Archive for July, 2011

Current sensor spacers

Saturday, July 23rd, 2011

I'm using Amploc brand Hall effect sensors (most easily found as a GRT accessory) instead of shunts to measure alternator current in my airplane. You pass the bus wire through the middle of the sensor, which is a bit less than an inch in diameter:

The wire doesn't have to be centered inside the sensor, but I still wanted to come up with a way to secure it and make it look nicer. This just looks sloppy to me:

I went up to Airparts and bought some 1" diameter nylon rod, then turned it down on my lathe so it would exactly fit the inside diameter of the current sensors. Having a lathe is great, even if I am only barely competent at using it.

With a bit chucked in the tailstock, I center-drilled the nylon to fit the diameter of the wire. After I took this photo, I parted off what I needed and cleaned up the ends a bit.

Voila, now it's a perfect fit:

I made one spacer with a 3/8" hole to fit the #4 wire from the main alternator, and another one with a 5/16" hole to fit three turns of #10 wire for the secondary alternator. With a Hall effect sensor, running multiple turns of wire through the sensor gives you a current reading that's multiplied by the number of turns, which your engine monitor then divides back down to give you the true current value. This is a handy way to increase the accuracy of the measurement, as long as your EFIS supports it, which mine does. Since these are 100A sensors, three turns of wire from the 20+ amp standby alternator should be just about right.

I glued the spacers into the sensors with E6000. Nylon is resistant to most glues, but I'm hoping this stuff will grab hold of the roughed-up surface.

Next: Off to Oshkosh for a week…

Exhaust pipe supports

Sunday, July 10th, 2011

One of the million little things surrounding the engine that I've been needing to figure out is the exhaust pipe supports, sometimes also referred to as exhaust hangers. These are made from two pieces of stainless tubing that are each flattened at one end, joined by a piece of rubber fuel hose that's clamped in between. Here is the single page of xeroxed handwritten instructions that was included in the kit, so you can get an idea of the end goal:

The idea is that these will support the cantilevered exhaust pipes, while also providing some amount of shock absorption. I temporarily used clear vinyl hose while fitting them, so I could see how the separation between tubes was working out. You don't want the tubes to touch, since it would negate the vibration-absorbing properties of the hose; you also don't want them to be too far apart, since the hose will collapse when the airstream tries to push upwards on the exhaust. I shot for a gap of 1/4"-3/8" per advice from Larry Vetterman.

To help prevent the hose from slipping, I roughed up the ends of the tubes with a file, and lightly expanded the tube mouths with my flaring tool.

I put the lower cowl on so I could move the pipes around on their ball joints to center them in the cowl outlet and get 3/4" of clearance all around:

It's hard to photograph the exhaust hangers since there's so much other stuff in the way. It's also hard to install them when there's so much other stuff in the way! My throttle cable was in the way of one of them, and since there's no way to move it, I had to anchor the exhaust supports to the engine differently from how it's shown in the drawing.

A view from the floor, looking up at the bottom of the engine, while fitting the transverse support tubes. The exhaust hangers are supposed to go straight up, but with the throttle cable in the way I had to angle them inward to provide adequate clearance.

Closeup of the top end – little steel tabs attach the tube ends to convenient bolts on the oil sump:

This photo of the finished transverse support also shows how I had to bend the tabs on the pipe clamps to get the angles I needed:

I wish I could have supported the pipes farther aft, but this was as far back as I could get the clamps to go without causing the starboard support to hit the throttle cable, or creating a big gap between the tubes that made the hangers too floppy. Longer tubes would have helped here – perhaps I'll make some replacements someday, but for now we'll see how these work.

Oil cooler plumbing

Saturday, July 2nd, 2011

The firewall forward kit came with a pair of hoses for the oil cooler, but they were the wrong length and I wanted to use a different type of hose anyway. I wish I would have thought to delete them from the order and save some money, but oh well. Anyway, to figure out the correct length to order a set of replacement hoses, I used some plastic ice maker tubing of a convenient diameter. I played with different hose routing and fittings, and decided that a 45-degree fitting was needed on the bottom, and a right-angle fitting worked best on top.

Once I had the lengths, I called up Precision Hose in Tulsa and ordered two Stratoflex 124J hoses with integral firesleeve, with #8 fittings, 13" flare to flare. They cost darn near a hundred dollars each, but they are just as nicely made as everything else I've ordered from PHT.

Notice how much longer the ones from the FWF kit are. Even if they'd been the right size, I'd have also had to buy and install a separate firesleeve on each of the factory hoses anyway. My rule is to always use firesleeve on important hoses forward of the firewall that carry flammable fluids.

The top hose returns oil to a 45-degree fitting on the engine (necessary to clear the oil filter adapter) from from a 90-degree fitting on the oil cooler:

There's over an inch of clearance between the hose and engine mount all the way around. The bend is a little tighter than I'd like, but it seems like it will work.

It's hard to photograph the bottom hose, which supplies oil from the engine to the cooler:

From certain angles it looks like the hose rubs against the prop governor bracket, but if you look at it from the starboard side you can see that there's enough clearance. These parts don't move relative to one another, so anything over a quarter inch is plenty.

The blue aluminum fittings shown in the photos above were installed hand-tight, and only for fitting purposes. For the real deal I wanted to use steel fittings (black) since best practice favors the use of steel over aluminum here.

I drew some temporary guide lines on the oil cooler to help me figure out how to clock the fittings when I tightened them for good. Trying to clock pipe fittings to the right angle can be a frustrating challenge sometimes.

When tightening the oil cooler fittings, it's important to use a wrench on the boss that surrounds the fitting, rather than torquing away on the fragile oil cooler body:

I installed the steel fittings with Loctite 567 thread sealant, and tightened them two full turns past hand-tight:

One more test fit to make sure the fittings were clocked at the right angle, and then I put the oil cooler and hoses on the shelf for later. The next time I install them on the airplane, it will hopefully be for all time!

While I was at it, I riveted the missing nutplate to the angle on the rear baffle where the oil cooler brace attaches. Go ahead, try to guess how I squeezed the rivets on that nutplate in the foreground:

Just kidding, I totally drilled out the rivets holding the angle to the baffle, attached the nutplate on the bench, and then riveted it all back together. Doh, should have done a little more planning ahead.