Aeronautics and astronautics – Aircraft – heavier-than-air – Helicopter or auto-rotating wing sustained – i.e. – gyroplanes
Reexamination Certificate
2003-02-03
2004-01-13
Carone, Michael J. (Department: 3641)
Aeronautics and astronautics
Aircraft, heavier-than-air
Helicopter or auto-rotating wing sustained, i.e., gyroplanes
C244S017130, C244S017250, C244S017270, C416S140000, C416S106000
Reexamination Certificate
active
06676074
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a drag damper for a rotary-wing aircraft rotor comprising a hub and at least two blades each of which is connected to the hub by a corresponding connection device. The drag damper is intended for fitting between two elements, one of which is a flapping mass comprising one of the blades and the corresponding connection device, and the other element is one of the hub (in the conventional configuration) and an adjacent flapping mass on the rotor (in the so-called inter-blade configuration). In use, the drag damper will dampen the relative angular drag movements between said two elements, namely: in the first case (conventional configuration), the angular drag movements of said flapping mass relative to the hub, i.e. the angular deflections of the blade, and more generally of the corresponding flapping mass, about its drag axis, which is substantially parallel to the axis of rotation of the rotor; and in the second case (inter-blade configuration), the relative angular drag movements between said two adjacent flapping masses, i.e. the relative angular deflections of the corresponding two adjacent blades of the rotor about their drag axes, which are substantially parallel to the axis of rotation of the rotor.
The rotor is more particularly a helicopter main rotor subject to the instability phenomena known as ground resonance and air resonance, although a conventional tail rotor may also be equipped with drag dampers according to the invention.
BACKGROUND TO THE INVENTION
On rotors of the hinged type, the device connecting a blade to the hub may be arranged as a means of securing the blade and hinging it to the hub, when the blade is connected by its root, possibly in the form of a fork, to the hub, or as a device which is substantially radial (relative to the rotor axis) generally termed a cuff, and fitted with yokes at the ends to be connected to the blade root on the one hand and on the other to means of securing and hinging, such as a spherical laminated stop, itself connecting it to the hub. On rotors of the semi-rigid type, this connecting device may be a flexible torsion arm, at the blade root, and surrounded by a torsionally rigid cuff integral with the blade root for controlling the blade in pitch, which is connected and hinged to the hub by this flexible torsion arm.
Numerous different embodiments of drag dampers are known, particularly dampers which are hydraulic, hydro-pneumatic, laminated with at least one layer of visco-elastic material stressed between two rigid fittings, or comprising combinations of these different means, these drag dampers comprising means of elastic return of defined stiffness and damping, when they are fitted to helicopter main rotors, to combat the resonance phenomena mentioned above.
It is a well-known practice to design helicopter rotor blades and therefore the corresponding flapping masses having a natural drag frequency, also termed first drag mode or natural drag mode, which is different from the nominal rotation frequency at which the rotor is designed to be driven.
More generally, to avoid in particular fatigue problems resulting from the dynamic stresses in the blades and the fuselage, and problems of vibration levels in the fuselage, it is essential to position correctly the natural frequencies of the blades in flapping, torsion and drag relative to the nominal rotation speed of the rotor and its harmonics (multiples).
This results from the fact that a helicopter rotor constitutes a powerful vibration generator. Because of the variable angles of incidence and speeds of rotor blades and also of helicopters, alternating loads of aerodynamic origin are developed particularly in the blades of rotors, and give rise in the latter to stresses as well as reactions on the attachments, particularly of the blades to the hubs. From this there result alternating loads and moments on the rotor heads, and the development of high vibration and stress levels in fuselages. The response of each blade, the stresses to which this blade is subjected and the loads which this blade transmits to the hub at the centre of the rotor are all the greater as at least one natural frequency of the blade (in drag, flapping and torsion) is close to the rotation frequency &OHgr; of the rotor or one of its harmonics n&OHgr; (where n is a whole number).
The dynamic characteristics of the rotor blades are therefore chosen to obtain suitable dynamic matching by ensuring that their natural vibration frequencies in flapping, drag and torsion are correctly positioned relative to the nominal rotation frequency &OHgr; of the rotor and its multiples n &OHgr;, which is why it is necessary to observe certain simple rules for positioning the frequencies, and in particular two essential rules. The first rule is to avoid positioning a natural vibration frequency in flapping, drag or torsion on or very close to a harmonic of the rotation speed n&OHgr; (where n≧1), and the second rule is to endeavour as far as possible to position only one of these three natural frequencies between two adjacent harmonics n&OHgr; and (n+1) &OHgr; of the rotation speed in order to avoid coupling. In addition to these two essential rules, it is imperative to follow recommendations proper to each type of deformation in flapping, drag or torsion.
Concerning the recommendations relating particularly to the drag is modes for hinged or semi-rigid (semi-hinged) rotors, the first drag mode (or natural drag frequency) is at the origin of ground resonance and air resonance problems due to coupling with modes of the helicopter structure.
On a rotor with blades hinged in drag, the angular frequency or pulsatance of the first drag mode is given by the expression:
ω
δ
=
Ω
(
e
·
M
δ
I
δ
)
1
/
2
where e is the drag eccentricity of each blade, M&dgr; is the static moment of the flapping mass (blade+device connecting it to the hub) relative to the hinge (drag axis) and I&dgr; is the inertia of the flapping mass relative to this drag hinge.
On a semi-rigid rotor, the first drag mode of a blade or flapping mass depends on the characteristics not only of the blade or flapping mass but also of the hub. The pulsatance of the first drag mode is then given by the expression:
ω
δ
=
Ω
(
e
·
M
δ
I
δ
+
k
δ
Ω
2
I
δ
)
1
/
2
where k&dgr; is the stiffness of the drag damper fitted between the blade or corresponding flapping mass and the hub of the rotor.
The positioning of the first drag mode of a blade or of the corresponding flapping mass depends upon the modes of the helicopter structure on the ground (fuselage mass, inertia, stiffness of the landing gear and of any tyres which may be fitted to it), these modes of the structure being generally determined by specific tests, adjustment of the first drag mode being obtained by altering the term k&dgr; representing the stiffness of the drag damper.
As a general rule, as the upper limit of the first drag mode &ohgr;&dgr;, a value close to three-quarters of the nominal rotation frequency &OHgr; of the rotor is taken, so as not to introduce excessively high stresses in the blades of the rotors.
For this reason, when the rotor is started up or stopped, and also at the end of a landing by the helicopter in autorotation, the instantaneous speed of rotation of the rotor intersects the resonance drag frequency situated below the nominal speed. Because of this, and also because of the fairly large range of variations in rotor rotation speeds which are authorised for helicopters in flight, it is necessary to increase damping at the natural vibration frequencies of the blades in drag, and if necessary to reduce this natural frequency by means of drag dampers, which is the reason why these dampers are also termed frequency adapters, the aim being that the blades should be sufficiently damped in
Legendre Philippe
Zoppitelli Elio
Carone Michael J.
Eurocopter
Semunegus L.
Sturm & Fix LLP
LandOfFree
Dual piston drag damper for rotary-wing aircraft rotor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dual piston drag damper for rotary-wing aircraft rotor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dual piston drag damper for rotary-wing aircraft rotor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3258078