enginehistory.org

[sithomas]

Can anyone shed some light on why the R-2600 took only a small amount of development to get right and was relatively reliable, yet the R-3350 was a dog for so many years?

From what I have been able to determine, Wright commenced development of the R-2600 in November 1935. It passed its type test on June 10, 1937 with a take-off rating of 1500 hp. It was being used in the Boeing Clippers in 1938 and seems to have been reliable. (Bearing in mind that Wright and Pratt & Whitney seem to have different views on what reliable means.)

The R-3350 commenced design work in January 1936 and first ran in May 1937. It passed its type test in 1939 and first flew in 1941. It doesn't seem to have been reliable at all during the war years.

[kmccutcheon]

The R-2600’s designer, Rudolph Daub, did the preliminary R-3350 design. However, he was reassigned to the ill-fated R-2160 Tornado project and the designers who took over the R-3350 project removed many of the features Daub had included.
The real problem was with Curtiss-Wright’s owners and top managers, who were money-grubbing investment bankers that understand neither their products nor customers. They were loath to spend money on research and development, which led Wright Aeronautical Division president Frederick B. Rentschler to resign in 1924 and form Pratt & Whitney Aircraft in 1925. The C-W owners and top managers also understaffed and underpaid their engineering department, middle-managers and factory workers, which led to morale problems and substandard work. They also prioritized work on engines for which they had customers, and very few were interested in the R-3350 until the B-29 was ordered.
The R-3350 came off the drawing board with four major problems:
1) It had a destructive secondary vibration;
2) Its propeller reduction gear failed continually;
3) It backfired and caught fire;
4) It overheated.
All these except the overheating were evident during test stand running from the very start, but C-W ignored them or explained them away.
The first two problems seriously impacted the B-29 schedule, for although the type test was passed in 1939, these problems were not adequately fixed until the spring of 1944! In both cases, C-W denied it had a problem, tried to blame others for the difficulties, and continually applied band-aids rather than real fixes.
The backfiring stemmed from a poor match of supercharger impeller, supercharger diffuser and distribution manifold. This created unpredictable standing waves in the induction system, which caused some cylinders to receive much more air/fuel mix than others. The lean cylinders would backfire, which would sometimes ignite the magnesium supercharger and accessory housings. In-flight magnesium fires could rarely be extinguished. This condition was improved when 9 branched induction tubes and a matching supercharger induction manifold replaced the original 18-tube design
Overheating appeared early in the flight test program and was the result of the poor mixture distribution already discussed, poorly-designed loose-fitting inter-cylinder baffles, and insufficient cooling air reaching the rear-cylinder-row exhaust ports. Forward-facing exhaust ports on the front cylinder row were also a factor. C-W partially addressed the overheating by supplying retrofit kits with re-designed baffles, but B-29 engines were always prone to overheating.
After WWII ended, C-W convinced the U.S. Navy to pay for re-engineering the R-3350 to produce the turbo-compound version, which was practically a completely new design. This engine, though still temperamental, achieved a specific fuel consumption of 0.38 lb/hp/hr, the best specific fuel consumption of any gasoline-burning aircraft engine.
_________________
Kimble D. McCutcheon

[sithomas]

Thanks for the highly informative answer. The post war redesign really explains how it went from being so bad to being useful.

Surprising that even after Rentschler et al left, the CW board still did not clean up their act. I read the article regarding the dubious inspectors/corruption at the R-3350 factory but I thought it was just that factory, seems that the entire CW upper echelon were playing the same game. Rather disappointing considering the importance of aero engines to the war effort.

I read that Sam Heron worked at Wright from 1926 to 1928, and it seems that someone in CW understood the importance of air cooling and was able to convince Heron to move over to CW. Although it seems that once they had him, CW annoyed him enough for him to depart again fairly quickly back to the Army Air Corps.

[kmccutcheon]

[sithomas]

You are correct - I was referring to the Lockland article. ( http://www.enginehistory.org/Piston/Wright/R-2600/R-2600Lockland.shtml )
I just re-checked and I was incorrect about it being a R-3350 factory, at that time it was building the R-2600.

[rinkol]

It seems that there is a big step involved in going from 14 to 18 cylinders due to problems involving:
- mechanical resonances
- mixture distribution
- cooling.

Perhaps the initial success of the R-2600 contributed to a sense of complacency.

The Soviets also seem to have been fairly successful in extrapolating Wright technology acquired with their R-1820 license to a 14 cylinder configuration (ASh-82), but found that developing a satisfactory 18 cylinder configuration was a time consuming exercise. However, this may have been at least partly a result of having more short term pressures to focus on the development and production of the 14 cylinder engines - long range strategic bombers were not a major priority for them before the mid-1940s.

That said, I completely agree that there were definitely problems concerning the organizational culture. I'm sure that these were a major factor in the later departure of Wright from the the aircraft engine business, even though the company seemed to be able to get development and procurement contracts for some time. The same situation seemed to apply to Westinghouse.

I spent my professional career working for a government R&D organization in an unrelated technical area, but often saw situations in my own organization and its industry partners where it seemed that the people who lacked technical understanding were the ones who made the decisions that mattered. It was sometimes possible to get way with this for a while, on occasion miracles did happen, but most of the time problems ended up being papered over until reality couldn't be ignored.

[jschauer]

Having flown behind both versions of the 3350 my opinion is that the later version of the 3350, ie 3350-26 or -95 is the greatest radial ever built. I have not flown behind the larger P&W 2800 or 4360 yet, so my opinion is somewhat biased.