Overheated Merlins And What To Do About Them

[MIflyer]

No doubt that everyone here has read many times of instances when Merlin engined aircraft were hit in the radiator that the pilot had a very few minutes to either land or bail out before the airplane went up in flames.

But I read of an incident in a book I cannot locate right now where that did not happen. I believe it was Don Blakeslee's unit, flying a long range escort mission in the Mustang when one airplane took a hit in the radiator and the pilot prepared to bail out. Blakeslee told him not to bail out but to throttle back and start pumping the primer to inject raw fuel into the engine to keep it cool. It worked, and the pilot flew all the way back to Great Britain, pumping the primer all the way. By the time he got home his glove was worn out and his hand was worn down to the bone, but he made it.

I have wondered why I never read of any other cases of pilots saving the airplane by using the primer. It turns out that the P-51B Mustangs being flown on that mission had manual primers. You grabbed a knob, pulled the plunger out and then pushed it back in. But the later P-51's had electric primers where you pushed on a button for a second or two. I can only assume that the electric primer would not operate after the engine was running, since there would be no reason to use it under those conditions.

In fact, one thing you check very carefully on a light aircraft with a manual primer is to make sure it is locked before takeoff, since having it come loose can cause the engine to quit. One trick in cold weather is to prime the engine and leave the primer open, pulled out, so to increase the fuel flow while at low RPM and keep the engine running until it warms up. Obviously you have to be careful to close it and lock it before taking off.

However I have never read of a Spitfire pilot cooling a Merlin by pumping the primer, even though the pilot's manual for the Mk IX shows it has a manual pump. I wonder if it has to do with the carburetor on the Spits being the float type rather than the Bendix pressure injection carbs used on the American Merlins from the very beginning?

 

[Dash119]

No doubt that everyone here has read many times of instances when Merlin engined aircraft were hit in the radiator that the pilot had a very few minutes to either land or bail out before the airplane went up in flames.

But I read of an incident in a book I cannot locate right now where that did not happen. I believe it was Don Blakeslee's unit, flying a long range escort mission in the Mustang when one airplane took a hit in the radiator and the pilot prepared to bail out. Blakeslee told him not to bail out but to throttle back and start pumping the primer to inject raw fuel into the engine to keep it cool. It worked, and the pilot flew all the way back to Great Britain, pumping the primer all the way. By the time he got home his glove was worn out and his hand was worn down to the bone, but he made it.

I have wondered why I never read of any other cases of pilots saving the airplane by using the primer. It turns out that the P-51B Mustangs being flown on that mission had manual primers. You grabbed a knob, pulled the plunger out and then pushed it back in. But the later P-51's had electric primers where you pushed on a button for a second or two. I can only assume that the electric primer would not operate after the engine was running, since there would be no reason to use it under those conditions.

In fact, one thing you check very carefully on a light aircraft with a manual primer is to make sure it is locked before takeoff, since having it come loose can cause the engine to quit. One trick in cold weather is to prime the engine and leave the primer open, pulled out, so to increase the fuel flow while at low RPM and keep the engine running until it warms up. Obviously you have to be careful to close it and lock it before taking off.

However I have never read of a Spitfire pilot cooling a Merlin by pumping the primer, even though the pilot's manual for the Mk IX shows it has a manual pump. I wonder if it has to do with the carburetor on the Spits being the float type rather than the Bendix pressure injection carbs used on the American Merlins from the very beginning?

Okay, I'm not an engineer or an A&P, and I don't play either on TV...

That being said, dumping raw fuel into the induction trunk causes the engine to run cool enough that it can continue to run without proper coolant? Raw fuel in the induction trunk will lower the induction temperature, but I'm not seeing how that effects the temperature of the block and the heads. Additionally I would think that too much raw fuel introduced into the induction trunk in flight will choke the engine. If this event really occurred, I am inclined to think the damage to the cooling system was not that severe.

Hopefully someone with real knowledge of such things will chime in.

 

[fastmongrel]

I recall reading that the Rolls Royce R engine in the Supermarine S6 racers was run very very rich to keep the engine cool enough. Mind you that wasn't AVGAS fuel it was a toxic mix of chemicals.

 

[MIflyer]

Air-cooled engines are known to be in reality "fuel cooled." They generally run at much richer mixture ratios than do liquid cooled engines, which is the main reason they are no longer popular in automobiles. Aside from the heat of vaporization of the fuel, rich mixtures run much cooler than do leaner ones.

Of course the event probably occurred at high altitude and no doubt the pilot throttled back, probably below 50% power.

 

[GregP]

The Germans had Hans Ekkehard Bob.

In one action, his Bf 109 was hit in the radiator. He set minimum climb power and started for home, When the engine got near redline hot, he throttled back to idle and glided. When he got close to the water, he went back to minumum climb power and repeated the glide when it again got near redline. By doing this repeatedly, he made it home.

The technique became know in the Luftwaffe as "Bobbing."

 

[MIflyer]

Yep, given that the DB was fuel injected, making it run very rich was not an option.

Goring once said, "One of our biggest problems is that Luftwaffe pilots want to sleep in their own bed at night." They would get shot up attacking USAAF bombers and then ignore the many alternate airfields available and always try to make it back to their home field, and crash along the way. During the BoB I do not think that RAF pilots were at all averse to landing at any available airfield if they got shot up, but of course they were over their own country.

 

[rinkol]

Okay, I'm not an engineer or an A&P, and I don't play either on TV...

That being said, dumping raw fuel into the induction trunk causes the engine to run cool enough that it can continue to run without proper coolant? Raw fuel in the induction trunk will lower the induction temperature, but I'm not seeing how that effects the temperature of the block and the heads. Additionally I would think that too much raw fuel introduced into the induction trunk in flight will choke the engine. If this event really occurred, I am inclined to think the damage to the cooling system was not that severe.

Hopefully someone with real knowledge of such things will chime in.

If the oil cooling system is intact, that will carry a proportion of the engine heat away and this might let you get away with a rich mixture for some time.

 

[pbehn]

Yep, given that the DB was fuel injected, making it run very rich was not an option.

Goring once said, "One of our biggest problems is that Luftwaffe pilots want to sleep in their own bed at night." They would get shot up attacking USAAF bombers and then ignore the many alternate airfields available and always try to make it back to their home field, and crash along the way. During the BoB I do not think that RAF pilots were at all averse to landing at any available airfield if they got shot up, but of course they were over their own country.

That is one way of looking at it, in the Battle of Britain pilots frequently just landed where they could but their aircraft were designed to land on grass fields and the airfield was a grass field too. An RAF pilot knew there were other aircraft at home all he had to do was get there, was it the same for a L/W pilot and if it took him days to get there would he be accused of cowardice?

 

[ThomasP]

I do not know if it is true, but I have read that the Merlin relied on oil cooling of the engine to a significantly higher degree than other liquid cooled engines of similar power. Even if this is true, it would not necessarily mean that it would make enough difference to allow the engine to keep on running for a significantly additional amount of time with the liquid cooling system non-functional.

 

[pbehn]

I do not know if it is true, but I have read that the Merlin relied on oil cooling of the engine to a significantly higher degree than other liquid cooled engines of similar power. Even if this is true, it would not necessarily mean that it would make enough difference to allow the engine to keep on running for a significantly additional amount of time with the liquid cooling system non-functional.

It quite possibly is true, but the Merlin eventually reached 2000BHP and at that output you take any cooling you can. The big USA and UK air cooled engines also relied heavily on oil cooling as part of the whole setup.

 

[MIflyer]

Well, in the P-51H they took the oil cooler out of the radiator airscoop and located it in the engine compartment, which improved engine cooling considerably, eliminating the problem of the engine overheating while idling on the ground. So the oil cooler could not have required much airflow, even with the high powered -9 engine.

 

[wuzak]

However I have never read of a Spitfire pilot cooling a Merlin by pumping the primer, even though the pilot's manual for the Mk IX shows it has a manual pump. I wonder if it has to do with the carburetor on the Spits being the float type rather than the Bendix pressure injection carbs used on the American Merlins from the very beginning?

Later Merlins in Spitfires had pressure injection carbs. The Merlin 66, for example was fitted with one.

 

[MIflyer]

Later Merlins in Spitfires had pressure injection carbs. The Merlin 66, for example was fitted with one.

Yes, that is what I understand. I do not know just when that change was incorporated. I assume that the MkXVI built with the Packard Merlins were equipped that way from the first, but that was pretty late in the war.

 

[Shortround6]

Well, in the P-51H they took the oil cooler out of the radiator airscoop and located it in the engine compartment, which improved engine cooling considerably, eliminating the problem of the engine overheating while idling on the ground. So the oil cooler could not have required much airflow, even with the high powered -9 engine.

Hard to say, perhaps someone has a better description but one website claims the P-51H used a heat exchanger for the oil cooler using glycol (or glycol/water?) that was shared with either the normal cooling system or the supercharger intercooler. Maybe I am reading it wrong?
ANyone have a manual for the "H"?

an oil cooler is going to require a fair amount of airflow. about 20-25% of the airflow of the radiator on a liquid cooled engine.

 

[MiTasol]

Okay, I'm not an engineer or an A&P, and I don't play either on TV...

That being said, dumping raw fuel into the induction trunk causes the engine to run cool enough that it can continue to run without proper coolant? Raw fuel in the induction trunk will lower the induction temperature, but I'm not seeing how that effects the temperature of the block and the heads. Additionally I would think that too much raw fuel introduced into the induction trunk in flight will choke the engine. If this event really occurred, I am inclined to think the damage to the cooling system was not that severe.

Hopefully someone with real knowledge of such things will chime in.

At take off all aircraft piston engine run almost maximum rich to cool the engine while there is minimal airflow to provide cooling. Broken down to the most basic level the cooling mainly comes from the heat energy needed to vapourise the fuel in the cylinders.

The down side is increased wear as the additional fuel removes much of the oil film from the cylinder lining. I expect whether you can cool the engine for an extended period like that would depend on not only very low power but also many other factors starting with OAT.

I had a customer years ago who kept cracking cylinders and then the crank case on a Cessna 210. After every failure I would grill him on how he operated the engine. After that engine died at under 300 hrs I insisted on going on the test flight with the new engine. At the end of the runway he bought the power up to maximum and then started leaning the mixture while watching the EGT gauge. I bashed his hand (VERY hard) to force the mixture full rich again and then told him to taxi back to the hangar as I had no intention of flying with a suicidal nut job.

At the hangar he demanded to know what I meant so I told him very bluntly to go and read his basic pilot technical training notes on engine fuel flow and how engines use fuel to cool at high power and low airflow. Leaning will provide more TO power but causes shock heating and over stresses the engine causing cylinder and crankcase cracking.

We kept his business and he later referred to the incident as his $30,000 lesson on why you must read, understand and remember the manuals

 

[Admiral Beez]

No doubt that everyone here has read many times of instances when Merlin engined aircraft were hit in the radiator that the pilot had a very few minutes to either land or bail out before the airplane went up in flames.

Is that what happened? If the radiator fails on my car the engine doesn't ignite a fire, instead the engine seizes and stops running.

 

[pbehn]

Is that what happened? If the radiator fails on my car the engine doesn't ignite a fire, instead the engine seizes and stops running.

The glycol coolant was flammable until the Merlin Mk X was introduced which used a glycol water mix. Quote Merlin development might have stagnated after 1940, any further increases in power needed a more efficient means of transferring the heat away from the engine. Rolls Royce responded with a mixture of water and Ethylene Glycol which was put under pressure. This mixture also reduced the fire risk associated with using pure Ethylene Glycol. This system was first used in the Merlin XII used in the Spitfire Mk II. The rapid introduction of this system was only made possible by everything Rolls Royce had learnt about pressurized cooling when developing the Goshawk and early Merlin condenser systems. and …. Introduced with the Merlin X, the use of 70-30% water glycol coolant mix, improved engine reliability and removed the fire hazard and reduced oil leaks compared with earlier marks.

 

[Shortround6]

Air-cooled engines are known to be in reality "fuel cooled." They generally run at much richer mixture ratios than do liquid cooled engines, which is the main reason they are no longer popular in automobiles. Aside from the heat of vaporization of the fuel, rich mixtures run much cooler than do leaner ones

In a car the carburetor or injection system takes care of the fuel mixture. Unless you have a manual "choke" there is no way for a regular driver (no computer plugged into the car's computer) to change the factory settings at a particular throttle setting.

AIrcraft differ, sometimes considerably, there were manual rich/lean settings, not often used on fighters as the pilot is too busy. Some multi engine planes used them because they had a flight engineer who took care of the engines.

An aircraft air cooled engine could be set up to use a considerable amount of fuel as "coolant" at take-off or full power, in rich mixture.
However in cruise settings when in lean (auto lean) an air cooled engine was very close to a liquid cooled engine in amount of fuel used per horsepower hour. Some air cooled engines were better than a few liquid cooled ones.

Some fighters were set up with one 'stick" controlling a number things, like throttle, prop pitch/rpm, mixture. Made for less work in combat, but less flexibility when not in combat.

Just because a certain 'trick' or technique was used on one plane doesn't mean it could be used on other planes or even later versions of the same plane.
P-40s went from pretty much manual everything including no boost limiter to a one stick control on the last models.

amount of heat dissipated by the oil circuit varied from engine to engine. Merlins needed bigger oil coolers than Allisons for the same amount of power I believe, I could have that backwards. In any case the P-40F used a different radiator and oil cooler setup than a P-40E or P-40K.

Running the fuel primer (or pumping as if your live depended on it) to provide extra cooling at low power settings might work. it might not depending on amount of damage to the cooling system, outside temperature, power setting of the engine and temperature of the engine when the whole situation started. Also altitude, Being able to trade altitude for distance while keeping the engine just ticking over to extend the glide is a lot different than loosing the cooling system at 200 ft on a hot summer day with the engine just out of the red zone.

 

[tomo pauk]

Hard to say, perhaps someone has a better description but one website claims the P-51H used a heat exchanger for the oil cooler using glycol (or glycol/water?) that was shared with either the normal cooling system or the supercharger intercooler. Maybe I am reading it wrong?
ANyone have a manual for the "H"?

an oil cooler is going to require a fair amount of airflow. about 20-25% of the airflow of the radiator on a liquid cooled engine.

The P-51J (2-stage V-1710 engine) have had the heat exchanger system to cater for oil cooling.
At avialogs, there is a host of manuals free for download, including many of P-51 versions. P-51H section: link
At P-51H, the oil system cooling was served by the afercooler cooling circuit via heat exchanger.

 

[MIflyer]

Yes, in the EAA article on the P-51H I thought they said that used the aftercooler for oil cooling, You want the oil temp to be around 180-200F to keep the water burned off and not over maybe 220F to keep the oil from being damaged. Liquid cooled engines had problems with oil getting sludge and water due to too low an oil temp. Under the cowling the only two alternatives for oil cooling are the aftercooler and the carb air intake duct and you normally would not want to heat up the air before it entered a supercharged engine.