Cooling a Rotax 912
in a Rans S-7
2004 S7S on 1260 Lotus
What does cooling a 912 entail?
First, it is important to keep in mind that there are many variations of the S-7 equipment depending on the kit year and subsequent updates. Coolant rads started out under the pilot’s seat when the 582 was the engine of choice, then were moved backward a couple of feet under the passenger seat to compensate for the 912 weight and then were mounted forward of the firewall on the S model. Late 90’s models also have the rad under the pilot.
As with any engine, the Rotax 912 installation must be able to dissipate the heat produced. Because of its unique design with water cooled heads and finned cylinders, the 912 gets rid of heat via air over the fins, heat transferred to the coolant and to the oil as well as, of course, through the exhaust.
For anyone seeing their first 912 installation in a Rans (and likely in many other aircraft), the first noticeable difference from a certified aircraft with a Lycoming is the lack of a pressure cooling setup where all the air entering the cowl inlets at the front is forced to flow over the cylinders by baffles around the perimeter of the engine which prevent incoming air from escaping around the sides, front or back of the engine. In these aircraft, all the incoming air flows over the cylinders or forced through an oil cooler, usually from the top down, and out the aft bottom of the cowl. In a Rans, air does enter at the front but is allowed to find any route it wants to get to the exit. Some people feel that this contributes to reduced need for carb heat because the carbs may be getting this pre warmed air all the time.
Some 912 installations do have a form of pressurized setup by means of a fiberglass molded shroud on top of the engine with an air inlet tube feeding it. This item can be seen on Rotax parts diagrams. Others who wanted more direct airflow over the cylinders have used scat tubing to route air there. An example of this method is shown later.
Based on the much smaller cooling fin size on a 912 compared to a Lycoming, it is clear that the 912 has less of its cooling requirement from the air over the cylinders. Thus the undirected blast of air can be adequate.
Cooling the coolant and oil does demand more finesse.
Keeping the oil cool For more discussion on keeping oil and coolant hot (thermostats), click here
Let’s look at oil cooling first. Here is a picture of some oil coolers that have been installed in the Rans S7:
There is information on the Earl’s (or Jegs Racing) site showing the heat transfer capabilities different cooler designs and the Earls style like the 3 right hand ones are the best.
Oil coolers in the early 90’s were the S tube type like the one on the left and were mounted horizontally above the engine with no direct air hitting them. Because this type and placement was quite ineffective, some people added a second one in series but that also was inadequate. By the mid 90’s Rans was shipping an Earls style cooler (like the one with the red tape on it in the picture, 13 rows) which is one of the better designs, mounted vertically in the left air inlet nostril. This was an improvement over two of the “S” tube style but was barely adequate on an 80hp in high temperature days. The 100hp engines required more and especially if operating on floats with slower air speeds. Thus Rans began shipping the larger cooler shown second from right above (same width but 16 rows).
Reducing oil temps:
Larger oil cooler
Most people with an early S7 who want to lower the oil temp will simply go to the larger cooler. This will drop the temp by about 25F. The main steel mounting bracket needs extensions as do the other 3 clamping rods and outer S bracket. It will also sit closer to the exhaust pipe so a heat shield baffle is important.
On this 94 S7 the oil cooler developed a leak, probably because it was mounted without the encasing brackets, so I replaced it with the larger one off the S7S:
Rather than the AN3 bolts tapped into the aluminum spacer tubes, I use larger tubes and 10/32 threaded rod right through.
Does the S7 have the optimal oil cooler position?
The stock oil cooler location in the left nostril has several disadvantages:
1. It prevents the cooling air from hitting the cylinders directly like it does on the right side.
2. Heated air from the rad is allowed to enter the engine compartment.
3. Putting a rad perpendicular to the slip stream without any ducting is inefficient.
4. The back side of any rad should be in an area of low pressure, not the high pressure created by air entering the right nostril.
5. The rad almost touches the exhaust stack below it.
Mounting the oil cooler under the engine:
Several of the above disadvantages can be eliminated by mounting the cooler below the engine and feeding it air trough a scoop. The bulkier cowl on the S-7S lends itself to this solution. Below is the extra large rad shown above mounted under the 912.
Notice the aluminum baffles to keep exhaust pipe heat away from the oil cooler, hoses and the oil filter. Also notice the use of constant tension hose clamps
That’s a Permacool thermostat in the oil lines with a 194F waxstat installed.
The 912 crankcase has four 8mm tapped holes on the bottom sides of the case. The front two are unused and that’s where the forward mounts on the oil cooler attach. The rear two holes on the S7-S are used for the motor mount so I added tabs to attach to the aft mounts on the oil cooler. These aft mounts are basically ½” oil line hose about 5” long with a metal tube slipped on before flattening to bolt to the tabs.
A small scoop feeds the oil cooler:
One radical mod:
Positioning the oil cooler vertically in the air stream with no ducting is not optimal. A more elegant approach is to re-position the rad and shroud it to force all the incoming air to go through the rad and route the warmed air out of the cowl. To accomplish this I used a wood and foam male mold to produce the fiberglass holder for the oil cooler with a small air inlet. This is the smaller oil cooler used on early S7 short tails.
This shows the rad mounted in the fiberglass duct with the cowl off:
With the cowls mounted and the rad fastened down there is good clearance from the manifold and throttle/choke cables.
Below is the front view:
Here is the finished cowl with a louver above the oil cooler to help pull out the heated air.:
The duct occupies a little over ½ of the air inlet opening leaving a good blast of air for the cylinders.
This picture also shows a top rear cowl exit louver which helps to reduce temps inside the cowl after shut down.
Monitoring temps inside the cowl showed a significant temperature increase after shutdown. The louver at the top rear of the cowl allows this excess heat to flow out.
This position for the oil cooler does create extra work to remove or replace the top cowl. Using ½ turn fasteners instead of bolts would reduce this extra effort.
The smaller black scoop on the side directs air over the muffler and the top yellow scoop provides air for the cabin heater using the same size Earl’s as the oil cooler.
Before the changes and with an oil and water thermostat, on a 30C+ (90F+) degree day, oil temps were 270 or so and water (cylinder probe) 215. Now without thermostats and with the new set up, temps are 230 and 180. Probably ½ this benefit was due to removing the thermostats (The Permacool thermostat has a problem in hot weather) and the rest due to the changes.
Ensuring that the air exits easily from the cowl is quite important.
Rans used a shallow cowl air exit flange (around the exhaust) on early S7 aircraft but most feel that enlarging it (like on the S model below) increases air flow without enlarging the inlet openings.
Coolant rad design and placement or keeping the coolant cool
Just as with oil cooling, the design of the coolant system has also evolved in the S-7 over the years. The coolant rad started out under the belly like this:
Hoses from the engine connect to aluminum tubes that run through the firewall, along the floor boards, then down to the external rad. This placement is a good design with a small frontal area, angled rad and an area of low pressure where the air exits the rad. This view illustrates this a little better:
In fact, this position reflects widely accepted theories on air flow over a radiator that go back to the design of the cooling system on a P-51 Mustang. High speed air is fed into a relatively small area scoop (compared to the area of the rad), is allowed to expand and slow down before it goes through the fins of the rad and then exhausts in an area of lower pressure.
Here is an S-7 rad before it is bolted on the belly. Air enters in that section between the rad pipes (about 20 sq “?), the area of the rad is about 235sq”. The nose down slope helps to get the exit side of the rad out of the high pressure slip stream thus creating an area of lower pressure for the heated air to enter.
Below is a 2004 S-7S. Notice there is no coolant rad visible.
On the S model, Rans dropped the belly rad and went to a smaller rad within the cowl shown on the right below. The S-7 rad is about 237.5 square inches, while the S rad is only 67.5 square inches. The S-7 rad is tubular (34x ¼” id? tubes) with all the coolant apparently going down one half of the rad (through 17 tubes) then coming back along the other half. The S rad is a plate setup with each of the 11 plates having a cross section area of about 1/16 x 3/4”. The 1” wide plates have two small rectangular coolant paths that measure very aproximately 1/16” x 3/8” These two styles are also seen in oil coolers.
This much smaller rad is mounted horizontally just ahead of the firewall near the outlet air path.
Here is a comparison of flow area and surface area of tubes:
Surface area of small rad: 11 plates at 1.25” x 2 sides x 13.5” long = 371 sq”
Surface area of larger rad: 34 tubes x diameter of aprox .33x 3.14 x length of 19” = 669 sq”
Flow area of small rad = 11 x .06 x .75 (all aproximate) = .5 sq”
Flow area of large rad = 17 x .125 x .125 x 3.14 = .83 sq”
Interesting to note that the cross sectional area of a 1” hose is .79 sq” so it would seem that the small rad also restricts the flow.
These numbers suggest that the larger rad should be much more effective yet it was not overcooling any 912.
This smaller rad is positioned close to the cowl air exit opening at the base of the firewall. There is no additional baffling (on a stock S-7S) to force air through the rad although some owners have added a baffle to prevent some air from escaping between the rad and muffler. Frankly, I find it curious that this much smaller rad, without the benefit of the direct slip stream air, is at all adequate, however, in moderate temperatures, it does work.
In this modified cowl S model, the rad is angled down so that the forward edge touches the bottom of the cowl to force exiting air to go through the rad rather than around it:
Some people have found that providing additional air entering the engine compartment contributes to overall lower temps. The builder of one S-7 chose to leave off the large spinner and its’ back plate to allow air to enter around the prop hub. Another builder advocates cutting two tennis ball sized holes below the spinner like this:
On the other hand, ensuring the cowl exit air is not constrained has proven useful in other aircraft. The exit flange on the cowl above is really quite small and while some have found modest increases don’t do much, here is an example of one that dramatically reduced cylinder head and oil temps on an 80hp on floats. This solution has a flange depth of 3”.
One builder decided to make an even more dramatic change.
Here is an interesting look at airflow mods on an Avid: http://avidflyer.wikia.com/wiki/912_Cooling which directs air right to the cylinders. It started out with some different cooling issues than we have on the S7 however.
See details on cabin heat and thermostats