Quiet.

This is coolbert:

"low-observable technology isn't limited just to cutting an aircraft's radar signature and can involve other factors, such reducing aircraft's infrared or visual profiles or cutting how much noise it makes in flight."

Stealth warplane but not radar-evading. Looked for cannot be seen. Listened for cannot be heard.

A legacy of the Vietnam War that never was totally discarded. Far from it.

"The U.S. Coast Guard May Be Flying a Unique Stealthy Spy Plane"

"The service's Manned Covert Surveillance Aircraft project has connections to decades of secretive work on 'quiet aircraft.'"

From The Drive and the article BY JOSEPH TREVITHICK.

Stealth "quiet" warplane YQ-3A in Vietnam a qualified success. But never enough of them and too late to make a real difference. Too bad!

That most recent "quiet" aircraft the American Coast Guard Manned Covert Surveillance Aircraft (MCSA). Varieties of MCSA type aircraft been in the inventory for since Vietnam and have apparently done yeoman service. Amazing.

Silenced and "quiet" aircraft having a military or quasi-military mission to include but I might suggest hardly limited to:

* HO-3. 

* QT-2PC.

* RG-8A.  

* RU-38A. 

* RU-38B.  

Army requirements spurred initial research and development into "quiet' stealth warplanes. U.S. Army please recall at one time had more aircraft than the Air Force and more boats than the Navy!

coolbert.