firewall forward

After an email exchange with Leon at Pipistrel, I used my dial caliper and his reference of 42 mm up from the bottom surface of the fuselage to mark the bottom hole for the engine mount at the center of the lower firewall hard point. I drilled an 8 mm hole there, inserted an 8 mm bolt through the engine mount and held the mount against the firewall with the lower bolt in place.

I marked the firewall material around the lower engine mounting point, then removed the mount and cut the material away so that the mount will bear directly on the firewall. Then I placed the engine mount in position with the lower bolt in place, drilled each of the remaining mounting bolt holes, and inserted a bolt in each one after it was drilled. My long 8 mm drill bit, which I had purchased previously, was necessary to drill the upper mounting holes, because they are located close to the fuselage sides that extend forward of the firewall.

Starting with the lower bolt centered resulted in the engine mount rotated toward the right side of the aircraft one degree when I checked the mount alignment against the wing mounting tubes in the cockpit. I decided that the one degree difference is OK.

Note that at this point the lower mounting bolt is inserted from the front of the mount, not the rear of the firewall. That makes it easier to remove and replace the mount while drilling the holes initially. Next I will remove the mount and reinsert the bolt from the aft side of the firewall, with epoxy and cotton flock to hold the bolt in place while mounting the engine.

I marked around the engine mounts onto the firewall material, then removed the mount from the firewall. Then I cut away the fire barrier material at the mounting points.

I then attached the engine mount to the engine with the rubber shock mounts, tightening the nuts onto the bolts until they bottomed out onto the mount bushings and using blue Loctite 243 on the nuts.

I unpacked, inventoried and studied the firewall-mounted components and fittings. After getting a general idea of where things belong, I used the firewall sketches from Pipistrel and double checked them against the components to be mounted, then started drilling holes.

First, I drilled the holes for the oil tank bracket. The measurements from Pipistrel indicate that the bracket is rotated a few degrees from vertical, clockwise as viewed from the cockpit, so that's how I drilled the holes, checking them against the bracket for proper fit. It appears that the oil tank bracket will be mounted with four M5 x 16 hex bolts.

Then I measured the hole locations for the electrical panel. I drilled the two lower holes first, then held the electrical panel in place with two bolts in those holes and checked the position of the upper holes. I found that the hole locations in the panel matched the lateral dimensions in the Pipistrel sketch, but both were about 1 cm higher on the firewall. So, I match-drilled those holes using the electrical panel as the drill guide.

I also concluded that the lower left electrical panel mounting cap screw is intended to protrude through the firewall and used to mount a hose clamp for the fuel return line. Therefore, the cap screw for that location is longer (M5 x 24) than the other three mounting screws (M5 x 16).

I laid out the holes for the heat duct and deflector, which penetrates the firewall through the center of the electrical panel. The heater is simply a hole in the firewall which passes hot air coming through the radiator, with a pilot-operated stopper covering the cockpit side of the hole. I checked the hole positions against the electrical panel and the heat deflector mounting bracket and confirmed that they were in the right position. I did this by drilling a 1/16" hole through the center of the heat duct opening, then mounting the electrical panel temporarily to ensure that the hole would be centered in the panel opening and that the heat deflector would be properly positioned in the hole.

I then drilled the two deflector mounting holes and the hole for the heat duct. A 2 1/8" hole saw would have been about right for the heat duct, but I had left my hole saws at my new house construction site, so I drilled out most of the opening with a 7/8" hole saw that I had on hand, then finished the perimeter to match the carbon heat duct using my rotary grinding tool.

There are two large holes, one 30 mm diameter and one 24 mm diameter, which are for electrical wire bundles that pass through plastic fittings inserted into the openings. The positions of these openings match corresponding holes in the electrical panel. I did not confirm the position of these openings before drilling them, but fortunately they matched the electrical panel holes very well. I used a 7/8" hole saw for the smaller hole, which is about 22 mm, but which matched the plastic fitting very well. I used the same hole saw for the larger opening, enlarging the hole with my rotary tool until it matched the larger fitting.

Between these two holes is a 10 mm hole for the propeller control cable housing. It also matches a hole in the electrical panel.

I sprayed contact adhesive on the firewall, fuselage sides and the portion of the fuselage above the firewall. I also sprayed contact adhesive on the firewall insulation blanket. After allowing the adhesive to become dry to the touch, I applied the insulation blanket to the firewall. Then I sprayed the carpet pieces for the sides of the fuselage on either side of the firewall and the carpet strip that goes on the bottoms side of the fuselage overhang above the firewall. I installed the side carpet pieces first, then the one above the firewall.

Finally, I sprayed the aft side of the insulation blanket and the back side of the firewall carpet with adhesive. After letting it dry, I pushed the carpet into place. It didn't adhere very well in places, but I figure it will be secured by the electrical panel and the other equipment that gets bolted through the carpet.

I had asked Leon at the factory how the washers are to be used on the engine mount bolts. He explained that a lock washer and a large washer go under the head of the bolt behind the firewall, then two washers under the nut when attaching the engine mount.

Then I rigged up a way to get the lower engine mount bolt through the firewall from the back side, using the fuel hose opening in the bottom of the firewall for access. I had some brass tubing on hand, which I flattened and bent into a suitable shape, then temporarily taped the bolt head to the end.

Dave Adams, another Pipistrel builder, used a different method for getting the lower engine mount bolt into position. He had a flat chain saw wrench on hand which he bent 90 degrees. He riveted a retaining strip to the wrench and used it to get the bolt and washers into position.

Then I used my soldering iron to cut through both the insulation blanket and the carpet at the large hole where the heat duct goes. I pushed the carbon heat duct in place with the flange on top of the carpet.

I opened holes in the insulation blanket and carpet at the remaining holes, using my soldering iron to melt through the material.

Then I mounted the heat diverter using two M5x16 socket head cap screws and large washers. On the front side of the firewall I inserted blind mounting nuts, which seat flush with the surface, extend into the hole and have three teeth to bite into the material to keep them from turning. I used blue Loctite 243 on the bolts.

Next I installed the through-firewall plastic threaded wire bundle fittings. They were not long enough to extend beyond the carpet, so I remove enough carpet material around the hole to permit installation of the nut on the end of the fitting on the back side of the firewall.

December 15, 2007 0.9 hours - Install oil tank bracket and electrical panel

I determined that the bolts for the oil tank bracket are not long enough to penetrate the carpet, insulation and firewall to engage the threads in the oil tank bracket, so I remove some carpet and insulation around the four mounting holes so that the washers under the bolt heads will rest directly on the aft side of the firewall. Then I fastened the oil tank bracket in position, using M5x16 hex head bolts, with large washers under the bolt heads. I used blue Loctite 243 on the threads.

Finally, I mounted the electrical panel to the aft side of the firewall, using blind mounting nuts on the front of the firewall. I used M5x16 socket head screws on three of the mounting points, and an M5x24 screw in the lower left mounting point. This screw extends through the front side of the firewall, and will be used later to install a hose clamp to hold the fuel return line.

Some of these installations are out of the sequence shown in the kit manual, but I believe it will be easier if I complete as much of the work as possible on both sides of the firewall before I hang the engine, rather than try to work around the engine after it is installed.

I studied the kit photos and determined how the various engine sensor and electrical wires are routed, then tied them to the engine mount in what I believe to be the correct routing. This is in preparation for hanging the engine on the firewall, at which point the wires will pass through the firewall to the electrical panel and instrument panel.

I studied the photos and the kit manual to clarify how the fuel supply and return lines are to be completed on the front side of the firewall.

First, I located the short fuel lines and attached T-fitting that connect the ground fueling valve into the supply line. I cut lengths of fire sleeve for this assembly.

I determined where the ground fueling valve will be attached to the fuselage flange around the firewall. This is mid-way between the left rudder cable fittings. This gave me a reference for the length of the fuel supply line that emerges from the lower center of the firewall, and I cut that line to its final length. Then I cut a length of fire sleeve to cover the supply line from the T-fitting to approximately 2 cm behind the front surface of the firewall.

I slid the remaining fire sleeve over the fuel return line until it penetrated the firewall opening, then secured it with a 20 mm hose clamp, a washer and an M5 nylon lock nut onto the end of the bolt that secures the lower left mounting point of the electrical panel. I used Loctite 243 on the nut. Then I tied the supply and return lines together with a cable tie.

Then I drilled a hole for the ground fueling valve in the fuselage lip at the location shown in the kit photographs. I used my rotary tool to grind away the step in the firewall-to-fuselage flange to create a level surface on which to mount the valve, and to reduce the material thickness enough to permit the valve to be assembled through the flange.

I screwed the fuel hose fitting into the top of the valve, applying Loctite 542 to the threads. Then I inserted the lower fuel hose fitting through a washer, inserted the fitting through the fuselage flange and screwed it into the valve, also using Loctite 542 on the threads. I will let the Loctite harden before attaching the fuel supply hose to the valve.

I mounted the gascolator on the bracket that is attached to the lower left engine mount attach point. This is accomplished using two 1 cm spacers (Pipistrel part 1033003), two M6 x 30 hex bolts, two 6 mm flat washers, two 6 mm lock washers and two plain nuts. In addition to the lock washers, I put blue Loctite 243 on the nuts.

I then cut a length of fuel hose and fire sleeve from the end of the fuel supply line and attached the fuel flow sensor to the outlet of the gascolator, using cable ties at the fittings.

I routed the fuel flow sensor wire down the engine mount and followed the other wires to the point where it will go through the firewall, securing the wires to the engine mount tubes with cable ties. The kit manual shows a length of fire sleeve split and wrapped around the fuel slow sensor, secured with cable ties; however, I plan to protect it with Aircraft Spruce Heat Proof Tape part number 09-31500. This is on order from Aircraft Spruce, so I will install it when it arrives.

I connected the large radiator hose to the water pump using a 25-40 mm hose clamp.

I mounted the radiator between the supports on the top of the engine mount, using four M6 x 16 hex bolts and 6 mm flat washers. I used Loctite 243 on the bolts.

I connected the long radiator hose to the upper radiator fitting adjacent to the filler cap, using a 25-40 mm hose clamp.

Finally, I connected the blue plastic tubing from the radiator overflow bottle to the overflow fitting on the radiator. I used my heat gun to soften the tubing slightly, then worked it over the fitting and secured it with a cable tie.