1 Title of invention

Elastomeric Restrainer for Limiting the Flapping of Main Rotor Blade of Rotorcraft.

2 Field of invention

The present invention belongs to mechanical systems in general and in particular relates to a flap restrainer system of a rotor blade for an articulated main rotor system of a rotorcraft to prevent excessive out of plane displacement in upward direction.

3 Background of Invention

In rotorcrafts, rotor system consists of plurality of rotor blades, which generates the required lift, thrust and control forces during flight. There are different types of rotor system in rotorcrafts and mainly classified as rigid, semi-rigid and articulated type which are based on the method of blade attachment to the hub and the way in which the blade attachment hinges are designed. In articulated type of rotor system, the hinge stiffness is very low compared to a semi-rigid/ rigid rotor system such as hingeless and bearingless rotors where the hinges are provided in the rotor blade itself, by tailoring the stiffness of composite beam at the blade root, which is known as virtual hinge. Whereas the articulated rotors with elastomeric bearings have inherent low hinge stiffness due to the elastomeric bearing configuration, hence the blades are subject to downward displacement due to its self weight during the rotor at rest or at low speed which is well known as blade drooping. Hence suitable device is required to keep the blade in a horizontal plane in the absence of centrifugal force or low.

Similarly the upward motion of the blade under gusty winds to be restricted to a minimum angle, otherwise the blades may flap high and fall on droop stopper and damage it and associated parts, also the blade may interfere with rotorcraft structure and causes damage to the blade as well as the structural parts at the time of start or stop of the rotor during which the centrifugal force is low and insufficient to keep the blades in the plane of rotation, hence a flap restrainer is provided in the articulated rotor system in
addition to droop stopper, further the flap restrainer to be retracted during flight to allow the necessary blade dynamic flapping, which are well known from the following prior arts.

US patent No. 2,719,593 titled Combined Anti-Flapping and Droop Stop by Ralph P. Alex teaches about a combined anti-flapping and droop stop which use a centrifugally operated latching member with a tension spring, for each blade, to limit the flapping motion of the blades with respect to the horizontal plane of rotation. US patent No: 5,007,799 titled Flapping Restrainer Device for Rotorcraft Rotor Blades and Rotor Head Comprising said Device invented by Rene L. Mouille and Robert J, Suzzi, explains about a droop stopper comprising a reciprocal ring and upper restrainer for each blade comprises of a stop operated by the governor weight which is connected using a lever pivoted in the hub. US patent No.5,588,801 titled Flap Stops Device with Retractable Upper Stops Ring, and Rotor Head Including it, by Sylvie J.Commelin and Jean J. Mondet is an improved art compared to the previous one and it describes the device comprise anti-cone stops also known as flap stops which are projected outwards from the common ring that can rotate about the rotor axis having fly-weights to operate the flap stops from original position to retracted position and further mechanical springs are used to bring back the stopper to the original position when rotor speed reduces to the pre-defined value. Different configurations of flap stops with various fly-weight and spring arrangements are discussed in this embodiment. US patent No: 7,462,015 B2 titled Rotorcraft Rotor with Hinged Blades and Lockable Retractable Flapping Abutments is an another improvement over the previous art with an additional feature on flap abutment for locking the same to avoid untimely operation of flap restrainer as disclosed in this art. US patent No: 9,682,775 B2 titled Rotorcraft Rotor Including a Flapping Abutment Mechanism, and a Rotorcraft by Fabien Massal and Alain Eberhard describes a hook shaped restrainer for restraining the displacement of blades attached to the hub through spherical bearings.
However, it can be noticed that in all above inventions, mechanical spring and fly-mass are necessary to operate the flap restrainer. The spring with fly-mass assembly has the tendency to slip from its position, when abutment happens under the combined angle of flap, lead-lag (in-plane motion of blade) and pitching (feathering motion of blade).

4 Summary of Present Invention

The present invention describes about application of elastomeric element in the flap restrainer which avoids the requirement of mechanical spring and fly-mass for actuation of flap restrainer and hence better reliability of the system. The elastomeric flap restrainer also provides better impact characteristics which is inherent to elastomers, when blade abutment happens on the flap restrainer. The usage of elastomer reduces the impact force on the flap restrainer and hence the resulting bending moment in the rotor system parts particularly in the hub arm 4 which is one of the critical part in the rotor system. Also the elastomeric flap restrainer assembly avoids the slipping tendency of the flap restrainer under the combined motion (flap, lead-lag and pitching) of the blade during starting of rotor.

Further, to achieve better reliability of the flap restrainer device compared, one with mechanical spring and fly-mass as mentioned in the prior arts, it is thought of improve the design with incorporating elastomeric elements in the flap restrainer, since the elastomeric elements have better life compared to mechanical springs. Also, elastomeric flap restrainer prevent fly-mass slipping from its position due to its inherent stiffness and ensures the intended operations.

5 Brief Description of Drawings

Figure-1 is schematic layout of droop stopper 5 and flap restrainer outer ring 7 in ground mode i.e rotor is at rest or in low speed, at this condition, brackets 6 and 6a which are attached to hub arm 4 rests on droop stopper ring 5a due to self weight of blade which is attached to hub arm 4 and at this
condition flap restrainer is at extended position where flap stop projection 7a is closer to bracket 4a with a small clearance of 101.

Figure-2 is schematic layout showing the displacement of droop stopper ring 5a and resulting abutment of bracket 4a on flap restrainer projection 7a during a gusty wind impacts the blade at starting or stopping of rotor, during which the blade is restricted for a limited flaping angle of 102 in the upward idrection 102a.

Figure-3 is schematic layout of flap restrainer outer ring 7 in retracted condition, where abutment 7b come in front of bracket 4a of hub arm 4 during flight mode which allows blade coning up to flap angle of 103 in the upward direction 103a in which the gap between flap restrainer surface 7b and bracket 4a is maintained to be adequate such that blade 9 will not interfere with the flap restrainer outer ring 7 during flight.

Figure-4 is layout of rotor head 1 with elastomeric flap restrainer assembly 10 which is mounted concentric to rotor axis 1a passes through the center of rotor drive shaft 8.

Figure-5 is layout of elastomeric flap restrainer assembly 10 comprising elastomeric flap restrainer sub-assembly 14, electro-mechanical rotary actuator 11 and its mounting arrangement which consists of top plate 12, bracket 13, mounting base 15, bolts 16 ; it also shows the slip ring 17 which is used to power the electro-mechanical actuator 11.

Figure-6 is schematic layout of electro-mechanical rotary actuator 11 with external spline 11a on the actuator drive shaft.

Figure-7 is partial cut section of elastomeric flap stopper assembly 10a mounted on hub plate 2 which comprises the elastomeric flap restrainer sub-assembly 14 in which said electro-mechanical rotary actuator mounted in the internal bore 8a of the rotor drive shaft 8, to reduce the overall height of rotor system hence the rotorcraft, and also show the scheme for mounting the slip ring 21 to power electro-mechanical actuator 11.
Figure-8 is layout of actuation scheme to change the position of elastomeric flap restrainer 14a from ground mode to flight mode and vice-versa through a central arm 25 having a internal spline feature 25a which is engaged with spline 11a in the said electro-mechanical actuator 11 and having plurality of connecting rods 26 which connect the arms 25b extended from central arm 25 to outer ring 7 of the flap restrainer 14a.

Figure-9 is layout of elastomeric elements 30 in elastomeric flap restrainer 14a.

Figure-10a is initial position of elastomeric flap restrainer 14a in ground mode, in which elastomeric elements 30 are not torsionally loaded and the elastomeric flap restrainer 14a is in extended position.

Figure-1 Ob is enlarged view showing the orientation of elastomeric layers 31 of the elastomeric element 30 in the elastomeric flap restrainer 14a.

Figure-10c is displaced position of elastomeric flap restrainer 14a during retraction i.e flight mode, in which elastomeric elements 30 are torsionally loaded, hence they are displaced to an angle of 109 from initial angle of 108 as shown in FIGURE-10a.

6 Detailed Description of Invention

The embodiment described hereunder discloses about a flap restrainer system that control or limit the excessive out of plane displacement of the rotor blades to prevent the damage to the rotor system. This artifact also discloses the method of operation of the proposed rotor blade flap restrainer system.

The concept of droop stopper and flap restrainer are illustrated through Figures 1 to 3. With reference to Figure-1, rotor head 1 consists of a hub plate 2 where the plurality of elastomeric bearings 3 are mounted. The elastomeric bearing connects the rotor blade 9 (as shown in Figure-4) to hub 2 through hub arm 4. The blades are articulated about the center 3a of spherical elastomeric bearing 3 which act as a hinge. When the rotor is not
rotating or at low speed, the hub arm 4 which is connected to rotor blade 9 rests on droop stop ring 5a which is mounted in radially floating manner in the droop stopper 5. This droop stopper 5 keep the plurality of blades in a horizontal plane by resting them on this floating ring 5a through brackets 6, 6a and hub arm 4 in a well known manner.

In case of sudden gusty wind at the time of rotor starts rotating, the blades oscillate and hence the hub arm 4 which connected to the blade, flaps up and down, during this time one or two of the blade(s) pushes the droop stopper ring 5a towards rotor center through hub arm 4 and brackets 6 and 6a, and one or two blade(s) in the opposite side, moves the ring 5a outwards as shown in Figure-2, this causes the hub arm 4 to move upwards and get restricted at flap restrainer projection 7a. A small clearance 101 as shown in Figure-1 is maintained between flap restrainer 7a and bracket 4a which allows the blade flapping up to an angle of 102 as shown in Figure-2 which is welt within the allowed flap angle considering the blade interference with structure.

Figure-3 shows the retracted position, also known as flight mode, in such condition the outer ring 7 of flap restrainer during flight, activated by fly-mass arrangement by centrifugal force, which brings the flap restrainer surface 7b in front of bracket 4a by the rotation of flap restrainer outer ring 7 about axis of rotor la by which it provides sufficient clearance for blade coning angle of 103 which is the position of the blade during flight, under the action of resultant of centrifugaf force and lift. Retraction of the flap restrainer to ground mode i.e the initial position of the flap restrainer 7, is achieved through a mechanical spring in the reduced centrifugal force field of fly-mass while lowering the rotor speed which are well understood from the given prior arts.

With referecne to current invention, rotor head 1 is equipped with an elastomeric flap restrainer assembly 10 to limit blade flap displacement, considering the advantages over the flap restrainers with mechanical
springs. The details of the said invention and advantages are evident from the following description with reference to the relevant drawings.

As shown in Figure-4, the elastomeric flap restrainer assembly 10 is mounted on the hub plate 2 which is part of rotor head 1. The rotor head 1 is driven by rotor drive shaft 8. With reference to Figure-5, flap restrainer assembly 10 is having a rotary actuator 11 to actuate the flap restrainer sub-assembly 14 from ground mode to flight mode. The rotary actuator 11 as shown in Figure-6 is mounted on the top plate 12 by using mounting bolts 16. Top plate 12 is connected to the hub plate 2 through brackets 13 and mounting base 15, such that there is no relative motion between the hub plate 2 and top plate 12. The electro-mechanical rotary actuator 11 is powered by a slip ring 17 which is well known for transferring current from a non-rotating member to rotating member such as rotor system and vice-versa as shown in Figure-5.

With reference to Figure-5, Figure-6 and Figure-7; the rotary actuator 11; installion is not limited to mount on top of the rotor hub but it can also be installed inside the shaft as Figure-7 within thr scope of the invention proposed here in this artifact.

With reference to Figure-7, to optimize the height of the rotor system and hence the rotorcraft, the said electro-mechanical rotary actuator is mounted inside rotor drive shaft 8 by using inner housing 18 which is mounted on hub plate 2. Further to power the said electro-mechanical actuator 11, slip ring 17 with power cable 19 is mounted co-axially using support tube 20 and slip ring housing 21. The slip ring 17 is supported on a bearing 22 which is mounted inside the slip ring housing 21 and secured using a bearing lock cover 23 and this complete arrangement along with elastomeric flap restrainer sub-assembly 14 form the elastomeric flap restrainer assembly 10a. A hollow tube 24 with power cables, attached to a non-rotating member, inputs power to the slip ring as shown in Figure-7.
The externa! spline feature 11a given in the rotary actuator 11 as shown in FIGURE-6, drives the central arm 25 which is having a internal spline feature 25a as shown in Figure-8. From Figure-8, it is evident that the rotary motion of central arm 25 is transferred to flap restrainer outer ring 7 through connecting rod 26 , spacer 27 and shear bolts 28 and 29. This arrangement relieves the rotary actuator from lateral loads exerted during abutment of bracket 4a on flap restrainer projection 7a since the elastomeric elements are compressible, otherwise this radial load is transferred to electromechanical rotary actuator 11 and damages it, if arm 25b is connected directly to flap restrainer outer ring 7. By the said arrangement only torque is transferred from electro-mechanical rotary actuator 11 to outer ring 7 of the said flap restrainer 14a and radial loads are relieved by the swinging motion of connecting rod 26 in a horizontal plane during abutment. A spacer 27 is provided to maintain the connecting rod 26 in a horizontal plane, however to avoid bending moments in the connecting rod 26 due to misalignment, if any, spherical bearings are provided at the end of arm 25b and connecting rod end 26a in a known manner.

The elastomeric flap restrainer 14a for a five bladed articulated rotor head 1 is shown in Figure-9. However the application of the embodiment is not limited to five bladed rotor, the scope of the disclosure in this artifact but it is an example to illustrate the invention. Plurality of segmented elastomeric elements 30 are bonded between outer ring 7 and inner housing 18 as shown in Figure-9. The inner housing 18 is attached to hub 2 such that there is no relative motion between inner housing 18 and hub plate 2. When the blade abutment happens on the projection 7a of the outer ring 7, the impact load is transferred to the corresponding elastomeric element 30. Since the loads are reacted at the elastomeric element 30 which consists of elastomeric layers 30a as shown in Figure-10a and Figure-10b which have its inherent flexibility obtained by virtue of the rubber and steel shim combination in a well known manner, the reaction at the blade 9 and hence the hub arm 4 which is attached to the blade are largely relived from the
bending moments arise due to the restriction of motion and hence better life of the said components.

Figure-1 Oa and Figure-1 Oc show the un-deflected and deflected position of elastomeric element 30 in ground mode and flight mode respectively. The stiffness of the elastomeric elements 30 are tailored such that jt should have higher stiffness along radial direction (axis perpendicular to curvature 30a as shown in Figure-1 Ob), axial direction (along thickness 104 as shown in Figure-9) and cocking directions (out of plane rotation with reference to the plane passing through lines AA and BB as shown in Figure-10a), compared to the torsional stiffness about the axis perpendicular to the above mentioned plane and coinciding with rotor axis 1a. The advantage of tailoring the stiffness properties helps to design the elastomeric flap restrainer to achieve no slippage of flap restrainer untimely during the combined motion of flap, lead-lag and pitching of blade during starting of rotor. Also the better fatigue characteristics and maintenance free nature of the elastomeric elements made them a better candidate over the mechanical springs/ bearings which have less life and maintenance intensive compared to elastomeric springs/bearings.

Also, minimizing the torsional stiffness of the elastomeric elements 30 reduces the torque requirement of said electro-mechanical rotary actuator 9, The parameters which effect the torsional stiffness of elastomeric elements 30 are thickness 104 as shown in Figure-9, outer radius 105, inner radius 106, included angle 107 and number of elastomeric elements 30 as shown in Figure-10a and also shear modulus of elastomeric element 30. Hence the configuration of flap restrainer 14a is optimized for the said parameters to realize minimal torque requirement of said, electromechanical actuator 11 to achieve the optimum size and weight.

The flap restrainer 14a has two positions, an extended position is said as ground mode which is shown in Figure-1 and Figure-10a; and the other position is retracted one said as flight mode which is shown in Figure-3 and
Figure-1 Oc. Where flap restrainer abutment 7a rotated from its ground mode 108 to flight mode 109 about the rotor axis 1a, by an electro-mechanical rotary actuator 11. Initially the flap restrainer 14a is in ground mode 108 in which the flap restrainer is in extended position to prevent any unintentional flap movements of blade 9 due to gust, if any, until rotor speed reaches to a pre-defined Rotational Speed. Then the electro-mechanical rotary actuator is actuated to retract the flap restrainer to flight mode as shown in Figure-10c as soon as the rotor rotational speed crosses the pre-defined RPM. Thereafter, electro-mechanical actuator 11 is kept in locked condition to flight mode, in which the elastomeric flap restrainer 14a allows the blade coning upward without any interference with flap restrainer as it is desired by the rotorcraft flying condition. The elastomeric flap restrainer 14a is retained in flight mode for the entire flight duration and as long as the rotational speed of the rotor is above the pre-defined RPM. Since the actuator is kept in the locked position, even in the event of power loss, if any, it would not affect the position of elastomeric flap restrainer 14a to ensure the flight safety. The actuator is actuated again to ground mode, when the rotor RPM decreases to the pre-defined RPM, only after landing of rotorcraft.

The particulars disclosed above are representations of the invention, it is also possible that the invention may be modified and practiced in equivalent but different ways or arrangements by one who is skilled in the art with the help of the details described in this manuscript. Therefore, all such variations are considered within the scope and spirit of the invention. Accordingly, the protection is sought.