Very interesting post from Mike, and a few thoughts occur, as this is a general discussion... These are by way of looking at general factors, expectation and analysis, not specific or engineering comment (which I'm not qualified or knowledgeable about!) and a riff on, not a criticism of Mike's comments.
T-28mike wrote:
I am still not a firm believer in all of the "upgrades" and new manufactured parts. In my experience there seems to be as many (or more) problems with the "new and improved" models.
That's an interesting thought, but if it's subjective, of no value (you've
got to quantify it, for the comparison to have any value, rather than being a variation of "the new stuff's not as good as old stuff..." twitch. But can you? I don't believe the data field is big enough or reliable enough.)
Also, more critically, there are a couple of changing vectors here - i.e. the expectation and performances then are not the same as now - they've developed in a direction over the time elapsed.
Firstly W.W.II era kit was not designed for longevity or low intensity long-term ops, but since then, stuff is being expected to perform for a long time - many, many more cycles, but at lower power, weights and altitudes. Hence failures the original designers would have been
wrong to have designed for, like
aged magnesium casting failures.
The second vector is that the expectation of reliable performance from W.W.II to the present has radically changed (car servicing and regular maintenance is the good comparison for that, from regular owner participation and care to no-touch today). Looking at that, it's clear that W.W.II (or 1950s) expectations of reliability are well below modern benchmarks.
That said it depends on data. Talking to someone responsible for a Merlin recently, he said that some information is shared readily, some not. I expect that data of
old originated and
mod-originated failure rates are too patchy to really draw meaningful conclusions from in general.
T-28mike wrote:
It is not limited to just the Warbird stuff either. Lycoming has been building the same basic flat motor since the earth cooled, and the new ones seem to have more AD's against them than the old ones do. Come to think of it, that may apply to Cessna 172's as well.
An obvious though that pops up here is that even without the greater, modern risk aversion culture (which is another way of saying failures are less expected or allowed now) the expectation we all carry is not actually an accurate memory of the performance that old engines gave.
In short, what used to be "Oh, they do that." is now "There's an AD for that."
T-28mike wrote:
Engineers..... You can tell them from 20 feet, but up close, you can't tell them anything.
Generally asking questions and listening to what's left out as what you're told is well worthwhile, I've found, albeit on a non-decisive active engineering basis.
But the shocking report on the Thunder City EE Lightning accident and a recent example of a warbird being engine run on stands for retraction tests shows that self-confidence still often exceeds training, laws, sense and good practice.
T-28mike wrote:
Get back to me in 70 years and if these upgrades are still in reliable use, I might be convinced that they are ok.
Good historical principle; that's certainly one way of testing what music's of merit! However it's of little use in a practical way, now, as we know. Whether you are talking engineering, aircraft colour schemes or publishing an article, sooner or later you have to commit to a decision, and handle the results, with a degree of honesty and humbly, being prepared to revise your practice from criticism or (hopefully minor) failures. Of course the engineering choices are potentially dangerous, while the paint and articles are unlikely to let someone down fatally.
And that's another crunch. With something like the Merlin engine, running a design like that today is a very different set of parameters than its design. It's a 1930s engine, with 1930s expectations built in, and consequences refined out over the following years. You can change nothing, or undertake RR suggested changes
or even add modifications beyond that; in all cases there are risks to do with the complex interactions of those elements with other elements. You are, however giving it all
lots of the fourth dimension - time - to bite you in, and given long enough, everything will go wrong. Generally, that should be a long time after the engine is checked over or rebuilt.
Just a few thoughts,
Regards,
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