Positioning bridge guide and its utilisation for the nozzle support pipe of a turboprop

The invention relates to the field of turboprops used in aeronautics, and more precisely the relative positioning of the heat shield liner and the nozzle support pipe for a thrusters, in particular a thrusters with cooling bypass air flow.

In military type turboprops, using cooling bypass air flow, it is necessary to be able to constantly position the heat shield liner at the level of the nozzle relative to the nozzle support pipe. In other words, this involves centering the heat shield liner. In order to achieve this, bridge guides are used, spaced around the whole circumference of the internal envelope, that is the heat shield liner. Reference can be made to figure 1 without taking into account the shape of the bridge guides 3 placed between the heat shield liner 1 and the nozzle support pipe envelope 2 . Furthermore, these bridge guides must also fulfill a diaphragm function relative to the bypass air flow circulating between the internal protection, meaning the heat shield liner and the external envelope, meaning the nozzle support pipe. Finally, they can contribute to preventing heat shield liner resonance modes below a certain frequency.

In general, the bridge guides are riveted onto the heat shield liner, spot welding sometimes causing tearing. The main inconvenience of this solution is the lack of flexibility. In fact, under a compression load due to a thermal expansion difference and a pressure differential between the nozzle support pipe and the heat shield liner, the bridge guide uprights are rapidly butted and the maximum admissible stress is quickly reached. The risks associated with this lack of flexibility are, amongst other things, the fact that the deformation may be transmitted to the nozzle support pipe and the heat shield liner. But these parts are complex, expensive and, if damaged, are liable to cause malfunctions of the turboprop. In particular, there is the possibility of implosion of the heat shield liner bringing about partial obstruction of the engine main exhaust flow, local dishing of the nozzle support pipe with associated ruptures.

A solution using very small height bridge guides fixed on a circular diaphragm riveted onto the liner is more flexible in operation. On the other hand, the thermal gradient between the diaphragm upstream and downstream is not satisfactory for the engine structure. In fact, thermal fatigue has been noted, provoking longitudinal and then circumferential edge cracks leading up to loss of sector portions.

Taking into account these different techniques and comments, the aim of the invention is to remedy the above-mentioned inconveniences and to propose another type of bridge guide, knowing that the bridge guides must ensure three principal functions. The first consists ideally of touching the nozzle support pipe as soon as possible, in order to centre the heat shield liner. It can be noted that increasing the number of supports makes it possible to increase the buckling capacity of the heat shield liner. A second function is to allow the best possible calibration of the diaphragm sections in the bypass air flow. And finally, the third function is absorption of significant multi-directional and differential expansions at the level of the turboprop.

As a result, a first aim of the invention is a positioning bridge guide with two parts related to each other, these two parts being able to undergo variations concerning their relative position, the bridge guide being constituted of a metallic bar principally comprising:

two lugs constituted by the two bar ends, that must be in contact with a first of the two parts; a central cambered part connecting the two lugs, and having at least one cambered summit zone constituting at least one contact zone with the second of the two parts.

According to the invention, the cambered central part has an inversed central camber, thus defining two summit zones constituting two contact points with the second of the two parts.

In a particular embodiment of the invention, with the aim of allowing a certain amount of circumferential expansion, a single one of the two lugs is fixed on the first of the two parts, the other lug not being in contact with it, thus allowing tangential movement relative to the first of the two parts.

In this case, it is interesting to produce this assembly having a slot in the second lug in the longitudinal direction of the bar constituting the bridge piece.

In the case, it is advantageous to complete this assembly by means of a T-shaped guide pin, integral with the first part and whose T-shaped foot is placed in the slot.

A preferred embodiment consists also of providing the junction between the cambered central part and the lugs with a low radius of curvature.

A second particular aim of the invention is a device for positioning and adaptation to the differential expansions between the heat shield liner and the nozzle support pipe envelope of a turboprop.

According to the invention, bridge pieces are used comprising the characteristics described above and fixed on the heat shield liner.

The invention and its different technical characteristics will be better understood by reading the following non-limiting description that is accompanied by four figures respectively representing:

FIG. 1 , the circumferential positioning in the utilisation of bridge pieces according to the invention for a turboprop;

FIG. 2 , the longitudinal positioning of bridge pieces according to the invention in this same utilisation for a turboprop;

FIG. 3 , in cross-section, a bridge piece according to the invention positioned for utilisation in a turboprop; and

FIG. 4 , in isometric perspective, a bridge piece according to the invention.

FIG. 1 shows the circumferential settings of bridge pieces 3 according to the invention, on a heat shield liner 1 of a turboprop nozzle. The two thin broken lines 2 represent the outer positions of the nozzle support pipe envelope. It can be seen that the space between the heat shield liner 1 and the cooling air bypass flow pipe envelope 2 is of the order of 10 mm. Consequently, the bridge pieces 3 are relatively flat. On the other hand, they must be able to support crushing of the order of 15 to 25% in height, while still ensuring sufficient mechanical hold.

FIG. 2 shows the longitudinal positioning of bridge pieces on the heat shield liner 1 of a turboprop nozzle. Thus it is possible and simple to produce several bridge piece 3 rows or rings over the whole length of this part in order to ensure the positioning of the cooling air bypass pipe envelope along its entire length.

FIG. 3 shows in more detail the shape of the bridge pieces 3 , especially their cambered central part 8 surrounded by two lugs 5 A and 5 B. The latter are in contact with the heat shield liner 1 .

The shape of the cambered central part 8 is thus characterised by two summit zones 4 extending to the lugs 5 A and 5 B. The two summit zones 4 each constitute a privileged contact point with the air bypass flow pipe envelope 2 (in this FIG. 3 , the latter's position is shown in thin broken lines). In other words, the cambered central part 8 shows in inversed camber relative to the curvature of the summit zones 4 . This geometry with inversed central cambering makes it possible to obtain clearly higher bending capacity for each bridge piece. In fact, the bridge pieces according to prior art usage only have very low flexibility on the bridge piece uprights.

Another characteristic complementing the inversed camber of the cambered central part 8 is the low radius of curvature of the joins 9 between this cambered central part 8 and lugs 5 A and 5 B. In this way, the bending zone in the cambered central part 8 of the bridge piece is optimized. Bending can thus take place because of the inversed camber orienting the bending by privileging one deformation direction. Furthermore, this allows utilisation of the geometry instability. It is thus possible to absorb a radial differential expansion between two parts through tangential bending.

The bridge piece as a whole is in contact with the heat shield liner 1 through fixation of the left lug 5 A by means of a rivet 6 and by means of a guide pin 7 that allows lug 5 B a certain degree of freedom relative to the heat shield liner 1 . These details are more visible in FIG. 4 , which shows the bridge piece and its two lugs 5 A and 5 B.

The left lug 5 A is provided with fixation holes 10 to be fixed integrally to the heat shield liner 1 . It is to be noted here that the bridge piece is made from a relatively wide metallic bar. The right lug 5 B is provided with a slot 11 whose axis is parallel to the longitudinal axis of the bar constituting the bridge piece. A guide pin 7 is fixed in the heat shield liner and is T-shaped. The head of the T extends above the slot 11 whereas the foot of the T is inserted inside it. In this way, the bridge piece can undergo length variations in the longitudinal direction, due to significant temperature differences relative to those undergone by the heat shield liner 1 . In fact, the guide pin 7 can move in the longitudinal direction of the slot 11 and allow relative movement of the heat shield liner in this bridge piece direction. Furthermore, this link with a degree of freedom limits stresses in the bridge piece.

Thus, the bridge piece plays its role of damper in an optimised way and so limits the risks of damaging the heat shield liner and the nozzle support pipe envelope, that are very much more expensive parts than the proposed bridge piece. In fact, it is the latter that absorbs almost all the whole of the deformations during the turbojet operation. Thus it plays the role of a fuse. It can also be seen that the buckling capacity of the heat shield liner is increased by using such bridge pieces.

Referring once again to FIG. 1 , it is understood that the use of a large number of bridge pieces 3 , spaced around the whole circumference of a heat shield liner 1 allows optimised relative positioning of the latter relative to the nozzle support pipe envelope.