The invention relates to a flap stops device for a main rotor of a rotorcraft such as a helicopter, and deals more particularly with a device with anti-cone stops or upper flap stops which can be retracted in flight, for a main rotor of the hinged type for any helicopter, and in particular for a helicopter of high tonnage, in which each main rotor is equipped with blades of high mass and large dimensions.
The flap stops device according to the invention is more specifically, although not exclusively, intended to equip heavy helicopters, in which the blades of each main rotor have to be able to be folded up, and employed in operating conditions causing them to encounter strong and gusting winds, as is the case with heavy helicopters on board surface ships, on the decks of which the blades of the main rotors have to be manoeuvred in terms of folding up and in terms of deployment, automatically or manually, while the ships are in motion.
In order to limit the angular flap or downwards bending of the flapping masses, each mainly composed of a blade, of a part for attaching the blade to the hub, such as a sleeve, and of various other components necessary for operation, such as pins and pitch levers, on hinged main rotors of helicopters under the effect of the self weight of the flapping masses, at low rotational speeds of the rotors and when the rotors are stationary, the rotor heads are generally equipped with flap stops known as "droop restrainer stops", and many different embodiments of droop restrainer stops devices have already been proposed.
In one embodiment which is well known and particularly popular owing to its advantages as regards the simplicity and low cost of the structure, and as regards the high degree of safety and reliability of operation, the device comprises a central droop restrainer stop common to all the blades and including a rigid so-called "droop restrainer" ring mounted so that it can move radially with respect to the rotor mast, and against which, when the rotor is stationary or at low rotational speeds of the rotor, the bearing tracks of rigid lower bearing shoes bear, each of which shoes is secured to a lower part of the root of a blade or of a member for linking the blade to the hub of the rotor.
When the rotor is turning on the ground at a speed above a given threshold, corresponding to a speed which is sufficient to allow the rotor to be controlled effectively using the flight controls or, in flight, for example when it is turning at its nominal speed, the laterally shiftable droop restrainer ring allows the disc of the rotor to be inclined using the cyclic pitch control, because the blade entering the lowest position in its rotation about the axis of the rotor pushes the droop restrainer ring back on the opposite side towards a blade entering the highest position, so that the droop restrainer ring can shift laterally without opposing the movements of the blades.
With the rotor halted, the droop restrainer ring also allows the blades to be controlled in terms of pitch in order to control the free excursion of the pitch control, because the control of a change in the pitch of the blades bearing on the droop restrainer ring via their lower bearing shoe causes the droop restrainer ring to be driven through a small rotation by the blades with slight gliding of the bearing surfaces.
However, a droop restrainer stops device with a droop restrainer ring as described for example with reference to FIGS. 3 and 5 of Patent FR 2,427,251exhibits a drawback when a gust of wind, for example, arrives during the critical phases of starting-up or stopping the rotor, when its rotational speed is below the aforementioned threshold, and thus when the centrifugal force is therefore not yet high enough to stabilize the blades in their plane of rotation. Under these conditions, what happens is that the gust of wind may push one of the blades hard downwards, so that it presses violently against the droop restrainer ring which, not being subjected to sufficient reaction forces from the other blades, assumes an extreme radially offset position in which it does not prevent the violently pushed-down blade from reaching a dangerous position of great inclination and possibly from striking or chopping off by its end the tail boom or cabin of the helicopter. Likewise, and still when the rotor is stationary or at low speed, a blade may be lifted by a strong gust of wind and assume a large cone angle, then fall back heavily onto the droop restrainer ring, damaging the latter and/or the lower bearing shoe of the blade, which itself is subjected to an instantaneous bending moment which is very much higher than the loads which it can normally withstand, and this may lead to it becoming unserviceable.
In general, starting from the position of equilibrium of the flapping masses, in which there is equilibrium between the static moments of the flapping masses and the reactional moments of contact of the droop restrainer stop tracks with the droop restrainer ring, which is free in the radial plane with respect to the axis of the rotor and ensures equilibrium of the whole by being subjected to forces of contact of all of the blades (which forces are opposed in pairs in a four-bladed rotor), if, for whatever reason, one of the flapping masses pivots upwards, equilibrium is lost. The droop restrainer ring which is unloaded on one side is driven onto this side by the opposite flapping mass which, thus freed, droops downwards, pivoting about its axis of flap. This downwards movement of one blade puts the crew, and the helicopter, and nearby equipment in great danger.
In addition to the wind, the reasons which may cause the equilibrium of the flapping masses to be lost, are circumstances which induce forces on the flapping masses, such as movements of the ship and/or the handling of the helicopter and/or the folding-up or deploying of the blades. When a blade is folded from the front towards the back of the helicopter, equilibrium is lost when the centre of gravity of the corresponding flapping mass changes to that side of the flap axis of this blade where the rotor axis is located, which reverses the direction of its static moment, and therefore causes the flapping masses to lose equilibrium. The blade opposite the folded-up blade bends rapidly downwards until it is halted by a flight flap stop, which was not designed for this. During deployment of a blade, the loss of equilibrium of the flapping masses causes a tilting which is the opposite of the aforementioned one and just as dangerous.
In order to overcome this drawback, without abandoning the use of a droop restrainer stop with droop restrainer ring, Patents FR 2,434,079 and FR 2,636,914 propose a flap stops device combining droop restrainer stops with droop restrainer ring and lower bearing shoes for the blades, and upper stops limiting the upward angular excursions of the blades. In such a device, when a blade bends downwards, when the rotor is stationary or turning at low speed, the lower bearing shoe of this blade radially pushes against the droop restrainer ring which itself pushes against the lower bearing shoe of at least one of the other blades of the rotor, in the raising direction. However, this raising is limited by the upper flap stops which, when the rotor is stationary or turning at low speed, are still in the working position. The droop restrainer stops or lower stops therefore cooperate with the upper stops in such a way as to reduce the risk of damage to the rotor.
In FR 2,434,079, the anti-cone stops comprise, for each blade, on the one hand an upper bearing shoe secured to the part for attaching the blade to the hub and, on the other hand, an upper stop carried by the hub and which can be retracted or moved out of the way by the action of centrifugal force to which it is subjected when the rotor is turning with a rotational speed above the given threshold and sufficient to allow the rotor to be controlled by the flight controls of the helicopter. This upper stop is a cranked lever pivoting about a pin parallel to the axis of the rotor and acted upon by a return spring so that in the "ground" position, the spring keeps the lever such that its first arm, constituting the bearing stop proper, is in a position for halting the upper bearing shoe of the corresponding blade and so that its second arm, carrying a flyweight sensitive to the action of centrifugal force in order to cause the lever to pivot against the return spring, bears, over part of its length, against a ring supporting the pivot of the crank lever. The two arms of the latter form an angle such that the centrifugal force applied to the flyweight when the rotor is turning tends to overcome the return force of the spring and to cause the lever to pivot in a direction such that its first arm is moved away from the upper bearing shoe facing it on the corresponding blade.
In FR 2,636,914, the anti-cone stops also include, for each blade of the rotor, an upper bearing member secured to the part for attaching the blade to the hub, and an independent upper stop which can be retracted automatically by centrifugal force, comprising a lever mounted so that it can tilt on a pivot secured to the hub, and a first arm of which is shaped into a stop finger, while a second arm carries a flyweight and exhibits a bearing part intended to come against a bearing surface secured to the hub, as well as a return spring returning the lever towards a "ground" position in which the bearing part of the second arm of the lever rests against the bearing surface of the hub and the stop finger of the lever is directed towards the corresponding upper bearing member, so as to limit the upwards excursions of the blade.
In these two known embodiments, the stops carried by the hub are individual and specific to each blade, so the device includes a large number of components. Furthermore, in FR 2,636,914, each pivoting anti-cone stop is mounted inside a sleeve or clevis for attaching the blade to the hub, and this poses problems of accessibility for maintenance.
In these two known embodiments, the devices are suitable for the main rotors of helicopters of low or medium tonnage, and exhibit structures with which there are associated risks of breakage of the anti-cone stops and risks of inadvertent retraction when they are mounted on the rotors of heavy helicopters, even if these embodiments remain compatible with the possibilities regarding bulk, complexity, and permissible mass in such rotors.
In particular, inadvertent retraction or breakage of an anti-cone stop, which may also be the origin of a loss of equilibrium of the flapping masses, may result not only from the abnormal forces mentioned hereinabove, via the flapping masses, but also from forces such as those transmitted by manoeuvring the stick during manual folding of a blade, or inertial loading on a retraction flyweight in the event of vertical impact, of the step impulse type, when, as in FR 2,636,914, the anti-cone stop pivots about a pin perpendicular to the axis of the rotor.