The "Sierra Sue" on display at the Western Museum of Flight.


Previous display of the S-1 did not show it to advantage.


Detail of top picture shows 'Northrop Turbo-pusher' monicker.


Northrop's initial contender for the AX program was a
V-tailed, turbine pusher design directly inspired by the S-1.

NORTHROP 'Turbo-pusher'
(SIERRA S-1 'Sportplane' a.k.a. "Sierra Sue")

Type: experimental pusher propeller aircraft

Powerplant: 1 x 85 hp (63 kW) Continental C-85 pusher

Significant date: 1948 (manufacture), 1968 (Northrop tests)

Designed and flown by Ron Beattie and Walt Fellers, the Sierra S-1 Sportplane (also found as the Beattie-Fellers S-1) was an experimental aircraft of unusual configuration built in the US in 1948 to investigate the advantages of a pusher propeller configuration. Apart from this engine installation, the aircraft was unusual in having an X-shaped tail incorporating ruddervators on the upper fins. The wing was mounted midway up the fuselage and was unswept. For some reason, the Sierra S-1 appears on civil register as being built in 1953, perhaps indicating a rebuild that might have justified a new FAA approval.

Only one example of the midget racer was manufactured by Hugh Crawford and C. Roger Keeney's Acme Aircraft Co. of Torrence, Calif., and christened "Sierra Sue", but it never competed. Acme relocated in Los Angeles County, briefly marketed a Longren-designed Convair L-13 conversion known as the 'Centaur', performed the conversion of a Ryan 'Navion' into the X-16 prototype of Riley's D-16 'Twin Navion', and became Sierradyne Inc. in 1953. The Sierra S-1 (or Sierradyne S-1) was later used by Sierradyne in the '60s as a flying technology demonstrator to test and promote Northrop's and Dr Werner Pfenninger's boundary layer control concepts, which found their realization in the X-21 program.

The S-1 (now rechristened the Northrop 'Turbo-pusher') then became the company's flying demonstrator for the Air Force’s AX close-support aircraft design competition. Northrop believed that the pusher system eliminated torque problems found in single-engine puller designs. In the early stages of the competition, turboprops were being considered for the AX, although the prototypes eventually built under contract had turbofans. The AX Competition led to the manufacture of two prototypes, Northrop’s A-9A and Fairchild Republic’s A-10A. After an extensive flight test program, the Fairchild Republic design won the competition and made history as the 'Thunderbolt II' (popularly known as the "Warthog").

Although extensively used by Northrop, the 'Turbo-Pusher' was never officially considered a Northrop aircraft and remained the property of Sierradyne. On November 24, 1967, it was substantially damaged during a demonstration at Langley AFB when its pilot, unfamiliar with the aircraft, caused overload failure by improper operation of the brakes and/or the flight controls, leading the aircraft to swerve and roll on landing as the gear collapsed. It was later donated to the Western Museum of Flight located in Hawthorne, Calif., at the Jack Northrop Field, where it still resides in restored condition, right next to the Northrop N-16 (JB-1) 'Bat' and not too far from the AX contender it helped develop, the unfortunate A-9.

Population: 1 (c/n 1) [N12K]

Specs:
Length: 18 ft 0 in (5.49 m)
Wingspan: 20 ft 2 in (6.15 m)
Empty: 590 lb (268 kg)
Maximum speed: 200 mph (320 km/h)
Comparable aircraft: Göppingen Gö 9

Crew/passengers: 2

Main sources:
- The Sierra Sue page at the WMOF
- Brian Johnstone's photograph on MyAviation.net
- NTBS Identification Report on N12K accident
- The Acme Sierra page at OpenTopia
- The Sierra Sue page at Airpower.Callihan.cc