Sunday, October 30, 2016

Fuel tank senders

This weekend I made good progress on finishing up my second flap as well as starting the final assemble on the first tank. The flap went well as I figured out the optimal bucking situation from the first flap. Also, the pro-sealing of the Trailing Edge went much smoother/quicker/cleaner than the first one.


With the flap clecoed to the table while the pro-seal dries I started the final assembly on the first tank. I squeezed the end rib wet, but started the other method where you let the sealant cure while the clecos are holding pressure and then shoot wet rivets after a couple of days of curing. I have gone back and forth on the pros/cons of shooting wet rivets vs letting the sealant cure and what finally did me in was while I was placing the first full rib I really did not want to be rushing to shoot rivets. My take is that I increase the risk of shooting bad rivets when I am in a hurry, so I am going to go with the Rick6a Method (at least for this tank). About the only thing that made sense to me against this approach was someones comment on the forum about how the clecos would not provide enough tension in this method and there would be too much pro-seal stuck between the rib and the skin. When you let it cure like that and come back to shoot rivets over cured sealant that the compression of cured sealant leaves a weaker mechanical connection between the rib and the skin due to the compressible material between the two. My intuition is that is a valid statement. My intuition also says that bad rivets due to rushing also make a poor mechanical connection. So cured sealant and wet rivets it is.

After 3 ribs I ran out of clecos. (I still have the flap in the jig, so I guess I am going to need to wait until I finish that up this week before I continue on the tank.) 

Since I was waiting for sealant to dry I thought it would be a good time to start taking care of the fuel senders. I am going to go with the standard float sensors for this build. I really don't trust fuel senders all that much anyways, so I might as well stick with the standard ones. I am not sure why I do this to myself, but I always seem to start with the right side of the plane even when the plans call out the left assemblies. In doing that, I guess I am more careful to read, re-read, and re-read the plans because I am always double checking left vs right. Also, it is always nice to put together the second assembly per the plans and have no incorrect part numbers.

In the case of fuel senders, this cannot be more true. I started with the right float arm bending. I remember reading someones post about the right float arm needing a couple of more bends than the left in order to clear one of the tank stiffeners.  I mocked up the float arm in some TIG wire so I did not screw up the real arm.

Now I am normally pretty good with building up pictures in my head, but I will say, when I started making bends for the right float arm I got really lost. What it came down to was the senders themselves. Per the diagram and the plans the LH side gets the 385B-F and the RH side gets the 385C-F.

That is fine and dandy, but for some reason I would have guessed that if there was a left and right sender that they would be symmetrical inverses of each other. If you look carefully at the sequence of photos below you will see that the operation of both senders are identical to each other. Also, I am a little ashamed of these photos considering I do own a real multi-meter (Fluke). Weird. I would have thought one would have been backwards from the other for some reason.

So what is the deal? I am a little saddened that it took me a good 20 minutes to figure it out. It wasn't until I tried screwing the 385C-F into the right rib before I figured out what was going on. If you look at the left photo, you will see that the hole pattern on the flanges are 180 degrees out from each other. That is beginning to make sense now.

I had to dig into the wonderful inter-tubes in order to verify what I was seeing. The Je-Mo build log had a great write-up on this. The following images are from his site:

Left tank (the sender orientation identical to the build plans)

Right tank. If you look carefully here, the float arm comes in from the forward side of the tank opposed to the left tank where the float arm comes in from the aft side of the tank. 

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