The present invention relates to load detection in a landing gear for an aircraft.
It is desirable to monitor the loading applied to a landing gear during an overload landing in order to determine whether the designed strength level has been exceeded, or is in danger of being exceeded.
It is also desirable to monitor the landing load during a normal landing in order to assess the fatigue loading applied to the gear.
According to the invention, a landing gear for an aircraft comprises a shock absorber strut with upper and lower telescoping portions, the upper portion being connectable to the airframe of the aircraft; an arm to extend fore and aft relative to the aircraft and carrying a landing wheel, and pivotally connected by a main pivot to the lower portion of the shock absorber strut; and a load reacting unit connected between the arm and the shock absorber strut for reacting to load applied between the arm and shock absorber strut on landing, and an indicator for monitoring the load applied to the load reacting unit on landing.
The invention recognises that friction from sliding bearings in the shock absorber strut make a significant difference to the load for a given shock absorber internal pressure (or alternatively the shock absorber pressure may be different for the same applied load, depending upon bearing friction). Bearing friction is highly variable, depending upon the coefficient of friction and the load applied normal to the bearings.
Thus the invention recognises that more accurate measurement of load can be made by measuring the load in the load reacting unit, which is less affected by friction than the shock absorber strut. That is, in a fluid pressure load reacting unit the pressure is much more closely related to the load applied to the arm than is the pressure in the shock absorber strut.
The load reacting unit may be pin-jointed, or mounted on spherical bearings at each end, resulting in minimal loading normal to its sliding bearings.
The fluid may be air (or another gas), but more preferably is a liquid.
In the embodiments described below, the arm, may be the arm of semi-levered landing gear, and/or a bogie beam with fore and aft landing wheels.
The invention will now be described by way of example with reference to the accompanying drawings in which:
The landing gear illustrated in
A load reacting unit 9 is connected between the upper portion 2 of the main strut and the forward section of the bogie beam 4 to control the angular position of the bogie beam during taxiing, take-off and landing. The unit 9 comprises an outer cylinder casing 10 with a closed upper end carrying an upper connector 11 which is pivotally connected at 12 to the upper end 2 of the main strut. A piston rod 23 extends from the lower end of the cylinder casing 10 and carries a lower connector 13 which is pivotally connected at 14 to the front end of the bogie beam between the axle 6 and the main pivot 5. A side stay (not shown) is connected between the upper portion 2 of the shock absorber strut and the aircraft, and moves with the strut when the landing gear is moved to a stowed position in the aircraft by a stowing actuator (not shown).
The load reacting unit 9 has a piston 15 within the cylinder casing 10 that divides the internal space into a high pressure chamber 16 on one side of the piston and a low pressure chamber 17 on the other side of the piston with an internal shoulder 18 between the two chambers 16, 17 against which the piston is urged by high pressure fluid within the chamber 16. The piston rod 23 extends through an axial aperture 19 in the piston and carries a retainer 20 at its inner end to hold the piston 15 captive on the rod 23. A tensile force applied between the connectors 11, 13 will cause the retainer to engage the piston 15 and for the two to move together in acting against the high pressure fluid, which flows in a hydraulic control circuit including a high pressure port 21 with a flow restrictor. When the applied tensile force is released, the rod 23 and piston 15 return to the position shown in
In an alternative embodiment of the invention, movement of the piston 15 and piston rod 23 with the tensile load may be accommodated by valving in the piston to allow the flow of oil between the chambers 16, 17 either side of the piston, the flow being more restricted when the unit is extending, compared with the flow when the unit is retracting.
In operation, the bogie beam 4 is tilted into the position shown in
Thus on landing, the load reacting unit 9 is pressurised, and the pressure can be taken as indicative of the landing load. Therefore, a landing load indicator 60 is provided in the unit 9 to be responsive to the internal fluid pressure. The indicator 60 shown in
The bobbin 62 is preferably painted a bright colour to increase its visibility once operated. The bobbin 62 may be reset by manual depression or, if needed, a special tool or key may be provided.
The invention makes use of the close correlation between the pressure in the load reaction unit 9 and the load on the aft axle 4, on touchdown. The bearing friction for the auxiliary actuator 9 will be small, because it is mounted on spherical bearings 12, 14 at each end. Friction at the bogie pivot 5 and seal friction will also exist, but can be shown to be small. Furthermore the bogie will always be rotating in the same direction during initial compression on landing. Therefore the results can be compensated to account for the mean friction value, with the only remaining error being due to a variation in friction.
The appropriate pressure threshold value at which the indicator operates may be determined after taking into account the normal aircraft attitude and landing gear geometry, acceptable landing gear load limits, and an allowance for variations or tolerances.
A single indicator 60 may be provided in the load reaction unit 9 to give an indication of the occurrence of an overload condition during landing. Alternatively, two or more indicators 60 may be provided, each set to operate at a different internal fluid pressure so that different load thresholds can be indicated. For example, two indicators can between them define an indicator range with one operating at a lower pressure and the other at a higher pressure. Different indicators may serve to trigger different inspection and safety procedures. Alternatively, multiple indicators 60 may be set to operate at slightly different pressures to reduce error margins due to tolerances.
In an alternative embodiment of the invention, the indicator 60 may comprise a pressure sensor rather than a mechanical pressure threshold indicator. Means may be provided to monitor pressure variations and to indicate when a threshold has been exceeded.
The output of the pressure sensor is received by a processor 75 which records the raw pressure data in a store 73. Simple geometry determines a relationship between the load in the unit 9 and the load applied to the aft axle by using a method such as comparing moments about the shock absorber to bogie pivot. This relationship is predetermined and stored in a recorded data store 72, enabling the processor 75 to derive a value indicative of the load applied to the aft axle from the pressure data and store the aft axle load values in the store 73.
The processor 75 may be set to determine whether the output of the pressure sensor, or a value derived therefrom (such as the aft axle load) has exceeded a pre-set threshold.
A potential drawback is that the moment about the bogie pivot 5 depends upon the direction of the load at the aft axle as well as its magnitude. Thus the processor 73 makes assumptions about the load direction based upon real-time data received from sensors such as aircraft attitude sensor 71, and pre-recorded data such as runway coefficient of friction, rolling stock inertia stored in pre-recorded data store 72. In this way, the processor 75 calculates the direction of the load applied to the aft axle and processes the measured direction to calculate a load value.
Also, the movement about the bogie axle 5 is dependent upon the coefficient of friction between the tyre of the wheel 7 and the runway on touchdown. In order to compensate for this variation, the recorded data store 72 may include a database of airport runway coefficient of friction values compensated for weather conditions. Alternatively, the system may make use of coefficient of friction data transmitted by another aircraft having just landed on the same runway. Alternatively, tyre spin-up rates may be measured, for example, by comparing spin-up time with ground speed and known rolling stock inertia, to determine coefficient of friction. Alternatively, coefficient of friction may be determined through the drag loading on a landing gear.
The measurement of load in the load reacting unit 9 (and hence aft axle load) may be used as part of a comprehensive load measuring system. The load measurement may be processed by the processor 75 in combination with the results from other sensors in order to obtain a more complete indication of all the loads applied.
A display device 74 can display a variety of indications, including the raw pressure data, the aft axle load data, or simply an indication of an overload landing when the pressure has exceeded the pre-set threshold.
Another embodiment of the invention shown in
Another embodiment of the invention is illustrated in
In another embodiment of the invention, the landing load indicator in
In other embodiments of the invention, the pitch trimmer 80 in
Number | Date | Country | Kind |
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0719732.0 | Oct 2007 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2008/063743 | 10/13/2008 | WO | 00 | 8/6/2010 |