Luigi Stipa
Luigi Stipa | |
|---|---|
 Luigi Stipa, likely photographed during the 1920s or 1930s | |
| Born | Template:Birth date |
| Died | 9 January 1992 (aged 91) Ancona, Italy |
| Occupation(s) | Aeronautical engineer, hydraulic engineer, civil engineer, aircraft designer |
| Years active | fl. ca. 1920s–1930s |
Luigi Stipa (born 30 November 1900 – died 9 January 1992) was an Italian engineer specializing in aeronautics, hydraulics, and civil engineering, as well as an aircraft designer. He is credited with inventing the "intubed propeller," a design some aviation experts consider a forerunner to the turbofan engine.
Early life and career
Born in Appignano del Tronto, Italy on 30 November 1900, Stipa interrupted his education to serve in the Italian Army's Bersaglieri Corps during World War I. Following his military service, he obtained degrees in aeronautical engineering, hydraulic engineering, and civil engineering. He subsequently joined the Italian Air Ministry, where he ultimately advanced to become the general inspector of the Engineering Division within the Regia Aeronautica (Italian Royal Air Force).
Intubed propeller
During the 1920s, Stipa employed his background in hydraulic engineering to formulate a theory aimed at enhancing aircraft efficiency through improved airflow management. He observed that according to principles of fluid dynamics—specifically Bernoulli's principle—a fluid’s velocity increases when passing through a narrowing tube. Applying this concept to aerodynamics, he proposed increasing an aircraft engine’s efficiency by channeling its propeller wash through a specially designed Venturi tube. This innovation, which he termed an "intubed propeller," involved constructing the fuselage of a single-engine airplane as a tubular structure enclosing both the propeller and engine nacelle. The propeller matched the tube’s diameter, and its slipstream exited at the rear end of the tube, corresponding to the fuselage’s trailing edge.
Stipa dedicated several years to mathematically analyzing this concept. His research concluded that shaping the Venturi tube’s inner surface like an airfoil maximized efficiency. He also identified optimal parameters for the propeller’s shape, its placement relative to the tube’s leading edge, and its rotational speed. Although he envisioned employing the intubed propeller primarily on large, multi-engine, flying wing aircraft—designing several models accordingly—he regarded building a single-engine experimental prototype as an essential initial step toward validating his idea.
After publishing his findings in Italy’s Rivista Aeronautica ("Aeronautical Review"), Stipa requested support from the Air Ministry to construct a prototype aircraft demonstrating his design. The Italian Fascist government, keen on showcasing technological advancements for propaganda purposes and particularly interested in aviation innovation, endorsed the project. Consequently, they commissioned Caproni Aviation Corporation in 1932 to develop the prototype.
The Stipa-Caproni
The experimental aircraft known as the Stipa-Caproni took to the skies for the first time on October 7, 1932. Although its design appeared quite unconventional and awkward, it successfully demonstrated Stipa's theory that an intubed propeller could enhance engine efficiency. Additionally, the tubular airfoil structure contributed to a superior climb performance when compared to traditional airplanes with similar engine power and wing loading. This aircraft also featured a notably low landing speed and operated with less noise than typical planes of the era. To improve control, its rudder and elevators were positioned within the propeller's slipstream inside an opening at the tube’s trailing edge, resulting in exceptionally stable handling characteristics during flight.
However, a significant limitation of the Stipa-Caproni was that the intubed propeller generated considerable aerodynamic drag, which largely offset the efficiency advantages gained. Nonetheless, Stipa regarded this aircraft primarily as an experimental platform and likely did not consider the drag issue critical for his larger-scale, multi-engine flying wing projects.
Following Caproni's initial evaluation, the Regia Aeronautica assumed control of the Stipa-Caproni and conducted further limited testing. They ultimately chose not to pursue development because it did not offer any substantial performance improvements over conventional aircraft designs.
Stipa's later work and influence
Although the Regia Aeronautica showed little enthusiasm for advancing the intubed propeller concept, the Italian government promoted Stipa’s achievement publicly. In 1938, Stipa secured patents for his intubed propeller in Germany, Italy, and the United States. His research was disseminated across France, Germany, Italy, the United Kingdom, and America, where it attracted attention from organizations such as the National Advisory Committee for Aeronautics. The tests conducted by the Regia Aeronautica also stimulated academic interest in this propulsion approach.
During the 1930s in France, the design of the ANF- Mureaux BN.4—an advanced multi-engine night bomber—was influenced by Stipa’s intubed propeller concept; however, this project was cancelled in 1936 before any aircraft were constructed. In Germany, Ludwig Kort developed the Kort nozzle in 1934—a type of ducted fan akin to Stipa’s intubed propeller—which remains in use today. The German Heinkel T fighter also exhibited design elements reminiscent of Stipa’s ideas. Meanwhile, none of Stipa’s Italian flying wing designs featuring intubed propellers were ever realized in production. Nevertheless, in 1940, the Caproni Campini N.1, an experimental aircraft powered by a motorjet and derived from the intubed propeller principle, was introduced despite its impracticality.
Stipa maintained that his intubed propeller invention laid the groundwork for the jet engine, asserting that Germany’s use of a pulse jet engine on the V-1 flying bomb during World War II infringed on his German patent—although this pulse-jet technology bore little direct relation to his own concept.
Passing away on January 9, 1992, Stipa remained resentful over what he perceived as a lack of proper acknowledgment for his role in inventing jet propulsion. Some aviation historians partly concur with his assessment, recognizing that modern turbofan engines incorporate elements traceable to his intubed propeller design.
Notes
References
- Guttman, Robert. "Caproni Flying Barrel: Luigi Stipa's 'Intubed Propeller' as a Precursor to the Jet Engine." Aviation History. March 2010. ISSN 1076-8858.
- Johnson, Charles F. Patent US1864912, filed June 28, 1930; issued 1932.