18-07: Fuel Sender Wire Bending

 There was a good thread about bending the fuel sender wires which seemed to indicate that the dimensions on the diagram on page 18-10 for the bends on the fuel float wires were not correct, and you could end up with the wires contacting the web of the first rib. 

Indeed, if you check the dimensions on the printed page, they don't tally with the actual dimensions noted on the page:

So the question is, which on is the correct dimension? I decided to make up 2 templates, one at the 4" and one at 3.89" on each arm and see how they fit in the tank. 

Here is the 4" template in the LEFT tank - it does not hit the 1st rib. 

Here is the 3.9" template in the LEFT tank - it also does not hit the 1st rib. 

So as you can see, they both fit! So how can other people have been having issues? Based on this, i decided to bend both wires to 4" and see how it fit. 

To bend the wires, i used my template to make a mark on each wire, and bent it using a small vice. 

With only some minor tweaking, the Left float wire fit fine! 

So far i had been working on the LEFT tank - the right tank is essentially the same, however the fuel sender units are installed mirror to each other - i.e. on the left, the swing arm is on the aft of the tank, whereas on the right tank it is on the forward side. This effectively means that the arc of the wire is further forward in the right tank than on the left. 

To make sure the right tank wire should be the same as the left tank, i thought i better test the wire in that sender. 

As you can see, the 4" dimension does not work for both tanks! As such, for the right wire i decided to use the 3.89" dimension for the arms - and this was ALMOST spot on, with only some minor tweaking of the wire needed.

The right needed only some minor tweaking.

Fuel Sender Checkout

On page 18-07 the instructions call for adding a lock washer under the head of one of the attach screws in order to ensure a good electrical ground between the fuel sender flange and the aircraft skin. The manual pictures the lock washer between the head of the bolt and the outside face of the flange of the fuel sender. I guess the theory is that the bolt provides the ground, through the nutplate and through it's attach rivets to the rib. This is fine, except the rib has also been installed with sealant, so how good a ground would this really be?

The plans on page 18-07 also call for a 1/32" bed of sealant between inside face of the fuel sender unit flange and the rib - this is to allow a blade to be inserted later, should the fuel sender unit need to be removed. 

Basically, the important thing is that i can take a reading of the resistance between the flange of the fuel sender and the centre screw, and send this back to my EFIS, while also having a gap full of sealant between the sender plate and the rib. 

SO! knowing all this, i am proceeding as follows with the fuel sender install:

• I will use a lock washer on each and every bolt, between the fuel sender plate and the rib. This will give me the requisite 1/32" of sealant gap, while also allowing me to tighten down each bolt properly.
• I will install a #8 screw terminal and ground wire under the head of one of the bolts, then use a lock washer between this on the fuel sender plate outside face. I will run this ground wire all the way back to my EFIS along with the wire from the centre screw. 

So i made up the ground wires and installed them for the dry test fit of the senders (the actual fitting of the senders will occur after the Aft Baffle is installed)

According to a post by Scott McDaniels on the VAF Forums, who works at Van's the fuel sender should have a resistance reading between 250ohms at empty to 20ohms at the full position, so i decided to test this. 

I got 248/247 when empty and 30/30 when full, which i think is within a good margin of error.

Left tank - FULL

Left tank - EMPTY

Right tank - FULL

Right tank - EMPTY

Plans Section 18 - Revision 1

On the 26 March 21, Van's released Revision 1 to Section 18, changing the fuel wire bend lengths from 4" to 3 21/32", and 3 3/4". As shown above, the 4" leg lengths did interfere with the fuel tank rib, but i was able to adjust the angles to ensure there was no interference with the fuel sender floats. The new published dimensions shown below would have been good to know, however I am happy i have addressed the Revision 1 changes in the install of the fuel tank sender wires.