I believe there are three major factors that influence the future of any engine: availability, cost and dependability.
The availability of the Pratt & Whitney R-1340, as a long-term power source, has been a concern of operators for as long as I can remember. The concern stemmed from the fact that there were only 34,966 of these engines produced. Production ceased around 1960 in favor of the more technologically advanced jet engine. People worried about running out of replacement parts such as cranks, cams, etc. I remember when Pratt & Whitney R-1340 crankshafts cost less than $1,000. The problem is that no one will produce a crankshaft that they have to charge $8,500 for when a surplus unit is available for $1,000 - and can you blame them? However, now that U/S crankshafts cost around $6,500 to 7,000 it's no longer unfeasible to produce and sell new crankshafts.
An operator should do a weight and balance check on the peace of mind vs. the cost of using new or used serviceable parts. These parts include new crankshafts ($8,500 each), new cams ($3,150), new pistons (exchange at $750 per set), new shaft bearings (roughly $650 each) and a limited quantity of new surplus cylinder stud assemblies (around $2,000 each).
This leads us into discussing the cost of a total new engine. Our basic overhaul cost for an R-1340 is $13,750 plus any rejected parts you may have, or new parts you want that aren't normally included in the overhaul. As with almost everything else, this cost has increased steadily over the years. For example, in 1972 I purchased a new Oldsmobile Cutlass Supreme for $4,153 at 4 percent interest - have you priced one lately?! A well-built 1340 (at $26,000) still stack up fairly well against an equivalent horsepower used jet engine conversion (anywhere from $110,000 to $230,000 installed). Most of these used serviceable jet engines will give you 2,000 to 4,000 hours of service before major maintenance must be performed.
New radial engine parts will not extend the official recommended time between overhaul (T.B.O) of the engine, however they will increase the probability of reaching the recommended T.B.O. with out incident. This brings up the topic of dependability. There are many factors that influence an engine's dependability. Maintenance definitely affects dependability. If, during the course of performing maintenance, you find a cracked cylinder and change it (allowing the engine to reach T.B.O. "uneventfully"), you had a "dependable" engine. If however, you miss the cracked head and the engine fails (resulting in a forced landing and damaged plane), you have an "undependable" engine. Both engines had the same defect, but maintenance made the difference in the engine's dependability.
If you find a streaking nozzle in PT6 before it burns a hole in your vane ring (resulting in a forced replacement of $30,000 of turbine blades and related parts), your PT6 will seem a lot more dependable than it would if you had to replace them.
When choosing an overhauler, please don't let price be the only consideration. Take the time to go through the shop you're considering to get a feel for their commitment to quality control. Check the cleanliness of the shop - chances are if they're faithful to the little things like keeping their work area clean, they're faithful to the truly important things also.
Regarding time between overhauls, I recommend that operators examine there seasonal flying requirements, decide on a target number of hours between overhauls, and stick to it as closely as possible. Examples are:
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400 hrs/year - 2 to 3 seasons (800 - 1200 hrs.)
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500 hrs/year - 2 seasons (1000)
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700 hrs/year - 1 full season, or seasons if primarily using ULV/LV applications
What about different types of crop work? Pratt & Whitney recommend ag time is 1,000 hours. But keep in mind that 900 hours of rice work is much harder on the average engine than 1,300 hours of ULV wheat work.
Concerning fuel and oil usage, we do not recommend the use of automotive gasoline in radial engines. This is largely because approved auto fuel (meets ASTM D4814-90A specifications) simply cannot be dependably purchased on a continuing basis. We feel if you have to use auto has, mix it with low-lead 100 av. gas in at least a 50-50 ratio.
Virtually any approved aviation oil will get the job done. Our customers have reported good results using many different grades of oil, with one exception: maintaining sufficient pressure in hot weather is sometimes difficult when using 15W50 wt. oil. Basically, how well the oil is filtered and how often it's changed seem more important than what grade or brand of oil you use.
In conclusion, it's clear to me that the future of the radial engine will hinge more on the engine's economic feasibility to the ag operator than on any other factor. The American economy of supply and demand will ensure parts availability for the R-1340 as long as it is a profitable engine to operate.
