Firewall Forward

March 28, 2013

Fuel Flow Transducer, or the "Red Cube" - 10.0 Hours

If you read my last entry from nearly 3 weeks ago, you saw how I jumped into this box of engine sensors and other parts and immediately started spinning my wheels. Well, that wheel-spinning has continued as I try to get some answers and clear direction on a few things, so I can make some progress. I decided to tackle the Fuel Flow Transducer. Also known as the infamous "Red Cube", because that's exactly what it looks like. I did my usual thing... go online and check the forums and other builder websites to see how others have done this. Well, this is where it really starts getting confusing. It's hard to find consensus! People have done a lot of different things with this part. So I went to my local builders, friends of mine, and started asking questions about where they mounted theirs. I visited some hangers and looked at some RV's with the cowling off. This is where I learned that the Red Cube apparently hasn't been around all that long, because many people who have been flying for a while don't have one. They have a fuel flow measuring device of some kind, but it's not the red cube! I also quickly learned that there are many ways that people have mounted this part. Just what a newbie like me needs... no clear direction or consensus! One friend of mine recommended putting it behind the firewall in the cabin. There are pros and cons. As with anything related to fuel, you introduce a potential failure point when you put something in the fuel line. Some say you don't want it in the cabin because if it leaks, you have fuel leaking in the cabin. Others have pointed out the advantages of that scenario, saying that if it leaks and it's in the cabin, at least you'll know it immediately because you'll smell fuel and you can take steps to get it fixed before something serious happens. What happens if it's forward of the firewall and it leaks? How soon will you find it? Sounds like a fire in the making to me. Not good. So I didn't know what to think. Honestly, I've been spinning my wheels over this one part for nearly 3 weeks now. At this rate, I'll never get my airplane off the ground.

Well, finally, the old adage "when all else fails, read the directions" came home to me. I thought, why didn't I go right straight to the horse's mouth and see what the manufacturer of the red cube says? So I started reading everything I could find from the company that makes this thing. They don't give any certain directions either, but they were quite adamant about making sure that you install this unit downstream from ALL fuel pumps. They had stern-sounding warnings about this, and some other warnings as well. The unit should be mounted upright, as seen below. So much for putting it behind the firewall, or even considering that idea. I couldn't see where I had any room in the cabin for it anyway. It should be mounted securely so it doesn't vibrate and shake around. For that reason, I don't like the idea of mounting it to my engine, even though others seem to have done so, and successfully at that. So I took heed. Some people have mounted it on the engine, but I didn't like the idea of it shaking around so much, and I can't see how to properly support it on the engine. If you've done yours that way, my hats' off to you and that's great. I just can't see how to do it myself. So it comes down to mounting it on the firewall, on a secure bracket of some sort. Okay, so where on the firewall? The fuel line coming out of the engine-driven fuel pump will go to the red cube, and from there to the servo on the bottom of the engine. So it makes sense to put it on the left side, near the engine's fuel pump. Vans drawings show an optional large battery can be mounted here, so I don't think it will be in the way of anything. Once I realized how the flow of fuel finally reaches the engine, I found a friend online who came up with this location and this bracket for his, and I copied it almost exactly.

I fabricated this little bracket and bent the angles and drilled the holes. The hole in the middle of the flat top surface is just a lightening hole. There are four bolt holes, even though only two are needed. I did this so that I can mount the cube either way... fuel flowing right to left, or vice-versa by just turning the cube around 180°, depending on how the hoses fit and how it all comes together. I can mount it either way. The top surface of this bracket ended up being a bit wavy, so I put in a small flute on the front edge. As you can see, two rivets on the left will hold it to the firewall stiffener angle behind the firewall, and two rivets on the right to the brake lines stiffener, making for a very solid mounting. I had to drill out those 4 rivets, obviously, and then I drilled two new holes on the top flange. Yes, there's a third hole drilled there that I'm not using, if you look close. It was only after this hole was pre-drilled (only through the bracket!) that I realized it was too close to the underlying stiffener on the other side of the firewall to get a rivet properly hammered in place. So I simply drilled another hole. No harm. Below, you can see my trial fitting of the red cube to it's new mounting bracket.

Here's the final installation of the bracket, finished and riveted to the firewall:

Painted Lycoming Grey to match the engine, of course.

And here's the red cube, finally mounted in place. Only finger-tight for now, depending on how the hoses fit, and how the left vs. right fuel flow works out. I may end up rotating it around 180°. Oh! One more thing. I had to order the hardware. The bolts, washers, castle nuts and cotter pins didn't come with the cube, so I had to order them and wait for delivery. All summed up, I've been working on this one item for several weeks now. It's ridiculous. You can imagine how good it feels to have it in place so I can move on to other things.

Jamie even made a rare shop appearance, to see what I was so excited about. I showed it to her and told her the whole story. She liked it!

Adios, Amigo!

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