Aeronautics and astronautics – Aircraft – heavier-than-air – Helicopter or auto-rotating wing sustained – i.e. – gyroplanes
Reexamination Certificate
2003-03-20
2004-03-30
Swiatek, Robert P. (Department: 3643)
Aeronautics and astronautics
Aircraft, heavier-than-air
Helicopter or auto-rotating wing sustained, i.e., gyroplanes
C244S00700B, C244S039000, C416S151000
Reexamination Certificate
active
06712313
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a rotary-wing aircraft rotor with constant velocity drive, in particular for a convertible aircraft supporting two generally fixed wings and at least one tilting rotor.
Although the constant velocity drive rotor of the invention can be used as a helicopter rotor, in particular as a tail or anti-torque rotor, a particularly advantageous application of such a constant velocity drive rotor consists in fitting it to convertible aircraft with tilting rotors, particularly of the type known and described in FR 2 791 319, FR 2 791 634 and FR 2 798 359, to which reference may advantageously be made for further details.
Briefly, such a convertible aircraft with tilting rotors generally comprises, as shown schematically in
FIG. 1
, a fuselage
1
, of the aircraft fuselage type, supporting two fixed wings
2
, for example high wings, raised with respect to the fuselage
1
, each wing
2
itself supporting a power pod
3
, housing a power unit driving in rotation a rotor
4
, represented schematically by the plane of rotation of the rotor blades, via a transmission (not shown in FIG.
1
), a rear reduction gear unit of which is directly driven by the power unit and housed like the latter in the stationary rear part
5
of the power pod
3
. The front part
6
of the power pod
3
, housing a rotor mast and a rotor hub, as well as a front reduction gear unit driving the rotor mast in rotation, is mounted so as to pivot with the rotor
4
, so that it can pivot from an aeroplane configuration, in which the rotor
4
operates as a propeller at the front of an engine pod
5
-
6
facing into the relative wind, to a helicopter configuration, in which the rotor
4
′ operates as a helicopter main lifting rotor at the upper end of the front pivoting part of the pod in the upright position
6
′ above the corresponding wing
2
, this latter configuration being used for take-off and landing of the convertible aircraft which, after transition from the helicopter configuration to the aeroplane configuration, is able to move in forward flight like an aircraft.
As a variant, the pods
3
may pivot totally with the rotor
4
with respect to the fixed wings
2
.
BACKGROUND TO THE INVENTION
On rotary-wing aircraft rotors, it is known that, since the introduction of the flapping hinge on autogyro and helicopter rotors, tilting the rotor where coning is present, whether this tilting is desired and generated by controlling the cyclic pitch or the unwanted result of the asymmetry between an advancing blade and a retreating blade, causes stresses in the drive plane of the blades which tend to cause the blades to accelerate and decelerate in the course of a revolution of the rotor. These variations in speed are caused by Coriolis forces, and may be illustrated simply by the fact that the trajectory of the blade tips, viewed in a plane perpendicular to the drive axis, is an eccentric ellipse, the angular rate of travel of which is constant and, consequently, the peripheral speed of which varies over a revolution. This acceleration and deceleration of the blades over a revolution of rotation has a disastrous effect on the lives of the rotor components, due to the fact that these variations in speed generate stresses which are all the more substantial as the rigidity of the rotor components is high.
Conversely, it is known that great flexibility about the drag axis of the blades has a highly beneficial effect on the dynamic stresses to which the blades and the components of the rotor hub are subjected, which is why the introduction of the flapping hinge has been accompanied by the introduction of the drag hinge.
These improvements to the original rotary-wing aircraft rotor concepts have led to a rotor fully articulated in pitch, flapping and drag, the main disadvantage of which was to be subject to instability of the ground resonance or air resonance type, which made it necessary to develop and use drag dampers, also known as frequency adapters, or again elastic return drag struts with built-in damping. On helicopter rotors, these drag dampers are arranged in the plane of rotation of the rotor, between the blades and the hub of the rotor in a conventional configuration, or between adjacent blades of the rotor in the inter-blade configuration. In both cases, the presence of the drag dampers increases the aerodynamic drag of the rotor, in particular at the hub and where the hub is connected to the blades, which reduces the overall performance of the helicopter.
On a convertible aircraft of the tilting rotor type presented above, in which the speed of travel in the aeroplane mode is far higher than that of a helicopter, and on which drag dampers, mounted as on a helicopter rotor, would be head on to the wind, this reduction in performance would be far more appreciable, which is why designers of convertible aircraft of this type have endeavoured, for the design of the rotors, to retain hubs which are extremely rigid in drag (known as stiff-in-plane rotors), with no drag dampers, the natural drag frequency of which is greater than the nominal frequency of rotation of the rotor, which eliminates any risk of instability in drag, even in the absence of drag dampers.
However, it is known that rotors which are rigid in drag have the major disadvantage of generating very high stresses when the rotors are tilted. On convertible aircraft, the importance attached to producing rotors of high aerodynamic efficiency, and therefore with no drag dampers, has led to the development of hubs which are not sensitive to Coriolis forces. A particular feature of these hubs, which include hubs with a universal joint drive, is that tilting of the rotor is accompanied by tilting of the drive axis of the latter. Because of this, the rotor drive axis is always perpendicular to the rotor plane, and the trajectory described by the blades always remains a circle in a plane perpendicular to the drive axis of the rotor. This type of drive has been used, for example, on prototype convertible aircraft, particularly the XV15 aircraft.
However, a known particular feature of universal joints is that they are not of the constant velocity type, which manifests itself by the fact that the output speed of these joints is not always equal to the input speed. This speed distortion occurs when the drive and output axes are not co-linear, i.e. in the application considered to driving a rotor in rotation, when cyclic flapping is present. In the simplest configuration of a universal joint, the latter comprises a spider, the joints of which, by one arm of the spider to a driving shaft and by the other arm of the spider to a driven shaft, allow the driven or output shaft to swivel relative to the driving or input shaft. It is known that these speed variations caused by such a universal joint, and transmitted to the driven shaft, correspond to acceleration and deceleration which, over one revolution of rotation of the universal joint, appear twice. The speed of the driven shaft is therefore not constant, but varies at a frequency equal to twice the frequency of rotation of the shafts.
To eliminate these speed variations, which are responsible for very substantial inertial forces, in the case of a rotary-wing aircraft rotor, which affect the hub as a whole and are prejudicial to the durability of the mechanical assemblies constituting the hub or associated with the latter, several constant velocity drive systems have been proposed, particularly so-called Clemens drive links, composed of assemblies of two branches hinged respectively to the driving and driven shafts and connected by a swivel, and also tripod joints, for which transmission of movement is provided by means of balls moving in axial grooves machined in the driving and driven shafts.
These arrangements are used to ensure that the drive point is always situated in a plane bisecting the axes of the driving and driven shafts. As the distances from this point to the axes of the two shafts are then identical, the speeds of rotation of the two shafts are st
Cornille Eric
Zoppitelli Elio
Eurocopter
Sturm & Fix LLP
Swiatek Robert P.
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