Liberty L-6

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Liberty L-6
Liberty L-6 aircraft engine on display at the National Museum of the United States Air Force
Type Piston aero engine
National origin United States
Manufacturer Wright Aeronautical, Thomas-Morse Aircraft
Designed by Jesse G. Vincent and Elbert J. Hall
First run about 1917
Major applications Engineering Division TW-1
Number built 52
Variants Liberty L-4, Liberty L-8, Liberty L-12

The Liberty L-6 was a six-cylinder water-cooled inline aircraft engine developed in the United States during World War I.

Design and development[edit]

Fokker D.VII with Liberty L-6 engine fitted for trials

The Liberty L-6, which developed 200-215 hp, was built by the Thomas-Morse Aircraft Corp. and Wright Aeronautical Corp. Since it was based on the same engine design as the more successful Liberty L-12 V-12 liquid-cooled aviation engine, the L-6's resemblance to the Mercedes D.III German aviation engine, the source for the Liberty V-12's own cylinder and valvetrain design, resulted in the American L-6 engine design bearing a close visual resemblance to the German straight-six aviation powerplant in a number of respects, with at least one L-6 even being mounted postwar into a captured Fokker D.VII fighter for testing in the US. The Liberty L-6's smaller displacement of some 825 cu. in. (13.5 liters) versus the late-war German Mercedes D.IIIaü's 903 cubic inches (14.8 liters) do not seem to have handicapped the American straight-six design, however, as the "Liberty Six" possessed a 5.42:1 CR, while the D.IIIaü had only a 4.64:1 CR, explaining a good bit of the American powerplant's output level — in addition to the 735.5 watt level for German Pferdestärke metric horsepower, versus the then solely-American-based SAE organization's standard of almost 746 watts per one horsepower.

Since the L-6 was too large for mail airplanes and other engines were available, the L-6 was canceled after only 52 had been built. In 1920 10 more L-6 engines were ordered, designated L-825, several of which were installed in the Curtiss PN-1, (only two built), and the Engineering Division TW-1, (only six built).


Rear view of restored Liberty Six engine

Data from U.S. Army Air Service Information Circular - Performance Test of Fokker D-VII with Liberty Six Engine

General characteristics

  • Type: Six-cylinder inline piston engine
  • Bore: 5.00 in (127 mm)
  • Stroke: 7.00 in (177.8 mm)
  • Displacement: 824.67 in³ (13.51 litres)
  • Dry weight: 567.5 lb (257.4 kg)



  • Power output: 215 hp (162.6 kW) @ 1,700 rpm
  • Compression ratio: 5.42:1
  • Fuel consumption: 0.49 lb/HP-hour
  • Oil consumption: 0.0256 lb/HP-hour

See also[edit]

Comparable engines
Related lists


This article incorporates text from, a public domain work of the United States Government.