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View Full Version : Subaru/ Merlin- Lycoming/ R3350


rv6ejguy
10-31-2006, 01:21 AM
This is a follow up to the discussion a couple weeks ago on the liquid cooled vs. air cooled - small vs. big after a bit of research.

I got in touch with several people who flew or worked on the R3350 in both military civil service and was lucky enough to talk to a pilot/ engineer who still flies the Martin Mars water bombers up here.

OK TBO of 3500 hours in civil service. Well depends what you mean by TBO. The R3350 started out in 1944 with about a 30 hour lifespan in the severe duty on the B29 hauling heavy bomb loads to Japan. This was very hard on them at high power settings and in high blower for hours. Steady improvements were made post war to these engines. It appears as though Wright set the TBO at 2000 hours initially in the early '50s and increased this to 3500 hours in the early '60s when they started a SOAP program. My sources said few if any R3350s remained on wing for this period as the oil analysis showed problems well before this time and many jugs were replaced along the way. The jugs were often damaged by the sodium cooled exhaust valves disintegrating. All the people I contacted said that when operated in high blower, engine life plummeted by about 50%.

The US Forest Service sets the TBO on their R3350s in the Neptune at 1600 hours today and say they routinely make it there with no jugs replaced. Locked in low blower and limited to 51 inches dry.

The Martin Mars water bombers have their TBOs set at 800 hours. They are not turbo compound models. Steve Wall said only 2 engines ever made it that far with no jugs replaced. They have a problem with master rod thrust bushings disintegrating. The overhaul costs on these engines is over $200,000! Again this is a hard life with heavy loads but they limit manifold pressure and high blower is locked out.

In military service on the ASW mission and C-121 intelligence gathering at low altitude, the lifespan was pretty good due to low power settings and most of the time in low blower.

In civil use on the Super Connie, experiences seem at both ends of the spectrum and maybe time has made some forget the facts a bit. One quote stuck out: "... I don't remember ever having a flight of more than six hours that I landed with all four running. With the TC engines, the Connie became known as the Worlds Fastest Tri-motor".

The following link should provide a few chuckles. This is from a line mechanic with Quantas on the Connie: http://www.lockheed.adastron.com/constellation/da3.htm

Airline use was a hard life in high blower most of the time. The man hours per flight hour going into a four engined airplane were staggering and why the jet was such a leap forward despite the very high fuel consumption.

My piece of personal trivia on these engines involves living on an RCAF base in the '60s and having a four engined Argus take off over the house with full wet power-3700hp X 4. What a noise. The ground shook! :cool:

Now the 1650 cubic inch Merlin in airline service in the same era. Used by TCA, CPA and BOAC. Again life started out grim. TCA had 20 in flight shutdowns in one month! With takeoff power set at 1660hp at a staggering 71 inches. Cruise power was initially set at 40 inches and 1100hp. Engine life on wing was 200-450 hours in most cases. With a lowering of cruise power to 950hp, life picked up considerably. Over a one year period and 957 ocean crossings, 2 engines made it past 2000 hours, 7 made it to 1750 and the average life on wing was 654 hours (no jug changes on the Merlin). TBO was set at 1250 hours although this was just as meaningless as with the Wrights.

Initial problems were with the intercooler pump seals, compressor surging, coolant leaks and erroneous fire warning lights. Rolls Royce offered TCA a "won't be sorry" warranty on their Merlins- 6000 hours or 3 years. They would pick up the tab on any unreasonable wear or failures. RR probably lost money on this one! The Merlins on the North Stars were in a power egg which had rads and almost all parts attached in one piece for quick removal and replacement. Good idea. They needed it. It appears that very few Merlins had catastrophic failures- indeed, one was held at full takeoff power for 5 hours in a flight test until the oil supply ran low- pretty tough. Coolant leaks caused most of the shutdowns.

Research showed that the Pratt R-2800 was the engine to have in this era. Better reliability and fuel consumption than either the Merlin or the Wright. The Pratt 4360 was horrible by all accounts I found. In any case, the jet quickly replaced them all.

Well a lot has changed in 50 years. Subarus appear to hold their coolant more reliably than Merlins and don't require much maintenance. If they need work, they need to come out of the airframe like the Merlin. The Sube is about half the size of the Lycoming just like the Merlin was half the size of the radials.

The O-320/360 Lycoming could be compared to the R-2800 perhaps and generally reaches its TBO with few problems. Major work like jug replacements can be done while still mounted in the airframe. Where the radials used barrels of oil (literally) and the Merlin was topped up with 2 quart tins, the Lyco uses the occasional quart and the Sube nothing between changes.

Some similarities here. Hope you found this interesting.