GB1591689A

GB1591689A - Oleo-leg primarily for an aircraft undercarriage

Info

Publication number: GB1591689A
Application number: GB1337878A
Authority: GB
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1977-04-09
Filing date: 1978-04-05
Publication date: 1981-06-24
Also published as: DE2715895B1; DE2715895C2; FR2386427A1; FR2386427B1

Description

(54) AN OLEO-LEG PRIMARILY FOR AN AIRCRAFT UNDERCARRIAGE (71) We, MESSERSCHMITT BOLKOW-BLOHM Gesellschaft mit beschränkter Haftung, of 8000 Miinchen, German Federal Republic a Company organised and existing under the laws of the German Federal Republic, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an oleo-strut, primarly, although not exclusively, suitable for an aircraft undercarrriage. Such struts comprise a piston, piston rod and cylinder, with a ground-engaging wheel mounted on the lower end of the piston rod and a pressure chamber of the cylinder being connected through one or more valves with a source of pressurised fluid.

For aircraft undercarriages oil-and-air systems are used almost exclusively. The single-chamber strut which proves adequate for many purposes and which is therefore the type chiefly used consists mainly of an air spring with an oil damper connected in parallel therewith. The drawbacks of this system are that the static spring characteristic is not optimum, the damping factor is only satisfactory for a descent speed, and when high load peaks occur, when the aircraft is running over short undulations in the ground, the system reacts to excessive spring stroke speeds. In cases in which single-chamber oleos are inadequate more complex constructions are used which offer spring characteristics better adapted to the purpose.This may be achieved by the addition of a further air chamber or the use of throttles of variable cross section with changes governed either by the pressure or by the travel distance. The more satisfactory dynamic characteristics obtained are at the cost of increased complexity but the system does ensure a closer approach to the optimum dynamic characteristics than is possible with a single-chamber oleo strut although ideal conditions can never actually be attained. One of the reasons is that the components used are of a mechanical nature and follow their own particular laws.

The spring characteristics remain confined to a combination of polytropes and throttle curves.

This invention seeks to provide an oleo strut in which the spring characteristics and damping characteristics can be controlled in such a manner that they correspond to predetermined values which can be formulated mathematically.

According to this invention there is provided an oleo strut comprising a cylinder, a piston reciprocable in the cylinder and defining therewithin on one side of the piston a chamber for pressurised fluid which is communicable via valve means with a source of pressurised fluid and a piston rod extending from the other side of the piston and having mounted at its free end a ground-engaging wheel, wherein means is provided for comparing the prevailing fluid pressure in the pressure chamber with a predetermined pressure value and for relieving pressurised fluid from the pressure chamber or for connecting the pressure chamber with said source to tend to eliminate any discrepancy between the compared values.

An advantage is that one and the same leg will adjust itself quickly to variations of load, ground conditions and the like involved in any one of a number of different situations in use.

The invention is further explained in conjunction with embodiments shown by way of an example in the accompanying drawings, wherein: - Figure I shows an oleo strut controlled by a servo-valve, and Figure 2 shows an oleo strut controlled by a system of valves.

Figure 1 shows a basic diagram of a preferred embodiment of oleo strut according to the invention, with a single-chamber of the known type, mainly comprising a cylinder 1 forming an integral part of the aircraft, a piston rod 2, separating piston 4, an air chamber 14 an oil pressure chamber 5 and a ground wheel 3. A load sensing device comprises a regulating piston 6, a servovalve 7, a pressure oil connection 19 with a pressure accumulator 13, a servo amplifier 8, a summation point 9 and a pressure pick-up 10, and a control means comprises a pre-programmable electronic computing circuit 12 with a displacement pick-up 11 and at least one further input 22.

In the event of a sudden shock load the piston rod 2 enters the cylinder 1 fixed to the aircraft structure and oil from the chamber 5 passes the throttle point 15 and the air enclosed in the chamber 14 is compressed by the piston 4. Oil pressure resulting from the throttle effect and from the air compression then builds up in the chamber 5. When the load is stationary the piston rod 2 assumes a position in relation to the cylinder 1 which results from part of the weight of the aircraft and the compression curve of the air. In the electronic computing circuit 12 the functions of the desired spring-damping characteristic are predetermined and by using the travel of the oleo strut and further parameters 22, the relevant reference load for the strut is continuously determined. This required load then serves as the control signal 16 for the operation of regulating the load of the strut.

The load 17 occurring is measured, compared at a summation point 9 with the reference load 16 and the load difference is fed through a servo amplifier 8 to the servo-valve 7 in such a way that in the event of an excessive load on the oleo strut oil passes out from the strut while if the load is too low oil flows in. The load actually occuring is thus caused to follow-up the reference load. If the available quantity of oil pressure medium is insufficient for the spring expansion process, then the quantity still required is supplied by the pressure accumulator 13, in which process a nonreturn valve 21 prevents hydraulic liquid from flowing back into the pressure supply system.

Figure 2 shows a further embodiment of the invention. This purely hyraulic oleo strut has no air chamber and comprises a cylinder 30 forming an integral part of the aircraft, a piston rod 31, a displacement pick-up 32, a pressure pick-up 33, an overload throttle 34, a magnetic valve 35 and four throttle points 36, which can be opened electrically. On compression of the oleo strut the piston rod 31 is pushed into the cylinder 30 by the load on the undercarriage. In this process the required value of the load is continuously determined, by means of the computing circuit 37 and using the travel stroke of the oleo strut given by the displacement pick-up 32. The required value is conveyed to the electronic control system.In the latter the reference value is compared with the actual value indicated by the pressure pick-up 33, and by a suitable circuit logic system 38, the exact number of throttles 36 is opened that is required to ensure that when the hydraulic fluid flows out of the chamber 41 and through the throttle 36 into the return conduit 40, the oleo strut will receive a load which is very close to the required reference load. In the event of particularly violent shock loads the overload throttle 34 opens, in order to ensure that the strut will not be subjected to a load in excess of a certain preselected maximum. The non-return valve 39 prevents hydraulic liquid from flowing back into the supply conduit 42.By the provision of the throttles, which can be electrically opened, the oleo strut characteristic preselected by the computing circuit 37 is approached by a graded function, the greater the number of throttles 36, the more accurate is this approach.

In an alternative version of the invention further parameters may be fed to the inputs 22, 43. This enables the aircraft to be lowered, for example, for loading or for the nose to be raised in order to assist take-off.

Furthermore by feeding in a correction value derived from the vertical descent speed the landing gear can be automatically programmed to certain loads to be expected during loading.

In a further embodiment of the invention a purely hydraulic oleo strut, such as is shown in Figure 2, has a servo-valve with two outputs which lead to the chambers 41 and 44. The compression movement of the oleo strut is thus hydraulically assisted, which is of advantage in the event of very rapid compression movements.

In a further advantageous embodiment of the invention the computing circuit 12, 37, is fed with a signal provided by a position pick-up. This ensures that when taxiing a turn it will not cause the aircraft to roll.

The use of an oleo strut according to the invention is not confined to aircraft undercarriages but can be used wherever such a strut is required.

WHAT WE CLAIM IS: 1. An oleo strut comprising a cylinder, a piston reciprocable in the cylinder and defining therewithin on one side of the piston a chamber for pressurised fluid which is communicable via valve means with a source of pressurised fluid and a piston rod extending from the other side of the piston

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. a system of valves. Figure 1 shows a basic diagram of a preferred embodiment of oleo strut according to the invention, with a single-chamber of the known type, mainly comprising a cylinder 1 forming an integral part of the aircraft, a piston rod 2, separating piston 4, an air chamber 14 an oil pressure chamber 5 and a ground wheel 3. A load sensing device comprises a regulating piston 6, a servovalve 7, a pressure oil connection 19 with a pressure accumulator 13, a servo amplifier 8, a summation point 9 and a pressure pick-up 10, and a control means comprises a pre-programmable electronic computing circuit 12 with a displacement pick-up 11 and at least one further input 22. In the event of a sudden shock load the piston rod 2 enters the cylinder 1 fixed to the aircraft structure and oil from the chamber 5 passes the throttle point 15 and the air enclosed in the chamber 14 is compressed by the piston 4. Oil pressure resulting from the throttle effect and from the air compression then builds up in the chamber 5. When the load is stationary the piston rod 2 assumes a position in relation to the cylinder 1 which results from part of the weight of the aircraft and the compression curve of the air. In the electronic computing circuit 12 the functions of the desired spring-damping characteristic are predetermined and by using the travel of the oleo strut and further parameters 22, the relevant reference load for the strut is continuously determined. This required load then serves as the control signal 16 for the operation of regulating the load of the strut. The load 17 occurring is measured, compared at a summation point 9 with the reference load 16 and the load difference is fed through a servo amplifier 8 to the servo-valve 7 in such a way that in the event of an excessive load on the oleo strut oil passes out from the strut while if the load is too low oil flows in. The load actually occuring is thus caused to follow-up the reference load. If the available quantity of oil pressure medium is insufficient for the spring expansion process, then the quantity still required is supplied by the pressure accumulator 13, in which process a nonreturn valve 21 prevents hydraulic liquid from flowing back into the pressure supply system. Figure 2 shows a further embodiment of the invention. This purely hyraulic oleo strut has no air chamber and comprises a cylinder 30 forming an integral part of the aircraft, a piston rod 31, a displacement pick-up 32, a pressure pick-up 33, an overload throttle 34, a magnetic valve 35 and four throttle points 36, which can be opened electrically. On compression of the oleo strut the piston rod 31 is pushed into the cylinder 30 by the load on the undercarriage. In this process the required value of the load is continuously determined, by means of the computing circuit 37 and using the travel stroke of the oleo strut given by the displacement pick-up 32. The required value is conveyed to the electronic control system.In the latter the reference value is compared with the actual value indicated by the pressure pick-up 33, and by a suitable circuit logic system 38, the exact number of throttles 36 is opened that is required to ensure that when the hydraulic fluid flows out of the chamber 41 and through the throttle 36 into the return conduit 40, the oleo strut will receive a load which is very close to the required reference load. In the event of particularly violent shock loads the overload throttle 34 opens, in order to ensure that the strut will not be subjected to a load in excess of a certain preselected maximum. The non-return valve 39 prevents hydraulic liquid from flowing back into the supply conduit 42.By the provision of the throttles, which can be electrically opened, the oleo strut characteristic preselected by the computing circuit 37 is approached by a graded function, the greater the number of throttles 36, the more accurate is this approach. In an alternative version of the invention further parameters may be fed to the inputs 22, 43. This enables the aircraft to be lowered, for example, for loading or for the nose to be raised in order to assist take-off. Furthermore by feeding in a correction value derived from the vertical descent speed the landing gear can be automatically programmed to certain loads to be expected during loading. In a further embodiment of the invention a purely hydraulic oleo strut, such as is shown in Figure 2, has a servo-valve with two outputs which lead to the chambers 41 and 44. The compression movement of the oleo strut is thus hydraulically assisted, which is of advantage in the event of very rapid compression movements. In a further advantageous embodiment of the invention the computing circuit 12, 37, is fed with a signal provided by a position pick-up. This ensures that when taxiing a turn it will not cause the aircraft to roll. The use of an oleo strut according to the invention is not confined to aircraft undercarriages but can be used wherever such a strut is required. WHAT WE CLAIM IS:

1. An oleo strut comprising a cylinder, a piston reciprocable in the cylinder and defining therewithin on one side of the piston a chamber for pressurised fluid which is communicable via valve means with a source of pressurised fluid and a piston rod extending from the other side of the piston

and having mounted at its free end a ground-engaging wheel, wherein means is provided for comparing the prevailing fluid pressure in the pressure chamber with a predetermined pressure value and for relieving pressurised fluid from the pressure chamber or for connecting the pressure chamber with said source to tend to eliminate any discrepancy between the compared values.

2. An oleo strut as claimed in claim 1, wherein the predetermined pressure value is determined by a control signal which is variable in response to variations of the position of the piston in the cylinder.

3. An oleo strut as claimed in either preceding claim, wherein the predetermined pressure value is established by a preprogrammable electronic control means.

4. An oleo strut as claimed in claim 3, wherein variations in the volume of pressurised fluid in the pressure chamber are controlled by a servo-valve which responds to a control signal from said control means.

5. An oleo strut as claimed in claim 4, wherein an amplifier and a summation means are interposed between the control means and the servo-valve, the summation means being connected to means monitoring pressure in the cylinder.

6. An oleo strut as claimed in any one of claims 1 - 3, wherein fluid pressure in the pressure chamber is variable by the sequential operation of a series of valves.

7. An oleo strut substantially as hereinbefore described with reference to and as diagrammatically illustrated in Figure 1 or Figure 2 of the accompanying drawings.