How Hot Is Too Hot?

[check6]

Yes, plenty of space behind the radiator. Getting to it is another matter.

The coolant lines are 3/4" I.D. And the radiator is a dual pass design.

Tex,

There are a couple of things that I would do if it were my project. I would increase the coolant line diameters going to and from the radiator. The 3/4” I.D. lines that are adequate for a Rotax are too small for a larger displacement engine. I would use at least a 1 1/4” tube. More horsepower means more heat to reject. It may be difficult to replace these long tubes in a completed aircraft, but the only way to keep the temps in check is push a high volume of coolant thru the radiator. The fin dimension of the radiator in the S51 is 22”x18”x3”. Built by Griffin, it is a two pass radiator built specifically designed for the S51 with the inlet and outlet tubes being 1 1/2” diameter. This radiator can keep the coolant temps of a 600 HP engine controlled during a two minute static thrust check at takeoff power. If your radiator is of similar size, your coolant temp problem isn’t the radiator assuming that you can flow enough coolant thru it.



Note the bung at the top of the end tank. It is there to bleed air out of the system, both when filling the coolant system and during flight. A dash 4 line is attached and routed to the coolant expansion tank at the highest point of the coolant system. This line continuously “burbs” the air out of the system. Air bubbles are constantly being generated in the coolant system both by the impeller of the pump and by the mechanism of heat transfer. The pump impeller is like a “cuisinart” as it produces flow of the coolant. At higher speeds it can cavitate creating minute bubbles.
The other source of bubbles entering the system is the interface of the coolant cavities in the engine and the liquid coolant during heat transfer. Called nucleate boiling, it occurs when the surface temperature is hotter than the saturated fluid temperature. Think of heating up water in a pot on the stove. When the water temperature approaches the boiling point, tiny bubbles begin to form on the bottom of the pot, detach and float to the top. These bubble forming displace the liquid coolant. This is why the pressure in a closed system increases as the coolant temperature rises.
The method engineers employed to control the air in the system was to provide an area away from the heat source to allow the bubbles to escape out of solution. Up until the late 70s automobile manufacturers used a coolant expansion tank. This tank was on top of the radiator fins, the highest point of the engine’s coolant system. The radiator cap on this tank was a pressure relief valve preventing an over pressure situation which would blow a head gasket or a rubber radiator hose. Air bubbles in the coolant would float to the top of the liquid coolant and enter the expansion chamber (tank) on the top of the radiator. This is the reason why you should never fill a radiator completely up to the neck. If the pressure became too great the radiator cap would release the pressure to the atmosphere creating the classic cloud of steam coming from under the hood. Auto manufacturers in the 80s, having to deal with higher engine temps due to emission requirements and eliminating the possibility of puking a toxic fluid onto Mother Earth, began to employ coolant recovery tanks which dealt with the expansion do to air entrapment. These tanks are made of plastic and were connected to the coolant system at the radiator. When the coolant expanded it flowed down a siphon tube into the recovery tank where the air was removed from the liquid. When the coolant cools it’s volume decreases creating a vacuum which sucks the coolant out of the tank “recovering” it for use. These tanks must always contain enough liquid to keep the siphon tube covered in order for it to work properly. Cars equipped with recovery tanks rarely, but do occasionally, boil over causing a cloud of steam. The coolant expansion tank and coolant recovery tank do the same thing, except that the expansion tank will vent coolant overboard.
The cause of your projectile coolant vomiting was most likely due to an air pocket. When the surface of the coolant chamber where the air pocket occurred got hot enough, the liquid coolant “flashed” to steam. When the liquid flashes to steam, its volume expands 1700 times the volume of the liquid, causing an instantaneous pressure spike. This pressure finds the weakest link of the coolant system to vent to the atmosphere.
Got a little long winded here, but if was me trying to solve this problem, I would increase the size of the coolant lines, install a bleed line on the top of the radiator, make sure that the coolant recovery tank was half full when cold and fill the coolant system with the aircraft in a level attitude. Consider a vacuum fill system for eliminating the air prior filling with coolant.

 

[Pops]

1 1/4" coolant lines for the little Buick 215 ci, 200 HP, aluminum block V-8 in the 1968 VW Bug.

 

[Hephaestus]

Some kind of electric blower etc for temporary ground testing may be warranted. Permanently mounted fans tend to me more trouble than they are worth in-flight, causing more restriction than flow. Been tried many times.

Depends on how you're working it wouldn't you agree?


You're not going to do fully shrouded - yes that would absolutely restrict airflow in this scenario


You're going to do something like this - likely more than one fan - goal is to increase pressure differential. Yes you loose some airflow mostly around the motor (and brackets if you go insane)

The issue is always execution and integration in an aircraft. This is no different from fuel, air, filters, electronics etc - all those things that there's never enough time or money to do right the first time.

None of this is new or groundbreaking - how many ieee papers exist on making this work? Almost every big city has one of those rad shops with a guy who knows the oddball configurations - ask your friend with a outlaw/sprint/formula whatever car - they always know who the guy to see is...

 

[Voidhawk9]

Depends on how you're working it wouldn't you agree?

Placing anything in the flow path will impede the flow past it. These aren't car-type cooling systems with oversized radiators designed for going (relatively) slowly. The history of using fans in this way on fixed-wing aircraft, at least in my reading and research, ends mostly in failure until the fan is removed. Less efficient, oversized rads (like a car) will manage OK, but I do not think that applies to the T-51.

 

[pictsidhe]

Just looked at the numbers for 3/4 lines, they are too small. What hp do you have, how long are the lines?

 

[Marc Bourget]

Both WSimpson and TFF raised good points that deserve greater development.

Overall, you have (not only) to ensure that the heads remain wetted, but you also have to (what I say in my cooling lectures) - "Turn off the Bubble Machine."

Points to keep in mind, it only takes a minor amount of Delta P at or before the water pump intake to generate bubbles that travel through the system and reduce heat transfer.

In the coolant stream, above a Delta T of 10 degrees, coolant starts to demonstrate layering or "immiscibility." It's not uncommon to have the sensor seeing 210 deg, when, 1/2" away, the coolant is streaming by above 270. If you doubt that, put a half-full glass in the freezer till just before going solid then slowly pour hot from the tap water and watch the refraction layer develop.

It's hard to maintain Reynolds Number turbulence in cast cooling passages. In some difficult installations that have coolant jackets with high spots or domed areas, the bubbles formed at the pump intake, or simply from flowing around a sharp corner, will collect in the high spots, and, as I call it, "the red zone." In some cases, a bleed line from the high spots (like those I've seen in some small block circle track racers in the center of the heads on the intake side) to the "header tank" ( a misnomer, it should be a "de-aerator tank") and some other trick features to get rid of the bubbles are necessary.

I'm not familiar with the LS376-495 engine, so much has developed since I was knowledgeable on small blocks but I'm sure I could give you good guidance if I knew more about the internals. Just a quick example, I've more than once sectioned a head on a Wells band saw to check the configuration of the cooling jacket in the head. I don't expect you to do this, but it would be helpful to have a better idea of the configuration of the cooling jacket. As for overheating the casting, I'd worry about the heads if aluminum. Cast Iron is generally easier to check for damage.

FWIW, I "learned" my cooling from an Engineer that developed cooling systems for GM Military Vehicle Operations. He had the task of developing cooling systems for the Israeli Defense Forces. They had the highest standards in the world. He had to demonstrate adequate cooling in 145deg. F ambient temperatures. That criteria happened to provide the IDF one of the "hidden advantages" of the 6 day war.

Onward and upward

Finally, I kinda jumped in towards the end of this thread and I apologize if, in my ignorance of what was discussed before, I "stepped on some toes"

 

[pictsidhe]

Googled it. You need a minimum of 1 1/4 lines, more if your pump moves a lot more water than required. A bigger pulley would fix that, though.
So, no air bleed at the high point and undersized lines... Perhaps you should tell us the core size while we are discussing this.

 

[BBerson]

He may already have the bigger pulley if that was how the factory did it and it works at flight rpm. Twenty feet of coolant and hose or pipe isn't light or easy to change now. I would see what they use and avoid ground runs.

 

[pictsidhe]

He may already have the bigger pulley if that was how the factory did it and it works at flight rpm. Twenty feet of coolant and hose or pipe isn't light or easy to change now. I would see what they use and avoid ground runs.

3/4 is just not going to work. With an optimistic 90F delta, it will have 18psi pressure drop on 20' of line. Too much. At a reasonable 50F delta, it will take 55psi to push enough water down 20' of 3/4 line.

 

[Marc Bourget]

Tex,

There are a couple of things that I would do if it were my project. I would increase the coolant line diameters going to and from the radiator. The 3/4” I.D. lines that are adequate for a Rotax are too small for a larger displacement engine. I would use at least a 1 1/4” tube. More horsepower means more heat to reject. It may be difficult to replace these long tubes in a completed aircraft, but the only way to keep the temps in check is push a high volume of coolant thru the radiator. The fin dimension of the radiator in the S51 is 22”x18”x3”. Built by Griffin, it is a two pass radiator built specifically designed for the S51 with the inlet and outlet tubes being 1 1/2” diameter. This radiator can keep the coolant temps of a 600 HP engine controlled during a two minute static thrust check at takeoff power. If your radiator is of similar size, your coolant temp problem isn’t the radiator assuming that you can flow enough coolant thru it.
View attachment 102666
View attachment 102667

Note the bung at the top of the end tank. It is there to bleed air out of the system, both when filling the coolant system and during flight. A dash 4 line is attached and routed to the coolant expansion tank at the highest point of the coolant system. This line continuously “burbs” the air out of the system. Air bubbles are constantly being generated in the coolant system both by the impeller of the pump and by the mechanism of heat transfer. The pump impeller is like a “cuisinart” as it produces flow of the coolant. At higher speeds it can cavitate creating minute bubbles.
The other source of bubbles entering the system is the interface of the coolant cavities in the engine and the liquid coolant during heat transfer. Called nucleate boiling, it occurs when the surface temperature is hotter than the saturated fluid temperature. Think of heating up water in a pot on the stove. When the water temperature approaches the boiling point, tiny bubbles begin to form on the bottom of the pot, detach and float to the top. These bubble forming displace the liquid coolant. This is why the pressure in a closed system increases as the coolant temperature rises.
The method engineers employed to control the air in the system was to provide an area away from the heat source to allow the bubbles to escape out of solution. Up until the late 70s automobile manufacturers used a coolant expansion tank. This tank was on top of the radiator fins, the highest point of the engine’s coolant system. The radiator cap on this tank was a pressure relief valve preventing an over pressure situation which would blow a head gasket or a rubber radiator hose. Air bubbles in the coolant would float to the top of the liquid coolant and enter the expansion chamber (tank) on the top of the radiator. This is the reason why you should never fill a radiator completely up to the neck. If the pressure became too great the radiator cap would release the pressure to the atmosphere creating the classic cloud of steam coming from under the hood. Auto manufacturers in the 80s, having to deal with higher engine temps due to emission requirements and eliminating the possibility of puking a toxic fluid onto Mother Earth, began to employ coolant recovery tanks which dealt with the expansion do to air entrapment. These tanks are made of plastic and were connected to the coolant system at the radiator. When the coolant expanded it flowed down a siphon tube into the recovery tank where the air was removed from the liquid. When the coolant cools it’s volume decreases creating a vacuum which sucks the coolant out of the tank “recovering” it for use. These tanks must always contain enough liquid to keep the siphon tube covered in order for it to work properly. Cars equipped with recovery tanks rarely, but do occasionally, boil over causing a cloud of steam. The coolant expansion tank and coolant recovery tank do the same thing, except that the expansion tank will vent coolant overboard.
These cause of your projectile coolant vomiting was most likely due to an air pocket. When the surface of the coolant chamber where the air pocket occurred got hot enough, the liquid coolant “flashed” to steam. When the liquid flashes to steam, its volume expands 1700 times the volume of the liquid, causing an instantaneous pressure spike. This pressure finds the weakest link of the coolant system to vent to the atmosphere.
Got a little long winded here, but if was me trying to solve this problem, I would increase the size of the coolant lines, install a bleed line on the top of the radiator, make sure that the coolant recovery tank was half full when cold and fill the coolant system with the aircraft in a level attitude. Consider a vacuum fill system for eliminating the air prior filling with coolant.

This is one of the posts I closed with the comment "I may have missed". As to increasing the size radiator hoses, anything that restricts flow can contribute to formation of bubbles. My notes are packed from a move, but we're talking, IIRC, inches of H2O Delta P. My "guy" gave me a manometer tester design used to measure that difference. The coolant recovery tank works with a "developed" coolant system, not necessarily with a one-off adaptation to an aircraft. A header tank with specific features to promote de-aeration would be advised.

Kudos to this poster!

 

[wsimpso1]

The overflow bottle is about 2 qt capacity. See the pictures, located in the upper right hand corner of the engine, the highest point in the system.

Initially, I was uneasy about having to get fancy about bleeding air, but it took a bit to be able to express why I feel that way. If you have to carefully get all of the air out of the system, any air (or other gases) that somehow remain or are created will find its way to local high spots, wherever that is. In a system where air or other gases do not automatically go to the pressure cap (continuously uphill from low spot to tank), they will accumulate at the local high spots. While an engine with boiling coolant - or worse one with a leaking head gasket - has a short life remaining, if the air/gases can vent out the expansion tank, you have a few more minutes before complete loss of power than if the engine traps air in the heads. In this way, any indication of reaching coolant boiling point must mean immediate diversion.

You have the expansion bottle in the right place - is it connected to the high spot in the engine cooling loops? Your photos do not show a connection on the highest spot on the forward face of the heads. Think about this a little - if air in the engine can not continuously travel up hill to hose toward the tank, air will still be trapped in the head. The ideal spots would be in both heads at the very highest spots in cooling loop. In a less than ideal circumstance, some small amount of air might still be trapped in the upper corners, but if all of the directly heated portions of the head are covered with circulating coolant, it might be OK.

I would look closely at the same model heads on a bench and locate the highest spot in the cooling jacket for an air bleed. Best would of course simply connect this spot to an expansion tank so the air (or emergency flow of coolant vapors and other gases) can leave the engine automatically.

This level of overheat in a 12 minute run at idle power is short indeed. Many dragsters run their several minutes of idling, a burnout, then the speed run with water in the engine but no radiators at all. The quick nature of your overheat does make me think that you either started the run with substantial air trapped in the system or you have failed a head gasket.

Billski

 

[TXFlyGuy]

My engine tuner (LS3 expert, Hutter Performance) is convinced we have air trapped in the system. I will meet with him this week as we seek solutions to the issue.

Thanks to all for the input. Each and every suggestion is being taken under consideration.

 

[Marc Bourget]

Trapped air in a liquid cooling system has been thoroughly researched and worked out. As Wsimpso1 said above (and referenced by me, earlier) you have to remove the air - or, avoid trapping air.

I'll repeat and re-emphasize, you have to work diligently to ensure that all instances of conditions that can cause aeration have to be addressed, or better, eliminated.

Purposefully sounding a bit harsh and judgmental (in order to motivate you if motivation isn't present) :^).

"If you ain't paying attention to this then you aren't responsibly interested in solving the problem."

I wish you only success, (not asking for anything back) and will back my opinions with advice from an additional (3rd, 4th,or) head - should you wish.

If I can save you time, $ or inconvenience, then I'm a winner too!

Onward and upward

 

[check6]

Tex,
Make the improvements necessary to enable your airplane to do this. This S51 has wheel chocks, the tail is held down by a bar through the fuselage lift tube chained to a tie down ring on the ramp and the landing gear struts have straps attached to a pickup truck. The coolant system in this airplane allows the engine to idle for 30 minutes without overheating or to hold takeoff power on the ground for two minutes. The last thing you need to worry about when you push the throttle up for the first flight is the engine. You spent a lot of time and money to get to this point. Take your time, make the necessary changes before you attempt to fly. It’s more than an engine, it’s a life support system.

 

[Mike0101]

Do you have rear steam vents? Given normal block orientation in your aircraft, air shouldn't be a problem. Corvettes sometimes needed nose jacked way up to burp LS engines.

 

[TXFlyGuy]

The factory vents are located at rear, towards the cockpit. There are two tabs that we will drill out and add purge lines to, at the propeller end.

Here is some feedback from a friend with a Jurca Mustang...

We have successfully run 25 minutes at 1000 rpm coolant temp 210 oil temp 200. Requires 2 qt Header tank ( pressurized) plus 2 qt Overflow Recover tank ( vented). Our system is 17 qt, the Header Tank when cold is filled with 1 qt, the Overflow Recover tank empty. After about 20 minutes the temps are in the 190’s and the coolant pressure approaching 15 psi. During the next five minutes the temps and pressure continues to rise, the until the 16 lb pressure cap allows venting into the Overflow Recover tank. Found the original 24oz Overflow Recover tank filled and overflowed from coolant expansion. New 2 qt Overflow Recover tank should be this week. We are doing our engine runs without the prop installed, so a small box fan is used in front of the belly scoop. We are not using a thermostat, have installed 2 six inch cooling fans in a sealed ducted area behind the radiator. Additionally we have removed the external bypass hose from manifold to water pump. These are some of our findings during our so far engine testing, (16) 10 to 30 minute runs so far

 

[Mike0101]

Who makes 6" fans? Spal? If this is in Texas heat, then those fans may not cut it. I've owned more then a few LS engines, and once up to temperature they need more cooling then you'd expect, even at low load. The box fan, if it's something you'd find by a dyno, then should be fine.

Is the bypass hose from front steam vents? If so, I can go to garage and look at Vette for correct plumbing. But 95% it needs to go back.

 

[TXFlyGuy]

The main coolant lines from the bulkhead to the radiator are 1 1/8" I.D.
We are going to add larger lines from the pump to the firewall. These are the 3/4" lines.
Plus a larger overflow bottle will be added, 2 qt size.

 

[TXFlyGuy]

Hutter Performance came out and borescoped the engine. All cylinders and pistons looked good, nearly like new.

My oil report just came back today from Blackstone Labs. Everything is good. No water or antifreeze in the oil. All other items were right in line for a new engine, after three hours run time.

This is a relief, as it shows the head gasket is not blown, and the cylinders do not have tell-tale scoring on the walls.

Plus we just did another compression check...all cylinders tested out at 200 psi.

 

[Pops]

Oh Happy Day. Wish you all the best.
Fell in love with P-51's when every Sunday after WW-2 the AG would practice making runs on a hill about 2 miles north of my Grandfathers farm . After Sunday dinner, I would go to the front porch and wait for the string of P-51's and my private airshow.
Dan