Reluctant Poster
Staff Sergeant
- 1,409
- Dec 6, 2006
As I recall there was an excellent article on R-3350 isses in the AEHS publication The Torque Meter. Also there is this AEHS article posted online:
Fighter Escorts of B-29's over Japan & Pacific
- Thread starter Zipper730
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Geoffrey Sinclair
Staff Sergeant
- 798
- Sep 30, 2021
You find WWII history in some interesting places. Journal of the American Statistical Association, V 41 no. 234, June 1946, pages 190 to 203. "Actuarial Analysis of the operating life of B-29 aircraft engines", by O L Altman and C G Goor.
Lots of obvious and not so obvious points.
1) R-3350's were in critical supply in 1944 and the first half of 1945. Supply problems were easing around the end of the war.
2) Air transportation was used to fly R-3350's to the US for overhaul, at best 2 R-3350's would fit in a cargo plane.
3) Overhauled engines had around 10% lower "life" before the next overhaul.
4) The -23 was the carburettor and the -57 the fuel injected versions. The -23 was modified to improve reliability.
5) The statisticians noted the standard USAAF methodology for forecasting engine life was only suitable for a reasonably static population. Not where there was a steady arrival of more strength using new aircraft.
In terms of engine life, operations in India were the worst, since each combat sortie required three reasonably rapid climbs, India to China, China to Japan, China to India. Next came training in the US, finally the best were the units in the Marianas.
Expected life prior to first overhaul, early operations from India, 163 hours, -23 engines. Using modified -23 engines this had risen to 280 hours by February/March 1945 for aircraft operating from India and 304 hours from the Marianas.
The figures for B-29s used in training were 221 hours and 310 hours versus the 163 and 280 hours figures above.
Operating from India a comparison between the modified and unmodified -23 engines showed 80% of the unmodified and 95.3% of the modified engines survived to over 100 hours, 33.9 of the unmodified and 81.5% of the modified engines survived to over 200 hours, 0.2% of the unmodified and 47.3% of the modified engines survived to over 300 hours.
In the Marianas, as of 20 November 1944 the average hours on each -23 engine removed was 91, by 20 January 1945 it was 151, as of 30 April it was 234. These figures include removals for engine model changes, modifications, accidents and battle damage. They are also under estimates of the normal engine lifetime because so many of the engines were new. The figures include new and overhauled engines, so it is either the number of hours since the engine was built for new engines or since overhaul for the overhauled engines.
Engine hours before removal as of 31 May was 259 hours, and 31 July 272 hours. These figures are for engines removed because of mechanical problems only.
Even in July the steady number of new B-29s arriving drove down the average engine hours per removed engine.
A study as of 31 July 1945 noted in the Marianas the -23 engines 96.8% of new and 92.5% of overhauled logged more than 100 hours before replacement, 87.5% and 75.7% respectively logged over 200 hours, 62.7 and 43.4 logged over 300 hours, 19% and 8% logged over 400 hours, none logged over 500 hours.
As noted above the training schools in the US went through R-3350 engines quicker than the combat units in the Marianas, for example 57.9% and 36.4% logged over 300 hours, but once this mark was passed the engines in the US held up more, so 24.6% and 10.4% logged over 400 hours, and 1.2% and 0.2% managed over 500 hours.
The fuel injected -57 engine had a higher time between overhauls, so in the above study 31.2% used in training logged over 400 hours, and 4.9% logged over 500 hours.
Lots of obvious and not so obvious points.
1) R-3350's were in critical supply in 1944 and the first half of 1945. Supply problems were easing around the end of the war.
2) Air transportation was used to fly R-3350's to the US for overhaul, at best 2 R-3350's would fit in a cargo plane.
3) Overhauled engines had around 10% lower "life" before the next overhaul.
4) The -23 was the carburettor and the -57 the fuel injected versions. The -23 was modified to improve reliability.
5) The statisticians noted the standard USAAF methodology for forecasting engine life was only suitable for a reasonably static population. Not where there was a steady arrival of more strength using new aircraft.
In terms of engine life, operations in India were the worst, since each combat sortie required three reasonably rapid climbs, India to China, China to Japan, China to India. Next came training in the US, finally the best were the units in the Marianas.
Expected life prior to first overhaul, early operations from India, 163 hours, -23 engines. Using modified -23 engines this had risen to 280 hours by February/March 1945 for aircraft operating from India and 304 hours from the Marianas.
The figures for B-29s used in training were 221 hours and 310 hours versus the 163 and 280 hours figures above.
Operating from India a comparison between the modified and unmodified -23 engines showed 80% of the unmodified and 95.3% of the modified engines survived to over 100 hours, 33.9 of the unmodified and 81.5% of the modified engines survived to over 200 hours, 0.2% of the unmodified and 47.3% of the modified engines survived to over 300 hours.
In the Marianas, as of 20 November 1944 the average hours on each -23 engine removed was 91, by 20 January 1945 it was 151, as of 30 April it was 234. These figures include removals for engine model changes, modifications, accidents and battle damage. They are also under estimates of the normal engine lifetime because so many of the engines were new. The figures include new and overhauled engines, so it is either the number of hours since the engine was built for new engines or since overhaul for the overhauled engines.
Engine hours before removal as of 31 May was 259 hours, and 31 July 272 hours. These figures are for engines removed because of mechanical problems only.
Even in July the steady number of new B-29s arriving drove down the average engine hours per removed engine.
A study as of 31 July 1945 noted in the Marianas the -23 engines 96.8% of new and 92.5% of overhauled logged more than 100 hours before replacement, 87.5% and 75.7% respectively logged over 200 hours, 62.7 and 43.4 logged over 300 hours, 19% and 8% logged over 400 hours, none logged over 500 hours.
As noted above the training schools in the US went through R-3350 engines quicker than the combat units in the Marianas, for example 57.9% and 36.4% logged over 300 hours, but once this mark was passed the engines in the US held up more, so 24.6% and 10.4% logged over 400 hours, and 1.2% and 0.2% managed over 500 hours.
The fuel injected -57 engine had a higher time between overhauls, so in the above study 31.2% used in training logged over 400 hours, and 4.9% logged over 500 hours.
FLYBOYJ
"THE GREAT GAZOO"
What I find funny with all these numbers, there is no indication on what type of overhaul we are talking about.
GregP
Major
How can you have a reasonably rapid climb from China-to-India and then from India-to-China?
Was it uphill both ways? Or did they climb over China and descend into Japan and then had to do the reverse on the way back? Either way there would only be two climbs ... from lower to over highest point, descend, and then back over the highest point - usually called "the Hump."
Was it uphill both ways? Or did they climb over China and descend into Japan and then had to do the reverse on the way back? Either way there would only be two climbs ... from lower to over highest point, descend, and then back over the highest point - usually called "the Hump."
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
It's possible, Greg - didn't our parents walk uphill both ways to school? 
special ed
1st Sergeant
- 5,005
- May 13, 2018
This is only B-29 engines. Was there hours time differences with the C-46's engines?
Shortround6
Major General
C-46????
I believe the C-46 used R-2800s?
There were a few C-69s (22?) but the low number was in part due to a number of problems with the R-3350 engines despite the much lower weight of the plane and the absence of turbo chargers.
Geoffrey Sinclair
Staff Sergeant
- 798
- Sep 30, 2021
No definition of overhaul beyond being sent to the US. They were statistics people, not mechanics.
In terms of engine life, operations in India were the worst, since each combat sortie required three reasonably rapid climbs, India to China, as the raids staged through China, Twentieth Air Force - Wikipedia Himalayas - Wikipedia so first leg, arrive in ChinaHow can you have a reasonably rapid climb from China-to-India and then from India-to-China?
It's possible, Greg - didn't our parents walk uphill both ways to school?
Second leg China to Japan (and hopefully return). Given the US had detected the Japanese were unhappy with them it was felt best to be at high altitude before crossing into Japanese controlled territory. World War II - Wikipedia after returning to the Chinese airfields came the final flight, China to India, as the raids staged through China, Twentieth Air Force - Wikipedia Himalayas - Wikipedia
FLYBOYJ
"THE GREAT GAZOO"
And that's a major problem when looking at some of these statistics! Sometimes that don't paint a real picture of true reliability. I know for a fact that during operations, if you had an aircraft with an engine not performing 100% due to a few bad cylinders, rather than replacing those cylinders (a "top" overhaul) on the aircraft, it may be decided to just pull the entire QEC (if one if readily available) and send that engine to an intermediate or depot level facility. If you want to talk complete overhaul, you're talking about overhauling or replacing all cylinders and splitting the crankcase and replacing or overhauling internal components (cam, crankshaft, bushings, etc.)
And I'll add for clarity - I'm just talking about radial engines
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Reluctant Poster
Staff Sergeant
- 1,409
- Dec 6, 2006
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FLYBOYJ
"THE GREAT GAZOO"
Good information but again does not indicate the level of overhaul from I can see
Geoffrey Sinclair
Staff Sergeant
- 798
- Sep 30, 2021
Enter deep dark space, where the largely forgotten copies of one off oddities relating to WWII history have made their way, fight off the spider cavalry and ant infantry, engage digger to remove the dust, locate copy of report. The article copy was originally taken to handle a German aviation fan who insisted the R-3350 made the Jumo 004 look reliable.
The authors were part of the Analysis and Plans Branch, A4, HQ AAF, the aim was to forecast the average lifetime of an engine, and so be able to set production targets. The USAAF figures were used to calculate the "Engines exposed to Contingency of Failure". This calculation follows the general lines as the calculations of human mortality tables. See H.S. Beers "Notes on Exposure Formulae" Transactions of the Actuarial Society of America, Vol. XLV, Page 41, and subsequent discussion.
The problem they were trying to overcome was the basic data being used were the ratios of the number of hours flown by *all* engines, including those not yet removed for overhaul. Until actuarial methods were adopted, engine life prior to removal was measured solely by the average number of hours flown by engines removed before overhaul. The latter method of necessity incorrectly showed low average engine life for many months after the introduction of a new type of engine for two reasons, 1) when *all* engines have flown a small number of hours the average hours flown by removed engines must be small, 2) the age distribution will be affected by the introduction of new aircraft into the group.
The statistics the report are from the USAAF. Using the figures to 31 July 1945. The measuring point was when an engine was removed from an aircraft using the hours flown at that point while noting why the engine was pulled, whether the removal was due to believed minor or major problem. The air force was after how long the average engine would remain on the aircraft it was fitted to.
As of Spring 1945 in the US monthly statistics being collected, 1. Number of installed engines on hand at start, arrivals, losses, departed and on hand at end. 2) Number of engines removed for overhaul for accident, modification or other arbitrary reason or for normal engine failure. Each major using organisation in the USAAF was required to report this data in 20 hour flying time groups, with time measured as the number of hours flown since manufacture or last overhaul. Data reported separately for new and overhauled engines and for each model of the R-3350.
The data was 1) number of engines removed for normal engine failure, 2) number of engines removed for accident, combat damage, modification or other arbitrary reason, 3) number of installed engines at start, 4) number of installed engines at end, 5) number of installed engines on aircraft that arrived during the period, 6) number of installed engines on aircraft lost during the period, 7) number of installed engines on aircraft that departed during the period.
FLYBOYJ
"THE GREAT GAZOO"
OK - and I read that - but again what IS NOT being shown, and I'll repeat one more time "is the type of overhaul."
Why is this important?!?!? Because it REALLY shows the wear and tear on the engine, RELIABILITY and what components are subject to that wear and tear.
These reports are wonderful reading! They expose a historical snapshot into combat operations, but what is lost in time is the fact that "OVERHAUL" covers a wide spectrum.
Again - you can have a "Top Overhaul" where you just remove problematic cylinder heads, replace the entire head or rework valves, seats, pushrods rocker arms and bushings. You can go further and remove pistons, pins and rings. You can replace every cylinder in the matter I describe (referred as a "Complete Top Overhaul"), or you can split the crankcase and replace the "lower end" of the engine (connecting rods, the master rod and related bushing and gears.)
Large round engines are subject to frequent cylinder head changes, just a fact of life...
I hope I have clearly made how wide open an "OVERHAUL" can be!!!
Now if I was a maintenance officer and had to try to maintain a high mission capable rate and "if" I had several "Quick Engine Change" (QEC) pods or spare engines available, rather than going through the time to pull "Jugs" (cylinder heads) and rework internal components, (which would probably not be counted as "an overhaul") it might be more advantageous, just to swap the whole engine. You may have an engine with the bottom end good but have 2 or 3 bad cylinders, but the engine will be removed for "OVERHAUL" and it seems the extent of overhaul of these engines removed are not indicated!
So as stated, these numbers are wonderful but at the end of the day they don't really paint a true degree of reliability on those engines sent to "overhaul" for non-combat issues.
From the internet
GregP
Major
It's possible, Greg - didn't our parents walk uphill both ways to school?
Now that you mention it, they DID! How did you KNOW that?
Reluctant Poster
Staff Sergeant
- 1,409
- Dec 6, 2006
Interesting excerpt from a letter from Arnold to Lemay in December 1944.
Those of you who are interested in the history of electronics and computers will be fascinated to note that the letter references a report by Mr. Schockley
Mr. Shockley is William Schockley, the co-inventor of the transistor and the founder of Silicon Valley
Those of you who are interested in the history of electronics and computers will be fascinated to note that the letter references a report by Mr. Schockley
Mr. Shockley is William Schockley, the co-inventor of the transistor and the founder of Silicon Valley
FLYBOYJ
"THE GREAT GAZOO"
OK - any further specific information?
Reluctant Poster
Staff Sergeant
- 1,409
- Dec 6, 2006
They were flying over the Himalayas, so yes it was uphill in both directions. The flight from China to Japan was a typical mission climb to target and descent back. 3 climbs to very high altitude for each mission. India to China (carrying full bomb load). China to edge of Japan. China back to India. The B-29 were based in India, China was only a staging base. Here is the mission plan for the first raid on Japan:
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Shortround6
Major General
Well, for Americans it is up hill from Denver to San Francisco both ways 
You can't coast from Denver to SF even if you did start 5,000ft higher.
You can't coast from Denver to SF even if you did start 5,000ft higher.
Thumpalumpacus
Major
A hump has two inclines, one on either side.
Reluctant Poster
Staff Sergeant
- 1,409
- Dec 6, 2006
Here is how the actual mission unfolded
Note that the very first B-29 raid on Japan was a night mission. One of the enduring myths of WWII is that Curtis Lemay came up with the idea of night bombing with B-29s. Night bombing was planned as a part of the repertoire before the B-29s first combat mission.
