First off, for truth in advertising, I have very little time in the T-6. However, I have spent a considerable amount of time lately reading just about everything I can get my hands on about operating the Six -- I'm hoping to spend a lot more time in it in the future.
You hit upon one of the challenging points for someone trying to learn to smartly operate this airframe: there is simply no one correct "book answer".
Here's what Skipper Hyle says about the differences in numbers between different operators of the T-6/SNJ/Harvard:
Quote:
One of the most interesting things about warbirds is their variety. The T-6 utilizes a Pratt & Whitney R-1340-AN-1 engine – all of them do. Yet, between the USAAC, USAAF, USN, RAF, and RCAF there are at least 7 sets of operational numbers that I know of. These variances are not just limited to oil temperatures and Manifold Pressure and RPM combinations – they also include airspeeds, flap speeds, and many other numbers. These airplanes all came off the same productions lines, for the most part. It’s amazing what military forces will go through to build pilots.
(source:
http://www.warbirdalley.com/articles/T6-sf.htm)
So, bottom line: there is no "right" answer so far as military tech data goes.
The problem is even more complicated than that, though. According to Randy Sohn, many military procedures were designed for safety and ease of use rather than on long life for their engines. Remember, the military of the 1940s and 1950s had a huge maintenance infrastructure to work on engines. Plus, overhauls weren't a $25,000 out-of-pocket expense for the pilots!!
Here's what Sohn says with respect to RPM vs Manifold pressure:
Quote:
In defense of the military we should remember that they were dealing with large numbers of three or four or five hundred hour pilots when these high RPM on downwind leg procedures were first promulgated. Given this level of experience they probably placed a higher priority on being ready for a missed approach. Also, and probably more importantly, the military didn't exactly overly concern themselves with cost or how many mechanics something took.
(source:
http://www.douglasdc3.com/sohn/3.htm)
So, what you read in a military manual MAY NOT BE what is most healthy for operating the engine. It may not be the best way to operate the engine to ensure long, reliable operating life. This is the crux of the reduced-power-takeoff discussion, too.
The idea behind a reduced power takeoff is that it supposedly limits the amount of strain on the engine during its most vulnerable point. Unfortunately, there may be unintended consequences to that reduced manifold pressure. Sohn addresses reduced power takeoffs here:
http://www.enginehistory.org/wbn/WBN11.pdf
It's even more complicated than THAT, though!! The warbird operating community doesn't even agree with itself on many of the things that Sohn says. If you ask 10 different pilots, owners, and maintainers of radial-powered airplanes, you'll probably get 3-4 different answers on what they do on their particular aircraft. The problem for a neophyte like me is that ALL of those 10 people are all very experienced and knowledgeable! Who do you believe??
Operating and caring for radials is becoming more of an art these days than a science. The goal of this art is to maximize safety in operation of the engine and maximize time between expensive overhauls.
So, good luck in trying to find a black-and-white answer to your question. The best you can do is take the military manuals as a starting point, and then get as many opinions as you can. Ultimately, you'll have to operate the airplane the way the owner wants it operated -- or, if you are the owner, you'll have to assess which of the experts you happen to agree with.
I'm very interested to see what the other WIX experts have to say on this topic. It's a great one to discuss.
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