IFMU,
One question that I would pose: If Flettner was so far ahead of his time, why hasn't Germany [or anyone successfully] picked up the synchrocopter torch and marched on with it?
A very valid question. It has also perplexed me for many years. The following web page is an unbiased (I hope) search for that answer; or combination of answers.
Intermeshing Configuration - Concerns
At this point in time, I have not committed to any configuration. The only commitment is that of finding Nick's elusive 'Free Lunch'. However, at this point in time, the Intermeshing and the Interleaving appear to be the most promising for the future, IMO.
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One might ask the same question of Sikorsky. They have known for 30-years what the pros and cons of their Coaxial ABC were. In addition, 30-years ago they stated the desired modifications to be made on a future craft.
Why, 30-years later, do they now think that it is a good idea?
Dave