The present invention refers to a low-noise aircraft, in particular of the pusher turboprop type equipped with three lifting surfaces.

Specifically, the present invention allows reducing the noise produced with the same, if not better, performances thanks to an inventive exhaust pipe that will be described in the present application.

It is currently common widespread practice to make aircrafts provided with a main bearing wing and a horizontal tail surface.

Alternatively, it is known to always provide a lifting main wing, but in combination with a surface, also lifting, arranged in front of the main wing.

Such a configuration is commonly called "canard".

The advantage of the "canard" configuration in comparison with the conventional one consists of the increase in maximum lift, for the same surface area. This derives from the fact that the balancing loads are obtained with upward aerodynamic forces instead of with the downward forces required by the conventional configuration .

However, most of this potential advantage of the "canard" configuration is lost due to the contrasting effects that the position of the barycentre, the area of the front lifting surface and the longitudinal distribution of the aerodynamic forces have on the requirements of stability and manoeuvrability.

Indeed a lifting surface in a front position has the same balancing capability as a conventional stabilizer in rear position, but its effects on stability are the opposite. Whereas a lifting surface in rear position has a stabilizing effect, a lifting surface in front position has a destabilizing effect.

In a "canard" aircraft, a well-defined destabilizing effect is acceptable to compensate for the forward position of the barycentre, in order to reduce the manoeuvring loads. As a result, however, for a given position of the barycentre, the front lifting surface must be large enough to generate the balancing forces in all flight conditions but at the same time must not exceed the maximum area required by the conditions of stability .

The two requirements can only be met simultaneously for an extremely small excursion area of the barycentre and accepting strict limitations on the maximum lift of the main wing, with the result of degrading the potential benefits of the "canard" configuration.

In practice, the increase in maximum lift that can be obtained with a canard configuration with two lifting surfaces is always of small entity.

Since the 1980s the Applicant has been proposing its solution for avoiding the typical drawbacks encountered of the "canard" configuration, but maintaining the potential distinctive characteristics thereof with respect to the conventional configuration.

Specifically, the Applicant has made an aircraft (see patent EP 0084686) comprising three lifting surfaces connected to the fuselage where the aforementioned lifting surfaces consist of:

1) a main lifting wing surface mounted on the fuselage in a position such that the centre of pressure of the wing surface itself falls behind the barycentre of the aircraft towards the rear tail point;

2) a front lifting surface mounted on the fuselage in front of the barycentre of the aircraft towards the front nose point;

3) a rear lifting surface mounted on the fuselage behind the main wing surface.

The powerplant is mounted on the main wing and comprises two turboprop units mounted on opposite sides with respect to the fuselage.

Each turboprop unit comprises a turbine engine that actuates a pusher propeller, a front air inlet and two rear exhaust pipes. Each exhaust pipe comprises a section inside the body of the turboprop engine, so as to intercept the exhaust gases, and an outer section arranged so that the exhaust opening faces the relative propeller .

Such an aircraft is shown in figures 1-3.

Since the propellers and the engines are located behind the cabins, the internal noise generated is per se less than conventional aircrafts with turbo propeller with two lifting surfaces.

However, due to the strong gradients in which the thrusting propellers work, the external noise is greater than desired.

The results of numerical simulations carried out by the Applicant have indeed demonstrated that the interaction of the exhaust gases with the rotating propellers represents the greatest cause of noise.

The solution to reduce such noise can therefore only consist of reducing the rotation speed of the propellers as well as reducing the speed of the exhaust gases .

In order not to compromise the performance of the aircraft, the reduction in rotation speed of the propellers can be pursued by adopting new propulsions and/or different profiles of the blades.

In order to reduce the speed of the exhaust gases the prior art suggests to widen the outlet opening. In this way, the expansion and therefore the acceleration of the gases expelled is reduced.

Indeed, as it comes from the isentropic one-dimensional equation given below, the increase in the outlet section corresponds to a lower speed of the gas in outlet .

In such an equation, the superscripts 1 and 2 mean the loaded and unloaded conditions, with A being the section of the duct and with M in Mach number.

However, it should be considered that a lowering of the speed leads to the reduction in thrust of the propeller.

Indeed, the speed of the exhaust gas plays an important role in the thrust equation given below.

F = Fei + F _j = m _a≠ (u _w - V) + m _a [(1 + ) u ₉ - V) + (p ₉ - p _a ) A ₉

In such an equation:

Fel = thrust of the propeller;

Fj= thrust of the jet

mael = mass flow of the propeller;

uw = speed downstream of the propeller; ma = mass flow processed by the engine;

f = ratio between fuel flow rate and air flow rate;

u9 = outlet speed of the exhaust;

V = flight speed;

p9 = static pressure at the outlet of the exhaust;

pa = ambient static pressure;

A9 = outlet area of the exhaust

Therefore, it is clear that the reduction in speed of the exhaust gases, required to reduce the noise produced, leads to the reduction in thrust and therefore to the reduction in performance in general. Moreover, increasing the outlet area has a negative impact on the transformation of the enthalpy into flow speed that occurs in the exhaust, since the reduction in expansion makes the limit layer of the duct's walls prone to detachment with dissipation of energy.

A convergent duct, indeed, determines a favourable pressure gradient that helps avoid the separation of the flow while, on the other hand, a non-convergent or divergent exhaust is more subject to detachments of the flow that reduce the efficiency of the exhaust nozzle. The following equations, with reference to the diagram shown in figure 4, show that the expansion represented in segment 5-9 of the thermodynamic cycle of the turbo propeller T-s takes the point 9 to a higher temperature and entropy, reducing the exhaust speed produced by the ressure ratio P5/P9.

such equations and in the diagram the factors indicated represent:

Cpg: specific heat;

n _n = exhaust efficiency

T05: total turbine outlet temperature;

T9: static exhaust outlet temperature;

T5: static turbine outlet temperature;

T9' : static outlet temperature of the isentropic exhaust at ambient pressure;

3 (3') : end of compression (isentropic);

4 (4' ) : end of combustion (ideal) ;

41 (41') : end of expansion (isentropic) at the outlet of the high-pressure turbine;

5 (5') : end of expansion (isentropic) at the outlet of the low-pressure turbine;

9 (9') : end of expansion (isentropic) of the exhaust.

Therefore, due to all of the above pointed out, the conflict between noise and thrust cannot be solved by the widening of the exhaust area.

Finally, the displacement of the exhaust towards the support of the propellers also does not seem to be a proper solution. Indeed, by doing so there is an increase in temperature that in turn would require heavy changes to the propulsion.

In conclusion, currently no effective solution is known, which allows reducing noise and maintaining or increasing the aircraft's performances.

Starting from such a prior art, the purpose of the present invention is to make an exhaust, in particular for aircrafts of the type described above, that is an alternative to known ones, particularly efficient and capable of reducing noise while ensuring the same, if not better, performances. According to the most general aspect of the invention the aforementioned purposes are accomplished thanks to a particular exhaust pipe the outer section of which is provided with at least one particular opening to allow the introduction of external air inside the exhaust pipe itself at the outlet opening.

It is known technique to make openings on nozzles for aircrafts capable of inletting and mixing cold external air with the flow of hot exhaust gases.

Examples of such perforated exhausts are described in US4007587, US3981449 and FR2508412.

However, such documents do not tackle the problem of how to lower the noise of the aircraft but rather deal with the problem of how to lower the temperature of the exhaust gases.

It is thus clear that a skilled person looking for a solution to the problem of reducing noise would not be led to seek a solution in documents in which, instead, the problem of lowering the temperature of the exhaust gases is dealt with.

However, hereafter it will come out that when the average skilled person looks into the aforementioned documents not relating to the problem of noise, in such documents he will not find any suggestion or hint to make the particular openings having innovative operation and shape as described in the present invention. Indeed, the openings according to the present invention are arranged so as to draw fresh air from the outside into areas where the flow meets a positive pressure gradient and configured so as to promote the entry in a direction tangent to the walls of the duct and to the main flow that licks them. This allows preserving the smooth running of the hot flow coming from the engine on the cold flow introduced from the slits, thus avoiding the mixing that in such an application would have a negative effect.

The constructive details, the operation of such an exhaust pipe and in general the characteristics and advantages of the invention will become clearer from the following description, given as an example and not for limiting purposes, referring to the attached schematic drawings, in which:

- figures 1-3 show the type of aircraft on which the invention of the present application has its maximum efficiency;

- figure 4 shows the thermodynamic cycle of the turbo propeller T-s;

- figure 5 shows the operation of the exhaust according to the present invention;

figures 6-9 show a comparison between the known exhaust and the one made according to the present invention;

- figure 10 shows the exhaust of the present invention installed on the aircraft.

With reference to the figures, reference numeral 10 shows an aircraft in which the exhaust 100 according to the present invention has its optimal application.

Before going into the details of the invention, i.e. into the constructive characteristics of the exhaust 100, a brief description of the aircraft shown in figures 1-3 will hereinafter follow.

Such an aircraft obviously is not the only type that can take advantage of the present invention, but without doubt it represents a very advantageous application thereof.

The aircraft is generically indicated with 10.

It comprises a fuselage 11 on which are mounted a main wing 12, a front tab 13 and a vertical tail plane 14 to which a horizontal tail plane 15 is connected in high position .

The aircraft 10 also comprises a powerplant mounted on the wing 12, which will be described hereafter.

The fuselage has a pure tapering shape without discontinuity from the nose 16 at the front to the aft 17.

The main wing 12 crosses the fuselage 11 at mid-height, in a longitudinal position roughly two thirds of the way along the length of the fuselage starting from the nose 16, behind the barycentre of the aircraft 10 towards the aft 17. It consists of a left half-wing 18 and a right half-wing 19, connected together so as to transversally form a dihedron of +2 degrees (inclined upwards) . On the left half-wing 18 a slider 20 and a high-lift device 21 are preferably installed and correspondingly on the right half-wing 19 a slider 22 and a high-lift device 23 are installed.

The front tab 13 is connected transversally and in a fixed manner to the fuselage 11, in front of the barycentre of the aircraft 10, close to the nose 16.

It has an area of less than one third of that of the wing 12 and is made up of a left half-wing 24 and a right half-wing 25, symmetrical with respect to the middle line of the aircraft 10, connected so as to transversally form a dihedron of -5 degrees (inclined downwards) .

On the half-wing 24 a high-lift device 26 is preferably installed and correspondingly on the right half-wing 25 a high-lift device 27 is installed.

The vertical tail plane 14 is connected to the rear end of the fuselage 11. It preferably includes a rudder 28 and direction compensators 29.

The horizontal tail plane 15 is arranged in a higher position with respect to the wing 12 and has an area of less than one third of the latter. It is made up of a left half-plane 30 and a right half-plane 31 that are symmetrical with respect to the middle line of the aircraft 10, rigidly connected to each other so as to form a negative dihedron (inclined downwards) . An elevator 32 is preferably installed on the left half- plane 30 and correspondingly an elevator 33 is installed on the right half-plane 31.

The powerplant consists of two turboprop groups 34 one mounted on the left half-wing 18 and the other one on the right half-wing 19, in symmetrical position with respect to the middle line of the aircraft 10. Each group 34 comprises a turbine engine 35 having front air intake 36 and rear exhaust pipes 100, which actuates a propeller 38, namely arranged rearwardly, on a plane parallel to the outlet edge of the half-wing, with axis parallel to the plane of symmetry of the aircraft 10. The aircraft 10 rests on the ground on a front, central landing gear device 39, retractable in flight, and on two lateral main rear landing gear devices 40, also retractable in flight.

Such an aircraft is intended for medium haul transportation of about ten passengers and the entire passenger-carrying part of its fuselage 11 is located in front of the main wing 12. The aircraft 10 thus has three lifting surfaces, i.e. the wing 12, the tab 13 and the horizontal tail plane 15, to obtain the aerodynamic lift and the balancing of the forces in play, necessary in all phases of flight. The main wing 12 generates the maximum part of the aerodynamic lift necessary for flight.

The front tab 13 contributes to the lift and to the balance of the aircraft.

The horizontal tail plane 15 contributes to the stability and to the balance of the aircraft and performs functions of longitudinal control and/or of trim through rotation of the elevators 32 and 33.

The vertical tail plane 14, on the other hand, ensures stability and direction control.

Thanks to the use of the three lifting surfaces, both the aerodynamic and structural benefits due to the average position in height of the wing 12 with respect to the fuselage 11 are exploited, without being penalized in the use of the volume of the fuselage 11 for the purposes of the payload.

Given the position of the wing 12 with respect to the fuselage 11, the main sources of noise, consisting of the turboprop groups 34, are in a position further back with respect to the passenger-carrying part of the fuselage 11 so as to improve the comfort in such a passenger-carrying part.

There is also a useful cabin volume greater than that offered by the existing aircrafts of similar categories, whilst still continuing to have absolutely better aerodynamic efficiency than that of current aircrafts with two lifting surfaces.

As well as the aerodynamic advantages, an important aspect of the configuration with three lifting surfaces consists of the possibility, not available for configurations with two lifting surfaces, of designing the general architecture of the aircraft without the typical need for a predetermined position of the barycentre .

This opens the way for more rational architectures, with consequent substantial reduction in weight and aerodynamic resistance, thus further reducing the size of the aircraft.

Basically, the aircraft 10 is smaller, lighter and more aerodynamically efficient than aircrafts of equivalent capacity currently available.

In view of all of these advantages, the Applicant has thus sought solutions capable of bringing further modifications to further reduce noisiness without however following the suggestions of the prior art, but instead overcoming a well-established technical drawback in the field of aircrafts.

Indeed, it is generally recognised that an exhaust pipe of the gases burnt in the propulsion system, operating in transonic regime, must be characterised by sections that along the development of said exhaust have continuously and progressively smaller areas proceeding from the inlet towards the outlet of the exhaust gases, otherwise the propulsion performance is reduced.

As demonstrated earlier, indeed, a simple widening of the exhaust area would have led to an unacceptable reduction in thrust performance.

For the aforementioned reasons, general design practice in the field of fixed-wing aircrafts does not provide the application of continuously divergent exhausts of burnt gases.

Instead, the present invention offers, in an inventive and revolutionary way, the definition of a subsonic exhaust with continuously divergent section, as described above, which on the one hand reduces the sound impact without penalizing the thrust characteristics thereof.

The recovery of the latter is achieved through the position of a series of openings for feeding and energizing the flow inside the pipe. The shape and arrangement of said openings has been identified in precise positions through accurate fluid-dynamic analysis .

The powerplant comprises two turboprop units 34 mounted on the main wing 12 on opposite sides with respect to the fuselage 11.

Each turboprop unit 34 comprises a turbine engine 35 that actuates a pusher propeller 38, a front air inlet 36 and two rear exhaust pipes 100.

Each exhaust pipe 100 comprises a section inside the turboprop unit 34 so as to intercept the exhaust gases of the turbine engine 35 and an outer section 102 arranged so that the exhaust opening 103 faces the relative propeller 38.

As stated earlier, such an exhaust pipe 100 is of the divergent type with substantially circular or elliptical section.

According to the invention the outer section 102 of each exhaust pipe 100 is provided with at least one opening 101 for introducing external air inside each exhaust pipe 100 at the exhaust opening 103.

Such at least one opening 101 for introducing external air inside each exhaust pipe 100 is made in the form of a circumferential slit of the outer section 102 of each exhaust pipe 100.

By the term "circumferential" we mean that the at least one opening 101 has a development substantially parallel to the outlet mouth of the gases and orthogonal to the longitudinal development of the exhaust .

In other words, these openings 101 represent a sort of splits of the orthogonal section of the exhaust at a precise distance from the outlet mouth of the gases. In the detail of figure 10 the exhaust opening 103 is shaped so as to make a closed curved line comprising a first substantially vertical section 104 and parallel to the turboprop unit 34, a second curved outer section 106 opposite the first substantially vertical section 104 and two curved connecting sections 105 between the first and second section 104, 106.

In such an embodiment of exhaust opening 102 the at least one opening 101 is in the form of a circumferential slit of the outer section 102 that develops at least partially in the curvilinear connecting section 105.

Preferably, a series of openings 101 are provided in the form of parallel circumferential slits arranged at increasing distance from the outlet mouth of the gases in which at least the slit nearest to the exhaust opening 103 develops from the curvilinear connecting section 105 at least partially up to the first substantially vertical section 104.

Even more preferably, two series of openings 101 are provided facing each other on the two upper and lower curvilinear connecting sections 105.

The operation of the exhaust of the present invention will now be described.

As shown in figure 5, from the openings 100 fresh and slow external air penetrates into the duct passed through by the hot gases at high speed.

The particular positioning of the openings as described above ensures a negative pressure gradient between outside and inside.

Such a pressure difference draws air from outside, necessarily occupying part of the inner volume of the exhaust .

Thus the effective area of the exhaust flow is reduced for the same exhaust opening, artificially creating a technical equivalent to a convergent geometry.

Moreover, the air introduced prevents the separation of the flow, generating a film of air between the walls and the hot exhaust flow.

This fact accelerates the flow close to the wall slowed down by the viscous effects of the limit layer, reducing the turbulence thereof.

Finally, these advantageous effects are shown in figures 6-9 where an exhaust without openings and an analogous one provided with the openings 100 are compared in terms of turbulence, speed and temperature. The table shown below indicates the advantages from a numerical point of view.

It has thus been seen that the invention achieves the purposes highlighted earlier.

In particular, the peculiar openings obtained on the exhausts on the one hand allow reducing the noise and on the other hand contribute to the thrust of the aircraft also under subsonic conditions with divergent exhaust pipes.

The present invention thus conceived can undergo numerous modifications and variants, all of which are covered by the same inventive concept; moreover, all of the details can be replaced by technically equivalent elements. In practice, the materials used, as well as their size, can be of any type according to the technical requirements.