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.
No comments:
Post a Comment