The May '04 of the Smithsonian's Air & Space magazine carries an interesting
article on the development of the QT-2 "spy plane" in the late 60s during the
Vietnam War.

The prototype test airplane was a Schweitzer 2-32 on which a Continental 0-200
engine was mounted on top of the fuselage behind the cockpit. The engine drove
a long shaft above the canopy to an 8-ft, 4-blade propeller. "QT" was the
designation for "Quiet Thrust". It could be barely heard at 250 ft, not at all
at 800 ft. Operational tests were apparently succesful, and production
airplanes using the technology of quiet engines were developed as the YO-3,
which bore only a very distant resemblance to the original Schweitzer 2-32.
Only the tail assembly shows the origin.

Cheer, Charles