FIELD

The present invention relates to shrouds for aircraft. More particularly but not exclusively, the invention relates to a shroud for minimising noise from an Unmanned Aerial Vehicle (UAV).

BACKGROUND

Many aircraft, such as UAVs, helicopters, vertical lift systems and fixed-wing aircrafts disadvantageously produce noise from their motors and propellers. As such, they may be a nuisance to the surrounding environment, adding to noise pollution.

Noise created by propellers is a particular problem for UAVs used in video and audio capture for filming. Current UAV audio capture for filming requires expensive and time- consuming post- processing to remove noise created by the UAVs.

SUMMARY

The invention may provide an improved shroud for reducing propeller noise, particularly for UAVs in the noise spectrum useful for audio capture, or to at least provide the public with a useful choice.

In a first aspect there is provided a shroud for an aircraft comprising:

a shroud body configured to at least partially enclose a propeller, and

a noise reducing material on or in an inner surface of the body adjacent to one or more tips of the propeller.

According to a further aspect there is provided a shroud for an aircraft comprising a shroud body configured to at least partially enclose a propeller having an interior surface provided with a plurality of baffles configured to dissipate sound by reflection and refraction.

According to a further aspect there is provided an aircraft comprising:

a. a first propeller,

b. a shroud at least partially surrounding the first propeller having a shape which redirects sound in a desired direction and absorbs sound; and

c. a second propeller configured to cancel sound from the first propeller. It is acknowledged that the terms "comprise", "comprises" and "comprising" may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, these terms are intended to have an inclusive meaning - i.e. they will be taken to mean an inclusion of the listed components which the use directly references, and possibly also of other non-specified components or elements.

Reference to any prior art in this specification does not constitute an admission that such prior art forms part of the common general knowledge. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description of embodiments given below, serve to explain the principles of the invention.

Figure la is a perspective view of a shroud according to one embodiment.

Figure lb is an underside perspective view of the shroud of Figure la. Figure lc is a perspective view of an interior section of a shroud having baffles in the form of an array of cones;

Figure 2a is a top view of a shroud according to another embodiment.

Figure 2b is a side view of the shroud of Figure 2a.

Figure 2c is a cross sectional view of the shroud of Figure 2a.

Figure 3a is a perspective view of a shroud according to another embodiment.

Figure 3b is a top view of the shroud of Figure 3a.

Figure 4 is a perspective view of a UAV according to one embodiment.

Figure 5a is an illustration of a sound camera image of an unshrouded UAV.

Figure 5b is an illustration of a sound camera image of a shrouded UAV.

Figure 6 is a diagram of active filtering according to one embodiment.

Figure 7 is a schematic diagram of filtering according to one embodiment.

DETAILED DESCRIPTION

Various embodiments of shrouds are described below, which may reduce sound produced by the aircraft propellers within the shroud, by absorbing sound waves, as well as reflecting and/or refracting the sound. This may reduce the sound heard by the public and/or allow any audio recording equipment mounted on the aircraft to more accurately record audio sounds.

Shrouds described below generally relate to cowlings, which at least partially surrounds an aircraft motor or propeller assembly. Noise reflection refers to changing the direction of the sound energy / waves. Noise refraction is also a change in soundwave direction as waves pass through a certain material. Sound absorption refers to the process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body. Figure la shows a perspective view of a shroud 100 according to one embodiment. Figure lb shows an underside perspective view of the shroud 100 of Figure la. The shroud 100 includes a body 102, with a top edge 104 and a bottom edge 106. I n the shown embodiment the body is substantially circular/ring-like. In other embodiments the body does not necessarily form a complete ring. The shroud body may be polygonal or toroidal.

An inner facing surface 108 of the shroud body includes a plurality of baffles in the form of a plurality of ribs 116. The ribs 116 of the inner facing surface 108 of the body 102 are configured to scatter energy from the propellers to avoid resonance. The ribs may be any suitable shape which achieves this purpose, for example they may be triangular or multisided forms in cross-section, such as half hexagons. In the shown embodiment, the ribs 116 are triangular projections extending the circumference of the inner facing surface 108. The ribs may be up to 40mm long, depending on the application. The ribs may be disposed at an angle between 0 to 45 degrees from perpendicular to the inner facing surface 108. Individual ribs may face at different angles, and may have different lengths. The ribs decrease in size from the middle to the ends as best illustrated in Figure 2c. I n this case the baffles are continuous ribs around the circumference of the inner facing surface 108, though in other embodiments the baffles may comprise several disjointed projections extending around the circumference. Such baffles need not be continuous ribs and could be in the form of cones, pyramids or other multi-faceted forms. Figure lc shows an embodiment in which a plurality of cones are provided around the inner facing surface 108 (only a section is illustrated although the cones would surround the inner facing surface 108). In this case a central row of large cones 121 has a row of medium sized cones 120 on one side and 122 on the other side with rows of small cones 119 and 123 as outer rows. Whilst the cones are in rows in this embodiment they may be offset or provided in other arrangements depending upon the shapes of the baffle elements and the sound to be reduced. An outer facing surface 109 is substantially smooth. The outer facing surface may be shaped as an airfoil, to aid the flight of the aircraft.

The shroud 100 includes cross members 110 which connect the body to an inner plate 114. The cross members 110 and inner plate 114 are located near the bottom edge 106 of the shroud body. I n this embodiment, the diameter of the bottom edge 106 of the shroud is slightly smaller than the diameter of the top edge 104 of the shroud body 102. The ratio of the top edge area to the bottom edge area may range from a 1:1 ratio to a 1:0.75 ratio (in other words, the bottom edge may be up to 25% smaller than the top edge).

The inner plate 114 of the embodiment shown in Figures la/lb includes a central hole 118. A motor with a dual bidirectional shaft (not shown) may be fitted to the inner plate 114 with its shaft passing through the hole 118 in the centre of inner plate 114 of the shroud 100.

The shroud, or individual parts of the shroud may consist of any suitable material, including but not limited to carbon fibre, nylon fibre, resin cloth, fibreglass, plastics or composites including natural fibres of plant or animal origin such as cotton or wool. Acoustically transparent epoxy resin and similar, such as Resinlab UR3010, may be used for surfaces where reflection is not desired. When reflection is desired, standard resins can be used. Individual parts of the assembly (such as the propeller, motor, shroud) may be encased in sound absorbing material, including but not limited to foam, rubber, resin cloth, fibreglass, plastics, acoustic nanomaterials or composites or materials made of natural fibres such as cotton or wool.

The shroud body may have any suitable thickness depending on the application, between 0.1mm to 50mm. The shroud body may be solid, or it may be hollow on the inside. If the shroud body is hollow, it may include bracing (e.g. a honeycomb structure or carbon fibre bracing), for support and/or to minimise vibration. The interior of the shroud body may be filled with aerogels, fibres or foams. The internal structure may be designed to absorb noise that passes though the inner facing surface 108 of the body 102 of the shroud.

Depending on the application, the shroud body 102 has a height of between 10mm (for small propellers) up to 300mm (for example for large counter rotating propellers). In other embodiment, the shroud top and/or bottom edges may include winglets and/or teeth protruding from the body. A lip/overhang may be formed on the top edge of the body.

Figure 2a shows a top view of a shroud according to another embodiment. The shroud is shown surrounding a propeller 202.

The propeller 202 may incorporate sound absorbing material on its leading edge, or it may entirely comprise a sound absorbing material. The shroud and propeller may be tailored to each other to reduce blade-vortex interaction. For example, the tip of the propeller blade may match the profile/curvature of the inner facing surface of the shroud in order to keep an equal distance from the shroud at all times. The propeller may include jagged tips, microabrasions, microgrooves, microindentations, winglets and/or edge serrations. The propeller may have additional mechanical alterations tailored to the shroud to reduce blade-vortex Interaction, including offset blades, variable length, shape morphing. The propeller blades may have any suitable length. The propeller produces a certain pitch, which changes according to the speed at which the propeller rotates. The inner facing surface 108 of the shroud 100 may be between 0.02mm to 2mm from the edge of propeller blades, essentially touching the propeller 202. The propeller tip may be surrounded in three planes (top, bottom and end) lined with sound-absorbing material, located to absorb sound produced by the propeller tips. The specifics of the sound absorbing material are described in detail below.

The shroud includes four cross members 110. In other embodiments, the shroud may include between one to eight cross members. The number of cross members used may related to the number of propeller blades to optimally minimise resonance issues. The cross members 110 may have a thickness of between 4mm to 60mm. The cross members 110 may be distributed equal distances from each other or in some other asymmetrical arrangement.

Figure 2b shows a side view of the shroud of Figure 2a. This shows the airfoil-like shape of the outer facing surface 109 of the shroud. Figure 2c shows a cross sectional view of the shroud of Figure 2a. The propeller and propeller motor 206 are mounted on the inner plate 114 of the shroud. The motor may be encased in a noise reducing material.

Sound Absorbing Material

Shrouds described in various embodiments may incorporate sound-absorbing material. For example with reference to the embodiment shown in Figures la/lb, the inner facing surface 108 of the shroud body 102 may incorporate sound absorbing material to directly absorb sound produced by the propeller. One example of suitable sound absorbing material is a ridged composite acoustic nanofibre. This may consist of an electrospun nanomaterial backed onto foams and other such materials that provide support and also have acoustic properties. The nanofibre can also have embedded materials within the electrospun material that have additional acoustic properties.

A wide spectrum of polymers and substrates may be used for the electrospun nanofiber material, such as Poly(methyl methacrylate) (PMMA), Polystyrene (PS), Nylon 66 (PA66), Nylon 6 (PA6), Nylon 46 (PA46), Polycaprolactone (PCL), Polyvinyl alcohol) (PVOH), Polyvinyl Butyral (PVB), Poly(ether sulfone) (PES), Polylactic Acid (PLA), Polyurethane (PU), Polyester (PET)

and Polyacrylonitrile (PAN). The nanofibers may have a diameter of between 50nm to 2000 nm. They may be formed as continuous fibres with a porosity of between 0.6 to 0.96. Up to 20,000 layers of nanofibers may be included in the sound absorbing material.

Examples of suitable backing material for the elecrospun nanomaterial include acoustically-transparent potting (epoxy) compounds. By rigidifying the sound absorbing material, smooth airflow is promoted, and a greater lining uniformity and a tighter shroud-to-propeller tolerance can be achieved. Thermoplastic polymer nanofibres may be sandwiched between two layers of substrate fabric. The substrate fabric could include non-wovens and mesh materials. The nanofibre/substrate materials can be used in conjunction with open-cell acoustic foams, such that layered sandwich structures may be used.

The material may have acoustic attenuation of between 0 and 100% across a frequency spectrum of 100Hz to 20kHz. Figure 3a shows a perspective view of a shroud according to another embodiment. Figure 3b shows a top view of the shroud of Figure 3a. In this embodiment, the top edge 104 and bottom edge 106 of the shroud body 102 have the same diameter. The inner facing surface 108 of the shroud body 102 does not include ribs. The inner facing surface 108 includes a slot insert 302. The slot insert may be lined with a sound absorbing material as described above. The slot insert 302 may have a width of between 10mm and 300 mm and may be of varying shape and profile to accommodate sound absorbent material and/or reflect or absorb sound. It may advantageously have a concave shape to reflect sound in desired directions. It may utilise the entire internal volume of the shroud or just a portion of it. Whilst the slot insert 302 in this example is relatively narrow in a region close to the path described by the propeller tips, it may be wider - even to the point of occupying nearly the entire height of the shroud. The slot insert 302 may be between 0.1 to 50mm deep and could occupy nearly the entire width of the shroud. Although only one slot is included in this embodiment further slots may be included. The slot insert 302 is located so as to absorb sound energy emitted directly from the tips of the propeller blades. The slot insert 302 may use the baffle structures described above - these may be provided on the surface of slot insert 302 with noise reducing material provided above the baffles.

In this embodiment, the cross members 110 may be aerofoil shaped in cross section, and/or may be angled or twisted. As shown in Figure 3a, the cross members twist from being substantially flat near the inner plate 114, to becoming vertical near the shroud body 102. The aerofoil form of the cross members reduces drag from the air moving past it and reduces buffeting or oscillation caused by the blades passing the cross members. Twisting the cross members reduces the spin on the air caused by the propeller blades.

The inner plate includes mounting holes 304. A motor may be mounted above or below the inner plate by screwing the motor to the mounting holes 304.

The shroud 100 also includes an attachment point 306 on the shroud body 102 which may be used for mounting purposes instead of or in addition to the inner plate 114. Figure 4 shows an unmanned aerial vehicle (UAV) including four shrouds 100. The UAV includes a central controller 402 connected to four propellers 202 via four arms 402. Each shroud 100 also includes an additional propeller 406 (shown extending from the shroud for illustration only as it is located within the shroud). Each propeller 406 may be counter rotating or may spin in the same direction as each propeller 202 but at a different speed relative to each propeller 202.

Figures 5a is an illustration of sound camera images of an unshrouded UAV 300. This shows how in an unshrouded aircraft the sound energy 301, 302 is spread all around the aircraft as illustrated by the dense shading above and below. This is used a baseline for comparison to shrouded designs. Figure 5b shows an illustration of a sound intensity image of a shrouded UAV. This shows how the majority of the sound is directed upwards as sound energy 301 and is attenuated below in sound energy 302. The shrouds may also include active noise reduction techniques. This may include noise cancellation and/or noise filtering.

Audio capture

Between 1 and 16 microphones may be provided to capture an audio signal of interest on the ground or other target. The microphones may be located on the arms of an aircraft, or near other sources of noise on the aircraft (for example on the gimbals below the drone). An array of microphones allows sound to be captured from multiple angles enabling out of axis sound to be filtered out more easily. The audio capture microphones may be omnidirectional, shotgun (unidirectional), cardoid, or a combination. They may be surrounded by wind socks/muffs. The microphones may include inbuilt active noise filtering. Microphones may be fixed to the UAV or they may be hanging/suspended from the aircraft by a tether. Noise Cancellation

The shrouds may include arrangements for active noise cancellation.

In order to do this the shroud may include one or more microphones (not shown) to capture the noise produced by the aircraft, and a mechanism for creating anti-noise to cancel out the noise produced by the aircraft. A controller may modify algorithms tailored to assembly's sound frequency.

The shroud may include a speaker to play back the anti-noise, or it may include an additional noise reducing propeller 406. The speed of the noise reducing propeller may be set by the controller such that it cancels out the noise from the propeller being shrouded. As well as providing direct thrust the noise reducing propeller may perform a noise cancelling function where the noise cancelling propeller is spaced a half wavelength of the sound to be cancelled away from a lift propeller. Two propellers spinning at the same speed (generating the same noise) with a gap between them equal to half the sound wave length (or the frequency peak with the most sound energy) will create destructive interference and cancel out the peak.

Modified least means squared (LMS) may be used to create anti-noise, and reduce sound by destructive interference.

Filtering

The shroud may include arrangement for active noise filtering. Active noise filtering differs from passive noise filtering in that the filtering strategy is able to be varied depending on the noise present or other variables. The microphone signal may be filtered in real-time (after shroud and propeller have directionally shifted and spectrally reduced the magnitude of the noise). The shroud is configured to direct the sound upwardly, away from microphones and audio equipment below the UAV. Any suitable filtering algorithm may be used, for example a modified least means square algorithm.

Between 1 and 16 microphones may be provided to capture sound on the aircraft/shroud. The microphones may be located on the arms of an aircraft, or near other sources of noise on the aircraft (for example on the gimbals below the drone).

The shroud may reduce the propeller wash near the microphones on the arms for example, and also create still and quieter zones on the drone where the microphones can be placed.

Figure 6 shows a diagram of active filtering according to one embodiment. This shows a noise source being filtered out from a signal-of-interest using an adaptive filter to produce a filtered output. In this case, "noise" means unwanted sound (i.e. the props + motor), signal of interest is the wanted recorded audio.

Figure 7 shows a schematic diagram of filtering according to one embodiment. Noise(s) and signal/s of interest are recorded by one or more microphones. The signals are pre- amplified and then digitized. Then unwanted noise is filtered out from the wanted signal of interest. The filtered signal may be stored/transmitted in digital and/or analog format.

Although embodiments described have focused on UAVs, the described shroud may be applied to other aircraft such as fixed wing flying devices or any other aircraft with propellers which are shroudable.

The shroud or parts of the shroud may be manufactured by any suitable method, including but not limited to 3D printing, reductive manufacturing (CNC machining), injection moulding or carbon / composite manufacturing. Advantages

One or more embodiments may shape the noise envelope created by an aircraft, by reflecting refracting and/or absorbing sound energy using a sound reducing material.

Sound absorbing material is located where sound directly impinges on it. Noise is focussed upwards, away from any sound recording microphones and away from locations where it may create a nuisance. The shroud offers improvements in aircraft thrust and efficiency. As a secondary benefit, the shroud protects operators, humans, farm animals, wildlife, the environment and the UAV asset from propeller damage.

While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the Applicant's general inventive concept.