Aeronautics and astronautics – Aircraft power plants – Arrangement
Reexamination Certificate
1999-09-30
2001-08-21
Jordan, Charles T. (Department: 3644)
Aeronautics and astronautics
Aircraft power plants
Arrangement
24, 24
Reexamination Certificate
active
06276633
ABSTRACT:
The invention relates to a convertible aircraft with tilting rotors, which is capable of operating in helicopter mode, in particular for the purpose of landing and taking off, in which case the rotors rotate about substantially vertical axes in order to lift the aircraft, and in aeroplane mode, in which case the rotors are tilted to operate as propellers, the aircraft comprising at least one fuselage, a fixed wing system comprising at least two wings extending laterally on either side of the fuselage and, in helicopter mode, a rotating wing system comprising at least two rotors which act as propellers in aeroplane mode, and each of which is mounted so that it can tilt about a pivot axis on a respective fixed wing and connected by a respective transmission to a respective engine supported by the corresponding wing, an inter-connecting shaft linking the two transmission in order to drive the two rotors in rotation by any one of the two engines should the other engine fail.
U.S. Pat. No. 5,054,716 describes a convertible aircraft of this type, on which each of the rotors, together with its control means, the corresponding engine and the corresponding transmission, constitutes a tilting unit housed in a pod or nacelle, with the exception of the blades and hub of the rotor, said pod or nacelle being pivotally mounted overhanging the end of a respective fixed wing.
This type of architecture has a number of disadvantages. In particular, not only must each engine be capable of operating in a substantially horizontal position (aeroplane mode), it must also be capable of being started, operated and halted in a substantially vertical position and ensure reliable operation when switching between aeroplane and helicopter modes (whilst the rotors are being tilted). Furthermore, all the cabling, flight control system, hydraulic and fuel lines as well as the inter-connecting shaft must be run through the pivot pin, which has to be supported by means of pivot bearings so that it overhangs at the tip of the wing, which bearings thus support a heavy load, and the design and operation of which are critical. Since the engines are generally turbo-engine units, the jets of gas discharged from their nozzle assist a lift thrust in helicopter mode but have the disadvantage of damaging the surface of the landing and takeoff areas. Furthermore, the fact that each fully pivoting pod needs to be so tightly packed makes it difficult to isolate vital components in this pod to prevent the risk of fire from the engine housed in this pod and the fact that the engine pivots with the pod increases the size of critical zones of the aircraft (surface area of the fuselage and tail) at risk from potential explosion of the rotating components of the turbo-engine.
Finally and above all, a design in which the pods tilt allows no freedom in terms of the position of the inter-connecting shaft, it being crucial for the power takeoffs thereof connecting the ends of this shaft to the transmissions in the tilting pods to be coaxial with the pivot axis. As a result of this architectural constraint, the pivot axis of each pod is positioned towards the rear of the aerodynamic profile of the corresponding wing, i.e. on the chord of the profile at a distance of approximately 55% to approximately 70% of the value of this chord starting from the leading edge of the wing and towards its trailing edge. As a result, the aerodynamic centre of the wings is then too far forward relative to the vertical and transverse plane passing through the centre of gravity of the aircraft in aeroplane mode, which must be compensated by adopting fixed, forward-swept wings (dipping towards the front) in order to move the aerodynamic centre farther back.
It is well known that forward-swept wings are a factor which increases the mass and cost of the fixed wing system and are also not conducive in aerodynamic terms, in particular with regard to the joint between the wings and fuselage, which also makes the inter-connecting shaft complex in structure and expensive.
In practice, in order to adapt to the forward-swept wings, the inter-connecting shaft is a segmented shaft, the structural segments of which are connected by couplings which allow for misalignments.
Another architecture has been proposed as a means of overcoming some of the disadvantages of convertible aircraft having rotors which tilt with their transmission gears and driving engine, in which each of the two transmission gears, mounted between a rotor and the corresponding engine, has a pivoting reduction gear unit linked to a non-pivoting reduction gear unit, whereby the pivoting reduction gear unit drives the corresponding rotor and is mounted so as to pivot with this rotor relative to the non-pivoting reduction gear unit, connected to the inter-connecting shaft and to the corresponding engine and fixed with this shaft and this engine on the corresponding fixed wing.
However, all the designs proposed using this architecture, where tilting rotors linked to engines fixed on the wings by kinematic chains which fold in, have forward-swept wings and as a result have the disadvantages outlined above, in other words the negative effects caused by the structural features on the one hand and by the aerodynamic and aero-elastic features on the other.
The structural aspects are essentially those relating to the complex structure of a forward-swept wing as well as the fuel tank it contains and the complex structure by which the wing is joined to the fuselage.
The aerodynamic and aero-elastic aspects relate to the fact that the aerodynamic centre of a forward-swept wing is shifted forward at high angles of incidence in flight, which tends to destabilise the aircraft on the one hand and, on the other, the fact that the forward sweep tends to amplify what is referred to as “stall flutter”, which may mean increasing the rigidity of the fixed wing system on certain aircraft, thereby increasing the mass.
The objective of the invention is to propose a convertible aircraft with tilting rotors, of the type outlined above, in which each transmission has a reduction gear unit which pivots with the corresponding rotor and is connected to a non-pivoting reduction gear rotor, in turn linked to the inter-connecting shaft as well as to the corresponding engine fixed to the corresponding wing, which does not have the disadvantages described above and, more generally, which is better suited to various practical requirements.
To this end, the convertible aircraft with tilting rotors proposed by the invention is characterised in that the fixed wings are wings with a zero sweep, on each of which the inter-connecting shaft is substantially rectilinear and substantially parallel with the pivot axis but offset from said pivot axis substantially perpendicular to a longitudinal axis of symmetry of the aircraft, the centre of gravity of which, in helicopter mode, is substantially contained in a vertical plane passing through the pivot axis whereas in aeroplane mode, the centre of gravity of the aircraft is substantially contained in or in proximity to another vertical plane passing through the straight line of the aerodynamic centres of the zero-sweep wings.
Since the wings have a zero sweep and have mounted on them the engines driving the rotors which pivot on these wings, significant gains are achieved in terms of mass and cost because of the simplified structure of the wings, the fuel tanks which they contain and their connection to the fuselage, as well as a reduction in the effects of “stall flutter” and better flight behaviour at high angles compared with the forward-swept wing designs previously proposed. Furthermore, the balances of the aircraft are more efficiently controlled, given the possibility of using wings with a zero sweep relative to fixed pods housing the engines fixed on these wings, which complies with the requisite balance condition in aeroplane mode, namely that the centre of gravity of the aircraft in aeroplane mode is substantially in or in immediate proximity to a vertical plane passing through the line o
Balayn Frederic
Magre Eric
Best Christian M.
Eurocopter
Jordan Charles T.
Piper Marbury Rudnick & Wolfe
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