Details on P-38F intercooler troubles

[Greyman]

Anyone able to elaborate on or give a rough timeline of the Lightning's intercooler issues?

Above is a very rough sketch based on a few documents. I'm trying to get a better grasp as to what went on.

USAAF Proof Dept. - 06 Mar 43 (AFDU air tests similar - 12 Aug 42)
Engineering Div. (est) - 19 Feb 43 (AFDU air tests similar - 26 Oct 42)
Lockheed data chart - 05 Aug 42 (British data sheet similar max speed point - 31 Aug 43)

 

[GregP]

The Lightning didn't have a lot of intercooler issues.

The early P-38s (through P-38G) used the leading edge of the wing as an intercooler. It had a cooling capacity of 1,050 HP. If you were asking for 1,050 or fewer HP, you had no intercooler issue. If you were using more power, the engine temp climbed continuously. At some point, it failed. Pilot error, not intercooler issue.

You COULD use more power, but you had to keep an eye on the engine temps, and back off when indicated.

With the P-38J, the intercooler could cool the entire power output capacity of any Allison installed.

Early P-38s basically had 5 issues:
1) No pilot training. They'd cruise into the battle area at low speed, low rpm, and low MP, with gun sights off. If they were attacked, they had to a) throttle back, b) come up on RPM, c) throttle forward, and d) turn on gun sights ... and by that time, they were shot down. It took some real combat flights to sort out tactics like coming into a battle area primed and ready to fight.

2) The early intakes were too smooth on the 3-cylinder units bolted to the heads. It took about 6 months to come up with turbulators to stop the outer 2 cylinders from running rich while in inner one ran lean.

3) British gasoline had a LOT more aromatics (20%) than U.S. gasoline (2%), and they were unable to duplicate the issue (since we ran U.S. gas) until someone shipped over a load of British gas. Once it was duplicated on the test stand, they realized all Allisons were jetted incorrectly for British gas. Unsurprisingly, the early Merlins we ran over here had the same issue in reverse. Once understood, the cure was a simple jetting change.

4) The worst one, as far as crew comfort goes, was the fact that early P-38s used a standard heater muff to heat the cockpit. The air was COLD when it got to the cockpit. A simple switch to an electric cockpit heater solved the issue.

5) The P-38 had the lowest critical Mach number of any major fighter in the ETO, but nobody understood why. Dive flaps helped a lot, but were NOT a solution. In fact, it was never solved completely. Solution would require redesign of the wing-pod and wing-boom intersection.

Early on, the Allison gave trouble over Germany. After the things above were fixed, it would NOT have done so, and proved very reliable in the MTO and PTO areas when it was largely relocated there. You may recall it was the mount of out two top aces. With the P-38L and boosted ailerons, it was a very good roller, even at high speeds.

 

[Greyman]

3) British gasoline had a LOT more aromatics (20%) than U.S. gasoline (2%), and they were unable to duplicate the issue (since we ran U.S. gas) until someone shipped over a load of British gas. Once it was duplicated on the test stand, they realized all Allisons were jetted incorrectly for British gas. Unsurprisingly, the early Merlins we ran over here had the same issue in reverse. Once understood, the cure was a simple jetting change.

Do you know if this would cause the speed or full throttle height being hindered? I find this cross-Atlantic problem interesting as it it somewhat born out by the graph/dates. IE:

The US figures get progressively worse as the Lightning goes from stateside testing to actual in-service examples. Then ...
The UK figures get progressively better as the issues are identified and fixed, finally bringing the Brit data sheet full circle and agreeing with Lockheed's initial figure.

 

[MIflyer]

The early leading edge intercoolers were inadequate, especially with the increased power of the later engines. There were no fins either externally or internally to help with the heat transfer, and they added a coat of paint that did not help, either.

The P-38H was power limited by the leading edge intercoolers.

For the chin type air-to-air intercoolers there was nothing to keep them from overcooling the air, and the real problems started. At the 9th Photo Recon Squadron they partially blocked off the intercooler exhaust exit, since their missions tended to be at high altitude and they did not need to adjust the intercooler temperature in flight. The 9th also removed the Allison manifold pressure regulator since that tended to fight against the turbo regulation, a bad feature for a photo recon aircraft.

Eventually the J and L models had shutters added that enabled the pilot to control the intercooler temperature.

The obvious solution would have been to use a liquid cooled intercooler/aftercooler like the Merlin 60 series, but no one seemed to think of that other than Sir Stanley Hooker.

 

[GregP]

They weren't inadequate or limited. They had a known heat dissipating capacity of 1,050 HP. More dissipation required a redesign, and that happened, if a bit late.

The USAAC approved the design and it was built as specified. Anything inadequate was there because it was approved for production. The cooling capacity is rather well documented. In service, according to former P-38 pilots, the "extra" HP was reserved for use at higher altitudes where going to full throttle would not exceed 1,080 Hp per engine. So, if was used operatiolnally as a high-altitude reserve rather than to get better performance down lower.

Practically speaking, people tend to "forget" limitations like that in combat, and the handwriting was on the wall as far as cooling capacity was concerned. But, the USAAC was not exactly a forward-thinking organization when the P-38 was approved for production. It took combat experience and reported problems to get their attention. Once addressed, the issues were corrected in a reasonable time, had a war not been in progress.

About British fuel, there was nothing wrong with British or U.S. fuels. They were simply comprised of different mixes of substances. Once understood, correct jetting was not difficult. I would NOT want to be flying a P-38 jetted for the U.S.A. on British fuel at the time though, or vice versa. As I said, the British Merlin aircraft we tested over here had the same issue in reverse. Once understood, this became a non-issue. By early 1944, nobody was having trouble on either side with jetting, and the issue quietly died.

 

[Shortround6]

The P-38 had two cooling issues.

One was the radiators in the rear of the booms and the other was the inter-coolers for the turbos.

At some point they put larger radiators (and ducts) to handle the higher power engines (1425-1600hp engines need bigger radiators than 1150hp engines).

This allowed higher HP at low levels where the turbos weren't compressing the air as much (heating it beyond the inter-coolers capacity)

See; http://www.wwiiaircraftperformance.org/p-38/P-38E_41-1983_PHQ-M-19-1313-A.pdf

Note that temeratures going into the carb could be over 44 degrees C and as high as 60 degrees C without side air being form -24 to -35 degrees C. and this was for 1150hp engines

The P-38 certainly did need the better inter-coolers to allow the higher power ratings at altitude.

The US and the British had come up with a common fuel specification well before the P-38s had trouble in late 1943. However there were at least 3 different specifications for 100/130 if not 4 or 5. The days of British 20% aromatics and American 2% aromatics were long gone by 1943.
These revised specifications were all about increasing the amount of 100/130 fuel from the same amount of feed stocks. As time went on more lead was allowed to be used (went from 3.0cc per US gallon to 4.0cc to 4.6cc) and different amounts (greater amounts) of other compounds (like certain aromatics, there are nearly a dozen that can be used in fuel) were also allowed to increase production.
There were dozens of fuel blends if not hundreds. Please note that a fuel "blend" is NOT a specification. A fuel "blend" or batch has to meet a specification to be accepted. It not only has to meet the 100/130 running test but have certain evaporation rates, fuel vapor pressure, certain allowable levels of varnish and a number of other criteria. AS long a "blend" meet these criteria it was accepted regardless of the actual chemical composition (within reason) Fuel was coming from dozens of refineries and from dozens of oil fields (feed stocks). In 1943 there was no "British" and "American" Fuels.

 

[GregP]

Hi Shortround,

It didn't have intercooler issues. It did have problems ONLY if they used more than 1,050 HP, which was the approved level. It required less than 1,050 HP per side to achieve spec.

In real world use, it DID need more cooling, but it was NOT a design issue, it was a design spec that was met and the issue surfaced when more was desired, above spec.

The XP-38 had to make 360 mph at altitude (it did, and more), and climb to 20,000 feet within 5 minutes (it did, easily), within the intercooler limitations. It only became something of note when more was needed. The PROBLEM was that more was never specified when it as developed and produced. As originally produced, it met the spec and then some.

Doesn't matter if you call it a blend or whatever. British fuel was different enough from ours to require re-jetting, simple and factual. It was accomplished. Is that so hard to understand? Once corrected, there was no issue with fuel since they tested it regularly after the issue cropped up the first time, and were never surprised again by fuel issues during the war.

 

[MIflyer]

They did not put in larger radiators for the later P-38's with the higher powered engines. They used the same radiators but moved them further out into the airstream and added a lip that separated the low speed boundary layer air from the faster moving air. They got better cooling with the same size radiators.

Take a look at the earliest Mustang MK 1's and then the later P-51's. They also added a lip there.

Y'all might enjoy reading this: 9th PHOTO RECON SQUADRON

 

[Old Wizard]

 

[Robert Porter]

Nice read thanks!

 

[BiffF15]

Great story!

 

[soulezoo]

Just another of those wonderful "lost" stories most have never heard about.

Thanks for sharing!

 

[GregP]

Sorry, you are right. An intercooler IS a radiator, but not a coolant radiator, It cools the pressurized air before it gets to the carburetor.

The P-38L had an intercooler that was basically a radiator and not a leading edge compartment. The air entering each boom was divided into 3 areas or separate flows. The outer two were oil coolers and the middle one was intercooler air.