Repair MLG Hydraulic Cylinder v2

I hate doing things twice…. During landing gear retract testing, I discovered a small hydraulic fluid leak around the shaft end of the cylinder. Since I had replaced all the seals in the cylinder only a few months previous, I was curious to see what the issue was.

I removed the cylinder and took it apart, and on a very close inspection found that the seating surface for the poly-pak shaft seal had a raised area. Microscope inspection revealed that the very thin metal between the NPT tapped area for the fitting was pushed toward the seating surface and that there was a very small hole that was allowing hydraulic fluid to leak. No way to fix this – had to order a new end piece from the factory.

Hydraulic Pump Modifications

It turns out that the version of hydraulic pump supplied to Velocity is configured with the UP side as the high pressure side and the DN side as the low pressure side. The pump itself is symmetrical, producing full pressure independent of rotation direction. Changing the DN side to be the high pressure side requires swapping the internal pressure relief valves at the base of the pump, and recalibrating them for the desired pressure.

As an aside, simply switching the hydraulic lines between the UP and DN side will not work, because the internal valve configuration of the pump requires that hydraulic cylinder retraction (which is our gear up) occurs on the DN side of the pump. The difference lies in the excess hydraulic fluid volume that must be supplied to the cylinders when the UP side of the pump runs (our gear down). Confusing? You betcha.

Scott S. offered to do this PRV swap and calibration at the factory, but I had the required equipment to do it and adjust the PRV settings.

First Landing Gear Swing Test

The nose and main landing gear hydraulic systems were connected to the hydraulic pump and power was supplied to the hydraulic pump bus. The emergency gear operation switch (in the pilot’s emergency panel) was wired up to allow testing of the gear retraction and extension.

As the video shows, the hydraulic pump pressure relief valve opens before the high pressure set point on the solenoid switch is reached. The result is that the hydraulic pump continues to run after the gear is up, and the “screeching” noise is the pump’s DN-side PRV opening.

Further investigation is required.

Nose Landing Gear Door Modifications

A few modifications were made to the nose landing gear doors and their open/close linkage.

During landing gear extend/retract testing it was difficult to find a nose gear door adjustment that satisfactorily held the doors shut, but opened promptly enough to prevent the doors from rubbing the tire on extension. I decided to remove the raised foam area on the doors to help prevent interference with the nose tire. The foam was sculpted and formed with EZ-poxy with flow, and then covered with carbon fiber for extra strength.

The rod ends on the nose gear door closing linkage were replaced with more sturdy and larger -3 versions that will not pop apart. I used left-threaded rod ends on the doors, and tapped an aluminum rod to act as a pushrod whose length can be adjusted by twisting it like a turnbuckle.

MLG Cable Attach

Because space in the keel is at a premium, I designed a smaller gizmo that attaches the MLG cables to the hydraulic cylinder. Fabrication of these parts are beyond the scope of the tools I have in my shop, so I set this out for fabrication – ouch$.

The new device has several features. It has an aluminum cylinder that fits over the actuator shaft, but it is threaded on a 1/4″-28 rod and so the MLG retraction limit can be adjusted by simply screwing the cylinder to the desired location and then locking it in place with a jam nut. In addition, the MLG cable rod ends fit into the device and tension/travel can be independently adjusted with nuts. Once adjusted, the nuts are secured with safety wire.

Main Gear Cylinder Leaks

When trying to swing the gear with compressed nitrogen, it was apparent that the main landing gear hydraulic cylinder was leaking. After pressurization and retraction, if the cylinder was valved off, the gear legs would slowly fall down. The leak seemed to be around the actuator shaft and not internal leakage around the piston.

Since I had to do the same teardown and seal replacement on the nose gear actuator, I decided to pull the MLG cylinder out and replace all of its seals too.

Hydraulic Pump and Lines

The landing gear hydraulic pump was mounted onto the MLG bulkhead with a thick aluminum backing plate. Bolts enter from the aft side, pass through the backing plate, and then screw into the hydraulic pump body. The pump reservoir is about 2 cm off the floor of the fuselage.

After making a couple of flexible hydraulic lines that run from the pump to the fittings on the MLG hydraulic cylinder, I started to worry about the hydraulic lines fouling the MLG retraction cables. One solution would’ve been to fabricate rigid hydraulic lines, but I decided to separate the keel space vertically into an upper deck and a lower deck. The brake lines, which are rigid tubing, and the MLG cables will occupy the lower deck, an the aileron torque tube, hydraulic lines, and wiring will be on the upper deck. A thin layup was made with 2x BID on aircraft plywood, and this was shaped to be the divider. An aluminum tunnel was made to cover the aft end of the gear cables, and to provide a transition for the hydraulic lines to enter the keel.

Landing Gear Dump Valve

I decided to put the landing gear hydraulic dump valve inside the keel, where it will be accessed by opening a door just under the pilot’s right elbow. I plan to also put the main fuel shutoff knob here. This location is above the aileron torque tube, and should not interfere with other stuff in the keel.

An aluminum bracket was made to hold the valve itself, and a drilled aluminum backing plate was glassed onto the exterior wall of the keel to be a hard point for the valve.

Landing Gear Retraction Test

With the main landing gear filled and primed, and with the brake lines attached to the gear legs, I did a retraction test to make sure that there was still no interference with the MLG cutouts in the fuselage (which had to be widened slightly), and the aluminum tubing for the header tank vent and drain. The trusty nitrogen cylinder was used to do the retraction, and all was successful.