Idle-Thrust Versus Zero-Thrust Stalls in the C-12C

The C-12C was modified with a new 4-bladed swept propeller and a redesigned inboard leading edge. The end result was lower stall speeds and better takeoff and landing performance. Reports of a stall speed discrepancy began shortly after the Air Force received their first modified aircraft. The magnitude of the issue didn't sink in until a functional check flight at Edwards AFB showed that the zero-flap, idle-thrust stall speed was essentially the same as the charted V1 speed. A 4-year investigation ensued that included a historical investigation, a limited stall test and a student Test Management Project where zero-thrust and idle thrust stall speeds were investigated. The investigation settled the perceived stall speed discrepancy, identified an apparent misinterpretation of FAA part 23 stall paragraphs, and looked into the history of FAA part 23 definitions for various takeoff speeds.