With the popularity of small cabin monoplane taildraggers like those of Piper, Aeronca, and Taylorcraft setting the tone, a new company, Interstate Aircraft and Engineering, entered the field by 1940.
Interstate’s two-place Cadet general aviation aircraft was thoroughly traditional: A high wing mounted to a welded-steel tube fuselage, all covered in fabric and powered by available air-cooled engines like the 65-horsepower Continental A65.
Interstate, already known as a manufacturer of components for aircraft, envisioned the Cadet as a logical choice for the Civilian Pilot Training Program (CPTP) begun in 1938 in an effort to increase the pool of available fliers in America as European countries were already doing.
The first Cadets came off the line at El Segundo, California, in December 1940, marking the first production of light aircraft of this type on the west coast, according to aviation historian Joseph Juptner.
More than 300 S-1-A Cadets were delivered into 1942. S-1As used 65-, 85-, and 90-horsepower Franklin air-cooled engines, as well as 65-horsepower Continentals.
Juptner described the handling characteristics of the Cadet, designed for the rigors of new fliers in the CPTP, as being light but stable, saying it “plowed through rough air like a small freight train.”
The Interstate S-1-A with the Continental A-65 engine was 24 feet in length, with a wingspan of 35 feet, 6 inches. The wing’s NACA 23012 airfoil had a five-foot chord. Stalling speed was 36 mph. Cruising at 98 mph, the S-1-A could top out at 107 mph. It had a service ceiling of 14,500 feet. When fitted with the Franklin 85-horsepower engine, the S-1-A was said to cruise at 108. The price at the factory was listed at $2,095.
The welded tube fuselage structure incorporates the Warren truss, a style developed for bridges in the 1800s that features equilateral triangles between horizontal members. This weight-saving load-bearing structure appears in other aircraft of the era, including the Piper Cub. The wing panels use spruce spars with built-up Alclad aluminum ribs.
In wartime 1942, Interstate evolved its S-1-A into the S-1-B as a civilian airplane, or the O-63 as an Army observation/liaison aircraft of the Grasshopper genre. Piper, Aeronca, Taylor, and Stinson all went to war with their own Grasshoppers. The XO-63’s Franklin engine was now rated at 100 horsepower.
The O-63 was the last Army aircraft to be identified as an O for Observation machine. Soon, it and the other O-birds became L for Liaison aircraft. The Interstate version became the L-6. To accommodate the Army’s liaison and spotting missions, the L-6 embraced substantially more glazing, angled to provide a downward view for the observer and pilot.
The Army ordered 250 L-6s from Interstate in April 1943. In addition to Interstate’s home plant in California, a second assembly facility in the midwest facilitated deliveries east of the Rocky Mountains.
Construction of military L-6s was finished in April 1945. By May, the war in Europe was over and victory in the Pacific was only months away. Surplus L-6s, sometimes called S-1-B1, entered general aviation after the war.
The National Museum of the U.S. Air Force displays an Interstate L-6. The museum’s website says the L-6 faced overheating problems that were partially remedied. The U.S. did not send its L-6s overseas, using them for liaison, utility, and training tasks domestically.
Thus ended Interstate’s adventure in aircraft design and construction. Rights, tooling, and inventory were sold to Harlow Aircraft in 1945. Subsequently Call Aircraft Co. (CallAir) bought the assets, later selling the Interstate rights again.
Interstate had another wartime claim to fame with its TDR twin-engine, optionally piloted, drone for the U.S. Navy. And that’s a story for another day…
(The author is indebted to Joseph Juptner’s pioneering work in documenting type-certificated aircraft of the United States in his book series “U.S. Civil Aircraft.” For this article, Juptner’s Volume 8, covering Approved Type Certificates 701-800 was helpful.)
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