IMPROVED AIRCRAFT

Abstract: This invention relates to an aircraft having three lift surfaces, and precisely a main wing lift surface located in an intermediate position of the aircraft and two less extend­ ed lift surfaces positioned, respectively, in a position ahead of the main wing surface and astern thereof.

Description: This invention relates to an aircraft having three lift surfaces. Normally, aircraft of the three lift surface type, like that according to the preamble of claim 1 and described in EP―A―00 33 053 or FR―A―2 254 481, though having an aerodynamic efficiency theoretically greater than that of aircraft of the two lift surface type, in practice present problems due to "aerodynamic interference" between the three lift surfaces which problems can at least in part jeopardize such superiority. In order to have an optimal "functioning" of each lift surface both from the standpoint of lift, and of resistance and of other aerodynamic parameters, it is requisite that the air flow that strikes the lift surface be as little disturbed as possible so as not to alter the laminar pattern of the fluid threads on the lift surface, and this applies particularly to the air flow which strikes the upper face of the lift surface. In the case of three lift surface aircraft, the front lift surface can give rise to vortices and, in general, disturbance in the air flow that strikes the successive main and rear lift surfaces, just as the main lift surface can create disturbances in the air flow that strikes the successive rear lift surface, altering the aforesaid laminar pattern of the fluid threads and causing a diminution of the aero-dynamic efficiency of the aircraft. With reference to EP―A―00 33 053, it is to be noted that the reciprocal position of the front lift surface and the rear lift surface is such that the front lift surface disturbs the air flow that strikes the rear lift surface. With reference to FR―A―1 154 481, there is inteference, in particular, between main and rear lift surfaces. DE-B-2 328 293 shows a watercraft with a front control surface, an intermediate lift surface and rear lift surface. The front control surface is substantially on the same plane of the intermediate lift surface and consequently aerodynamic interferences occur between these surfaces. DE-A-1 803 132 shows an aircraft with a front, intermediate, and rear lift surfaces or wings, disposed in a scaled manner, the front wing and the rear wing being respectively disposed at the lowest and the highest position. The wing span progressively reduces from the tail point to the nose point. Such aircraft configuration adopts artificial means for reaching a high aerodynamic efficiency. This configuration involves structural complications and limitations of the aircraft, which reduce the aerodynamic benefits. An object of the present invention is to obviate the aforesaid problems. Said object is achieved by a three lift surface aircraft comprising a fuselage extending from a forward bow point to a rearward tail point, an intermediate lift surface mounted on said fuselage in an intermediate longitudinal position between the forward bow point and the rearward tail point and generally at fuselage half-height, a front lift surface and a rear lift surface mounted on said fuselage respectively forward and rearward with respect to the intermediate lift surface, the front lift surface lying at a lower level than the intermediate lift surface and the intermediate lift surface lying at a lower level than the rear lift surface, characterized in that: the intermediate lift surface has a substantially larger lift area than the front and rear lift surfaces and is mounted on said fuselage in such a position that its centre of pressure falls behind the centre of gravity of the aircraft in the direction towards said rearward tail point, the front lift surface is mounted on the fuselage at said forward bow point ahead of the centre of gravity, the rear lift surface is mounted at the top of a vertical tail assembly extending upward from the tail point of the fulelage. In order that the features and the advantages of the present invention may be better understood the description will be reported hereinafter of an exemplary nonlimiting embodiment thereof, as illustrated in the accompanying drawings, wherein: FIGURE 1 is a plan view of an aircraft constructed according to the present invention, FIGURE 2 is a side elevational view of the aircraft of Figure 1, FIGURE 3 is a front view of the aircraft made according to Figure 1, and FIGURE 4 is a side view of the aircraft of Figure 1, and, besides it, for comparison, of a conventional aircraft with two lift surfaces. The aircraft made according to the invention is generally indicated at 10. It is comprised of a fuselage 11, whereon there are mounted a main wing 12. a front slat 13 and a vertical tail plane 14 to which is secured, in a position near the top of the plane 14, a horizontal tail plane 15. The aircraft 10 also comprises propelling means mounted on the wing 12, which will be described hereinafter. The fuselage has a pure tapered outline without any discontinuity and running from the nose 16, ahead, to the tail portion 17, astern. The main wing 12 is placed across the fuselage 11 roughly at the level of the half-height thereof and at a position about two thirds of the fuselage length starting from the nose 16, behind the centre of gravity of the aircraft 10 in the direction towards the tail portion 17. Said wing 12 is composed of a left half-wing 18 and a right half-wing 19, which are connected together so as to form, transversally, a dihedron angle -2° wide (inclined upward). On the left half-wing 18 there are mounted an aileron 20 and a flap 21 and, correspondingly, on the right half-wing 19 there are mounted an aileron 22 and a flap 23. The front slat 13 is transversally connected in a fixed manner to the fuselage 11, ahead of the centre of gravity of the aircraft 10, in the vicinity of the nose 16, in a position lower than that of the wing 12 and with the angle of incidence wider than the angle of incidence of the main wing 12. The slat 13 has a superficial area which is narrower than one third of that of the wing 12 and is composed of a left half-slat 24 and a right half-slat 25, which are symmetrical relative to the central axis of the aircraft 10 and are connected together so as to form, transversally, a dihedron angle of -5° (inclined downward). On the left half-slat 24 there is installed a flap 26 and, correspondingly, a flap 27 is mounted on the right half-slat 25. The vertical tail plane 14 is connected to the rear end of the fuselage 11. It comprises a rudder 28 and directional adjustment tabs 29. The horizontal tail plane 15 is thus arranged at a position which is at a level higher than that of the wing 12 and has a superficial area which is less than one third that of the area of the main wing 12. Said plane 15 is composed of a left half-plane 30 and a right half-plane 31, which are symmetrical relative to the central axis of the aircraft 10 and are rigidly connected together so as to form a negative dihedron angle (inclined downward). On the left half-plane 30 an elevator 32 is installed, and, correspondingly, an elevator 33 is installed on the right half-plane 31. The propelling motive means are formed by two turbopropeller units 34, one of which is mounted on the left half-wing 18 and the other on the right half-wing 19 in positions which are symmetrical relative to the centre line of the aircraft 10. Each of the units 34 comprises a turbine engine 35 with a front air intake 36 and exhaust tubes at the rear, said engine driving a thrust propeller 38, that is, a propeller which is positioned behind the trailing edge of the half-wing, with its axis on a plane which is parallel to the plane of symmetry of the aircraft 10. The aircraft 10 rests on the ground on a central front landing gear (39), which is retractable in flight and on two landing gears 40, which are placed at the rear and laterally and are also retractable in flight. Said aircraft is especially suitable for the medium-range transportation of some ten passengers and the entire passenger compartment within the fuselage 11 is positioned ahead of the main wing 12. The aircraft 10, as described and illustrated herein, thus exploits three lift surfaces, that is, the wing 12, the tab 13 and the horizontal tail plane 15, to obtain the aerodynamic support and the equilibrium of the forces which are active, as it is necessary during all the stages of the flight. The main wing 12 generates the predominant fraction of the aerodynamic lift which is required in flight. The front tab 13 contributes to the lift and the equilibrium of the aircraft. The horizontal tail plane 15 contributes to the stability and the equilibrium of the aircraft and fulfils a task of longitudinal control and/or trimming by the rotation of the elevators 32 and 33. The vertical tail plane 14, instead, provides both the stability and the directional control. The three lift surfaces permit to redress the defects in the stability and the longitudinal control of the "canard" configuration, but without resorting to artificial means for increasing the lift by adopting a power increase and/or artificial means for improving the stability. The reason therefor is the availability of an additional degree of freedom, as provided by the third lift surface, so as to fulfil at the same time the stability and manoeuverability requirements. As a matter of fact, the configuration having three lift surfaces makes it possible to determine, for each position of the centre of gravity, a distribution of areas among the three surfaces such as to be able to exploit the contribution of the aerodynamic forces directed upward with a view to balancing the aircraft, while concurrently fulfilling the stability conditions. In addition, if the function of controlling the longitudinal equilibrium is entrusted to the rear lift surface (tail plane 15), the stall of the aircraft does not reduce the power of the elevators. The configuration, as described herein, having three lift surfaces, concurrently provides the potential favourable features for the "canard" configuration, as compared with the conventional one. To give an evidence of this fact, Figure 4 shows the aerodynamic forces, represented by the vectors F₁, F₂, F₃, acting upon the aircraft 10 having three lift surfaces, in comparison with the aerodynamic forces, as represented by the vectors H₁, H₂, which are activated upon a conventional aircraft 50 of the same class but with two lift surfaces only, in the flight condition which is characterized by the angle of stall incidence for which the vectors have the following modules: F₁ = 0.9 g; F₂ = 0.2 g; F₃ = -0.1 g; H₁ = 1.3 g; H₂ = -0.3 g. In this flight condition, both the configurations must provide the maximum possible lift, so as to reduce the lift surface which is required to obtain the desired stall speed. A reduction of the lift surface will originate, in fact, an increase of the efficiency at the cruising speed. The ratio of the lift surface which is required for the aircraft 10 according to the invention, to the surface which is required for the conventional aircraft 50 is equal to the ratio of the sum of the magnitudes of the forces F₁, F₂, F₃ to the sum of the forces (in terms of magnitudes) H₁ and H₂. From the comparison of the two configurations shown in Figure 4, one obtains for the aircraft 10 a saving of the overall required lift surface as great as 25% over the conventional aircraft 50. The particular "stepwise" arrangement of the aforesaid lift surface, i.e. the main lift surface 12 at the half-height of the aircraft fuselage, the front lift surface 13 at a low level beneath the fuselage half-height, and the rear lift surface 15 at the top of the vertical tail assembly 14, confers high aerodynamic efficiency to the aircraft 10. In effect, and as can be deduced from Figures 2 and 3 of the drawings, with this "stepwise" arrangement none of the lift surfaces in any substantial way disturbs the air flow that strikes the successive lift surface or surfaces, and even less the flow that strikes the upper part of the successive lift surface or surfaces, inasmuch as the preceding lift surface is always arranged lower than the following one. In this way the problems mentioned in the introduction of the present specification are solved. By virtue of the use of the three lift surfaces, one exploits the benefits, both from the aerodynamic standpoint and from the viewpoint of the structural characteristics, which are originated by the midway positioning of the wing 12 as to its level relative to the fuselage 11, without having to suffer from reductions of the fuselage volume reserved for the payload. On account of the positioning of the wing 12 relative to the fuselage 11, the main sources of noise, that is, the turbopropeller units 34, are in a position which is shifted backwards relative to the passenger compartment of the fuselage 11, so as to improve the comfort in said compartment. A useful compartment volume becomes thus available, which is wider than that provided by the contemporary aircraft of the same class, while having aerodynamic efficiencies which are positively improved over those afforded by the present aircraft having two lift surfaces. In addition to the aerodynamic advantages, an important aspect of the configuration with three lift surfaces is. the possibility, which cannot be afforded by the configurations having two lift surfaces, of being able to design the general architecture of the aircraft without the typical requirement of a predetermined position of the centre of gravity. This fact opens the way to more rational architectural designs, with a consequent substantial reductions of the weight, and of the aerodynamic resistance, so that the dimensions of the machine can further be reduced. Finally, the aircraft 10 is smaller, lighter and aerodynamically more efficient than the aircraft of the same class as available today. It is obvious that the present exemplary embodiment does not set any limitations to possible modifications and/or additions. The idea of the three lift surfaces disposed in the mentioned "stepwise" arrangement can be applied to aircraft having dimensions and tasks other than those described herein, while obtaining the same advantages. Geometrical variations of the shape, the dimensions, both absolute and relative, of the three lift surfaces, can be introduced to optimize the configuration for the operative requirements of the individual aircraft, since what has been shown in the accompanying drawings has been originated solely by the particular class of aircraft described herein. In addition, at least one portion of the surface of the tab 13 can be rotated about an axis normal to the vertical plane of symmetry of the fuselage thus contributing towards the functions of longitudinal control and/or trimming. Lastly, it can be movable relative to the fuselage, or the two half-tabs 24 and 25 can effect rotations relative to one another. At least one portion of the entire surface of the horizontal tail plane 15 can also be rotated about an axis normal to the vertical plane of symmetry of the fuselage. In connection with the motorized propelling means, one may provide two turbopropeller units arranged like the units 34, but with pulling propellers placed ahead of the main wing. As a rule, it is possible to adopt propelling means consisting of conventional propellers, or turbo-jets, mounted on the fuselage ahead or astern, or in an intermediate position, or also mounted on the wings. In the case of conventional propellers, it can be envisaged to place the engines in the fuselage and the propellers on the wings, by connecting the latter to the engines by means of adequate linkages. Lastly, there is to be added that the approach suggesting three lift surfaces disposed in the mentioned "stepwise" arrangement can be adopted also on aircraft having no propelling means of their own, such as the gliders.

Claims: 1. A three lift surface aircraft comprising a fuselage (11) extending from a forward bow point (16) to a rearward tail point (17), an intermediate lift surface (12) mounted on said fuselage (11) in an intermediate longitudinal position between the forward bow point (16) and the rearward tail point (17) and generally at fuselage half-height, a front lift surface (13) and a rear lift surface (15) mounted on said fuselage (11) respectively forward and rearward with respect to the intermediate lift surface (12), the front lift surface (13) lying at a lower level than the intermediate lift surface and the intermediate lift surface lying at a lower level than the rear lift surface, characterized in that : the intermediate lift surface (12) has a substantially larger lift area than the front and rear lift surfaces (13,15) and is mounted on said fuselage (11) in such a position that its centre of pressure falls behind the centre of gravity of the aircraft in the direction towards said rearward tail point (17), the front lift surface (13) is mounted on the fuselage (11) at said forward bow point (16) ahead of the centre of gravity, the rear lift surface (15) is mounted at the top of a vertical tail assembly (14) extending upward from the tail point (17) of the fuselage (11). 2. Aircraft according to claim 1, wherein said intermediate lift surface forms a transverse positive dihedron and said front lift surface forms a transverse negative dihedron. 3. Aircraft according to claim 1 or 2, wherein said intermediate lift surface (12) is provided with ailerons (20, 22) and flaps (21, 23), said front lift surface (13) is provided with flaps (26, 27), said rear lift surface is provided with elevators (32, 33), and said vertical tail assembly (14) is provided with a rudder (28). 4. Aircraft according to one of the preceding claims, wherein said front lift surface (13) is rigidly connected to the fuselage (11). 5. Aircraft according to anyone of claims 1, 2, 3, wherein said front lift surface (13) is movable with respect to the fuselage (11). 6. Aircraft according to anyone of claims 1, 2 or 3, wherein at least one portion of said front lift surface (13) is rotatable about an axis normal to the vertical plane of symmetry of the fuselage (11). 7. Aircraft according to claim 6, wherein said front lift surface is split into two portions (24, 25) which can be rotated with respect to one another about an axis normal to the vertical plane of symmetry of the fuselage (11). 8. Aircraft according to anyone of claims 1, 2,3,4,6, wherein said front lift surface (13) has an angle of incidence wider than that of said intermediate lift surface (12). 9. Aircraft according to anyone of the preceding claims, provided with propelling means (34). 10. Aircraft according to claim 9, wherein said propelling means (34) comprise conventional propellers (38). 11. Aircraft according to claims 9 or 10, werein said propelling means (34) are mounted on said intermediate lift surface (12). 12. Aircraft according to claim 11, wherein said propelling means (34) are composed of turbo-propeller units having their propellers (38) arranged behind said main lift surface (12). 13. Aircraft according to claim 10, wherein said propelling means comprise motive members arranged in said fuselage and propellers mounted on said intermediate lift surface, and mechanically connected to said motive members.. 14. Aircraft according to claim 9, wherein said propelling means are turbo-jet means. 15. Aircraft according to claim 1, comprising in combination: said fuselage (11) having a minimum aerodynamic resistance and the external outline of which is absolutely devoid of any curvature discontinuities, a main wing (12) constituting said intermediate lift surface, equipped with wing flaps (21,23) and ailerons (20,22), and forming a transverse positive dihedron, a front tab (13) constituting said front lift surface, rigidly connected to the front end of said fuselage (11) and having an area less than one third of the area of said main wing (12), composed of two symmetrical half-tabs (24, 25) relative to the centre line of the aircraft and rigidly connected together so as to form a transverse negative dihedron positioned beneath the plane of said main wing (12), said front tab (13) being equipped with wing flaps (26,27), a vertical tail plane (14),constituting said vertical tail assembly, equipped with a rudder (28), installed behind said main wing (12), a horizontal tail plane (15) constituting said rear lift surface having an area less than one third of the area of said main wing (12), consisting of two half-planes (30, 31) which are symmetrical relative to the centre line of the aircraft and are connected rigidly together so as to form a transverse negative dihedron mounted in elevation on said vertical tail plane, said horizontal tail plane (15) being equipped with elevators (32, 33), two turbo-propeller engines (34) which drive propellers (38) positioned behind said main wing (12), installed on said wing in a position symmetrical relative to the middle plane of the aircraft, a passenger compartment placed in said fuselage (11) ahead of said main wing (12), a front landing gear (39) which can fully be retracted within the fuselage (11), and two main rear landing gears (40) which can completely be retracted within said fuselage (11) behind the passenger compartment thereof.

Inventors: MAZZONI, ALESSANDRO
Applicants: INDUSTRIE AERONAUTICHE E MECCANICHE RINALDO PIAGGIO S.P.A.
Application year: 1982
Main classification: B64C39
IPC codes: B64C39, A63F7, H05F3