Overheating
Posted: Sat Aug 13, 2022 7:03 am
by Skippydiesel
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.
Re: Overheating
Posted: Mon Aug 15, 2022 4:05 am
by lakespookie
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.
Re: Overheating
Posted: Mon Aug 15, 2022 9:36 am
by 13brv3
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
Re: Overheating
Posted: Mon Aug 15, 2022 7:13 pm
by Skippydiesel
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)
Re: Overheating
Posted: Mon Aug 15, 2022 8:03 pm
by 13brv3
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
Re: Overheating
Posted: Mon Aug 15, 2022 11:17 pm
by Skippydiesel
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.
Re: Overheating
Posted: Tue Aug 16, 2022 2:46 am
by peter anson
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.
Re: Overheating
Posted: Tue Aug 16, 2022 4:14 am
by Murray Parr
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).
Re: Overheating
Posted: Tue Aug 16, 2022 5:23 am
by Skippydiesel
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.
Re: Overheating
Posted: Tue Aug 16, 2022 5:30 am
by Skippydiesel
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.
Re: Overheating
Posted: Tue Aug 16, 2022 7:02 am
by Murray Parr
[quote]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/quote]
My radiator is also at the lowest point in the system, however, the hose orientation has one hose high and the other diagnally across from that at the low point in the radiator but the radiator is not horizontal and there is definately a large area inside the radiator that traps air and no matter how hot or how much flow it has, the air can never escape that environment. Just a thought given how quickly yours is heating up
Re: Overheating
Posted: Tue Aug 16, 2022 7:41 am
by Skippydiesel
My radiator is also at the lowest point in the system, however, the hose orientation has one hose high and the other diagnally across from that at the low point in the radiator but the radiator is not horizontal and there is definately a large area inside the radiator that traps air and no matter how hot or how much flow it has, the air can never escape that environment. Just a thought given how quickly yours is heating up[/quote]
Persuasive argument - how would I go about diagnosing an air bubble in the radiator?
Then if I find that this is the case - how would I remove it, without making the modifications that you have mentioned ?
Re: Overheating
Posted: Wed Aug 17, 2022 4:33 am
by Murray Parr
As long as one of the hose connections on the radiator is at the highest point, the radiator shouldn’t have any air in it. Also make sure none of the hoses have high spots in them. If your system has a hose connection on the radiator that isn’t at the high point then you can get the air out by either tilting the plane or if that isn’t possible you could try removing the radiator from its mounts and manouvre it around to get the air out that way. Once the air is removed it shouldn’t get air back in it again until you empty it in the future. If you think your system is free of air then maybe look into wether your temp guages are accurate.
Another thing to look at is make sure you have enough exit air volume
Re: Overheating
Posted: Wed Aug 17, 2022 8:22 am
by Skippydiesel
“Another thing to look at is make sure you have enough exit air volume”
Murray - in a sort of way this is where I think the problem is - insufficient air getting into the cowling, leading to insufficient cooling air moving through the radiator. The focus at the moment is experimenting with temporary fixed cowl flap, to try & induce greater exit air flow.
Re: Overheating
Posted: Wed Aug 17, 2022 8:26 am
by Skippydiesel
“Another thing to look at is make sure you have enough exit air volume”
Murray - in a sort of way this is where I think the problem is - insufficient air getting into the cowling, leading to insufficient cooling air moving through the radiator. The focus at the moment is experimenting with temporary fixed cowl flap, to try & induce greater exit air flow.
Oh! On the possible air bubble in the radiator - today, I strongly pulsed (squeezed) both the in & out pipe,s to the radiator, with radiator cap off & on - no hint of air in system. Also carefully inspected installation to try & guess where air might be trapped. Cant envision any potential for trapped air.
Re: Overheating
Posted: Wed Aug 17, 2022 8:31 am
by Skippydiesel
I am going to place some digital thermometers I have, on the radiator inlet/outlet hose right where they enter/exit the raditor, to get a separate reading (estimate) on coolant temperature to compare with Dynon readings.
This wont happen until starter motor has been replaced - how long ???
Re: Overheating
Posted: Wed Aug 17, 2022 9:07 pm
by lakespookie
I really think you need to examine your inlet vs outlet area and how you are routing the radiator air volume, I know rotaxs are not primarily air cooled but i suspect that a lower cowl entrance when the cowl is on is causing a high pressure at the exit and limiting your overall airflow unless you have that volume of inlet air segregated from the upper plenum inlet and exhaust air. If you are running with no cowls you are making it worse. Also with the extra inlet i would confirm your exit to entrance ratio is 3 to 1 at a minumum.
Just out of curiosity what is your Radiator size/area? i think the Current factory experiment with the 1 week wonder used split rads to add extra cooling volume and also to simplify some of the concerns with plenum exit pressurization but i could be wrong and i dont necessarily trust my goldfish brain.
Re: Overheating
Posted: Thu Aug 18, 2022 2:36 am
by Skippydiesel
lakespookie wrote:I really think you need to examine your inlet vs outlet area and how you are routing the radiator air volume, I know rotaxs are not primarily air cooled but i suspect that a lower cowl entrance when the cowl is on is causing a high pressure at the exit and limiting your overall airflow unless you have that volume of inlet air segregated from the upper plenum inlet and exhaust air.
This is not a traditional front or even side mounted radiator (the largest Rotax supply) - its sort of belly mounted, in that it is located at the bottom of the firewall but designed to be incorporated into the cowling (https://webuildplanes.com/cliff-jarvis/). The concept is that of a pressurised/sealed cowlin, with all exit air going out via the radiator
If you are running with no cowls you are making it worse. .
The no cowl runs demonstrated that the cooling systems oil, coolant & fins, are all working - to the point where suboptimal operating temperatures were recorded
Also with the extra inlet i would confirm your exit to entrance ratio is 3 to 1 at a minumum
See above comment - we are trying to get away from this ratio (foolish?)
Just out of curiosity what is your Radiator size/area? i think the Current factory experiment with the 1 week wonder used split rads to add extra cooling volume and also to simplify some of the concerns with plenum exit pressurization but i could be wrong and i dont necessarily trust my goldfish brain.
I have seen split radiators on Europa aircraft - certainly worked but at considerable additional weight & complexity
We may end up with a pilot controlled cowl flap for ground ops & climb out, closing when reaching cruise altitude.
Re: Overheating
Posted: Sun Aug 21, 2022 4:14 am
by lakespookie
Cowl looks pretty but i suspect your prop Is not driving much air into the inlet based on the few pictures i could see and where the prop pitch starts. That does not help you much with ground running it really would help to see actual detailed pictures of your cooling layout and air flow even if its just a drawing. I looked through that link but i was not able to see the rad (I think maybe i saw it at the bottom)and obviously with the cowl on you cant really see the airflow pathway. If i remeber correctly and i dont trust my goldfish brain, the liquid cooling was something like 65 to 70% of the cooling on a rotax so directing airflow through the rad is critical. So having it bottom mounted is not the worst thing but you need to make sure your inlet air is being directed and forced through the rad not just near it.
This is one of those times where a picture or drawing showing the airflow pathway would do wonders. If you are just ramming air into the cowl with no direction then you are not going to have much cooling as air is not going to want to go through restrictions like a rad. I didnt see any baffling anywhere or some kind of plenum so i really have no way of adding any value with out a drawing or more pictures.
That being said that White is clean nice paint job.