Originally Posted by
low n' slow
What's right and wrong in this?
What you need to differentiate between, IMHO, is cases where there is a true bypass ratio, and cases where you are saying "as if it had" a bypass ratio.
You can make a assessment of the effect of bypass ratio change on the design and off-design characteristics of a turbojet/turbofan type engine - and incidentally, bypass ratios don't start at 2:1, there are cases where the bypass ration is between 1 and 2 - sometimes called, at least partly in jest, a "leaky turbojet".
You can also determine a "pseudo bypass ratio" for things like UDF and turboprops, and you'll find that the same design relationships hold true to a greater or lesser extent. So it's fair to say, when talking at a fairly high level, that a turboprop performs
in some respects as if had a bypass ratio of xx.
As long as you make the distinction between a "pure" bypass ratio and a "pseudo" bypass ratio I think you should be fine.
An analogy is often a useful teaching device - just be careful that it is clear that it is an analogy, and is of restricted application.
For example, I might say that a certain size of steel beam is as strong as if it were an oak beam of a certain size, to allow a student to comprehend the relative difference in strengths. As long as its clear that's all you are doing, no-one should ask silly questions about burning steel beams, or building rafts out of them, or ...