The Jacobs radial engine began life in 1930 as a 3-cylinder engine producing 55 hp. and a 7-cylinder producing 150 hp. The 3-cylinder was soon made redundant with the 7-cylinder becoming the engine that was developed over the years.

The L-4 design of 1934 replaced the exposed rocker arms with a fore-and-aft design. The L-4 (R-755) developed 225 hp from 757 cubic inches and was still in production in the 1970s. The 831 cubic inch L-5 (285 hp) produced from 1936, and the L-6 or R-915 (330 hp) were the largest American seven-cylinder engines until the post-war Cyclone Seven was built.

The Jacobs engine is unusual in that it uses a combination of magneto and battery ignition – one set of spark plugs is powered from the magneto, while the other is powered from the battery via a coil and distributor system. The engine was nicknamed “Shaky Jake.”

It was used extensively in the 1930s and 40s on the following aircraft:

• Cessna Crane,
• Avro Anson Mk. II,
• Waco AQC-6,
• Beech Staggerwing,
• Cessna 195,
• Howard DGA-12
• and the Fleet Fort.

The Museum’s Waco AQC-6 is maintained in flying condition and is powered by this Jacobs engine.