improvement of efficiency. Therein lies the rub.

Nice pun. I am proud of you :-)

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As a point of interest the 4-place shrouded pusher developed by Ryan
after WWII had the tips in contact with a compliant gasket embedded in
the shroud.

The plane was an all-composite design, by the way. Interesting in the
engineering sense but over-weight and too expensive. They did the
test flights
at Holtville and there's still a few films of it.

-R.S.Hoover