient temperature, due to Second Stiffening Ring 602 attempting to return to its smaller inside diameter, but being prevented from doing so due to the larger outside diameter of Second Stiffening Ring Notch 306, a shrink with an interference fit results, with radially compressive circumferential force being applied to Fan Case 302 by Second Stiffening Ring 602, and tensile circumferential force is applied to Second Stiffening Ring 602 by Fan Case 302. In one embodiment, the radially compressive forces may be centered on the axis of rotation defined by center line 107. Also, the radially compressive forces are applied continuously around the entire circumference of the Second Stiffening Ring 602 without interruption. Also, each stiffener ring is, in one embodiment, fabricated as a solid, unitary or one-piece continuous or seamless member forged or machined in a closed loop shape. In another embodiment, a stiffener ring may be fabricated using an open loop-shaped member and bonding the ends together by welding, for example, to form a closed loop shape.

[0031] It is appreciated that the stiffener rings may be located at other positions of a fan case, depending upon the application. It is further appreciated that the sizes, dimension, shapes, materials and clearances may vary, depending upon the particular application. It is appreciated that due to various factors such as nonperfect roundness, a stiffener ring such as the First Stiffening Ring 502 may not engage 100% of the circumference of the outer surface of the Fan Case 302. For example, the First Stiffening Ring 502 may contact 70% of the circumference of the outer surface of the Fan Case 302, but the amount of contact may vary, depending upon the particular application. Nonetheless, it is considered that the First Stiffening Ring 502 applies radially compressive forces along the length of the circumference of the inner circumferential surface of the First Stiffening Ring 502 when the First Stiffening Ring 502 is shrink interference fit to the Fan Case 302.It is further appreciated that in some applications, it may be appropriate to provide a liner material between a stiffener ring shrink interference fit to a fan case such that the compressive forces between the stiffener ring and the fan case are transmitted through the liner material. In one embodiment, the liner material may be made of compressible material. It is appreciated that the liner material may in other embodiments be relatively stiff or may have other properties.

[0032] FIG. 7A shows a cross section of a forging for a containment ring for the fan casing of FIG. 3 in an embodiment of the present description, and FIG. 7B shows the containment ring of FIG. 7A about to be shrink interference fit to the fan casing of FIG. 3 in an embodiment of the present description. Referring now to FIGS. 7A and 7B, Containment Ring Forging 700, after machining to a predetermined shape to match with Containment Ring Notch 310, yields Containment Ring 702, shown in dotted line outline in FIG. 7A. In this example, Containment Ring Forging 700 is forged from a ring of nickel-base super alloy, such as Inconel 718, in one piece. The forging weight for Containment Ring Forging 700 is approximately 467 pounds. After machining to the predetermined shape, Containment Ring 702 has a flight weight of approximately 138.1 pounds. It is appreciated that the containment ring may be located at other positions of a fan case, depending upon the application. It is further appreciated that the sizes, dimension, shapes, materials and clearances may vary, depending upon the particular application. For example, the Containment Ring 702 may be made of other super alloys, steel, titanium or other appropriate materials to contain the blade. Also, the containment ring is, in one embodiment, fabricated as a solid, unitary or one-piece continuous or seamless member forged or machined in a closed loop shape. In another embodiment, a containment ring may be fabricated using an open loop-shaped member and bonding the ends together by welding, for example, to form a closed loop shape.

[0033] In this example, Containment Ring 702, fabricated separately from the fan case 302, is shrink interference fit into Containment Ring Notch 310. At ambient air temperature the outside diameter of Containment Ring 702 will be slightly larger than the inside diameter of Containment Ring Notch 310. Fan Case 302 is heated, which causes Fan Case 302 to expand, increasing the inside diameter to a diameter that is greater than the outside diameter of Containment Ring 702, and giving rise to Containment Ring Clearance 704, enabling Containment Ring 702 to be positioned as shown in Containment Ring Notch 310. In this position, Fan Case 302 is allowed to cool, which shrinks in diameter and allows Containment Ring 702 to seat itself circumferentially into Containment Ring Notch 310. At ambient temperature, due to Fan Case 302 attempting to return to its smaller inside diameter, but being prevented from doing so due to the larger outside diameter of Containment Ring 702, a shrink with an interference fit results, with radially compressive circumferential force being applied to Containment Ring 702 by Fan Case 302, and tensile circumferential force is applied to Fan Case 302 by Containment Ring 702. In one embodiment, the radially compressive forces may be centered on the axis of rotation defined by center line 107 as schematically represented by arrows in FIG. 12. In one embodiment, the radially compressive forces are applied continuously around the entire circumference of the Containment Ring Notch 310 of the Fan case 202 without interruption. It is appreciated that due to various factors such as nonperfect roundness, a containment ring such as the Containment Ring 702 may not engage 100% of the circumference of the inner surface of the Fan Case 302. For example, the Containment Ring 702 may contact 70% of the circumference of the inner surface of the Fan Case 302, but the amount of contact may vary, depending upon the particular application. Nonetheless, it is considered that the Fan Case 302 applies radially compressive forces along the length of the circumference of the inner circumferential surface of the Containment Ring Notch 310 when the Fan Case 302 is shrink interference fit to the Containment Ring 702. It is further appreciated that in some applications, it may be appropriate to provide a liner material between a containment ring shrink interference fit to a fan case such that the compressive forces between the containment ring and the fan case are transmitted through the liner material. In one embodiment, the liner material may be made of compressible material. It is appreciated that the liner material may in other embodiments be relatively stiff or may have other properties.

[0034] For a fan case manufactured from composite material, Containment Ring 702 may be cooled with liquid nitrogen to reduce its outside diameter giving rise to Containment Ring Clearance 704, enabling Containment Ring 702 to be positioned as shown in Containment Ring Notch 310. In this position, Containment Ring 702 is allowed to warm up to ambient temperature, increasing in diameter, and seating itself circumferentially into Containment Ring Notch 310. At ambient temperature, due to Containment Ring 702 attempting to return to its greater outside diameter, but being prevented from doing so due to the smaller inside diameter of Containment Ring Notch 310, an interference fit results, with radially compressive circumferential force being applied to Containment Ring 702 by Fan Case 302, and tensile circumferential force is applied to Fan Case 302 by Containment Ring 702. One skilled in the art will recognize that a combination of heating Fan Case 302 along with cooling Containment Ring 702 may also be employed in certain situations to effect a shrink fit.

[0035] In one embodiment of the description, Containment Ring Notch 310 is machined circumferentially with a reverse taper such that the inside diameter of Fan Case 302 at point A is less than the inside diameter of Fan Case 302 at point B. The taper may vary from fan case to fan case, ranging from just greater than 0° for a cylindrical case to an appropriate degree that would depend upon the specific geometry of a conical fan case. Containment Ring 702 is machined circumferentially on its outside surface to match this same reverse taper. Even though Containment Ring 702 is shrink interference fit onto Fan Case 302, the taper can add extra security to retard the Containment Ring 702 from slipping axially on Fan Case 302.

[0036] In addition, the machining for Fan Case 302 may be done in a first direction, such as radially, and the machining for Containment Ring 702 may be done in a second direction, such as axially, which is more or less perpendicular to the first direction. Since machining leaves a spiral, or record, continuous groove on the machined surfaces, the grooves on each surface will align in a cross-hatch manner to each other, increasing the frictional forces between the two surfaces and reducing the potential for spinning of Containment Ring 702 within Containment Ring Notch 310. The plurality of grooves on Containment Ring 702, which may be made of a nickel-base super alloy, for example, can be harder than the plurality of grooves on Containment Ring Notch 310 of Fan Case 302, which may be made of titanium, or in other fan casings, possibly steel or aluminum, for example. The nickel-base super alloy grooves can dent into the softer titanium, steel, or aluminum grooves. Alternatively, Containment Ring 702 could simply be spot welded in one or more locations to Containment Ring Notch 310, or bolted to one or more flanges secured to Containment Ring Notch 310, to keep Containment Ring 702 from spinning in relation to Containment Ring Notch 310. Machining in cross directions may also be applied or omitted, as appropriate, depending upon the application.

[0037] FIG. 4 shows a cross section of a machine finished fan casing having two stiffener rings (FIGS. 5A, 5B, 6A, and 6B) and a containment ring (FIGS. 7A and 7B) that have been shrink interference fit to the fan casing of FIG. 3 in an embodiment of the present description. Referring now to FIG. 4, Containment Ring 702 can replicate part of the structure comparable to Second Containment Ring 212, and can obviate utilization of First Containment Ring 210 fully or partially, depending upon the application. By shrink interference fitting Containment Ring 702 on the inside diameter of Fan Case 302, as opposed to the outside, the harder super alloy of Containment Ring 702 can provide an initial striking surface should a blade break off. The softer titanium, steel, or aluminum, of Fan Case 302 on the outside of Containment Ring 702 can act as a shock absorber due to the different expansion rates between the two materials. As the super alloy of Containment Ring 702 begins to move, it can push against the titanium, steel, or aluminum of Fan Case 302 with a different coefficient of expansion. This is like having two nets close to each other. The super alloy of Containment Ring 702 can take the initial blow, and some of the force be transferred to the titanium, steel, or aluminum Fan Case 302 like a shock absorber.

[0038] First Stiffening Ring 502 and Second Stiffening Ring 602 are shown seated in First Stiffening Ring Notch 304 and Second Stiffening Ring Notch 306 respectfully. First Stiffening Ring 502 and Second Stiffening Ring 602 provide stiffening to prevent Fan Case 302 from deforming out-of-round, or going oval, during operation of the engine under temperature and load conditions.

[0039] In this particular example, Table 1 below shows a comparison of the forge and flight weights, and costs, of prior art Fan Case 202 compared to Fan Case 302 of the present description.

TABLE 1
Part/	Forge Wt.	Flight Wt.
Material	(lbs)	(lbs)	Cost/lb	Total Cost
Fan Case 202	3,347.0	975.2	$8.00	$26,776.00
Titanium
Fan Case 302	2,595.0	751.3	$8.00	$20,760.00
Titanium
First Stiffening Ring 502	154.0	40.5	$1.50	$231.00
Aluminum
Second Stiffening	148.0	40.6	$1.50	$222.00
Ring 602
Aluminum
Containment Ring 702	467.0	138.1	$7.00	$3,269.00
Inconel 718
Total	3,364.0	970.5	$7.28	$24,482.00
Savings	−17.0	4.7	$0.72	$2,294.00

[0040] Thus, in this example, though the forge weight is 17 pounds more, the flight weight is 4.7 pounds less. In addition, the average cost per pound of the materials for Fan Case 302 is $0.72 per pound less than that of Fan Case 202, resulting in a total savings of $2,294.00. Also, in this example, it is believed that Fan Case 302 is considerably stronger than Fan Case 202.

[0041] In other applications, the savings could be more significant. For example, for a fan casing that requires Kevlar® reinforcement, a fan casing of the present description may be sufficiently stronger so as to eliminate the need for the Kevlar® reinforcement, which would be a substantial savings in both materials cost and labor. The present description could also be used with Kevlar® reinforcement to attain higher fan case containment strength. For gas turbine jet engines that currently use steel or titanium for the fan casings, the present description may enable aluminum to be substituted for the steel or titanium, and the strength needed for containment provided for by the containment ring of nickel-base super alloy or other suitable material. Since the same volume of aluminum or titanium is about 30%-55% of the weight of the same volume of steel, substantial weight savings can result. This weight savings may translate into increased cargo carrying capability or reduced fuel costs or a combination of both.

[0042] In the gas turbine jet engine industry, the trend is toward making fan blades longer to increase thrust. The tips of the fan blades can rotate at supersonic speeds, while the base of the fan blades rotate at subsonic speeds. This can cause a harmonic vibration in the blades resulting in the tips of the blades breaking off. To counter this problem, instead of making straight fan blades, the blades are shaped more like wide paddles. These wider and longer blades result in more mass that must be contained within the fan casing. Also, as engines become more efficient, they tend to operate at hotter temperatures, potentially adding more difficulty to the containment problem. It is believed that the present description can greatly assist in meeting these challenges for greater fan case containment strength and potentially less overall weight and lower cost, as well as provide other features in addition to or instead of, depending upon the particular application.

[0043] FIG. 8 shows a schematic diagram of the airflow through a typical gas turbine jet engine. Referring now to FIG. 8, for a gas turbine jet engine not readily susceptible to backfiring, Air Flow 824 flows into Fan Case 806 and into Booster 808. High Pressure Compressor Rotor 810 compress Air Flow 824 as it enters Combustor 812. After passing through the high pressure and low pressure turbines, Air Flow 824 flows out of the back of Gas Turbine Jet Engine 800. The heated Air Flow 824 in Combustor 812 travels on through and out of the back of Gas Turbine Jet Engine 800.

[0044] FIG. 9 shows a schematic diagram of the airflow through a typical gas turbine jet engine susceptible to backfiring. Referring now to FIG. 9, in contrast to FIG. 8, Gas Turbine Jet Engine 900 is susceptible to backfiring, which causes a portion of Air Flow 924, Heated Air 926 represented by dotted lines, to flow backwards through Gas Turbine Jet Engine 900 and into the area of Fan 902. Heated Air 926 causes the temperature within Fan Case 906 to rise, which also elevates the temperature of Fan Case 906 itself in the area designated generally by Area 928. Temperature is of course one of the primary factors in determining from what material Fan Case 906 is to be constructed. When fan case temperatures rise above 800 degrees, aluminum may no longer be suitable. More expensive heat tolerant materials, such as steel, titanium, or super alloys, may be appropriate.

[0045] FIG. 10 shows a cross section of a forging for a fan casing for improved heat resistance in an embodiment of the present description. Referring now to FIG. 10, for gas turbine jet engines susceptible to backfiring, it is believed that a fan casing of the present description can be used to counteract the heat problems and improve the heat resistance of Fan Case 1002. Fan Case Forging 1000 is similar to Fan Case Forging 300 shown in FIG. 3. Fan Case Forging 1000 after machining yields Fan Case 1002, shown in dotted line outline, which is similar to Fan Case 302. In this example, Fan Case Forging 1000 is forged from aluminum in one piece.

[0046] Structural features of the machined Fan Case 1002 are similar to that shown in FIGS. 3 and 4 but also include Heat Resistance Ring Notch 1012 which circles the inside of Fan Case 1002 circumferentially at the location defined generally by Area 928 in FIG. 9. A ring of heat resistant material will, with a shrink interference fit in this example, be seated in Heat Resistance Ring Notch 1012. The section of Fan Case 1002 spanning Heat Resistance Ring Notch 1012 is the region where the heated air from backfiring can cause increased fan case temperatures.

[0047] It is appreciated that the heat resistance ring may be located at other positions of a fan case, depending upon the application. It is further appreciated that the sizes, dimension, shapes, materials and clearances may vary, depending upon the particular application.

[0048] As described above, the machining for Fan Case 1002 may be done in a first direction, such as radially, and the machining for Heat Resistance Ring 1112 may be done in a second direction, such as axially, which is more or less perpendicular to the first direction. Since machining can leave a spiral, or record, continuous groove on the machined surfaces, the grooves on each surface may align in a cross-hatch manner to each other, increasing the frictional forces between the two surfaces and reducing the potential for spinning of Heat Resistance Ring 1112 within Heat Resistance Ring Notch 1012. The plurality of grooves on Heat Resistance Ring 1112, which is made of titanium, may be harder than the plurality of grooves on Heat Resistance Ring Notch 1012 of Fan Case 1002, which is made of aluminum. The titanium grooves can dent into the softer aluminum grooves. Alternatively, Heat Resistance Ring 1112 could simply be spot welded in one or more locations to Heat Resistance Ring Notch 1012, or bolted to one or more flanges secured to Heat Resistance Ring Notch 1012, to keep Heat Resistance Ring 1112 from spinning in relation to Heat Resistance Ring Notch 1012. Machining in cross directions may not be needed in this case or may be applied in addition thereto.

[0049] FIG. 11 shows a cross section of a machine finished fan casing having a ring of heat resistant material that has been shrink interference fit to the fan casing of FIG. 10 in an embodiment of the present description. Referring now to FIG. 11, Heat Resistance Ring 1112 is shown shrink interference fit on the inside diameter of Fan Case 1002 in Heat Resistance Ring Notch 1012. In this example, Heat Resistance Ring 1112 is made of titanium, but may also be made of other materials that have a combination of heat resistant properties and strength properties to maintain the structural integrity required for Fan Case 1002 such as steel, steel alloy, or any number of commonly used aerospace super alloys. Heat Resistance Ring 1112 may be fabricated from titanium sheet material that is cut, bent into a cylindrical shape, welded along the seam, and formed to match the inside diameter of Heat Resistance Ring Notch 1012. Heat Resistance Ring 1112 may also be forged as described above.

[0050] The shrink with interference fit may be accomplished, as described above, by heating Fan Case 1002, causing it to expand in diameter and allowing Heat Resistance Ring 1112 to be slid into place, where upon cooling, Fan Case 1002 and Heat Resistance Ring 1112 apply force to each other in a shrink interference fit. Alternatively, Heat Resistance Ring 1112 may be cooled with liquid nitrogen, reducing its outside diameter and enabling Heat Resistance Ring 1112 to be slid into Heat Resistance Ring Notch 1012. Also, a combination of heating Fan Case 1002 and cooling Heat Resistance Ring 1112 may be employed to secure the shrink interference fit. The titanium of Heat Resistance Ring 1112 is not weakened structurally by the fan case temperatures, and serves as a buffer to the aluminum Fan Case 1002 due to the different expansion rates between the two materials. The titanium of Heat Resistance Ring 1112 is exposed to internal fan case temperatures, and some of the heat is transferred to the aluminum Fan Case 1002. The titanium provides the strength needed that the aluminum lacks at the higher temperatures. Containment Ring 1102 may be made from a super alloy. It is appreciated that due to various factors such as nonperfect roundness, a heat resistance ring such as the Heat Resistance Ring 1112 may not engage 100% of the circumference of the outer surface of the Fan Case 1002. For example, the Heat Resistance Ring 1112 may contact 70% of the circumference of the inner surface of the Fan Case 1002, but the amount of contact may vary, depending upon the particular application. Nonetheless, it is considered that the Fan Case 1002 applies radially compressive forces along the length of the circumference of the inner circumferential surface of the Heat Resistance Ring Notch 1012 when the Fan Case 1002 is shrink interference fit to the Heat Resistance Ring 1112. It is further appreciated that in some applications, it may be appropriate to provide a liner material between a heat resistance ring shrink interference fit to a fan case such that the compressive forces between the heat resistance ring and the fan case are transmitted through the liner material. In one embodiment, the liner material may be made of compressible material. It is appreciated that the liner material may in other embodiments be relatively stiff or may have other properties such as enhanced heat insulation properties to protect the fan case further.

[0051] First Stiffening Ring Notch 1104 and Second Stiffening Ring Notch 1106 may be made of aluminum, titanium, or steel, for example. Depending upon the specific gas turbine jet engine being considered, a containment ring and one or more stiffening rings may not be necessary with a heat resistance ring, and a heat resistance ring may not be necessary with a containment ring and one or more stiffening rings. The present description gives the engine designer many options regarding materials, weights, strengths, and heat resistance that can be combined to come up with an optimum design for a specific engine's goals and requirements. For example, the Fan Case 302 may be made of a relatively lightweight but relatively inexpensive material such as aluminum and the containment ring 702 may be made of a material having a relatively higher containment strength (such as a super alloy Inco 718) as compared to the material of the fan case. However, since the Containment Ring 702 may be made substantially smaller in mass than that of the Fan Case 302, the material of the Containment Ring 702 can be more expensive or more heavy than the material of the Fan Case 302 and yet achieve savings in overall weight or expense or both, depending upon the particular application. Similarly, the Fan Case 1002 may be made of a relatively low heat resistance material such as aluminum and the Heat Resistance Ring 1102 may be made of a material having a relatively higher heat resistance property (such as titanium, for example) as compared to the material of the fan case. However, since the Heat Resistance Ring 1112 may be made substantially smaller in mass than that of the Fan Case 1002, the material of the Heat Resistance Ring can be more expensive or more heavy than the material of the Fan Case 1002 and yet achieve savings in overall weight or expense or both, depending upon the particular application.

[0052] As previously mentioned, FIG. 12 schematically illustrates a containment ring 702 seated in a containment ring notch 310 formed in an inside surface of the fan case 302. Referring now to FIG. 13, it is appreciated that a containment ring 702a may be seated in a containment ring notch 310a formed in an outside surface of a fan case 302a as shown in FIG. 13. In this example, the containment ring 702a applies radially compressive forces directed toward the axis of rotation defined by center line 107 as schematically represented by arrows in FIG. 13. In one embodiment, the radially compressive forces are applied continuously around the entire circumference of the Containment Ring 702a without interruption.

[0053] As previously mentioned, the Stiffener Rings may be placed in a variety of locations along the fan case 302. FIG. 14 shows an example in which a pair of stiffener rings 502, 602 are placed opposite a containment ring 702b. As shown in the cross-sectional diagram of FIG. 15, the stiffener ring 502 and the containment ring 702b are each shrink interference fit to the fan case 302 with the fan case 302 positioned between the stiffener ring 502 and the containment ring 702b. The stiffener ring 602 is similarly shrink interference fit to the fan case 302 with the fan case 302 positioned between the stiffener ring 602 and the containment ring 702b. The stiffener rings 502, 602, in addition to their stiffening functions, may in this embodiment, provide further containment strength to supplement that of the containment ring 702b.

[0054] Again, the Fan Case 302 may be made of a relatively lightweight but relatively inexpensive material such as aluminum and the stiffener rings 502, 602 may be made of a material having a relatively higher containment or stiffening strength (such as a super alloy Inco 718 or other super alloys, steel, titanium or other appropriate materials) as compared to the material of the fan case. However, since the stiffener rings 502, 602 may be made substantially smaller in mass than that of the Fan Case 302, the material of the stiffener rings 502, 602 can be more expensive or more heavy than the material of the Fan Case 302 and yet achieve savings in overall weight or expense or both, depending upon the particular application.

[0055] In the described embodiments, the rings such as the rings 702, 702a, 702b, 502, 602, 1112 are each described as being seated in an associated notch. It appreciated that one or more of the rings fabricated separately from the fan case 302, may be affixed to the fan case to reinforce the fan case without utilizing an associated notch.

[0056] In the described embodiments, the rings such as the rings 702, 702a, 702b, 502, 602, 1112 are each described as being seated in a shrink interference fit with the fan case 302, 302a. It appreciated that one or more of the rings fabricated separately from the fan case 302, may be affixed to the fan case to reinforce the fan case such that radially compressive forces are applied between a ring and the fan case without utilizing a shrink interference fit.

[0057] Having provided the present description, it will be understood by those skilled in the art that many changes in construction and widely differing embodiments and applications of the description will suggest themselves without departing from the scope of the present description.