Dump Valve

This blog covers the installation of the hydraulic dump for the undercarriage.

Overview
The factory supplies dump valves which on the current factory aircraft are installed in the floor just in front of the nose wheel well. Without installing them, it was decided that the access would be a difficult for this builder when harnessed.

A Matco dual park brake valve would allow the emergency extension circuit to be activated using lockable cable mounted on the secondary panel. Of concern is the restriction created by the integrated check valves but as the undercarriage on the Tucano is not a free fall system and is pumped down in an emergency it would seem not to be an issue. The dual port design allows both the fluid from the up circuit to return to the pump reservoir while connecting the down circuit  to the hand pump in one operation.

Matco Valve

Cable T-Handle Button

A button operated cable was selected for the dump requiring a deliberate action to activate it, a  length of 6' was selected but on arrival it had a attachment at the end not apparent at the time of ordering - more later.




Installation
A 0.040'' x 2024-T3 aluminum plate was cut, alodined then riveted into the corner of the lower panel on the opposite side to the emergency pump using 4 x 3/32'' c/s pop rivets. A angle return was included allowing the fuel lines to share the same bracket.

A graphic for the panel was drawn with Draft Site, edited in Inkscape and saved as an EPS file for printing.

The Tucano spar has provision for two x 20 mm dia holes through the spar face with a suitable doubler both holes were drilled then edges polished with 400 grit aluminium open cut then installed a pair of 50 mm OD x 0.040'' 2024-T3 doubler fixed with four [4] 1/8'' pop rivets. 

Factory drilling details to suit design 

Note: This drilling is different to published photos but allows the dump valve to be located in the hydraulic bay.

Next the Matco valve was fitted onto the side of the rib adjacent on the starboard hole with the actuator lever located about the 20 mm od hole center while being secured using M4 x 30 mm screws via rivet nuts fitted previously to the rib. 

Dump valve installation
P clamp was installed at the end of bowden cable

P Clamp

The actuator cable was installed running down the side of the wheel well, then along the floor through existing openings in the seat mount and finally through the spar opening. 

The cable was secured using a series of alloy AN3 [4.8 mm ID] P clamps drilled to 6.35 mm to match the OD of the cable outer and then secured using M4 x 16 S/S countersunk screws fitted into rivet nuts installed into the floor panel.

Note: Running through the seat frame was not the prefered option but the available length of the cable created limitations for any alternative routes.

The six [6] foot cable was just long enough but the swaged fitting at the end useless a decision was made to remove it using a Dremel motor tool, it was slit on both sides and the sleeve removed then the threaded attachment cut leaving about 2 mm attached, finally steel eye was crimped onto the cable behind the lug allowing attachment to the valve lever. The eye was secured with a AN3 bolt with nylock nut with a custom spacer machined to clear the arm and to support the cable in it's correct location. 

Note: Fitting was sourced from Hobby & Engineering Supplies  Part # SM-EYE

One of the most time consuming task was the mix of NPT / ISO threads creating the need to manufacture custom nipples to connect the hand pump, shuttle valve using the metric hose connectors. 

Dump valve actuator

Note: These issues resulted from the installation of an alternative emergency hand pump  with remote dump valve.

The factory supplied 6 mm od hose was used for the high pressure circuit in preference to rigid line for ease of installation with only a slight weight penalty with nyon 6.35 mm od tube used in the low pressure supply to the hand pump.

All hoses were secured to the airframe as required with aluminium line separators used by the automotive industry. All fittings were fixed using M3 rivet nuts and pan head screws. The separators were lined with Kapton tape before assembly to protect the hose.

Note: All the high pressure fittings and hose were supplied by FREN TURBO Italy.

Comment
The suitability of the valve chosen cannot be verified until emergency dump tests are conducted but the the manufacturer replied to a query that all valves are tested to 1200 psi and with the hydraulic pump labeled at 800 psi [60 bar] maximum pressure this creates a 1.5 : 1 operating margin.

The button cable was an expensive choice and the builder would examine cheaper lockable option if doing it again

The Dremel motor is an overrated tool but the new snap on 1.5'' cutting discs combined with a flexible drive has made the tool useful for the tasks above, in fact there would be no other way to remove the end without it.