Air Duct Bonding Preparations Begin

Upon getting the air duct positioned in its approximate position, measurements given in the plans reveled the edge of the air duct that sits under the left side cowl hinge needed a little more trimming. A little Dremel trimming and hand sanding later and it was time to begin drilling holes. The goal here is to position the aft 6" or so of the air duct 27/32" below the edge of the cowling hinge and clamp it in place … recheck fit and then drill #40 holes through the lower cowl and into the upper flange on the air duct every 6 to 8" so the air duct can be secured to the lower cowl with Clecos.

Drilling a #40 hole through the lower cowl and into the upper flange of the air duct for the instillation of Clecos every 6" or so. A block of wood was used to keep slight pressure against the flange on the air duct to keep it tight to the lower cowl while drilling.


After the upper flange of the air duct is drilled and secured with Clecos, more measurements are checked and when all appears as good as it is going to get, the lower cowl is to be removed so the lower flange on the air duct can be drilled.

Drilling the lower flange on the air duct into the lower cowl.

At this point, the air duct is checked again by placing the duct interface onto the end of the duct. The duct interface is a frame that slips over the end if the air duct and will support a rubber gasket that will mate to the coolant radiator. The preliminary test fit looked good. Truth be told, the lower cowling is cycled on and off many times during the process for material trimming to obtain the best fit along with correct clearance measurements.


The reason for all the drilling and Clecos, is in preparation for bonding the air duct onto the lower cowl with Epoxy resin. The areas where the air duct contacts the lower cowl will be marked then the pieces will be separated for scuffing with sandpaper to insure a good bond when the Epoxy resin is mixed and applied along with some strips of glass cloth.  The builder will only get one shot at this …. so this is the time to perfect the fit so all the components play nice with one another before the Epoxy resin is mixed and the lay ups and bonding begin.

Oil Cooler Mounting Completed – Air Duct Instillation Begins

Yesterday, the first task for the work session was to match drill #12 the two nutplate holes in the upper flange of the previously clamped oil cooler box into the cooling duct.  After the drilling is completed, the oil cooler assembly is unclamped from the duct so the two nutplates can be riveted onto the upper flange of the oil cooler box … totally completing the oil cooler assembly.

With the oil cooler box centered and clamped to the air duct, the two nutplate holes in the upper flange of the oil cooler box were match drilled into the air duct using a #12 drill bit.


After the two nutplates are riveted in place, the oil cooler box is bolted onto the duct so the lower mounting holes can be installed. The RV-12’s oil cooler uses an ingenious method for mounting the lower flange of the oil cooler box onto the air duct, in that, the oil cooler’s lower flange has two slots which slip over conical guides bolted to the air duct. The conical guides will allow the lower cowl to be slipped away from the oil cooler box during lower cowl removal.

The two conical guides are to be positioned in the slots on the lower flange of the oil cooler box and used to match drill the air duct.


With the cooler bolted to the duct using the previously installed nutplates, the conical guides are placed in the slots on the lower flange of the cooler box and held in position while a #12 drill bit is inserted into the guides to drill the remaining two mounting holes into the fiberglass air duct. Once the drilling was completed, the conical guides were bolted in position. I ended up using an additional thin washer under each nut because the unthreaded shank of the bolt still showed beyond the single washer called for in the plans.

Completed mounting of the oil cooler assembly on the fiberglass air duct.

Air duct with the oil cooler box removed … note how conical guides still remain bolted in place.

To begin fitting the air duct, the lower cowling is to be installed and prior to slipping in the hinge pins on the left side, the air duct is slipped into position between the Rotax engine and the lower cowl for a test fitting.

Slipping the air duct into position between the engine and lower cowl.

Air duct in the approximate final position … a little tweaking is still necessary.

The plans have a few details that show the correct positioning at various locations and a few measurements that need to be met prior to moving on and bonding the air duct to the cowling. One point that could have been worded better in the instructions is that the forward edge of the air duct should butt up against the aft edge of the oval air inlet’s opening. This can be seen as a curved line directly under the prop flange in the following photo.

The forward edge of the air duct needs to butt up against the upper aft edge of the air inlet on the lower cowl. This can be seen as a dark curved line on the fiberglass under the prop flange.


At this point, once measurements have been met, the plans instruct the builder to begin drilling holes and securing the air duct to the lower cowling with Clecos. I did not feel comfortable doing that quite yet so spent a little more time tweaking the positioning and getting comfortable with all the steps that will be involved.

Preparing The Cooling Duct For The Oil Cooler

Now that the oil cooler is enclosed within the oil cooler box, the assembly needs to be mounted onto the cooling duct. The fiberglass cooling duct will eventually be bonded onto the lower cowling and in addition to being the mounting point for the oil cooler assembly, it will also create a tunnel to provide cooling air for the coolant radiator. However, prior to mounting the oil cooler assembly onto the fiberglass cooling duct, the fiberglass needs to be trimmed down to the scribe lines along the outer perimeter of the fiberglass duct and also the rectangular opening the oil cooler will mount onto. Of course, wouldn’t you know I would pick the first 90 degree day with high humidity to sit in the sun and grind, sand and file fiberglass all afternoon. I used the Dremel outfitted with a cutting wheel then sanding drums to get the fiberglass close to the trim scribe lines.

Sitting in the hot sun on a hot humid day using the Dremel outfitted with a sanding drum grinding away fiberglass to get close to the scribe lines in the area where the oil cooler will mount.


Once the Dremel work was completed, the cooling duct was hand sanded using a sanding block and tweaked in a couple of places with a file.

Hand sanding the edge of the fiberglass duct to the scribe lines.

After the sanding was completed, the opening in the duct was placed over the oil cooler and the overall fit was checked.

Overall fit is looking good at this point, just needs a couple of tweaks.

A couple of spots were tweaked a little and then the frame of the cooler box was clamped to the duct so the two nutplate holes on the top flange of the oil cooler box can be match drilled into the fiberglass duct.

The oil cooler assembly clamped to the duct after being centered in the opening and ready to be flipped over for drilling.

With the oil cooler assembly now clamped onto the fiberglass duct, it's in position to match drill the fiberglass duct using the two nutplate holes that can be seen on the top flange of the oil cooler box.


By this time it was getting late, so stopped short of drilling the holes so that will be the next task during the next work session.

Installing The Oil Cooler In The Oil Cooler Box

Yesterday, while waiting for the paint on “the comb” for the cabin heat Bowden cable to dry, decided to move on and begin working on the oil cooler. Many months ago, portions of the oil cooler box were riveted together and painted ... so yesterday the oil cooler was installed in the box. Two beads of high temperature silicone are to be laid down in the frame prior to inserting the oil cooler.

Two beads of high temperature silicone are to be laid down in the oil cooler box in line with the flat surfaces on the oil cooler my fingers are pointing at.


With the oil cooler seated in the beds of silicone, the two large nuts are tightened down on the threaded pipe protruding from the oil cooler to secure the oil cooler to the cooler box. Next, two AN fitting adapters are screwed into the threaded hole on the oil cooler. Because the adapter fittings have straight threads, fittings also receive a copper crush washer … in addition, the use of Loctite 243 (blue) thread locker is called for.

The adapter fittings that will screw into the oil cooler require copper crush washers along with an application of blue Loctite 243 thread locker.


Fortunately, these adapter fittings are shown in the Rotax Illustrated Parts Manual and a torque specification of 26 foot pounds is given for the fitting. A ¾" crows foot wrench was used to torque the fittings. Fortunately, I was able to keep the crows foot at 90 degrees to the torque wrench so performing a torque offset calculation was not necessary.

Using the ¾" crows foot wrench adapter on the torque wrench at 90 degrees meant no torque offset calculations were required, so the desired torque of 26 foot pounds was dialed in and both the fittings were torqued down.


Moving on, the FF-1213 bottom is riveted onto the oil cooler box after two dollops of high temperature silicone are placed between the tabs on the FF-1213 bottom and the oil cooler. To keep things neat, a little masking tape was placed on the oil cooler to mask off where the silicone was being applied.

Cooler FF-1213 bottom prior to being riveted … High temperature silicone is to be placed on the cooler under in the area under the tabs on the FF-1213 bottom prior to riveting.

Using the pneumatic rivet squeezer to rivet the FF-1213 bottom onto the oil cooler box which completes the assembly of the box.

Completed oil cooler box enclosing the oil cooler.

Cabin Heat Bowden Cable Puts Up A Fight

As mentioned briefly in the previous post, the Bowden cable for the cabin heat control was installed in the instrument panel, routed through the grommet in the firewall and around the right side of the Rotax 912 engine along the engine mounts per the plans.  The cable is supported off the engine mounts by utilizing pairs of Adel clamps at three locations. One of the Adel clamps is placed on the engine mount tubing and the other around the Bowden cable then the two clamps are bolted together. Sounds easy enough right? The first pair of Adel clamps on the upper right engine mount tube gave me quite a fight. There was just no room to compress the ears of the Adel clamps while trying to slip a bolt through the clamps yet alone start a nut. The main issue is the coolant hose and #3 cylinder’s exhaust pipe prevent a hand from getting onto the clamps, so only fingers can reach the clamps. I after fighting and fighting it became apparent, I could not do this task by myself so called Bernie for help.

The two Adel clamps and the cabin heat Bowden cable are hidden behind the #3 cylinders exhaust pipe. Between the exhaust pipe and the three black coolant hoses blocking hand access, it was very difficult to install the clamps at this location.


Friday, Bernie assisted with installing the Adel clamps in the tight spot. One idea that I came up with that ultimately worked was to make a loop out of safety wire that Bernie could place around the ears on the Adel clamps and pull the wire to compress the clamps while the bolt was inserted. I was using a nut driver reaching in from above along the backside of the engine and finally got the nut to thread onto the bolt. Sounds easy, but working out the details and finally getting the parts onto the airplane took at least an hour of trial and error.

Well buried, the two Adel clamps on the upper right engine mount tubing put up quite a fight … but with Bernie’s help we knocked it out in the 12^th round.


Bernie and I finished routing the cabin heat Bowden cable under the engine where an additional two pairs of Adel clamps are used to suspend the cable from the lower engine mount tubes as the Bowden cable traverses the bottom of the engine on its way to the cooler door. The two pairs of lower Adel clamps were a piece of cake to install compared to the upper clamps. Thanks for the assistance Bernie.

Two pairs of Adel clamps are used to suspend the Bowden cable for the cabin heat as the cable traverses the bottom of the engine on its way to the cooler door.


A suggestion to fellow builders: Consider installing the Bowden cable for the cabin heat prior to installing the exhaust system or cooling system. The Bowden cable can be installed without any difficulties … but would suggest leaving the bolts threaded onto the nuts, but not tightened down so the clamps can be moved or the cable positioned for best fit as necessary after the exhaust and coolant hoses are installed.


I could not quite finish up the task of connecting the Bowden cable to the cooler door because I forgot to prime and paint “the comb” that is installed on the Bowden cable between the two pairs of Adel clamps under the engine in the above photo. The comb in the following photo is used to add friction to the cable by weaving the cable through the fingers. The added friction is desired to combat the forces of air pressure on the cooler door creating unwanted movement.

Painted “comb” for the cabin heat Bowden cable. Weaving the cabin heat Bowen cable through the slots in “the comb” will add friction … this will combat the forces of air pressure on the cooler door.

Discovered Missing Threaded Hole On WD-1201 Nose Gear Weldment

Yesterday, the coolant radiator hoses were installed along with installing the pull cable for the cabin heat in the instrument panel (more on the firewall forward side of this topic in the next post).

Upper coolant hose fits nicely and clears the senders mounted on the firewall by a good margin.

Houston we have a problem! While installing the lower coolant hose, it was quickly discovered the WD-1201 nose gear weldment was missing a threaded hole. After the discovery, quite a bit of time was spent pouring over the plans at least three times at the hangar and a couple of times at the house to see if a step had been missed calling for drilling and tapping the hole. Nope … couldn’t find it. Finally a call was placed to Van’s and I was told I did not miss a step … the hole should have been drilled and tapped during manufacturing of the part … bummer.

This is a photo of the plans showing the hole location where an Adel clamp is to be mounted to support the lower coolant hose. The arrow is pointing to the problem area.

Photo of the front of the WD-1201 nose gear weldment … missing the threaded hole for an Adel clamp to support the coolant hose


Granted, I could just drill and tap the hole … but I don’t want to do that just for a clamp that supports a water hose because there is no good way to drill and tap the hole with the exhaust muffler installed and I really don’t want to remove it.

Lower radiator hose passing by the WD-1201 nose gear weldment the hole should be an inch or so above the hose.

I’ve been exploring options with Van’s that do not include the use of wire ties (one of their suggestions) for I would prefer a sturdy bracket that will not be affected over time by heat. Something along the lines of using the lightening holes in the weldment to mount a bracket or clamp to support the Adel clamp for the hose. After a little brainstorming,  I sent a photo of one idea utilizing the lightening holes in the WD-1201 weldment and the supporting description and received Van’s blessings today.

This is a side view of the WD-1201 nose gear weldment … notice the 1/2" lightening holes that are part of the gusset on the back side of the weldment. One of these holes (likely second from the bottom) will be used to secure a stainless hose clamp around the weldment.


Below is a photo that was sent to Vans from the DOG Aviation engineering department along with a detail of the proposed solution for the problem. Basically, a stainless steel hose clamp will be drilled with a #12 hole in the center so an AN525 washer head screw can be inserted from the backside so only a stud protrudes the clamp on the forward side. … as demonstrated by the screw in the photo below that the clamp is setting on and the lose screw which shows the head of the screw. The screw will either be welded to the clamp or locked with a jam nut on the front side using red Loctite. The hose clamp will be covered with a rubber protective covering as is the Adel clamp for the coolant hose in the photo. The stainless hose clamp will be passed through one of the lightening holes in the gusset on the WD-1201 nose gear weldment ... as viewed in the above photo. A piece of the thick hard rubber shown in the photo below will be placed under the head of the AN525 screw prior to tightening down the clamp so the head of the screw will not mar the powder coating on the WD-1201 weldment.

Proof of concept photo sent to Van’s for approval of the above solution to the missing hole in the WD-1201 nose gear weldment.


Now that the above idea has been approved, will round up the parts and work on making it happen. In the meantime, not having a mounting point for the lower coolant hose’s Adel clamp is not going to affect moving on … and future additions will not hinder access to the area. So at the moment will “Keep Calm and Build On”.


Return from the future: Discovered that the hose clamp in the above photo was becoming knife-edged on the lightening hole which in all likelihood would become a point of failure in the future. Another plan was devised and received Van's approval. The post covering the new mounting method can be seen at the following link.
http://www.dogaviation.com/2016/06/rethinking-lower-coolant-hoses-adel.html