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Hydraulic System

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Having elected to use the Infinity retractable main landing gear which is hydraulically activated it became obvious to use the hydraulic pump as a common source of motive power for the nose gear and the speed brake in addition to the main landing gear for which it had original been intended. It was important that its use, in these additional functions, did not adversely affect the operation in its primary function namely the actuation of the main landing gear. To understand the functionality of the full system it is necessary to understand the individual systems and their components.

Hydraulic Components

The following section describes the components used within the final design. This paragraph describes the function of each element.

Sequencing Valve SV1

SV1 is a 2-position, 3-way, spool-type, solenoid operated directional control valve. The diagram illustrates two boxes, a spring on the right hand end, and an actuating coil on the left hand end. The Infinity Aerospace kit includes this valve using a 12 volt 24 watt energizing coil. Un-energized, the valve behaves as indicated in the left hand box allowing fluid to pass in the direction of the arrow to the port on the left hand side of the box. Once the coil is energized the spool moves to the right against the action of the spring and allows the fluid flow according to the schematic of the right hand box. To the electrical engineer this component is equivalent to single pole double throw relay and can be treated as such in the system design.

These valves are not particularly good for internal leakage and have a specified maximum leakage of 82 cc/min at 3000 p.s.i....

Sequencing Valves SV2, SV3, & SV3

SV2, SV3, & SV4 are 2-position, 4-way, spool-type Solenoid operated directional control valves (Parker part number DS084NxxD012LS-4P). The diagram illustrates two boxes, a spring on the right hand end, and an actuating coil on the left hand end. These parts are unique to my design. I preferred having 8-32 screw and nut for the electrical attachment over the molded integrated wires that came with the Infinity aerospace sequencing valve. I also elected to use only 12 watt coils as I saw no benefit in having the larger current draw of the kit valve. Un-energized, the valve behaves as indicated in the left hand box preventing the passage of fluid to and from the pump to the hydraulic circuits. Once the coil is energized the spool moves to the right against the action of the spring and allows the fluid flow according to the schematic of the right hand box. To the electrical engineer this component is equivalent to double pole single throw relay and can be treated as such in the system design. Since I had the benefit of selecting an option I selected "Push and Twist Manual override" as this would allow me to effect the operation of the valve in the unlikely event of a coil or electrical failure. The valves are mounted under the pilot's seat so that emergency operation is possible in flight.

These valves are not particularly good for internal leakage and have a specified maximum leakage of 82 cc/min at 3000 p.s.i..

Check Valve CV1

CV1 is a simple check valve and allows fluid flow in only one direction. The illustration to the right will allow fluid flow from left to right but not right to left. The seal on these valves is very good and internal leakage in almost undetectable.

 

Dual Pilot Controlled Check Valves DPCCV1, DPCCV2, & DPCCV3

Dual Pilot Controlled Check Valves ( DPCCV's ) are simple passive devices (i.e. no solenoid is used for activation) that allow flow in a direction similar to that of CV1 however they will also allow return flow if there is hydraulic pressure being fed through the dotted line. The valves seal extremely well and will hold pressure for several months unlike the few minutes of the spool valves described above.

 

 

The simple circuit illustrated alongside is the circuit necessary to provide isolated hydraulic pressure to the speed brake and the nose gear. Without the DPCCV's the leakage through the two spool valves would cause a mutual interaction so that as a force is applied to the rod of the speed brake (a condition that is characteristic of the air loads on the speed brake) hydraulic fluid from the speed brake's piston would be transferred to the piston side of the nose gear and would cause the nose gear to be lowered whilst the speed brake would be slowly retracted. The introduction of the two DPCCV's isolate the circuits so that once the applied pressure from the pump is equal or lower than that of the circuit the check valves close and seal completely thereby giving total isolation with no creep.

Emergency Valve

The emergency valve is a design from Infinity Aerospace. The valve is usually left in its sealed position. In the event of an electrical failure causing the hydraulic pump to be inoperative, the pilot screws in the tap on the end of the valve. This punctures the end of the sealed CO2 cartridge and sends CO2 into the system, at a pressure of 850 p.s.i. in a direction corresponding to the curved arrow direction. The second arrow indicated that a path through the valve is also made available and this allows the fluid, from the main gear cylinder, to return to the reservoir.

Standard Configuration for the Infinity Gear

The standard hydraulic circuit offered by Infinity Aerospace is indicated to the right. In this configuration there should be no need for any DPCCV's unless the valves internal to the pump are not adequate.

In the retract mode hydraulic pressure is initially directed to the strut compression cylinders to compress the strut. The strut cylinder is backed by high pressure air used for the oleo suspension. This is achieved by activating SV1 at the same time as operating the hydraulic pump in the 'UP' direction. There is no need for any hydraulic return in this operation.

Once the strut is compressed SV1 is de-energized and the hydraulic pressure is diverted to the main gear shoulder brace. This action causes the gear leg to retract. Hydraulic fluid on the back side of the shoulder brace cylinder is directed back to the reservoir in without impedance.

To lower the gear the flow direction from the hydraulic pump is reversed and the sequencing process is also reversed however, if SV1 is energized before the gear leg (shoulder brace cylinder) has reached the full travel fluid may be trapped behind the piston of the shoulder brace cylinder and the gear leg may not be fully deployed. Infinity Aerospace overcome this using an electronic controller incorporating some timing circuits and a sensor in the shoulder brace.

Sine I was needing to modify the controller, or indeed dispense with the controller so that I could operate all of my hydraulic system from a single controller I sought to find an alternative method that would activate using the available hydraulic pressure. I included a check valve CV1 which performs this function.

In operation during the 'UP' cycle the valve does nothing. The SV1 is activated and the Oleo strut is compressed with a pressure of approximately 1000 p.s.i., SV1 is then de-energized and the pump provided hydraulic pressure to the shoulder brace cylinder. The cylinder requires approximately 450 p.s.i. to raise the gear so check valve CV1 remains closed throughout this operation. (note: the internal leakage of  SV1 will eventually cause the shoulder brace to under the same pressure as the Oleo strut.).

On the 'DOWN' cycle the SV1 is inactive and hydraulic pump reverses pumping fluid to the rod side of the shoulder brace cylinder whilst recovering fluid from the piston side of the same cylinder. The gear leg comes down under these conditions and the micro-switch on the shoulder brace changes state and activates SV1. The hydraulic pump continues to try to pump fluid to the rod side of the shoulder brace cylinder whilst recovering fluid from the Oleo strut as it un-compresses. The high pressure of the Oleo strut forces fluid to return to the reservoir (the pump tries to suck the fluid from the Oleo strut). Now if the shoulder brace micro-switch is incorrectly set, and SV1 is activated early, the fluid trapped behind the piston of the shoulder brace, is expelled through the check valve CV1 thus preventing the premature activation of SV1 causing incomplete operation of the shoulder brace. The benefit of this solution is that it is solves a hydraulic problem using the hydraulic pressure that is causing it.

Full System Schematic

The full hydraulic system for operating all threes systems is depicted below. I have created a hydraulic buss by connecting SV2, SV3 & SV4 in parallel combined with their respective DPCCV's. Notice that I have pressure gauges showing 'UP' & "DOWN' pressures for both the Main gear and the Nose gear. I did not feel that it was necessary to incorporate pressure gages for the speed brake and the pressures for this function are minimal.

 

 

Last Updated:   

Sunday September 10, 2006