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Common problem with home builds - my new Sonex/Rotax 912ULS is overheating.

She hasn't even taxied (forward motion may help) and the coolant (head temps) gets up to 120C - the oil 110C.

Under cowl temperatures are around 40C

Running with no cowling, sees well below optimum temperatures - we have deduced that the issue is lack of air flow through the cowling.

*In an effort to improve airflow we have tried a small (motorcycle style) fan on the coolant radiator - helped a little

Very painful surgery to the cowling is now being done but its all guess work. A small hole & temporary scoop (air in) has been created on the chin and a larger rectangular hole in the vicinity of the firewall (air out) has been created - considerable improvement but this is in our winter with ambient temperatures around 15C summers can get into the mid 40C's.

All suggestions considered.

Cooling is always an oddity, alot of times the lack of a cowling actually hurts rather than helps you as the cowling serves to direct airflow where you need it, i.e. through the cylinder fins and oil cooler etc, with out ideas as to how you have managed and located your oil cooler/s, and how you have managed the cowl outlets in general you want a 3 to 1 ratio of inlet to outlet, but in addition to that where you are ducting air matters as really the end goal is a pressure differential cold to hot which is why no cowling is actually worse fans can help a little but they can also hurt you as well depending on your layout.

When you say the temps get that high, at what power level? How long does it take? Nearly every aircraft engine will overheat on the ground eventually, even at idle. If this is just within 5 min or so, there's a problem. If it's an hour, maybe not.

Rusty

13brv3 wrote:When you say the temps get that high, at what power level? How long does it take? Nearly every aircraft engine will overheat on the ground eventually, even at idle. If this is just within 5 min or so, there's a problem. If it's an hour, maybe not.

Rusty

Thanks Rusty,

This is our winter - ambient during tests is about 15C, under cowl temps reach above 40C and it takes about 15 minutes (from cold engine) for the decision to shut down when coolant gets to 115 C & oil 105C.

Power - initially fast idle 2000rpm - moving to 3000 rpm for a short period (simulated ignition check power)

As a result of incremental cowling surgery, we seem to be making some headway - its taking longer for the temperatures to get up to shut down decision & under cowl is now in high 30's.

This is all good at the current temperatures but summers can get above 45C (not that I will be contemplating a take-off at this temp)

That does sound higher than I'd expect. I've built a few planes with one-off cooling setups, and the biggest unknown on the first flight is always cooling. The problem is that you really have to fly it to know, unless you can strap the plane down on a flatbed and drive it down the highway :-) Winter is certainly the time to try to get that first flight in.

For something of a reference, my 912UL has a small aluminum radiator, and the small Rotax oil cooler behind the radiator. I'm running Evans which doesn't cool as well as 50/50 mix. This is nowhere near an ideal arrangement, but you have to do what you can with a small (Onex) cowl. On my last flight, I can see in the logs that it was 10 min from engine start to takeoff, including the runup. That was way longer than usual for our quiet airport. Ambient was 90F (32C). At takeoff, oil temp was 217F (103C), and rose to a max of 235F (113C) at the end of the climb. CHTs were 169F (76C) at takeoff, rising to a max of 214F (101C). The only time I've ever had to shut down due to temp, was when I was making full power runs to balance the prop.

You may have mentioned this on a previous thread, but what are you using for the temp measurements? Are you sure it's correct? If your temps match ambient when the engine is cold, there probably isn't much chance they're wrong if it's a known EMS or gauge.

Do you have any thermostats for oil or coolant? If so, that would explain the quick rise, and hopefully they'll open up and allow full cooling as well.

Good luck
Rusty

Rusty; Great to hear/compare your experience with mine.

No oil/coolant thermostats - I don't think they are really required in Australia.

The biggest "issue" is that my Sonex Legacy has a rear (low on firewall) mounted radiator - The idea is that all of the cowling air must pass through it on its way out. Problem is that there is just not enough air going through the cowling.

Our (my Son & I) first attempt was to install a good quality motorcycle cooling fan, on one side of the radiator. My Son is a fanatical dirt bike rider and has found this idea to be very effective on his bike. Certainly slowed the cylinder head (coolant) temp rise but not by enough.

So we are in the process of cutting extra holes in the cowling.
First try was to increase air flow in by cutting a 30mm hole in front of the under engine (chin) mounted oil cooler complete with temporary cardboard air scoop - again some improvement but still not enough.
Next we have cut a rectangular hole 70 x 250mm just in front of the radiator (low on the cowl belly) with a temporary "lip" to create turbulence/low air pressure - this is intended to encourage air out. Not tested yet as starter motor has packed it in after only 6 starts & 3 hrs of running. We live in hope.

Skippydiesel wrote:She hasn't even taxied (forward motion may help) and the coolant (head temps) gets up to 120C - the oil 110C.

My experience is that even taxiing helps cool the engine.

One thing you may not have thought of is bleeding all the air out of the coolant. If you have any high spots where air can be trapped, the coolant flow will be diminished and if it happens to be in the radiator the air space will also reduce cooling capacity of the radiator. In my application, this was such a big factor that I actually had 2 bleed ports welded into the radiator, 1 for bleeding air out of the top and the other just for convenience for draining coolant at the bottom (no mess when changing coolant).

peter anson wrote:

Skippydiesel wrote:She hasn't even taxied (forward motion may help) and the coolant (head temps) gets up to 120C - the oil 110C.

My experience is that even taxiing helps cool the engine.

Agree Peter - I even noticed a difference by just turning the fuselage into wind. I guess the concern is that if we are having a temperature rise to near 120C (shut down) in winter , this can only get worse in summer conditions.

Murray Parr wrote:One thing you may not have thought of is bleeding all the air out of the coolant. If you have any high spots where air can be trapped, the coolant flow will be diminished and if it happens to be in the radiator the air space will also reduce cooling capacity of the radiator. In my application, this was such a big factor that I actually had 2 bleed ports welded into the radiator, 1 for bleeding air out of the top and the other just for convenience for draining coolant at the bottom (no mess when changing coolant).

Thanks for the comment Murray - the configuration of my cooling system (horizontal radiator at the lowest point) would suggest little chance of an "air bubble" anywhere. Couple this with about 6 run ups where temperatures reached almost to 120C - I would expect any entrapped air to have been forced to the highest point and then expelled. I have also checked the collection header every day before engine start - so far I have added about 200 ml to fill to the lip, suggesting that some air has been removed.