This invention relates to aircraft noise reduction apparatus, in particular to apparatus to reduce noise emanating from landing gear of the aircraft when deployed on approach to landing.
The invention has particular application to commercial aircraft when landing at airports in or near to built up areas where aircraft approach noise is an increasing nuisance with increasing frequency of use of airports and ever larger aircraft. Noise emanating from commercial aircraft, whether cargo aircraft and/or passenger carrying aircraft, is now an important environmental issue. The increasing frequency of use of airports has led to aircraft of increasing size being used. This in turn has led to greater noise being generated by each aircraft in its approaches to the airport. Modern high bypass ratio turbofan engines are now extremely quiet on approach to landing and dominant noise on some aircraft types, perceived at ground level, is starting to become the noise generated by landing gear and by high lift devices on the wings.
This invention addresses the problem of aircraft noise, as perceived at ground level, generated by landing gear. Although noise is likely to be induced by any element which creates turbulence in the airflow past the landing gear, those elements which have been found particularly likely to create high noise levels are as follows:—elements which act as Helmholz resonators such as hollow axles, apertures, fastener fittings, spindles, etc; also elements likely to create small vortices such as small protuberances, sharp edges; dresssings such as small pipes, tubes, cables, and coil springs, etc. All such features are to be found aplenty on modern landing gear and tend to be more prevalent in the region of steering gear and wheel axles.
Although wind tunnel tests have been carried out on landing gear fully enclosed by fairings to determine noise levels in the far field, such full enclosure fairings are normally completely impractical for everyday use on an aircraft. This is because articulation of the landing gear during deployment and retraction movement is prevented by the fairings and because very limited stowage space in the landing gear bay in most aircraft will not accommodate full enclosures owing to the space which they take up. Additionally, full enclosure fairings render daily visual inspection regimes and maintenance of the landing gear almost impossible. Firstly, this is because visible inspection through such fairings is generally impossible and secondly because regular removal of such fairings for inspection or maintenance is far too time consuming and difficult. To date therefore no practical solution to the problem of landing gear induced airframe noise has been found.
According to a first aspect of the invention there is provided aircraft noise reduction apparatus including at least one noise-reducing attachment for landing gear of the aircraft shaped for positioning on the landing gear to deflect air away from noise inducing components of the landing gear and to permit deflection, articulation movement and stowage of the landing gear with the attachment installed.
The attachments may comprise deflector screens adapted not to completely surround noise inducing components on the landing gear. The said lack of a complete surrounding being for the purposes of allowing the said access and foldability of the landing gear.
Induced noise is proportional to the sixth power of the relative velocity of the airflow and component around which the air flows, and attachments of the invention are positionable to deflect incident air away from noise inducing components, preferably into the free stream.
The attachments must withstand all aerodynamic loads and foreign object damage, and it will be appreciated that the use of attachments according to the invention may increase landing gear drag.
Preferably a said attachment is shaped such that when positioned on the landing gear, airflow over the attachment during flight leaves the attachment substantially in the free stream direction.
Preferably a said attachment is shaped to avoid or minimise an air stagnation region forming in front thereof when positioned on the landing gear and in use. Such stagnation regions tend to increase both drag and noise emanating downstream of the stagnation region. The term “in front of” as used herein refers to a position upstream in the airflow, during flight.
In regions of the landing gear where incident air is accelerated between components of the landing gear, such as struts, stays and actuators, at least one said attachment is preferably positioned to deflect incident air away from the said air-accelerating region. In this way any noise-inducing components of the landing gear positioned in the said air-accelerating region will not be subject to the passage of such accelerated high speed air past them.
The apparatus may include an axle attachment positionable in front of an axle for nose gear of the aircraft to deflect air away from noise inducing components such as landing gear dressings in the region of the axle.
The axle attachment may be shaped to shield items such as a towbar fitting on the nose gear and the said shaped portion is desirably separately removable or pivotable out of the way in order to provide access to such a towbar fitting without disturbing the remainder of the axle attachment. Cam fasteners or other quick release fittings are suitable for fastening the shaped portion.
The axle attachment may extend laterally into close proximity with the wheels and may include generally cylindrical wheel extensions extending between a central portion thereof and each wheel whereby to deflect air away from noise inducing components on the axle and at the same time to deflect air away from a cavity formed within each wheel.
The wheel extensions preferably extend into close proximity with a rim of each wheel. Alternatively the wheels may be fitted with inner hub caps or wheel fillers. Such fillers may be of say foam material formed in two opposing sections with wedge-shaped elements driven into position therebetween. The foam or like material is preferably covered on the outside with a weatherproof material.
The axle attachment may include upper extension portions for positioning on either side of a main leg of the nose gear to deflect air away from torque link attachments usually sited on the axle.
The apparatus may include a said attachment positionable in front of a steering mechanism for the landing gear. Such a steering mechanism has been found to be prone to noise inducement including, as it usually does, many sharp edges, apertures and sundry pipe and cable work.
Similarly, the axle attachment deflects air away from another region of the landing gear particularly prone to noise inducement, i.e. the joint between the main leg and the axle, again liable to include sundry sharp edged fittings and pipe and cable work.
For circular landing gear components such as main legs or actuators positioned immediately in front of noise inducing components, transition strips of tape may be attached to the circular component on either side thereof to encourage clean detachment of airflow therefrom and to encourage airflow away from a region immediately behind the circular component.
Where articulation and stowage permits it, components such as circular legs or actuators may be fitted with trailing extensions from either side thereof. Such extensions, in combination with the component, may then act as an attachment according to the invention to deflect air away from noise inducing components behind the said component with trailing extensions.
The said apparatus may include at least one said attachment positionable in front of an apertured landing gear strut or stay to deflect air away from edges of the aperture.
For a said strut or stay having surfaces extending approximately streamwise in the airflow a said attachment may be shaped to blend in airflow leaving the attachment with airflow passing over the said streamwise extending surfaces. Alternatively, where further noise inducing elements are positioned on the landing gear to the rear of the said attachment, the attachment may be flared or otherwise shaped also to deflect air away from the said rearward noise inducing elements. In this regard it has been determined that for an attachment to accelerate incident air is not unacceptable so long as the accelerated air does not pass over noise-inducing components.
A said attachment whilst being shaped to avoid any regions of air stagnation where possible may also be shaped or flattened as appropriate to avoid other elements of the landing gear or further attachments, during movement such as articulation and/or stowage of the landing gear.
Where the landing gear includes a wheeled bogie having a bogie beam the apparatus may include an attachment in the form of an undertray extending in front of and underneath the bogie beam to deflect air away from noise inducing elements in the region of the bogie beam, for example brake rods, bearings and their mountings, other fittings and sundry pipe and cable work.
An advantage of the undertray not extending above and behind the bogie beam is that inspection and maintenance access and air cooling are not thereby prevented by the attachment.
Where the bogie beam is tilted upwardly at the front, during approach, the area of the bogie beam left exposed is downstream of the undertray and thus the passage of high speed air past noise inducing components associated with the bogie beam is prevented by the undertray attachment.
Preferably the undertray includes at least one door therein to provide access to concealed items such as jacking points for the bogie.
The undertray may have a single curvature aerodynamic surface and may be of a width to permit air to pass between the undertray and wheels on the bogie to cool brakes. Preferably the undertray includes a forward upwardly extending aerodynamic surface blended smoothly into an under surface extending underneath and along the bogie beam, desirably with a further surface extending longitudinally of the bogie beam on either side of the undertray depending from lateral edges of the undertray, preferably orthogonal to the said aerodynamic surface. In this way air spilling over each lateral edge of the undertray in the region of wheel brakes will be able freely to flow past the brakes but will not be accelerated past the brakes by any funnelling effect of the surface contours of the undertray.
Where the bogie includes a pair of forward articulation links a said attachment may comprise a pair of articulating fairings, one mounted to each articulation link with an aerodynamically contoured interface being defined between the fairings. Preferably the said interface comprises a generally spherical element on one fairing co-operating with generally circular section tubing of the other. The spherical element desirably has its centre coincident with a pivot axis between the articulating links.
Where the landing gear includes a door attached thereto extending in a generally streamwise direction to one side of a main leg of the landing gear a said attachment may comprise a deflector fairing extending from the door in front of at least the main leg of the landing gear. Preferably the fairing is positionable in sealing engagement with the door and may additionally extend in front of any further struts, stays and/or actuators for the landing gear located in the vicinity of the main leg thereby to deflect air away from all said landing gear components. For a main landing gear having a drag stay extending upwardly in the vicinity of the main leg and a retraction actuator similarly extending upwardly in the vicinity of the main leg, the said fairing not only achieves deflection of the air away from any noise inducing fittings on the main leg, but also deflects air away from passages formed between any combination of the main leg, the actuator, the drag stay and the main leg door where air would otherwise be accelerated therethrough usually past sundry noise inducing fittings.
For a large aircraft the undertray may be in excess of 2 metres in length and is desirably constructed to possess adequate longitudinal and torsional stiffness to enable handling when off the aircraft.
The front portion of the undertray desirably has extension portions to extend around the lower articulation link fairing and thus provide a smooth airflow in this area, upon approach to landing. Generally a main landing gear with a bogie will approach landing with the rear wheels deflected downwardly and the front wheels deflected upwardly. The undertray and lower articulation link fairing thus desirably lie closely adjacent one another when the bogie is in approach configuration.
The undertray may be attachable to axles on the bogie, and/or to the bogie beam itself by means of band-type clamps adapted to pass around the axle/bogie beam and through at least one part-annular chamber formed in an extension piece fixed to the undertray.
In general, all attachments will be as easily removable as possible, employing either existing screw-type fasteners on the gear and/or clamps around items of the gear such as struts, stays and axles. Some attachments will inevitably need to be removed for maintenance, at some stage, and here ease of removal and re-fitting is essential. All attachments are also preferably retro-fittable to existing designs of landing gear.
According to a second aspect of the invention there is provided aircraft landing gear including apparatus according to the first aspect of the invention.
According to a third aspect of the invention there is provided an aircraft incorporating landing gear fitted with noise-reduction apparatus according to the first aspect of the invention.