Patent Description:
A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-pressure and temperature exhaust gas flow. The high-pressure and temperature exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section may include low and high pressure compressors, and the turbine section may also include low and high pressure turbines.

Airfoils in the turbine section are typically formed of a superalloy and may include thermal barrier coatings to extend temperature capability and lifetime. Ceramic matrix composite ("CMC") materials are also being considered for airfoils. Among other attractive properties, CMCs have high temperature resistance. Despite this attribute, however, there are unique challenges to implementing CMCs in airfoils.

<CIT> discloses a prior art gas turbine engine as set forth in the preamble of claim <NUM>.

From one aspect, there is provided a gas turbine engine as recited in claim <NUM>.

There is also provided a method of assembly as recited in claim <NUM>.

In this disclosure, like reference numerals designate like elements where appropriate and reference numerals with the addition of one-hundred or multiples thereof designate modified elements that are understood to incorporate the same features and benefits of the corresponding elements.

The engine parameters described above and those in this paragraph are measured at this condition unless otherwise specified. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about <NUM>, or more narrowly greater than or equal to <NUM>. The "Low corrected fan tip speed" as disclosed herein according to one non-limiting embodiment is less than about <NUM> ft / second (<NUM> meters/second), and can be greater than or equal to <NUM> ft / second (<NUM> meters/second).

Vanes in a turbine section of an engine typically include an airfoil section that extends between radially inner and outer platforms that bound the core gas path. In metallic alloy vanes, the airfoil sections are substantially centered on the platforms such that the platforms have near equal overhangs on the pressure side and the suction side of the airfoil section. The circumferential sides of the platforms serve as matefaces and are often used as a sealing interface between vanes, such as with a flat seal in a seal slot. In a ceramic matrix composite (CMC) vane, however, such matefaces and sealing configurations may cause duress from thermal gradients and interlaminar stresses that are not present in metallic vanes. Moreover, turbine vanes require constraints to inhibit motion when loaded by gas path and/or secondary flow forces. Attachment of CMC vanes in an engine and management of stresses, however, is challenging. Attachment features, such as hooks, that are typically used for metallic alloy vanes can result in inefficient loading if employed in CMCs, which may also be sensitive to stress directionality and distress conditions that differ from those of metallic vanes. Additionally, hooks, seal slots, variable thickness walls, gussets, complex-geometry investment casting cores, etc. that may be used in metallic alloy components are generally not acceptable or manufacturable with CMC materials.

As CMC vanes may be single-piece integral structures, there is also considerable difficulty in bending ceramic fiber plies from the airfoil section to form the platforms. For example, the ceramic fiber plies are first laid up to form the airfoil section. The fabric that overhangs the radial ends of the airfoil section is then draped in opposite directions so as to fan out and form the suction and pressure sides of the platforms. There can be considerable difficulty in bending the fiber plies in opposite directions without forming discontinuities from folds, kinks, wrinkles, or substantial unraveling of fibers. To address one or more of the above concerns, the examples set forth herein below disclose CMC vane arc segments that have single-sided platforms.

<FIG> illustrates a representative portion of the turbine section <NUM> of the engine <NUM>, including ceramic matrix composite CMC vane arc segments <NUM> (three shown) that are assembled in a circumferential row about the engine central longitudinal axis A (axis A is superimposed in <FIG>, along with radial direction RD and circumferential direction CD). <FIG> represents an axial view of a radial cross-section through several of the CMC vane arc segments <NUM>, and <FIG> shows an isolated view of a representative one of the CMC vane arc segments <NUM>.

Each CMC vane arc segment <NUM> includes an airfoil section <NUM> that defines first and second side walls <NUM>/<NUM>, leading and trailing ends 68a/68b, and first and second radial ends 70a/70b. In the examples herein, the first side wall <NUM> is a suction side of the airfoil section <NUM>, and the second side wall <NUM> is a pressure side of the airfoil section <NUM>. The side walls <NUM>/<NUM> and leading and trailing ends 68a/68b define an internal through-cavity <NUM> that may be used to convey cooling air to downstream cooling structures and components. In this example, at the first radial end 70a the airfoil section <NUM> has a first single-sided platform <NUM> projecting from the first side wall <NUM> in a circumferential direction away from the airfoil section <NUM>. In this example, at the second radial end 70b, the airfoil section <NUM> also has a second single-sided platform <NUM> that projects in the circumferential direction away from the first and second side walls <NUM>/<NUM>. As the first single-sided platform <NUM> projects from the first side wall <NUM> (suction side wall), the first single-sided platform <NUM> is a single-sided, suction side platform. Likewise, as the second single-sided platform <NUM> projects from the second side wall <NUM> (pressure side wall), the second single-sided platform <NUM> is a single-sided, pressure side platform. It is to be appreciated that in addition to the configuration shown various other configurations of the platforms are also contemplated, such as both platforms <NUM>/<NUM> being suction side, both platforms <NUM>/<NUM> being suction side, or the platform <NUM> being pressure side and the platform <NUM> being suction side.

The terms such as "inner" and "outer" refer to location with respect to the central engine axis A, i.e., radially inner or radially outer. Moreover, the terminology "first" and "second" as used herein is to differentiate that there are two architecturally distinct structures. It is to be further understood that the terms "first" and "second" are interchangeable in the embodiments herein in that a first component or feature could alternatively be termed as the second component or feature, and vice versa.

The CMC material <NUM> from which the CMC vane arc segments <NUM> are made (shown in a cutaway section in <FIG>) is comprised of a ceramic reinforcement 65a, which is usually continuous ceramic fibers, in a ceramic matrix 65b. Example ceramic matrices are silicon-containing ceramic, such as but not limited to, a silicon carbide (SiC) matrix or a silicon nitride (Si3N4) matrix. Example ceramic reinforcement of the CMC are silicon-containing ceramic fibers, such as but not limited to, silicon carbide (SiC) fiber or silicon nitride (Si3N4) fibers. The CMC may be, but is not limited to, a SiC/SiC ceramic matrix composite in which SiC fiber plies are disposed within a SiC matrix. The fiber plies have a fiber architecture, which refers to an ordered arrangement of the fiber tows relative to one another, such as a 2D woven ply or a 3D structure. The CMC vane arc segments <NUM> may be one-piece structures in which at least a portion of the fiber plies are continuous from the platform <NUM>, through the airfoil section <NUM>, and into the platform <NUM>.

The CMC vane arc segments <NUM> may be supported between inner and outer static supports 61a/61b (<FIG>). Each of the static supports 61a/61b may independently be, but are not limited to, an engine case, a full hoop ring, a ring arc segment, or an intermediate structure that is attached to any of these. In the illustrated example, there is no connection between adjacent CMC vane arc segments <NUM> in the circumferential row. Each CMC vane arc segment <NUM>, therefore, must carry its own aerodynamic loads by contact points or regions on single-sided platforms <NUM>/<NUM> where the loads are transmitted into the static supports 61a/61b. As the platforms <NUM>/<NUM> are on opposite sides of the airfoil section <NUM>, the line of action between the points or region where the loads are transmitted crosses the airfoil section <NUM> and represents a cross-corner loading state. A wheelbase, i.e., the distance between the cross-corner points or regions on the platforms <NUM>/<NUM> where the loads are transmitted to the static structures 61a/61b, determines the load-carrying capacity of the vane arc segment <NUM>. In general, increasing the wheelbase (length) corresponds to an increase in load-carrying capacity.

The increased wheelbase is further demonstrated in <FIG> against superimposed comparison double-sided platforms <NUM>. Lines L1 and L2 represent, respectively, the wheelbase lengths of the platforms <NUM>/<NUM> versus the platforms <NUM>. The wheelbase length L1 is greater than the wheelbase length L2, thereby representing the greater load-carrying capacity of the vane arc segment <NUM>. For instance, the wheelbase length L1 is greater than L2 by at least <NUM>% or at least <NUM>%. That is, since the platforms <NUM>/<NUM> are single-sided and thus facilitate bending of the ceramic fiber plies during fabrication, the platforms <NUM>/<NUM> can each be made longer in the circumferential direction than a double-sided platform. In contrast, if the platforms <NUM>/<NUM> were on the same side of the airfoil section <NUM>, the load-carrying capacity would be expected to be lower and may be lower than the double-sided platforms <NUM>. The platforms <NUM>/<NUM> may also exhibit increased flexibility due to the longer moment arm, which may facilitate reduction in sensitivities to manufacturing tolerances. Additionally, since the CMC vane arc segments <NUM> are not interconnected and, prior to assembly into the engine <NUM>, are separate pieces, the segments <NUM> can be fully pre-fabricated, including with any coatings machined features. Processes such as coating deposition and machining are easier to conduct while the segments <NUM> are separated, and singlets such as the segments <NUM> are generally easier to manufacture than doublets. For example, in vane doublets or triplets it may be difficult to for line-of-sight processes to access regions between the airfoil sections.

<FIG> illustrates an isolated view of another example CMC vane arc segment <NUM>, and <FIG> illustrates an axial view of a portion of a circumferential row of the CMC vane arc segments <NUM>. The CMC vane arc segments <NUM> are similar to the segments <NUM> except that at the first radial end 70a the second side wall <NUM> has a bearing surface <NUM>, enabling interconnection between the segments <NUM>. The bearing surface <NUM> is situated along the circumferential edge of the single-sided platform <NUM> to the pressure side of the airfoil section <NUM>. The bearing surface <NUM> may be a substantially flat and smooth face. For example, the bearing surface <NUM> may be formed by machining after densification of the CMC material <NUM>, or alternatively during the fiber ply layup process.

As shown in <FIG>, each of the CMC vane arc segments <NUM> is situated with the edge region of the single-sided platform <NUM> bearing, at least at times, against the bearing surface <NUM> of the next of the CMC vane arc segments <NUM> in the circumferential row, although actual contact may vary depending on conditions and loading state. In this manner, the CMC vane arc segments <NUM> are modestly interconnected and may act as a vane multiplet unit. In a vane multiplet unit, the segments <NUM> at the ends of the unit are not interconnected with the end segment <NUM> of the next unit in the circumferential row.

A vane multiplet unit can carry higher loads than a singlet configuration where each segment carries its own loads (e.g., the segments <NUM>). For instance, the interconnected CMC vane arc segments <NUM> generate an effective wheelbase that is greater the wheelbase that can be achieved in a vane singlet such as the segment <NUM>. As a result, the multiplet can carry greater aerodynamic loads and pressure loads for tangential onboard injectors. Further, with increased load carrying capability, mass may be reduced for lower weight and lower cost designs with enhanced life. Seals <NUM> may also be provided, respectively, between the static support 61b and the platform <NUM> of each of the CMC vane arc segments <NUM>. In this example, the seals <NUM> are arranged at locations that are radially opposite the bearing surfaces <NUM>. The seals <NUM> may serve to limit ingress of combustion gases from the core gas path and/or to provide a compressive pre-load on the CMC vane arc segments <NUM> once assembled. The seals <NUM> may be, but are not limited to, rope seals, C-springs, and E-springs.

As also shown in <FIG> the second side wall <NUM> at the first radial end 70a also includes a ledge <NUM> that borders the bearing surface <NUM>. The ledge <NUM> may act as a stop for the platform <NUM> that mates onto the bearing surface <NUM> and thus may bear the contact force between the segments <NUM>. The ledge <NUM> may also serve as a stiffening structure to provide a relatively rigid region to support loads from the platform <NUM> of the mating CMC vane arc segment <NUM>. Moreover, neither the bearing surface <NUM> nor the ledge <NUM> is cantilevered on a platform, as are mating features of a double-sided platform, which further facilitates stiffening.

Both the ledge <NUM> and the bearing surface <NUM> are contoured in the circumferential direction such that, along a chordal extent of the airfoil section <NUM>, the ledge <NUM> and bearing surface <NUM> track the peripheral shape of the airfoil section <NUM>. At or near the trailing end 68b, the portion of the platform <NUM> that contains the ledge <NUM> and bearing surface <NUM> may project circumferentially past the first side wall <NUM>. Such contouring further facilitates interconnection of the CMC vane arc segments <NUM> by limiting relative circumferential movement between adjacent CMC vane arc segments <NUM>. The opposite edge of the platform <NUM> that mates with the bearing surface <NUM> and ledge <NUM> of the next CMC vane arc segment <NUM> in the circumferential row is of complementary geometry to the contour of the bearing surface <NUM> and ledge <NUM> such that the edge of the platform <NUM> closely fits to the bearing surface <NUM> and ledge <NUM> of the mating segment <NUM>.

In the example shown, the second side wall <NUM> also includes, at the second radial end 70b, an additional bearing surface <NUM>. Each of the CMC vane arc segments <NUM> is situated with the edge region of the single-sided platform <NUM> bearing against the bearing surface <NUM> of the next of the CMC vane arc segments <NUM> in the circumferential row. In this manner, the CMC vane arc segments <NUM> are further interconnected. In this and in the further examples below, it is to be appreciated that the platforms <NUM>/<NUM> may be configured with both platforms <NUM>/<NUM> being pressure side, both platforms <NUM>/<NUM> being suction side, the platform <NUM> being pressure side and the platform <NUM> being suction side, or the platform <NUM> being pressure side and the platform <NUM> being suction side. The bearing surface <NUM>/<NUM> in each of the configurations will be on the opposite side from the respective platform <NUM>/<NUM>.

<FIG> illustrates an isolated view of another example CMC vane arc segment <NUM>, and <FIG> illustrates an axial view of a portion of a circumferential row of the CMC vane arc segments <NUM>. The CMC vane arc segments <NUM> are similar to the segments <NUM> except that the bearing surface <NUM> is multi-faceted. As shown, the bearing surface <NUM> includes a first facet 78a and a second facet 78b. The second facet 78b overhangs the internal cavity <NUM>. Additionally, the ledge <NUM> has a window 80a that opens to the internal cavity <NUM>. On the opposite side of the platform <NUM> from the first side wall <NUM>, the edge of the platform <NUM> includes a tab 74a projecting therefrom. The tab 74a is crooked to provide a hook structure. The tab 74a of each of the CMC vane arc segments <NUM> extends through the window 80a and bears against the second facet 78b of the next of the CMC vane arc segments <NUM> in the circumferential row. The hook structure of the tab 74a acts to catch at the second facet 78b, thereby interlocking the CMC vane arc segments <NUM> together to act as a multiplet unit. The interlocking limits movement in one circumferential direction. The CMC vane arc segments <NUM> at the end of the multiplet unit may be modified such that they do not interconnect with the end segments of the next multiplet unit in the circumferential row. For example, the end segments do not include the tab 74a, or other connecting structure in the further examples below.

<FIG> illustrates an isolated view of another example CMC vane arc segment <NUM>, and <FIG> illustrates an axial view of a portion of a circumferential row of the CMC vane arc segments <NUM>. The CMC vane arc segments <NUM> are similar to the segments <NUM> except that the bearing surface <NUM> includes one or more blind pin holes <NUM> and the platform <NUM> includes a platform pin hole 74b. The platform pin hole 74b of each of the CMC vane arc segments <NUM> is radially aligned with the blind pin hole <NUM> of the next of the CMC vane arc segments <NUM> in the circumferential row. A lock pin <NUM> extends through the aligned platform pin hole 74a and blind pin hole <NUM> to interlock the CMC vane arc segments <NUM> together. The lock pins <NUM> may further be anchored to the static structure 61b and may thus also serve as an anti-rotation feature that limits circumferential movement of the CMC vane arc segments <NUM>. Alternatively, a slot may be machined into the bulbous lobe that can accept a linear seal strip. The seal strip would be inserted into a corresponding slot in the static structure. In this configuration, this feature acts to improve sealing and as an anti-rotation device, instead of being an interlocking feature.

The holes 74a/<NUM> and pin or pins <NUM> may also be used in the example CMC vane arc segments <NUM>/<NUM>/<NUM> disclosed herein for additional interlocking of the CMC vane arc segments. It will also be noted that the CMC vane arc segments <NUM> include the windows 80a in the ledges <NUM>. Alternatively, if there are no structure that extend through the windows 80a, the windows 80a may be excluded such that the ledge <NUM> also extends along the edge of the internal cavity <NUM>, which may provide additional surface that can be used to facilitate sealing.

<FIG> illustrates an isolated view of another example CMC vane arc segment <NUM>, and <FIG> illustrates an axial view of a portion of a circumferential row of the CMC vane arc segments <NUM>. The CMC vane arc segments <NUM> are similar to the segments <NUM> except that the edge of the platform <NUM> opposite from the first side wall <NUM> includes a cantilevered arm 74c and there is a lap joint slot 74d in the radially outer face of the platform <NUM>. The cantilevered arm 74c of each of the CMC vane arc segments <NUM> is disposed in the lap joint slot 74d of the next of the CMC vane arc segments <NUM> in the circumferential row to form a lap joint there between that interlocks the CMC vane arc segments <NUM> as a full hoop structure.

In the illustrated example, the cantilevered arm 74c and lap joint slot 74d are located such that the cantilevered arm 74c is to extend through the window 80a and bridge across (as represented at dashed line <NUM>) the internal cavity <NUM> of the next of the CMC vane arc segments <NUM> in the circumferential row. Alternatively, the cantilevered arm 74c and lap joint slot 74d may be moved forward or aft so as to avoid bridging the cavity <NUM>, as bridging the cavity may interfere with air flow to the cavity. In a further example, the platform <NUM> includes multiple cantilevered arms 74c and lap joint slots 74d, for further interlocking.

As depicted in <FIG>, during the layup process to form the CMC vane arc segments disclosed herein, an over-sized, bulbous lobe <NUM> may be formed at the first radial end 70a of the second side wall <NUM>. For example, fiber plies 86a from the second side wall <NUM> divide and wrap around a filler material 86b (often colloquially referred to as a noodle) to form the lobe <NUM>. The filler material 86b may be formed of, but is not limited to, a CMC or a monolithic ceramic. The CMC or monolithic ceramic may be prefabricated prior to incorporation into the lobe <NUM> such that the fiber plies 86a are laid-up around the prefabricated CMC or monolithic ceramic.

The lobe <NUM> is then machined to form the features described herein above. For instance, as shown in <FIG>, the lobe <NUM> has been machined to provide the bearing surface <NUM> and the ledge <NUM>. In this regard, as the machining is conducted through the filler material <NUM> and fiber plies 86a, a portion of the bearing surface <NUM> is formed by the fiber plies 86a and another portion of the bearing surface <NUM> is formed by a filler material 86b. Similarly, the face of the ledge <NUM> may also be formed from both the fiber plies 86a and the filler material 86b.

As shown in <FIG>, the lobe <NUM> has been machined to provide the bearing surface <NUM>, including the facets 78a/78b. Again, as above, the facets 78a/78b may be formed from both the fiber plies 86a and the filler material 86b.

As shown in <FIG>, the lobe <NUM> has been machined to provide the bearing surface <NUM> and the blind pin hole <NUM>. In this example, the bearing surface <NUM> is formed from both the fiber plies 86a and the filler material 86b, and the blind pin hole <NUM> is exclusively in the filler material 86b. Having the blind pin hole <NUM> exclusively in the filler material 86b may facilitate machining, as removal of only a single material is required (for a monolithic ceramic).

In the examples of <FIG>, <FIG>, <FIG>, and <FIG>, the single-sided platforms <NUM>/<NUM> both extend to the suction side and the platform <NUM> overlaps the bearing surface <NUM> such that the edge of the platform <NUM> is radially outside of core gas path C. <FIG> demonstrate further examples that can be applied to any of the prior examples. In <FIG> the platform <NUM> extends off of the second side wall <NUM> to the pressure side and the bearing surface <NUM> is at the second radial end 70b of the first side wall <NUM>. The edge of the platform <NUM> overlaps the bearing surface <NUM> such that the edge of the platform <NUM> is radially outside of core gas path C. In <FIG>, the platform <NUM> also extends off of the second side wall <NUM> to the pressure side but the bearing surface <NUM> at the second radial end 70b of the first side wall <NUM> faces radially outwards instead of radially inwards. The edge of the platform <NUM> thus overlaps the bearing surface <NUM> such that the edge is radially inside of core gas path C. As will be appreciated from the further examples below, the configurations disclosed herein control how loads are transferred among the CMC vane arc segments and may this be used when designing a system to manipulate load management in the particular implementation.

<FIG>, <FIG> demonstrate states of deformation under load for different configurations. The dashed outlines represent the positions at rest, while the solid form lines represent a deflected state under load. In <FIG>, anti-rotation features <NUM> are provided at the radially outer side of the platform <NUM>. The anti-rotation features <NUM> are not particularly limited and may include pins flanges, or the like that are attached to the static structure 61b to prevent circumferential rotation of the CMC vane arc segments. Under aerodynamic loading the CMC vane arc segments tend to rotate about the features <NUM>. In doing so, the platform <NUM> applies pressure to the bearing surface <NUM> that drives the second side wall <NUM> radially outwards. This places the second side wall <NUM> in compression, which is desirable for CMC materials, and also drives the bearing surface <NUM> to seat into the platform <NUM> and tab 74a. The resultant state is that the CMC vane arc segments are interlocked.

In the example in <FIG>, the platform <NUM> is configured to be radially outside of the core gas path C. Under the aerodynamic loads the side walls <NUM>/<NUM> are in interlaminar compression, which is favorable. However, the deflection is such that the platform <NUM> separates from the bearing surface <NUM>, providing modest or even poor interlocking. Accordingly, this configuration may be used where strong interlocking is not required.

In contrast, the example in <FIG> is the same as in <FIG> except that the platform <NUM> is configured to be radially inside of the core gas path C. Again, under the aerodynamic loads the side walls <NUM>/<NUM> are favorably in interlaminar compression. However, rather than the separation as in the configuration of <FIG>, the bearing surface <NUM> is driven into contact with the platform <NUM>. Thus, there is less or no separation and stronger interlocking is provided.

<FIG> demonstrates full hoop interlocking. Under aerodynamic loads the platform <NUM> applies pressure to the bearing surface <NUM> that drives the second side wall <NUM> radially outwards. This places the second side wall <NUM> in compression, which is desirable for CMC materials, and also drives the bearing surface <NUM> to seat into the platform <NUM>. The cantilevered arm 74c also provide a large wheelbase. The resultant state is that the CMC vane arc segments are interlocked over the full hoop structure, i.e., every CMC vane arc segment is interlocked with the next segment in the circumferential row such that the segments act as a single unit that has a high load-carrying capacity.

<FIG> depicts a method of assembling the circumferential row of the CMC vane arc segments in the engine <NUM> through progressions I, II, III, and IV. Since the segments are interlocked with each other, they are not amendable to individual insertion into the circumferential row one-at-a-time. Rather, as shown at progression I, the CMC vane arc segments <NUM> are initially provided at circumferentially-spaced apart radial positions in a circumferential row about the central axis A. At progressions II and III the CMC vane arc segments <NUM> are moved radially inwardly toward the central axis A such that they become circumferentially closer together. Such movement may be conducted manually but more typically will be automated. During the movement, or at intervals between movement, alignment may be verified to ensure that the platforms <NUM>/<NUM> are in register with the bearing surfaces <NUM>/<NUM>. Finally, at progression IV, the CMC vane arc segments <NUM> are moved to final radial positions in which one or both of the platforms <NUM>/<NUM> come to bear against the respective bearing surfaces <NUM>/<NUM> of the adj acent CMC vane arc segment <NUM>. In further examples, where there is a relatively tight fit, the CMC vane arc segments <NUM> may be pre-loaded with a pre-stress. Tooling or other fixturing may be applied for the pre-loading. The pre-stress deflects the segments <NUM> and provides clearance for the segments <NUM> to fit together. The pre-stress is released after moving to the final radial positions such that the segments <NUM> elastically rebound toward their rest position, fully constrained by the static supports, and thereby interconnect together.

In general, there may be manufacturing benefits to producing a single-sided platform as disclosed herein. For example, fiber plies are transitioned from the airfoil to the single-sided platform in one direction rather than two directions as for a double-sided platform. Moreover, the fillet region between the airfoil section and the single-sided platform can be of higher quality due to the avoidance is discontinuities of the fiber plies during processing. Additionally, if there are attachment features that are to be provided in the single-sided platform the features only need to be placed on one side of the airfoil (instead of both). In double-sided platforms such features are often interrupted. A single-sided platform is also more flexible due than a double-sided platform due to the longer moment arm. In configurations that act as singlets such as that of <FIG>, the bending forces on the platform fillets may result in interlaminar compression which facilitate reducing the likelihood of delamination of the CMC material.

Interconnected segments that act as multiplets, such as the configurations of <FIG> and <FIG> can carry significantly more load than singlet configurations because the vanes combine to generate a much greater wheelbase that what can be achieved in a singlet vane. A multiplet unit facilitates installation into the engine <NUM> as a prefabricated unit. Interconnection of the CMC vane arc segments in a multiplet unit may provide high contact forces where the segments connect. Features such as the ledge <NUM> stiffen the platform <NUM> and may thus facilitate high contact forces to maintain tight interconnection. Some CMC vane designs require metal substructures for support of aerodynamic loads and loads due to a tangential onboard injector. A full hoop unit of interlocked CMC vane arc segments facilitates a further increase in load-carrying capability for supporting such loads without metal substructures.

Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this invention. In other words, a system designed according to an embodiment of this invention will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures.

Claim 1:
A gas turbine engine (<NUM>) comprising:
a plurality of ceramic matrix composite (CMC) vane arc segments (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>) arranged in a circumferential row, each of the CMC vane arc segments (<NUM>...<NUM>) including
an airfoil section (<NUM>) defining first and second side walls (<NUM>, <NUM>), leading and trailing ends (68a, 68b), and first and second radial ends (70a, 70b), the first and second side walls (<NUM>, <NUM>) and leading and trailing ends (68a, 68b) defining an internal cavity (<NUM>),
wherein, at the first radial end (70a), the airfoil section (<NUM>) has a platform (<NUM>) projecting in a circumferential direction from the first side wall (<NUM>) and, also at the first radial end (70a), the second side wall (<NUM>) having a bearing surface (<NUM>; <NUM>),
wherein the platform (<NUM>) of each of the CMC vane arc segments (<NUM>...<NUM>) in the circumferential row is situated to bear against the bearing surface (<NUM>) of the next of the CMC vane arc segments (<NUM>...<NUM>) in the circumferential row,
characterised in that
the platform is a single-sided platform (<NUM>), circumferentially terminating on one side at the bearing surface (<NUM>,<NUM>), and continuing circumferentially in opposite direction and then projecting radially to form the first side wall (<NUM>).