Title:  Monospar ST.25

The Monospar ST.25 was a 1930’s British twin-engined monoplane built by General Aircraft Ltd using the “monospar” wing construction patented by H J Steiger, the Swiss inventor of the concept. (How the structure works is explained in the manual.) The ST.25 was the final and largest of the Monospar series of aircraft, and between 1935 and 1939 around 60 examples were built.

This title includes the manufacturer’s Instruction manuals for the Monospar ST.25 aircraft, and the Pobjoy Niagara air-cooled radial aero engine which powers the ST.25, along with a NACA aircraft circular describing an earlier Monospar model. The files have been scanned from the original manuals and retain any colour pages.



This title contains:

  • Manufacturer produced Instruction Manual for the Monospar ST.25, titled “Maintenance, Rigging and General Instructions” (includes pilot’s notes), undated with approx. 20 pages of text and 10 illustrations.
  • Manufacturer produced Instruction Manual for Pobjoy Aero Engines, Niagara Series III, Cataract Series III and Niagara Series IV, titled “Installation, Care and Maintenance Manual”, document 1000.11.36 with approx. 88 pages of text and 15 illustrations.
  • NACA Aircraft Circular No 148, “Mono-Spar” civil aircraft. A technical descriptive circular dated July 1931 with approx. 11 pages. Although not specifically stated in the circular, this describes the first Monospar, the ST-3, but it gives a good description of the structural principles.