FIELD OF THE INVENTION

The present invention relates to propellers which can be folded, particularly but not exclusively for small hovering air vehicles (HAVs) or drones.

BACKGROUND ART

Market demand is increasing for hovering air vehicles (HAVs) that are small and light in weight. Many market applications are better served when an HAV is able to be packed for carriage,, and an HAV that is quick to stow and deploy is desirable, Nearly all HAVs have multiple propellers with two or more aerofoil blades; these propellers usually extend beyond the airframe and limit the reduction in volume of a packed HAV. One approach used to address this is to provide a propeller with blades rigidly fixed to the hub but which can be removed from the motor shaft for packing. However, this approach has disadvantages, in that propeller removal and replacement process takes time, and propellers may be damaged during removal or replacement and during transit.

Folding propellers are becoming more popular for such applications. Most folding propeller designs have a simple hinge at the end of each blade, where it Is joined to the propeller hub. On rotating the propeller hub with a motor, the inertia of the blade causes it to fly out under centripetal force to its operating, or deployed, position, which is at an approximately normal orientation to the hub. Folding propellers with simple hinges are prone to vibration in use. In a common form of vibration, the blades lead and lag relative to the norma! position on each rotation (lead-lag vibration), Lead-lag vibration often occurs in conjunction with vibration in the airframe on which the motor is mounted, particularly if the motor is on the end of an arm. Lead-lag vibration has many negative effects including blur in the imagery produced by a camera carried by the airframe, loss of propulsion efficiency, reduction in airframe life and audible noise.

A device can be included that locks the propellers in the open position for flight such that the propeller becomes fixed and cannot exhibit lead-lag vibration, A locking device takes time to deactivate and reactivate when folding and unfolding, propellers. Some locking devices activate automatically when the blades are manually straightened, but requires manual release for stowage. The locking device adds weight and increases manufacturing cost. For example, US 2016/0001879 sets out an arrangement in which individual propeller blades were fixable in either a deployed position or a folded position; in order to move the blades between these positions a spring-biased pin aligned parallel to the axis of rotation of the propeller has to be depressed and the individual biades moved from one position to the other. On reiease of the pin, the spring pushes the pin back into engagement with the edge of the blade, which is provided with locking cutouts which engage with the pin to hold the blade In position. Such an arrangement takes time to operate, particularly if the HAV has several separate propellers, and it can be awkward to arrange the blades simultaneously in the desired positions whilst simultaneously depressmg the pin, and it is also difficult to ensure that the blades do not move out of position when the pin is released.

To reduce the size of HAVs, particularly with multi-rotor HAVs such as quadrotors, in some designs, moveabie-arm airframes are used in conjunction with folding propellers. Moveable arm airframes are usually heavier than the equivalent fixed arm airframes due to the additional elements that permit arm movement. Moveable arm airframes are often less stiff than the equivalent fixed arm airframes and more prone to vibration.

The demand for HAVs that are lighter to carry and have longer flight endurance means that stiffening the airframe with the additional weight incurred therefrom is disadvantageous. HAVs often receive knocks on landing or in collisions. Such knocks often put a propeller out of balance by distortion or damage, An out of balance propeller is more likely to initiate lead-lag vibration,

Lead-Sag vibration of folding propellers with simple blade hinges is particularly common on smaller air vehicles which have relatively long propellers with low disk loading that benefit from higher aerodynamic propulsion efficiency to deliver longer endurance from the on~board stored energy.

There remains a need for small HAVs (of less than 250g mass) that can be packaged in as small a volume as possible and which can be deployed in the minimum amount of time when required. Such an HAV with an on-board camera may be carried by a single soldier on a patrol and used when needed to provide "flying binoculars" for the soldier to see around a corner or over a hill

SUMMARY OF THE INVENTION

The present invention therefore provides a propeller assembly comprising a propeller hub which can be driven about an axis and at least two propeller blades each mounted to the hub at a point distal from the axis by pivoting means allowing the propeller blades to pivot generally parallel to the said axis between at least: two positions relative to the hub, a first, folded position in which the propeller blade Is not aligned with the axis and the point and a second, deployed position in which the propeller blade is aligned with the axis and the point, in which the pivoting means is configured to allow the propeller blades to pivot freely relative to the hub when in the first position and, on driving the hub about the axis, to allow the propeller blades to pivot relative to the hub until the or each propeller blade is aligned with the axis and the point, the pivoting means comprising an elongate key pin of non- circular cross-section, the key pin in use having its length substantially parallel to the axis, and the pivoting means being adapted to hold the or each propeller blade in the second position for as long as the hub is driven.

The present invention is therefore predicated on allowing the inertia of the blade when the hub is driven to urge the blade from a folded position to a deployed position, whereupon a mechanism holds the propeller blade in the deployed position at least while the hub is driven. The mechanism may be one which relies solely on the inertia! effect arising from the continued rotation of the hub, so that the application of centripetal forces to constrain the blade to rotate holds the propeller blade in the deployed position movement, or there may be some locking mechanism which positively locks the propeller blade in the deployed position, such as a hi-stabte or over-centre mechanism, for example. Arrangements in accordance with the invention provide a propeller assembly which, without any additional manual operations, combines a fixed, seated propeller characteristic when spinning to create thrust with a folding characteristic when being stowed or deployed. This means that such propeller assemblies are preferable to a folding propeller with a locking device, and the absence of lead-lag vibration permits a lighter weight airframe design and a greater tolerance to propellers that have lost their balance. Furthermore, in an impact, a blade of the propeller can temporarily fold and this extra flexibility reduces the damage to the biade and increases the chance of the HAV staying aloft.

The pivoting means comprises an elongate key pin of non-circular cross-section, preferably hollow, where in use the key pin has its length parallel to the axis about which the propeller assembly rotates. This is a simple and inexpensive item to manufacture, and one which does not add significant!y to the weight of the assembly.

The hub may have a hub socket and the propeller biade a blade socket whereby the propeller blade is mounted to the hub via the key pin, one of the hub and blade sockets being a close fit around the cross-section of the key pin, and in which the other of the hub and blade sockets is a loose fit around the cross-section of the key pin. This makes the propeller assembly easy to put together, simply by inserting the pin into the tight socket, whilst also passing the key pin through the other socket, thus "capturing" It, yet leaving the loose socket free to rotate about the key pin (or the key pin to rotate In the loose socket).

The cross-section of the key pin may be generally circular, but with at least one section which deviates from the circular, the socket which is a loose fit around the key pin being generally circular but with at least one section which deviates from the circular in complementary configuration to the deviating section(s) of the key pin. The cross-section of the key pin may be symmetrical about an axis perpendicular to the elongate axis of the key pin; this means that the pin can be inserted into the sockets in either direction, which makes the arrangement more flexible to assemble. The pin may be generally circular with a protruding lobe, and the iobe may be generally trapezoidal in shape, the iegs of the trapezoid extending tangentially from the generally circular part of the cross-section of the key pin. The base of the trapezoid which is outermost relative to the key pin may be parallel to a tangent to the opposite side of the cross-sectional part of the key pin. The key pins may be formed integrally with the hub or with the propeller blades and joined thereto by an integrally-formed flexible strap, This allows the elements forming the propeller assembly to be manufactured (for example by 3D printing) in only 3 separate pieces, with each key pin connected to either the hub or to a blade and being protected from being detached or getting lost because they are attached to the larger elements _*

Additionally or alternatively the propeller assembly may comprise at least one pair of propeller biades which in use are on opposite sides of the hub, in which each propeller blade in a pair is connected to a magnet adjacent to the axis, and in which the magnets in each pair of propeller biades are arranged such that their field directions relative to the axis are of opposite polarity. This provides a positive magnetic locking means. The magnets are preferably disposed relative to the propeller blades such that in the first folded position the magnets are located further apart compared to when the propeller blades are in the second, deployed position, Such an arrangement allows the blades to move automatically to the second, deployed position on rotation of the propeller assembly, and it is a simple task to overcome the attraction between the magnets to fold the blades into the first position when required.

Further in addition or as a further alternative the propeller assembly may comprise an over-centre spring mechanism acting on the propeller blades and the hub, such that the first and second positions are stable, equilibrium positions and such that mechanical work has to be done to move the propeller blades between the first and second positions. The arrangement may be such that the work required to move between the first and second positions is less than that required to move between the second and first positions, so that rotational forces alone are sufficient to unfold the blades whereas folding the blades requires operator effort, or the arrangement can be such that operator input is required to move between positions. Suitably the over-centre mechanism may comprise a protrusion and a complementary recess which the protrusion can engage with, the protrusion and the recess being associated with a propeller blade and the propeller hub such that pivotal movement of the blade relative to the hub causes relative movement between the protrusion and the recess, and so that when the biades are in the second, deployed position the protrusion is engaged with the recess.

The invention extends to an air vehicle comprising a propelier assembly as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example and with reference to the accompanying figures, in which; Figures la, lb and lc are schematic plan views of a conventional rotary prapeiier assembly, Figure la shows the assembly with the propeller blades in a deployed position, Figure ib shows the propeiier blades folded, and Figure lc shows the depioyed blades and illustrates lead/lag vibration;

Figures 2a to 2c are schematic views of an embodiment of a propeiier assembly in accordance with the invention, Figure 2a is a perspective, exploded view, Figure 2b is a pian view and Figure 2c is a perspective view of the key pin forming part of the assembly;

Figure 3a is a schematic, cross-sectional view of a socket and Figure 3b is a schematic, cross-sectional view of the cross-section of the key pin of Figure 2c;

Figure 4a is a schematic, cross-sectional view of the socket and key pin of Figures 3a and 3b with the propeiier biades in the deployed, or extended position, and Figure 4b is a view with the blades in a folded position;

Figures 5a to Sc are views of a folding propeller assembly with a first mechanism for locking the propeiier blades in a depioyed position;

Figures 6a and 6b are views of a folding propeiier assembly with a second mechanism for locking the propeiier biades in a deployed position, and

Figures 7a and 7b are perspective views of an air vehicle incorporating a propelier assembly in accordance with the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure la shows a prior art propeiier assembly 100 in a deployed or extended position, Two propeller blades 102 are mounted onto a hub 101 and located with two pins 103 such that the blades can rotate freely around the pins, like a hinge, The two blades 102 are shown in an extended orientation, namely the orientation in which they provide thrust when being driven through the hub. The propeller assembly 100 is attached via the hub

101 to a mechanical driving element (not shown) which rotates the hub 101 and the blades

102 so as to generate thrust and/or lift. The mechanical driving element is usually a motor attached to a drive shaft (not shown) which drives the hub 101 around its central axis 104.

There are alternative ways of attaching a hub 101 to a mechanical driving element, such as push-fit, keying, a threaded nut, an elastic retainer and a bayonet. In direct drive implementations, the drive shaft is also the motor output shaft. In indirect drive implementations, there is gearing or one or a series of other power transfer mechanisms such as a drive belt which may or may not be geared. The different ways of driving a propeiier are well known to those skilled in the art,

Figure lb is a plan view of the propeller assembly 100 in the folded position. The two blades 102 are shown in a folded orientation that is the orientation in which they best arrange with the motor, airframe and any packaging such as a case. When in the folded orientation, the two blades 102 are in a near-parallel arrangement, and the blades can adopt any intermediate position when moving between the open and closed positions.

An HAV has one or more propeller assemblies 100, A quadrotor HAV, for example, has four propeiier assemblies 100, each with at least two propeller blades 102 (although it is usual to have two blades to a propeller assembly, there may be three, four or more, provided that the blades are spaced equally around the circumference of the assembly). In the stowed arrangement, the propellers 102 are folded. During deployment, there is no need for the user to spend time manually straightening each propeiier 102. When each propeller assembly 100 is spun-up through torque applied at the hub 101, the two blades 102 automatically extend outwards under centripetal acceleration of their inertia until they reach the extended position shown in Figure la. Figure lc is a plan view of a snapshot of a spinning propeller assembiy 100 with the simple hinging pins 103 illustrating the problem of lead-lag vibration (vibration in the plane of rotation). The propeller assembly 100 has two blades 102a, 102b. if photographs are taken at different times from above a spinning propeller assembiy 100 undergoing lead-lag vibration, then most of the photos will show the two blades misaligned and one of the photos is likely to resemble the lead-lag orientation shown in the drawing. In Figure lc, the DRIVE and rotation of the propeller 200 is anti-clockwise from the plan view, The blade 102a is exhibiting Lead in the drawing, and is angled ahead of the hub through its hinge 103; the blade 102b is exhibiting Lag and is angled behind the hub through its hinge 103. This misalignment of the blades causes vibration, which increases loadings on the hub and absorbs power from the means driving the propeller assembly. There are other modes of undesirable blade vibration. In one other mode, just one blade vibrates clockwise and counter-clockwise about its hinge. In another mode, both blades lead then both blades lag, giving a zig-zag shape, Figure 2a is an isometric view of a partially assembled central section of one embodiment of a propeller assembiy 200 in accordance with the present invention, Each propeller blade 202 is assembled to the hub 201 with a key pin 203, At both ends, the hub 201 has two parallel jaws 207. The inner end of the blade 202 has a tab 208 that is sized to slip between the jaws 207 of the hub 201 with negligible play and low friction. The jaws 207 have key-ways 212.

Figure 2b is a bottom view of a partially assembled centra! section of the propeller assembiy 200. The hub 201 has two key-ways 212 through the jaws 207 at either end of the hub 201. The tab 208 of the blade 202 has a keyhole 211.

Figure 2c is an isometric view of the key pin 203; it has an elongate shank 210 with the same outer cross-sectional shape as the key-way 212, the pin is arranged in the propeller assembiy such that its elongate axis is parallel to but distanced from the axis about which the propeller assembly is rotated, The shank 210 is hollow to save weight. There is a stop 209 to locate \t at the right depth in the hub 201, The key 210 is toieranced to provide a friction fit in the key-way 212 of the jaws 207 of the hub 201, but it is smaller than the keyhole 211 in the blade 202, as will be described, Figure 3a shows the geometry of the keyhole 211. There is a landing straight line 222, and there are two, angled guiding straight line sides 221 that connect to the straight landing side 222; together these form a trapezoidal shape expansion of the keyhole 211, with the angled sides 221 being the legs of the trapezoid and the landing side 222 forming the outer base of the trapezoid, The two, angled guiding straight lines 221 connect tangentiaily to a large circular arc 220, Together, the three sides 221, 222, 222 and the arc 220 form the edge of the keyhole 211. The angle of the guiding straight fines 221 to the norma! of the landing straight line 222 is a (alpha). The diameter of the circular arc 220 is D. Figure 3b is a cross-sectional view along the shank 210 of the key pin 203, and shows the geometry of the outer surface of the key pin 203. There is a butting straight line 232, There are two, angled locating straight lines 231 that connect to the butting straight line 232; together these form a trapezoidal shape which matches the same shape in the keyhole 211. There is a large circular arc. 230 that connects to the two, angled guiding straight iines 231. Together, the three lines 231, 232, 231 and the arc 230 form the key pin geometry. The angle of the locating straight lines 231 to the normal of the butting straight line 232 is a (alpha). The length of the cross-section of the key pin about its axis of symmetry (perpendicular to the elongate axis of the key pin) is L

Figure 4a shows the geometrical arrangement of the key pin 203 and key hole 211 in an extended or deployed location and orientation, which is the location and orientation of the propeller assembly 200 when it is being rotated at speed and providing thrust. In this location and orientation, the key pin 203 seats into the keyhole 211 such that the butting straight line 232 seats against the landing straight line 222 and the locating straight lines 231 seat against the guiding straight lines 221. In this location and orientation, the seating of the three line pairs (222-232, 221-231, 221-231) is held firmly in place by the forces induced by rotation of the propeller assembly: a force C acts on the outer edge of the key hole 211 due to rotation of the blade 202, whilst the key pin 203 exerts a force T inwardly towards the hub 201 and opposite to the force C as a result of the inertia of the rotatingbiade 202; the complementary shape and size of the protruding part of the key pin 203 and the inset part of the key hole 211 means that when the parts are seated together they are held in place so as to completely inhibit any incipient iead-iag vibration. It can be seen that this seating of the key pin 203 within the inset part of the key hole 211 physically holds the two blades 202 in the normal orientation and completely inhibits lead-lag vibration, After many experimental tests, it was found that the optimum angle a (alpha) is 26 degrees, although the present invention works adequately with o (alpha) less than or greater than 26 degrees, or with angles that are different on either side of the normal. It was also found experimentally that the angle of the guiding straight lines 221 to the normal of the landing straight line 222 could be slightly greater than the angle of the locating straight lines 231 to the normal of the butting straight line 232,

Referring to Figure 4b, the geometry of the key pin 203 and key hole 211 permits folding, for folding to occur freely, it is best that the maximum cross-sectional length of the key pin L is less than the diameter of the circular arc 220 D or friction will occur, which will prevent the free manual folding or automatic extending movements. When the propeller assembly 200 is stationary, a blade 202 in the extended location and orientation can be smoothly folded away, to the folded location and orientation. It will be appreciated that the propeller assembly 200 can be folded without any additional manual operations and the blades 202 remain seated when spinning and delivering thrust without exhibiting lead-lag vibration or other vibration modes.

This embodiment of the propeller assembly 200 may be assembled from five parts. Each part is manufactured with tolerances that are tight enough such that after assembly the whole propeller will be in balance and for many applications will not need to undergo a balancing process, The key pin 203 can be permanently bonded in position to the hub 201 using an adhesive or any other method of attaching it so that the key pin 203 will not work free in operation. The five parts are made of a plastic such as nylon but may also be made of a glass fibre reinforced composite or a carbon fibre reinforce composite amongst other materials. Each part may be moulded or 3D printed, For medium manufacturing quantities of thousands, the small parts such as the hubs could be 3D printed and the aerofoil blades for which the surface finish is important could be moulded. It will be understood by one skiiied in the art that the scope of this Invention is not limited by the. materials or the manufacturing processes employed in its production and testing,

The scope of this invention is not limited to a propeller assembly 200 with two blades and comprising five parts, but can comprise more than two blades and it can comprise any number of parts greater than 5. It can be provided with further degrees of freedom such as teetering. The jaws couid be provided on the blade 202 and the tab on the hub 201, The jaws 207 could be parailei or could taper towards the jaw tips if the jaw material is compliant. The key pin 203 could be manufactured with a bendabie length of retaining plastic to the hub 201 such that the retained key pin 203 can be pushed into the hub 201 and through the blade 202 such that the total part count is 3 not 5,

Figure 5a is an isometric view of a propeller assembly 500 with magnetic seating. Each blade SOS is gripped in a blade grip SOL A hub 506 supports the teeter S04 on a simple hinge 507. Each blade grip 501 folds on a simple hinge 503. Each blade grip 501 retains a cylindrical magnet 502. The cylindrical magnet 502 is normally bonded into the retaining housing of the blade grip 501. The propelfer assembly 500 is shown in the extended operating orientation for generating thrust when rotating. The two magnets S02 are arranged for flux flow generating a magnetic attraction force. The magnetic attraction force is sufficiently high to seat the blade grips 501 of the propeller assembly 5Q0 aligned to each other such that lead-lag vibration or other vibration modes do not occur. Referring now to Figure 5b, this shows the propelier assembly 500 with magnetic seating when folded. The propeller assembly 500 is folded manually by the user with the magnetic attraction force being broken during the folding operation. The magnet 502 typically has a larger diameter than thickness and is sized with enough attractive force to prevent lead-lag vibration. In a working prototype of a 5" (125 mm) long prop, the magnets used were made of Neodymium N42, had 3mm diameter and 2mm thickness with a combined pull of 0.54 kg; the magnets may be purchased as standard components from suitable suppliers.

Figure 5c is a side view of an extended propelier assembly 500 with magnetic seating at a teeter angle. The present invention is operable with additional degrees of freedom such as teetering. The propelier assembly 300 can be formed with the blade grip 501 and the biade 505 combined into one piece.

Figures 6a and 6b are isometric views of a propeller assembly 600 with pip and recess seating; in Figure 6a the blades are folded and in Figure 6b they are deployed, A pip 602 is mounted on a compliant support 609 that is mounted on the blade grip 601 which grips the biade 605. If 3D printing is used, then just one 3D printed part can combine the labelled parts 601, 602, 603 and 60S in a single manufactured part without the need for internal mating faces and fixings. The pip 602 Is preferably hemispherical in shape for at least the lower half but could be other geometry such as a cylindrical section, Each blade grip 601 is connected to the teeter 604 with a simple hinge 603 permitting folding. There are two symmetrica! ramp-pairs 610 on the teeter 604 either side of two recesses 608, A recess 608 is typically a cylindrical hole of smaller diameter than the spherical diameter of the pip 602,

This arrangement operates as follows, As the drive spins the propeller assembly 600 up to the rotational velocity required for the demanded propeller thrust, If the propeller assembly 600 is not already extended then the inertias of the blades 605 cause the blades to swing out to be in line with the axis between the two simple hinges 603. As a blade 605 swings out around the hinge 603, its pip 602 sides up a ramp 610 until it drops down and settle into the recess 608. Whilst rising up the ramp 610, the compliant support 609 bends and absorbs energy creating a down force onto the ramp. When the pip 602 reaches the recess 608, the spring energy stored in the compliant support 609 partially releases and seats the pip 602 into the recess 608, The down spring force of the compliant support 609 holding the pip 602 In the recess 608 for each blade 605 is sufficiently high to seat the blade grips 601 of the propeller assembly 600 aligned to each other such that lead-lag vibration or other blade vibration does not occur.

It will be appreciated by those skilled in the art, that a working device of the type shown in Figure 6 requires a working combination of several factors including for a propeller weight and length; the angle of the ramp 610, the vertical stiffness of the compliant support 609, the diameter of pip 602 and the diameter of the cylinder of the recess 608. For the case of a 5" (125 mm) propeller weighing 2.4g,, a working prototype was made with a ramp 610 angle of 13 degrees, a compliant support 609 spring stiffness of 30 g/m.m, a pip 603 hemisphere diameter of 1.5 mm and a recess 608 cylindrical diameter of 0,8 mm.

When the thrust of the propeller assembly 600 increases to hover thrust or higher, the tips of the blades raise marginally and the propeller arcs upwards. There is a slight Increase in the thrust downward of the pip 602 into the recess 608, which makes the arrangement more robust to inhibiting lead-lag vibration since the pip 602 must jump out of the recess 608 and regularly cross the recess 308 back and forth for lead-lag vibration to occur. The propeller assembly 600 can be provided with a variable slope ramp, starting with a steep ramp up,, flattening out and then dropping down into the recess,

Figure 7a shows a perspective view of an air vehicle 700 comprising four .propeller assemblies 701 (which in construction are as described In connection with Figure 2 above}, each of which supports two propeifer blades 702, The propeller assemblies 701 and the propeller biades 702 are shown in the deployed position for flight, the propeller assemblies ^, being mounted to the distal ends of arms 70S, which themselves are attached at their proximal ends to airframe 707, Airframe 707 contains the power supply and drive means (not shown) for the air vehicle 700, together with any payload, Figure 7 b shows the same air vehicle 700 with the arms 705 and the propeller assemblies in the folded position for carriage.

It wiii of course be understood that many variations may be made to the above- described embodiment without departing from the scope of the present invention. For example, the key pin shown in Figure 2 could be held tightly in the key hole in the blade, with the key ways 212 in the hub 201 being oversized to provide the loose fit (in which arrangement the protruding part of the key pin would point outwards rather than inwards as shown in the drawings). The key pin need not be symmetrical in cross-section: because the propeller assembly would normally be driven in only one rotational direction it may be advantageous to shape the key hole and the key pin asymmetrically, so as to provide positive engagement in one direction of rotation (the driven direction) but a less positive engagement in the opposite direction of rotation. The key pin and key hole geometries need not include straight lines, however they still require complementarily-shaped sections, such as a protrusion and a matching recess, which can engage and hold the biades in the deployed position when the propeller assembly is driven. It will be understood that the keypin and keyhole arrangement, the magnetic locking means and the over-centre locking mechanism are separate and alternative arrangements, but that they can be combined with each other in any combination.

Where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.