Author: bzvelocity

Building a Velocity XLRG-5

Sump Tank Modifications v3

The fuel selector valve will select which fuel tank (or aux tank) delivers fuel to the sump tank. I decided to add a 1/2″ inlet to the sump tank, level with the fuel selector valve, which will now be the fuel inlet. This required cutting a hole in the back of the sump tank and adding a new hard point. The rear cutout was glued back in place with floxxy-poxy and leak-checked.

I will use one of the existing fittings in the upper corners of the sump tank for fuel return from the engine fuel servo, and simply cap the other one. I added an emergency shutoff ball valve on the fuel feed line to the engine.

Fuel Selector Valve and Linkage

Based on the results of the previous fuel selector valve tests, I made the decision to put the fuel selector valve between the whale tail and the landing gear bulkhead, just above the MLG pulleys and between the right and left brake hydraulic lines.

The Andair valve I purchased (Right, Left, Off, and Aux) has four 1/2″ ports, and orientation is important for the fuel lines and for the shaft linkage. I fabricated an aluminum sheet metal mount that will mount to the landing gear bulkhead. The valve position is slightly right of center because the shaft runs in the right side of the keel.

I calculated that even a steel 1/4″ diameter shaft of the required length would have too much torsional deflection, so I opted to make the shaft from 3/8″ aluminum. The estimated torsional deflection is about 3 degrees. The tank selector is mounted just below the instrument panel, accessible by both pilot and copilot. From there, a shaft runs down to a 90 degree gearbox, and then the shaft runs aft all the way the the whale tail, supported by oillite bushings. A u-joint directs the shaft up to the selector valve.

It is pretty busy in this area with the battery and solenoids, hydraulic pump, brake lines, MLG retraction cables, and fuel selector valve. I still have to route the landing gear hydraulic lines and a cable conduit in the keel, so it will get even tighter.

Brake Lines

All of the brake lines were fabricated and installed. I used Aeroquip hose for the flexible portions around the rudder/brake pedals, and then -3 hard tubing from the canard bulkhead all the way back through the keel to the landing gear bulkhead. Exiting the keel at the whale tail, the two tubes make a bend outward and upwards, passing through the landing gear bulkhead on either side of the MLG pulleys. Elbow bulkhead connectors then make the transition to the flexible hose that continues down the gear legs to the brake assemblies on the wheels.

Aileron Torque Tube Bearings

While positioning and repositioning the aileron torque tube in the keel, I started thinking about how one might perform maintenance. I was also not happy with the way the build manual suggested to hold the torque tube bearings into their mounting angles.

I designed a very simple retainer ring that holds the bearing very nicely into the mounting bracket. I also modified the bracket by cutting out a portion of the bracket so that the torque tube can be removed simply by removing the three socket screws that retain the bearing, and then slipping the bearing out of the mounting bracket. Finally, installing nut plates on the mounting brackets makes their installation into the keel easier.

Fuel Selector Valve Testing

The plan is to install a fuel tank selector valve, inspired by Ron Stacey’s very nice Velocity. As Ron says – if you think about it, there are no certified aircraft that don’t have fuel selectors. Another advantage is that a selector valve provides a handy place to hook up an optional auxiliary fuel tank.

The options break down as follows:
A) Position the selector valve close to pilot, probably in the side of the keel. This requires routing fuel from the strakes up to the valve and then back to the sump tank. Because of the long runs, 1/2″ tubing is the minimum practical size (IO-550 engine). Thats lots of fuel line, and there is not much space in the keel for three parallel runs, especially back at the whale tail.
B) Position the selector valve somewhere behind the whale tail. This requires running some type of linkage (cables + pulleys or shafts + u-joints) between the selector knob and the valve itself, but eliminates the long runs of fuel tubing.

Another potential issue with option A is that when the aircraft is in a climb, the selector valve may be higher than the fuel tank. The sump tank is still below the fuel, so this represents a siphon condition – the fuel must run uphill to the valve, and then back downhill to the sump. This should be fine as long as there are no bubbles in the fuel line.

To test this, I set up the fuel valve with the correct distance from and with varying height with respect to a fuel tank, which was the correct height off the “floor”, and could be filled with varying heights of “fuel”. I used water for the tests, and converted the measured flow rates to estimated fuel flow by scaling with the viscosity ratio. The simulated sump tank was a graduated bowl, and for each simulation scenario I measured the time it took for one liter of water to flow into the sump.

The measurements indicate that the siphon works OK, and the maximum expected fuel flow rate (full tanks, level attitude) into the sump was about 46 GPH, in excess of what is required for an IO-550 at full power. This flow rate was only slightly decreased when changed to a steep climb. However, with the fuel tank at 1/4 full, the flow rate into the sump decreased to about 28 GPH, which is marginal for a high power setting. The most worrisome aspect was that introducing a bubble into the feed line from the fuel tank to the valve caused the flow to stop. This always happened if the attitude was climbing, and even sometimes if the attitude was level. The only way to reestablish flow was to change the attitude to a significant dive.

The bottom line is that putting the fuel selector valve forward is very risky without also installing a pump that can re-prime the fuel lines (or run all the time!) to prevent the loss of siphon. Not worth it…

Solenoids and Battery Boxes

I fabricated some aluminum supports for holding the hydraulic pump solenoids and the front and aft battery boxes with their solenoids. I have had very good experience with the Concorde 35 series lead-acid batteries in the Viking, so I thought I would use them in the Velocity.

The plan is to have an aft main battery for starting and providing power to engine and other accessories, and a forward battery in the nose compartment for the avionics. A switchable crosstie will allow charging of the nose battery from the alternator through an isolation rectifier. Both battery boxes will have main fuses and current-sensing shunts installed, as well as two solenoids each (aft: master and starter; nose: avionics master and crosstie).

Aileron Trim Actuator

The factory upgraded the aileron trim mechanism around the same time that they moved to the pushrod-bellcrank mechanism. The end plate at the “whale tail” that holds the aft end of the aileron torque tube was made smaller, and the old trim motor, string, and spring were eliminated. The replacement is a linear actuator with an integral spring.

There are no instructions from the factory for installation, so I plan to install the actuator on the pilots side.

Engine Mount Bolts

Scott Swing let me know that the engine mount for a TCM IO-550 is 24″ wide at the bottom, and that the bolts penetrate the firewall just above the fuselage floor. I had previously cut down my header tank to make more room in this area, and I was relieved to find that there appears to be sufficient space to accommodate the bolts and the large washers. The 12″ mark is aligned with the fuselage centerline, and the tape measure shows the approximate location where the holes will be.