Page 49: Cooling system

The Rotax 912 is an odd duck in that it has water-cooled heads and air-cooled (finned) cylinder barrels.  The water is actually a 50:50 mixture of antifreeze and distilled water.  From an engineering standpoint this is superior to the more typical air-cooled-only (Lycoming, Continental) arrangement.  Of course all the rejected heat ultimately goes to the air.  It just takes a different path with the Rotax.  

The components of the system which must be installed are the fiberglass duct which directs the ram air from just aft of the prop to the heat exchangers (shown again from the previous post), the oil cooler (air-oil heat exchanger), the "radiator" (a poor name since there's negligible radiation heat transfer) which is an air-water heat exchanger, a cabin-controlled door which can divert the hot air downstream of the radiator into the cockpit for heating (definitely needed here on the Front Range), and various hoses which transport the oil and water to various places.

The oil cooler is first bonded with high-temp RTV to an aluminum frame to which nut-plates have been riveted, allowing it to be attached to the bottom cowl.  The aft (left) end of the duct interfaces via a rubber seal (shown on the pic) with the radiator.

For the bonding process, the instructions call for a 20 lb weight to be used to ensure proper contact while the RTV cures.  I searched around the hangar weighing various things to use for this.

It turns out that a jug from a R-985 Pratt & Whitney radial from a twin Beech weighs almost exactly 20 lb.  I put the corresponding piston in the picture to show the beating it took when the engine swallowed a valve.  The jug (with cracked head) is sitting on a wooden plate atop the heat exchanger

The radiator (which doubles as a heater core) is also bonded to an aluminum frame which provides a flat surface against which the aft edge of the cooling duct interfaces.  The instructions specify that a 1/8th-inch gap should exit all the way around between the duct face and the aluminum frame.

A separate fiberglass piece bonded with epoxy (and RTV as a backup) to the duct itself makes this do-able.  The gap, which can be seen in the pic at right, was achieved by temporarily gluing inch-long segments of paint stir sticks from Lowe's (not Home Depot, too thick) around the aluminum and clamping everything together while it cured.  The rubber gap seal is bonded to the duct face later.  The seal can be seen in the first pic.

Hooking up the oil lines turned into an unexpected problem.  One line goes from the sump on the bottom of the engine to the oil tank (shown) at the top.  The ends of this particular hose are "clocked" by the supplier and the angle can't be changed.  The instructions warn not the twist the hose.  Looking at the pic at left, you can see that the female end can't possibly fit on the nipple on the tank without  significant twist.  I ordered a replacement ($288!) which fit perfectly.  The Mothership did give me a refund when I sent the bad one back.

Installing the oil lines, water hoses, gas lines and a couple of vent hoses in the tight quarters of the engine compartment was, in a couple of cases, a challenge.  Van's commonly requires the use of double Adel clamps, where one clamp attaches to a structural member, the other to a hose and the two are attached to each  other with a bolt, creating a sort of standoff.  I hate single Adel clamps, but two together requires the invention of new, stronger swear words.  In one case I didn't have a 3/8th-inch socket small enough to fit into the available space, requiring me to fabricate the wrench shown at right.  I've had to use it several times now.

In July I made my 33rd trip to Mecca (Oshkosh).  There I encountered this RV-12 with the best quality paint job I've seen on a -12.  If only I had a spare $20k!  As I've said before, I'm going to fly for a while with no paint, then wrap it in vinyl.  With wrap I can do it a bit at a time and if I screw something up I can easily remove the wrap and redo it.

Page 38: Finishing the cowling (good riddance!)

Cowling Par Deux

At the end of the previous post I described how I'd managed to somehow over trim the fiberglass, in spite of countless trial fitting iterations, along the seam where the top and bottom cowls join.  For a while I thought I'd come up with a solution.  Before drilling the rivet holes for the piano hinges which hold the

two halves together, I used clear packing tape to hold the cowl halves in alignment while drilling the upper cowling and piano hinge.  The pin had been inserted into the two hinge halves, holding everything in place.

It seemed to work well, drilling and clecoing front to back

With the drilling done and all the countersinking done in the fiberglass, I squeezed all the solid rivets and proceeded to install, for the first time, both cowlings with all pins inserted.

Since everything was riveted in place now, with no room for adjustments, the two curved pins along the aft edge of the top cowl were, to put it mildly, a (suppressing the urge to swear) problem.  They had been difficult before but now seemed impossible.  Dry Boelube helped, but not enough.  I ended up inserting some spare pin material into the hinges, chucking it up in my drill, and spinning it a bit.  This opened up the holes in the hinges enough to allow insertion of the curved pins.  The gaps between cowl halves looked good as did the gap between the prop spinner and the front face of the cowl.

What a clever builder I am!  I got to enjoy this fantasy until doing the next step: Fiberglass in the cooling duct.

The cooling duct as supplied is of necessity too large, requiring another iterative session of fit-mark-trim-fit-repeat.

The duct takes in the ram air just behind the prop and directs some of it to the oil cooler, which attaches to the duct, and the rest to the coolant heat exchanger which doubles as a heater core for cabin heat.

The fit between the duct and the lower cowl doesn't have to be perfect since the duct is epoxied to the cowl along with fiberglass cloth, making it easy to fill any gap.

The hard part is getting the duct to interface correctly with the face of the coolant heat exchanger which is attached to the airframe. A deformable rubber seal helps a little.

When the duct is satisfactorily fiberglassed to the lower cowl, before the epoxy sets up, the upper and lower cowls are attached to the airplane with all piano hinge pins and screws inserted.  This is supposed to ensure that the final fit is good.  The problem is that the duct is a large, rigid piece which inevitably distorts the lower cowl a bit, screwing up the perfect fit which I bragged about earlier.  Now the seems are no longer perfect.  C'est la vie.

At this point I fabricated and riveted in the oil cooler door, using the RV-12iS plans which include a nice holder for the oil cap riveted to the inside of the door.

The NACA duct used to cool the voltage regulator was fitted and glassed in.  I'm siffck of fiberglass.

Off Topic: more Colorado wild life.

My home in North Carolina was waterfront on beautiful Badin Lake, a 5500 acre reservoir 25 miles east of Charlotte.  There were quite a few nesting pairs of Bald Eagles there, so seeing them fly by was common, but never got old.  I never had one land on my house, however, until I moved to Colorado.

Page 38 (Page 37 iS): Cowling

From the start, I realized that the instructions in the build manual for the cowling installation made no sense to me.  I was supposed to fit the cowling before installing the engine, fitting the bottom cowl first, independent to the top cowl.  Most of the group wisdom contained in the Van's Air Force forums advised to install the engine first, which I did, so the cowling could be installed relative to the engine.  To me, the most visible indication of a good-fitting cowl is the way the top cowl fits relative to the spinner.  Any lack of concentricity is immediately obvious.  At this point, I was still following the

build manual for the ULS engine (my engine) which has the builder trim the top and bottom cowlings to the factory-marked scribe lines (marks in the gel coat) and proceed from there.  I did this and it turned out to be OK since trimming beyond the scribe lines was required in all cases (otherwise I'd have been screwed).  

I first trial fitted both top and bottom with duct tape to get a general feel for what had to happen, then proceeded with the top cowl first.  I leveled the top cowl as described in the manual, using a long spirit level and plumb bobs hanging from the front corners of the top cowl.  Amazingly, the floor in my hangar turned out to be level, making this part easier.  I checked that both main tires were pumped to the same pressure and that the wing tips were the same distance off the floor.

At this point I decided to check the procedure for cowl installation for the newer 912iS (fuel injected) airplanes and discovered that this new procedure closely mirrors what I had just done: engine first, top cowl next, then bottom cowl.  I wish I had read this before starting work on this section!  It would have spared me the unease and frustration I was feeling for making such a large deviation from the manual, only to find that I was unknowingly following the new procedure.

After leveling the top cowl, the next task was to set the gap between the front face of the cowl and the rear bulkhead on the spinner.  I decided to set the gap at 1/8th inch, which allowed me to once again use the handy 1/8th inch spacers provided free by Lowe's in the form of paint stir sticks.  Thin double-sided tape worked great here.

If I had wanted a slightly larger gap (the manual recommends 3/16th inch) I'd have used stir sticks from Home Depot which are a bit thicker.

I love the way the small gap looks.  At Oshkosh 2022 I checked the gaps and general fit of the cowls on all the -12s in attendance (and other RV models, as well) and found mostly larger gaps.  Supposedly, larger gaps make for easier cowling removal, but I haven't found it to be a problem (yet).

The small piece of aluminum angle clecoed to the top of the cowl forces the vertical alignment of the cowling and the spinner, although a hangar neighbor who built an RV-7 says the cowling will sag a bit after a few hours of operation screwing up that perfect alignment.  

The Sharpie drawing just in front of the pilot seat is where I'll make the cutout for the NACA duct used to cool the voltage regulator.

Next, the aft edge of the top cowl is block sanded to match the forward edge of the aluminum skin covering the instrument bay.  All the #40 holes for rivets attaching the cowl to the soon-to-be-installed piano hinges are marked and drilled.  One errant #40 hole is shown.  I could've staunched the bleeding with a cleco in the finger tip, I suppose.

The cowling is held in place with duct tape throughout this procedure.  As with anything dealing with fiberglass on this airplane, it's an iterative process: trial fit (with tape), mark, remove, sand, trial fit, etc.  All of the marking is done with Vis-a-Vis Wet Erase.  The marks stay on well until rubbed with a wet finger tip (or paper towel if you grew up somewhere other than South Carolina).

The aft part of the piano hinge to which the finished cowl attaches was installed much earlier in the build (in my case probably seven or eight years ago).  The forward half of the piano hinge, to which the fiberglass is to be riveted, was fabricated (fluted to match the curvature) and installed with its pin.

At this point I fabricated all the piano hinges and pins.  There are two on top (one shown in the pic), two on each side which attach the aft edges of the bottom cowl to the airframe, and one on each side which attaches the top cowl to the bottom cowl.

With the aft half of each hinge installed, I proceeded to trial fit the bottom cowl.  Lots of on, mark, off, sand, fit, repeat.

It all went fairly well once I switched to the 912iS instructions. My only screw-up so far in this section was sanding off too much fiberglass from the interface between the upper and lower cowling (shown with the hinge clecoed at left) on one side.  I'm trying to decide how to recover from that and I'll describe it in part II of the cowling writeup.

Off topic

More Colorado wildlife: A few elk showed up behind my house recently.

A few large males cut through my front yard.

They better be glad I no longer hunt!