Sensor wiring and routing

 I woke up yesterday and checked the stats on this blog, and it was thru the roof!! All thanks to my fellow EAA members logging in. It seems the EAA light sport magazine published a link to the EAA thread I started on the Robin.   Welcome all my EAA friends!!

I am currently waiting for my friend Ed  http://mmwauto.com/index.php/services.html to finish painting the horizontal/Vertical fin fairing , the canopy Yarmulke and the tip caps for the vert fin and rudder.Meanwhile I started installation of the engine wiring and instrumentation. I bought a set of all electronic gauges from Jim Weibe http://www.beliteaircraft.com/  I was particularly interested in his electronic fuel gauge. Its unfortunate that I did not plan for these gauges from the beginning. I could have incorporated a flat section into the fuel tank that would have accepted the sender. However, the method I came up should work just fine. The fuel sensor appears to be a variable capacitance sender. There is a outer tube that acts as one plate and a center 14 gauge wire that acts as the other plate. As the fuel enters this tube, the fuel will act as a variable dielectric and upset the balance of a capacitance bridge. The difference is read on the fuel gauge. There are two standard lengths of the senders, 12 and 24 inches. You can trim the probe to any length you need. The base of the probe mounts to a 3/16" thick plate that is drilled and tapped. To lessen the weight and add additional bond area, I drilled lightening holes between the threaded holes. I countersunk both faces so Flox, which will be forced through, will act as a positive lock. I started the process by filling the  threaded holes with carnuba wax. I coated the fasteners with carnuba and inserted them into the plate. I sanded the bond surface with 100 grit paper and then wiped the bond surface with MEK. This is then carefully set aside and not allowed to come into contact with any contaminants.

Fuel level sender base

This looks like a real mess, but after it cures, the mating surface will be ground clean and flat. a flox radius was swept all around the perimeter and inside. This serves to increase the bond area. One trick I like to use is to add 3M 777 spray into the resin before I mix the flox in. I have found that this dramatically increased the peel strength of the bond. This is a poor mans method of creating a 'Toughened Resin" system The all composite 787 uses a toughened resin system. The idea is to add elastomeric polymers to the resin matrix. The addition of an elastomer or rubber molecule into the matrix, allows the brittle epoxy to have a slight amount of give. Since all bond stresses concentrate on the edges, this additional give keeps the edge from delaminating or peeling away. The effect is dramatic. I used a toughened resin on the wing spar of the Robin and I conducted my own testing . Without the toughener, the peel strength was around 100 psi. With it, 1000 psi, a ten fold increase. Be forewarned, I have not tested this in the hot/wet environment and I understand that there is some degradation. There are commercial tackifiers available that are used in the VARTM industry that not only increase the peel strength, but also increase the hot/wet properties. I installed the cylinder temp sender and the EGT sender. I then routed the engine wiring. On the computer in the evening I will be laying out the pattern for the gauges. I am also looking for a lighter weight throttle quadrant. if I cant find one commercially available. I will design and build my own.

Wires after routing

This shows the sender installation.
I want to be able to remove the canopy when I am working on the plane and when it is being suspended from the hanger ceiling at my local airport. So that means that all wiring must terminate right at the canopy hinge with a multi pin plug. I am looking for that plug right now. I will need to route a Tygon tube from the wing pitot to the airspeed pressure sensor mounted behind the airspeed gauge. This tube will also have to route to the area of the plug. I need to figure out a quick disconnect not only at the canopy hinge, but at the wing root also.

keyhole slot

I had to add a Keyhole slot at the base of the tank fairing. so it could be removed without disconnecting the wires. I will have to add two more on both ends to accommodate the throttle cable.

Iso view of the engine installation

This shows the additional baffle plate I  added to firewall so I can seal the gap around the cheek cowls. I am using the stock carb. air filter, but I would like to find a more compact unit, as it is now, I have to bend the filter aft inside the cheek cowl. It will work, but I would like to find a better  filter.

So a little searching and some more scrounging and I should have the instruments all installed. It feels good to make final saftied connections. the last job before covering will be to re-rig the wings and mount the ailerons and route the aileron cables. Also today I bought 3 spring scales and I weighed the whole mess!! No estimating, because all of the parts are completed. assuming 18 lbs for complete cover and paint (should be conservative) I weight 243 lbs!!!! That is a huge relief, but this is the reason the prototype has no brakes or starter. however, my initial prop weighs 8 lbs, this will reduce to 1.5 lbs when I finally choose the correct diameter and pitch. I will ask CGS props to carve me a fixed spruce prop. If I  have time I will carve my own. I use the Fred Weick method he outlined in a NACA pub from 1929. Come think of it, I will carve my own and detail it in the blog, its really a very simple process and 90% of the work can be done on a band saw.