Acme S-1 Sierra: Torrance's native pusher airplane

As I've long recognized, the Western Museum of Flight in Torrance is ubiquitous for housing a variety of aircraft built in the Los Angeles basin, including those made by Northrop as well as Radioplane (renamed Northrop Ventura in 1962). However, unknown to most aviation enthusiasts, this museum happens to have a very exotic homebuilt airplane on display, built at the very airport in Torrance near which the Western Museum of Flight is located, the Acme S-1 Sierra. Therefore, I am dedicating this post to discussing this unusual homebuilt pusher-engine aircraft from Torrance.

The Acme S-1 Sierra (aka "Sierra Sue") on display at the Western Museum of Flight. Photographed by me on April 21, 2024.

In the late 1940s, two former employees for North American Aviation, Ron Beattie and Walt Fellers, who came to work for the Northrop company after the end of the World War II, proposed a single-seat high-performance aircraft to meet Goodyear Racing Plane specifications and investigate the advantages of an airplane utilizing a pusher propeller layout. This design featured a teardrop-shaped fuselage and a Y-shaped tail empennage with ruddervators on the upper fins, and it had straight wings midway up the fuselage, and large air scoops placed at the forward ends of the wing roots. It was 20 feet 2 in (6.14 meters) long with a wingspan of 18 feet (5.49 meters), an empty weight of 590 lb (268 kg), and a top speed of 200 mph (322 km/h), and power came from one 85 hp (62 kW) Continental O-85 4-cylinder horizontally opposed piston engine situated behind the cockpit and driving a tail-mounted two-bladed pusher propeller. The Acme Aircraft Company based in Torrance, California, was entrusted to build the aircraft, and when this plane was completed in late 1948 it was now called the Acme Sierra, bearing the civil registration N12K (although this aircraft was sometimes nicknamed "Sierra Sue"). The first flight of the Acme Sierra occurred on November 23, 1953, and when Acme Aircraft was renamed Sierradyne Incorporated that year, the Acme Sierra itself received the internal designation S-1.  

The Acme S-1 Sierra at an airfield in Hawthorne in 1967 after being acquired by Northrop for testing the configuration of the N-308 attack aircraft project and rebranded as Northrop Turbo-Pusher (courtesy of Aerofiles).

Although it was not entered in any air races, the S-1 Sierra obtained extensive aerodynamic data during flight testing suggesting some aerodynamic benefits of a pusher-engine aircraft. During the 1960s, it was used by Sierradyne for tests of the aerodynamic benefits of the boundary layer control concept advocated by Swiss-born American aerodynamicist Werner Pfenninger and supported by Northrop. When the US Air Force in the late 1960s began contemplating plans for a purpose-built ground attack aircraft (which led to the A-X requirement), Walt Fellers in 1967 conceived a pusher-engine proposal for Northrop for the A-X program, the N-308, and that same year Northrop acquired the S-1 Sierra for use as a technology demonstrator to test the pusher-engine layout of the N-308, re-labeling it as the Northrop Turbo-Pusher. The S-1 Sierra's final flight ended in mishap on November 24, 1967, when its pilot created so much overload failure by improper operation of the flight controls and/or airbrakes that he crashed-landed the aircraft after a demonstration flight at Langley AFB in Virginia.

By 1970, Northrop dropped the N-308 in favor of the twin-turbofan N-312 and N-320 proposals for the A-X program after the US Air Force judged a turbofan-powered ground attack aircraft to be much faster than its inventory of A-1E Skyraiders compared to a turboprop-powered design, and the N-320 was designated YA-9 after being selected by the Air Force along with the rival Fairchild Republic A-10 for prototyping (the YA-9 itself ended up losing the A-X competition to the A-10 in 1973, but that's another story). Meanwhile, the S-1 Sierra which had ended its flying career as a technology demonstrator for the N-308 was eventually spared from scrapping and found a home at the Western Museum of Flight, which was initially based in Hawthorne before relocating to its present location in Torrance, the very city where the S-1 Sierra was built. 

References:

Chong, T., 2016. Flying Wings & Radical Things: Northrop's Secret Aerospace Projects & Concepts 1939-1994. Forest Lake, MN: Specialty Press.

Underwood, J.W., and Caler, J., 1958. Experimental Light Aircraft and Midget Racers. Fallbrook, CA: Aero Publishers.