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The design & implementation of the Rotax engine/Sonex cooling system is possibly the least well understood (too few examples/hrs) part of the whole build.

For myself, current & future Rotax installers, I think it would be very helpful to see photos of cooling system (oil/coolant/barrel) sets ups.

Advice from owners regarding efficiency and ease of installation, would also be useful.

My Sonex has now been flying for about 25 hrs. The concept of an aft coolant radiator, using air exiting from the cowling has proven problematic. After much mucking about the temperatures are just under control but I would like "just" to be converted to well under control. I theorise that the restriction to air flow, by the hot radiator core, is reducing air entry to the cowling, thereby negatively impacting on all other cooling systems - I will be doing a big redesign in the near future.

The radiator will be moved forward (location yet to be decided)- the space it vacates will become an unobstructed air exit vent. This change will result in modifications to cowl shape/design (may be a whole new cowl).

I don't believe it!

???????Not one Rotax 912 UL/S drivers on the Forum have had any problems/solved with their cooling system (oil/coolant)???????????

I missed your earlier post, perhaps others missed it too.

I suspect you're not the only one to have cooling issues. I don't know how much sense it makes, but I'd try to understand how the RV-12 cooling works and try to set mine up similarly. They've obviously got much more time on their installations. I bet we've got a bunch of different one-off installations over here.

We used to have a guy who'd talk a lot about the science of liquid cooling and how to size inlets and outlets. I think he's still on Homebuilt Aircraft, but I haven't seen him here since Yahoo!.

Thanks Chris - I have "browsed" a lot of Rotax 9 installations - lots of ideas BUT I would like to retain the Sonex "look/profile" if possible.

While function is, for me, always ahead of form, I don't want any unnecessary vents/bumps/protrusions. To this end, it is important to get the location of heat exchangers just right, so as to minimise the number & size of vents (In & out).

My cowl exit vent is (for the time being) fixed in location & size, as is the two inlet "nostrils".

My coolant & oil radiators will need to be relocated from their present position and with them any added inlet vents.

Do you or any other readers, have any thoughts/advise on the concept of having coolant & oil radiators in air flow series (one behind the other). I ask because on my installation its the oil temperature which is the biggest issue - what if I mounted the oil cooler in front of the coolant radiator (or visa versa)? Could this work?

That´s how they do it on the composite french sports aircraft Dyn Aero MCR101 Ban Bi (quite a name!): one cooler behind the other. There are quite a few pics of such an aircraft on the web- it is really a ´smiling´cowling. I don´t know which cooler is up front, but I speculate it´s the coolant radiator.

Both coolers are in a common, tightly fitting, very narrow duct under the bottom cowling. That makes both coolers very deep and long, but not very high: quite different from what one normally sees in a 912-installation. Both coolers are out of house specially made for the aircraft manufacturer, and since it folded some years back I speculate it would be quite a task to get hold of new ones should that be required.

The system seems to work well and cools fine as long as the duct is kept free of debris (grass etc). To my knowledge there are no thermostats in the system- I know of owners who have experimented with outlet flaps to restrict airflow during low ambient temperatures.

Sorry no experience with the rotax, only thoughts here.

Your cruise speeds with the constant speed prop are likely to be quite a bit higher than most others 912s with a fixed pitch. So what other guys have done may not be optimum for you anyway.

I dont think blocking the cowl exit with a radiator is a good idea. I imagine the air entering the radiator would be a bit preheated and also restrict airflow. My thoughts on stacking the oil cooler and radiator are that, I would avoid it.
I would try to get cool air to the barrels and cool air to both the radiator and the oil cooler. Maybe a scat tube feed to plennums on rear mounted coolers.??

I would try to not reinvent the wheel and look to other designs for ideas. Designs that successfully use a 912, designs that have a 130 to 140kt cruise in a tight cowl.

If you can make a cowling from scratch you would be able to make fibreglass plennums for your coolers and barrels, you can then hopefully place the coolers where they fit.

Kai wrote:That´s how they do it on the composite french sports aircraft Dyn Aero MCR101 Ban Bi (quite a name!): one cooler behind the other. There are quite a few pics of such an aircraft on the web- it is really a ´smiling´cowling. I don´t know which cooler is up front, but I speculate it´s the coolant radiator.

Both coolers are in a common, tightly fitting, very narrow duct under the bottom cowling. That makes both coolers very deep and long, but not very high: quite different from what one normally sees in a 912-installation. Both coolers are out of house specially made for the aircraft manufacturer, and since it folded some years back I speculate it would be quite a task to get hold of new ones should that be required.

The system seems to work well and cools fine as long as the duct is kept free of debris (grass etc). To my knowledge there are no thermostats in the system- I know of owners who have experimented with outlet flaps to restrict airflow during low ambient temperatures.

Amazing engine installation - definitely some tangential thinking went into that design. Unfortunately I must work within the Sonex airframe/engine alignment . I agree with the likelihood that the oil cooler comes second in a series installation - oil taking longer to absorbed/give up heat.

sonex892. wrote:Sorry no experience with the rotax, only thoughts here.

Your cruise speeds with the constant speed prop are likely to be quite a bit higher than most others 912s with a fixed pitch. So what other guys have done may not be optimum for you anyway.

I dont think blocking the cowl exit with a radiator is a good idea. I imagine the air entering the radiator would be a bit preheated and also restrict airflow. My thoughts on stacking the oil cooler and radiator are that, I would avoid it.
I would try to get cool air to the barrels and cool air to both the radiator and the oil cooler. Maybe a scat tube feed to plennums on rear mounted coolers.??

I would try to not reinvent the wheel and look to other designs for ideas. Designs that successfully use a 912, designs that have a 130 to 140kt cruise in a tight cowl.

If you can make a cowling from scratch you would be able to make fibreglass plennums for your coolers and barrels, you can then hopefully place the coolers where they fit.

As is, I seem to be able to get to around 147 Knots TAS @ 6500 ft - probably get similar with a ground adjust optimised for high speed cruise but then TO/Climb would be anaemic.

"I dont think blocking the cowl exit with a radiator is a good idea" - well it turns out you are correct but that was the card I was dealt & the install was so neat/clever, that I thought I would give it a go.

"make fibreglass plennums for your coolers and barrels, you can then hopefully place the coolers where they fit." - not sure what you are saying here - oil/coolant cooler locator comes (in my mind) before cowling design

I am quite impressed by some of the Zenith 912 installations - the engine location & general lay out seems to be similar to the Sonex

Just to field something more to mull over:

When Edge tested out his original EFI setup, it was done on his Sonex Legacy (#037). This engine had a turbo, an aftercoolder, and a CS prop. See pic below (never mind the non standard windshield and canopy). Incidentally, this airplane originally had a VW Aeroconversions engine under its universal Sonex cowling, so this was the starting point for the conversion.

The fairly large coolant radiator that the 912 requires, was a bother to position under the existing original cowling- there was just not enough space with all that other hardware. The bright idea then came up to have two identical radiators manufactured, with a total grid identical to that of the standard radiator. It would then be fairly easy two find space for these two rads behind the existing cooling air intakes. Incidentally, these rads were then hooked up in series and installed with selv made brackets on the front of the gear housing, just behind the prop flange. The pic should give some idea of the setup.

The intake side of the cowling had to be slightly doctored, but this work was minor. To the best of my knowledge, there have been no cooling issues.