Cabin Heat Control Cable Creates Issues

Yesterday’s work session became a day of resolving issues. Now that the bonding of the air duct to the lower cowl is completed, the cowlings have been removed to continue with the instillation of the cabin heat control cable … which, by all rights, should be an easy task, but ended up being far from that. The cabin heat Bowden cable instillation was suspended at the step where the comb is installed … the comb is now painted, so it was installed on the Bowden cable and the last remaining pair of Adel clamps were tightened.

The comb is now installed on the cabin heat Bowden cable. The comb will add just enough friction to counter the effects of air pressure on the cooler door so the cable will not creep.


With the cabin heat cable in place and secured with Adel clamps, the next step in the plans involves cutting off the excess cable. Following Van’s instructions for cutting the length of the Bowden cable that controls the cabin heat cooler door led to an issue that took a little creativity to overcome. With the Bowden cable installed along the bottom of the engine mounts, the plans call for removing the center cable and cutting the Bowden cable’s outer housing at a point 2 27/32" from the Adel clamp closest to the cooler box.

Following Van’s plans, as one can see, the cable housing was cut to 2 27/32" from the Adel clamp.

The design is overly simplistic (in fact, too simplistic) … basically, the FF-1210 cable end is a hollow rod with threads at one end and a hole for a cotter pin drilled through the tube that captures a loop placed on the center cable. A rod end is then screwed all the way into the tube and wedges the center cable into the cotter pin thus locking the center cable. Even though the rod end is threaded, it is not used to make fine adjustments because it needs to be screwed all the way down to compress the center cable into the cotter pin. As the cabin heat knob is pulled the FF-1210 cable end slides up the Bowden cable pulling the cooler door open as it moves. Below is a photo that shows the parts and the drawing.

FF-1210 cable end, the cotter pin and rod end that locks the center cable onto the cotter pin.

Functionally, as the cabin heat knob is pulled in the cockpit, the inner cable is pulled causing the FF-1210 cable end to slide up the Bowden cable until it hits the Adel clamp. Simple except for one thing, the FF-1210 needs to be over the Bowden cable to begin with. When the rod end was in the FF-1210 cable end and attached to the cooler door and the cooler door positioned in the closed position, the FF-1210 cable end did not quite reach the outer portion of the Bowden cable so as the cabin heat knob was pulled, the FF-1210 cable end would snag on the edge of the Bowden cable not allowing the cable to move. This can be seen in the following photo.

The outer housing of the Bowden cable should already be barely inside the FF-1210 cable end when the cooled door is fully closed. As one can clearly see, that is not the case.


I don’t know why Van’s instructs the builder to cut the Bowden cable prior to test fitting the FF-1210 cable end. My suggestion to fellow builders: Prior to cutting the Bowden cable, make sure the outer housing of the cable will make it slightly inside the FF-1210 cable end with the cooler door closed.


What to do … what to do? I finally came up with an idea that worked remarkably well and has the added benefit of allowing the rod end to be adjustable. I measured the gap between the end of the cable’s outer housing and the FF-1210 cable end and added a little extra and purchased a set screw that was about that dimension, which was 5/16". The set screw was screwed into the FF-1210 cable end and used to secure the inner cable to the cotter pin …doing this allowed the rod end to be adjusted so the cooler door closed then the cabin heat knob in the cockpit is pushed in.

Parts prior to assembly … everything is the same except the set screw will be threaded into the FF-1210 cable end to secure the center cable to the cotter pin. This will allow the rod end to be adjustable to keep the cooler door closed when the cabin heat knob in the cockpit is pushed all the way in.

Installing the set screw that will push the Bowden cable’s center cable tightly against the cotter pin locking it in place.

The above modification worked out quite well as can be seen in this photo of the FF-1210 cable end shown with the cabin heat knob in the cockpit is pulled out … which pulled the FF-1210 cable end up the Bowden cable and into the Adel clamp, thus fully opening the cooler door.


With the cabin heat cable issue resolved, attention was turned to my other pressing issue … making the support for the coolant hose. Frequent readers of the DOG Aviation blog may recall it was discovered a threaded hole was missing in the nose gear weldment and a plan was devised and sent to Van’s for evaluation and subsequently approved by Van’s to use a hose clamp with a hole drilled in it so a washer head screw could be welded onto the inside of the clamp leaving a threaded stud protruding for attaching the Adel clamp which will support the coolant hose.


Have not taken any photos yet, but have amassed all the items necessary and did a mock instillation of the hose clamp to mark a good location to drill the #12 hole in the clamp. The hole was drilled so will take the hose clamp and an AN525 washer head screw to a welder today so the screw can be welded onto the hose clamp.

Air Duct Bonding To Lower Cowl Successful

Yesterday, the lower cowling was removed after letting the Epoxy resin cure for two days. The air duct and duct interface appear to be solidly bonded onto the lower cowl. There are a couple of spots that I will add a little more cloth to because one spot in a corner behind the duct interface has a small gap that air can get through and another location has an air bubble which is not to my liking, so will put down one more layer of glass over that spot.

Completed bonding of the air duct and duct interface to the lower cowl.

There is still a little more work necessary on the lower cowl before it is totally completed. All the Cleco holes still need to be filled with a flox mixture, so will do that when I tweak the previously mentioned spots. In addition, there is a round gasket (looks like a round door seal gasket) that will be glued onto the perimeter of the duct interface using high temperature silicone. The gasket will create the air seal between the duct interface and the frame around the radiator. The last step is installing a protective foil heat shield in the high heat areas near the muffler and exhaust pipes.

Bonding Of Air Duct To Bottom Cowl Completed

Today the bonding of the air duct and duct interface to the lower cowling was completed without any issues … well for the most part. Photos of the process were not taken because there was just not enough time to spare … plus a camera should be nowhere near Epoxy resin.


Fortunately, Bernie was available to give me a hand with the job for which I’m grateful because we needed to move fast. Bernie was only available during the afternoon and by then, the temperatures were well into the 80’s … which, for those who have used Epoxy resins already know, means the Epoxy sets up rapidly after getting mixed … especially when in large batches due to an anomaly called exotherm.


Prior to mixing the Epoxy resin, we read through the procedure a few times and did a dry run so we had the entire procedure fresh in our minds. The cloth pieces for the three layup locations were precut to the appropriate lengths and all the mating surfaces were wiped with Acetone and Isopropyl alcohol (not at the same time).


The first step is to brush on a thin coating of Epoxy resin onto all the surfaces that will be mated or receive layups. Next a large batch of Epoxy resin is mixed with cotton flox until the mixture thickens to the point it will not pour out of the mixing cup. The flox mixture is to be placed in a Ziploc bag and the corner of the bag cut so the flox mixture can be applied like icing on a cake using the Ziploc bag like a pastry bag. This is where we had some issues in that the first bag got a hole in it while being wiped off … the second bag ripped when being twisted to move the flox mixture out of the bag. I’m not sure what brand of bags was being used, but fortunately I had three. My suggestion to other builders … don’t use the thin sandwich Ziploc type bags, instead, use the heaver freezer bags. My hands were getting plenty warm from the Epoxy resin kicking off so do not know if that also contributed to the bags breaking so easily.


The only deviation made from the instructions involved applying the flox mixture onto the lower cowl instead of on the flange edges of the air duct that mate with the lower cowl. There was a well-defined area where the pieces will mate … so felt this way was much easier because the air duct could be lowered directly in place while Bernie spread the lower cowling a little. After all the parts were in place Clecos were installed to hold all the pieces together. The three areas that receive cloth layups were quickly knocked out.


When the Epoxy work is completed, the lower cowl is to be installed onto the fuselage along with the upper cowling using screws in every hole and all the hinge pins. This way the Epoxy resin will cure in the exact position the lower cowling needs to be in when totally secured.

Both the upper and lower cowls are placed in position using all the necessary mounting hardware so the cowls are in their natural position as the Epoxy resin cures.

Bernie taking a well-deserved rest after our fast passed Epoxy resin bonding and lay-up session.

It would have been nice if we could have been slightly neater and not had as many dribbles as we did, but as it was we just barely had enough time to finish as each batch of Epoxy was rapidly kicking off in the hot hangar.

Air Duct & Duct Interface Ready For Bonding

Today, the final touches were put on the duct interface after a little positioning tweaking and sanding the outboard flange a little to gain clearance between the duct interface and the lower cowl. The outboard flange on the duct interface was sanded down until a piece of paper could be slipped between the flange and the lower cowl. When the fit was acceptable, the top of the duct interface was drilled into the air duct using a #40 drill bit and secured with Clecos. Bernie was available to lend a hand, so he drilled the top holes while I reached in through the air inlet to press upward with a piece of wood to keep the air duct tight to the duct interface while Bernie drilled the two #40 holes for the Clecos.


At this point, the plans would have the builder remove the lower cowl and drill more holes for Clecos relying on blind faith the duct interface does not move. Decided this method was not to my liking … so I used a pneumatic 90 degree drill and reached way in through the air inlet while Bernie directed the drill placement while looking down from above so a single #40 hole could be drilled through the lower flange of the duct interface into the air duct. Bernie suggested using a pencil to draw a line adjacent to the duct interface … this turned out to be an excellent idea because the duct interface did want to move around when the lower cowl was off the fuselage.

After clamping the lower flange in position as dictated by the previously drawn pencil line a second hole was drilled in the bottom flange of the duct interface.

One can see the pencil line that was drawn when the cowl was on the fuselage. Having this line insured the duct interface was positioned correctly prior to drilling the second hole with the cowl off the fuselage.


Having the Cleco holes in the duct interface drilled, the lower cowl is removed so lines can be drawn where the flanges on the air duct meet with the lower cowl.  After the lines are drawn, the air duct and duct interface are removed for sanding. To insure a good epoxy resin bonding, 100 grit sandpaper is used to scuff up the areas that will be bonded or receive fiberglass cloth. I actually used 120 grit because it was handy and gave all the surfaces a good scuffing in multiple directions.

Scuffing up the air duct with 120 grit sandpaper.

Sanding the cowling where the bottom flange of the air duct will sit. Looking closely at the photo one can see the faint pencil marks that trace where the layups or bonding will take place.

Sanding of the lower cowl completed at the areas where the bonding or layups will occur.

Now that the pieces have been sanded, tomorrow will be the day for the bonding the pieces together making the air duct, duct interface and lower cowl as one. Prior to applying the Epoxy resin, the surfaces will receive a good cleaning with Acetone.

Fitting The Duct Interface

Today’s work activities continued on with the fitting of the air duct … a spacing of 5/16" is required between the aft edges of the air duct and the frame around the radiator (this measurement is made without the duct interface in place). This can be a little difficult to mark, but discovered a #6 nut was 5/16"  ... so used one to mark the air duct for more trimming.

Prior to installing the duct interface, the gap between the frame around the radiator and aft edges of the air duct needs to be 5/16". A #6 nut happens to be 5/16", so one was used to make the measurements.


After sanding away the excess material from the aft edges of the air duct, the duct interface was set in place to check the fit. A decision was made to offset the duct interface a little inboard so a little more material was removed from the air duct so the duct interface could be moved inboard a little more.

Test fitting the rectangular duct interface after offsetting the frame a little more inboard. The duct interface is the  termination point for the air duct.


At this point, the duct interface is to be placed over the end of the air duct and the bottom cowl set back on the fuselage and a ¼" spacing should be obtainable between the duct interface and the frame around the radiator. Prior to installing the lower cowl a ¼" thick paint stirring stick was cut into strips and taped on the frame around the radiator so the desired ¼" gap could be checked and visualized.

Pieces of a ¼" paint stirring stick were taped to the frame on the radiator to establish the desired gap between the radiator and the duct interface.


The goal was to be able to see just a little light between the duct interface and the blocks of wood before closing the gap by moving the interface aft against the wood spacers. There was a tiny gap all around except for the left side of the radiator. In that area, the duct interface was pushing on the spacer blocks … which upon further investigation, revealed the left side of the duct interface was touching the lower cowl and will require a little sanding. Of note for fellow builders … when the bottom cowl was sitting on the ground during the test fitting of the duct interface, this was not an issue. However, when on the fuselage with the hinge pins installed, the shape of the bottom cowl is pulled in a little tighter which created the loss of clearance between the duct interface and the bottom cowl.

Duct interface in place and if one were to look straight down, there is a small gap visible between the blocks of wood and the duct interface except for the outboard edge … this is because the duct interface is pushing on the lower cowl and requires a little sanding to ease the pressure.


Once the duct interface clearance is resolved on the outboard edge, the duct will be drilled and Clecoed to hold its position for the bonding process. The reason it is necessary to create an even ¼" gap between the duct interface and the radiator frame is because a gap gasket will be installed later to make a seal between the lower cowl assembly and the radiator to prevent the loss of air flow through the radiator.