Italy (1932)
Experimental Aircraft – One Prototype Built

In the history of aviation, there have been many projects that on paper promised outstanding flight capabilities, or offered other technical advantages. The time before the Second World War saw aviation advance at a breakneck pace, and is well known for such experiments. The so-called Stipa-Caproni was one such project, being an intriguing, and somewhat bizarre, experimental aircraft designed by Italian aeronautical engineer Luigi Stipa, and built by Caproni during the interwar period. It was characterized by its tubular fuselage, hence earning it the nickname Flying Barrel.

History

In 1927 a young Italian aircraft engineer Luigi Stipa began working on an unusual tube-shaped aircraft. Like many other aviation enthusiasts, Stipa was very interested in how aircraft could achieve better performance through exploring unorthodox construction methods. Thanks to his studies in thermodynamics, he was aware of the so-called Venturi effect, named after Italian physicist Giovanni Battista Venturi. In essence, this effect describes the reduction of fluid pressure and increasing velocity when it’s moving through a cylinder of decreased diameter. In theory, using this principle, a special type of aircraft could be created that could achieve significantly higher speeds than the conventional models of the time. Stipa theorized that for this purpose, such an aircraft would have to have a tube-shaped fuselage with the engine being positioned near the front. After finding it theoretically possible, he moved forward to test if the Venturi effect could be implemented in his airplane concept. For this purpose, he began a series of different tests inside a wing tunnel, carried out at the Aerodynamic Laboratory in Rome, from 1928 to 1931. The main focus of this testing period was to find the adequate shape, and leading edges, of the tube-shaped fuselage. This also included finding the right position of the engine, its position inside that tube, and the ideal propeller rotation speed. Following a series of wind tunnel tests, Stipa concluded that it was possible to build a full-scale prototype by using a single tube-shaped fuselage.

At the end of his research, he concluded that such a project was viable and set the task of building a working prototype. To gain interest in his project, he wrote about his work in the Rivista Aeronautica journal in 1931, and even built a small working replica. The next logical step was to write to the Italian Minister of Aviation, in the hope of getting approval for the realization of his project. Luckily for Stipa, his work came to the attention of General Luigi Crocco, the Air Ministry’s director. Stipa’s work was well received and the project received a green light. To test the concept, a working prototype had to be constructed.  It is important to note, that both Stipa and the Italian Air Ministry were aware that this project was merely to test his theories, and would not entail any further development of the prototype. In addition, both were aware that Stipa’s proposed principle was only practical on larger aircraft types.

For this purpose, the prototype was to be powered by a small 120-hp engine. The reason behind this decision lay in the fact that this aircraft was primarily built for evaluation and academic purposes. The Italian Air Ministry was not quite willing to invest huge monetary resources in it, beyond those necessary for the construction of the working prototype.

To help build the test aircraft, the Caproni aircraft manufacturer from Milan Taliedo was chosen. It was designated as Stipa-Caproni (sometimes referred to as Caproni-Stipa) referring to its designer and constructor. The prototype was built quickly and was ready for testing in October 1932.

It is perhaps a little surprising that such an unusual design would receive the necessary support for its realization. However, the exploration of new and unorthodox ideas in aviation was very popular in pre-war Europe. During the 1930s, Italy led the way in this aspect, perhaps even more than other countries, testing many unorthodox designs. What’s more, the Italian Fascist regime even encouraged different and unusual projects like this one, although many of them did not produce any meaningful results.

Technical specification

The Stipa-Caproni was a two-seater, mixed-construction aircraft, designed to have the simplest and thus cheapest fuselage. Its fuselage consisted of a tube which internally consisted of two large wooden round-shaped rings at the nose, followed by a series of similar but smaller rings. All of them were then connected with horizontal ribs which in turn were covered in fabric. The outer wooden rings served as the foundation, on which the wing and the cockpit would be connected. The fuselage design was, in effect, a large tube shaped airfoil.

The wings were mounted centrally on each side of the fuselage. These had a simple wooden construction, and were covered in fabric. They were also connected to the fuselage through metal bracing wires, which as a consequence increased the aircraft’s drag.

To the rear, a fairly large tail assembly was placed. During the design work of this aircraft, Stipa intentionally placed the rear control surfaces as close to the slipstream as possible. He hoped that this arrangement would greatly improve the aircraft’s handling and maneuverability.

On top of the fuselage, an elevated two-seat cockpit was placed. These were top-open with a small windshield placed in front of each position. There were also a pair of small doors that opened on the left side to give access to the seats.

The 120-hp de Havilland Gypsy III engine was placed inside this fuselage. It was centrally positioned and suspended using several steel bars that held it strongly in place. This was necessary to do so, as a weaker mounting could potentially endanger the aircraft during flight. The engine propeller was the almost the same diameter as the tube-shaped fuselage.

Given its overall design, and the position of the propellers inside the fuselage, the landing wheels were small and quite close to the ground. It consisted of three fixed road wheels. Two larger on the front and one smaller on the rear. Initially, wheel fairings were used but at some point, and for unclear reasons, these were removed.

Testing and Final Fate

With this project approved, a prototype was constructed and air tested in October 1932 at the experimental field at Monte Celio near Rome. Despite its odd design, the prototype was able to take to the sky without any major problems. Furthermore, it made several successful flights around Taliedo and Guidnia. It was even presented to the Italian Air Force for future test flights. During this period the aircraft was jokingly nicknamed Flying Barrel or Aereo Botte (Eng. Wooden wine barrel aircraft) or Aereo Barile (Eng. Fuel-Barrel aircraft).

The weight of the aircraft during these flights was 800 kg (1,874 lb), while the calculated wing loading was 44,73 kg/m² (9,16 lb sq.ft.). The maximum speed achieved was 133 km/h (83 mph), and it needed 40 minutes to climb at a height of 3, 000 m. It needed an 800 m long airfield to be able to take to the sky.

Despite Stipa’s hopes that the position and shape of the tail control surfaces would improve its mobility, several problems were noted by the test pilots. Firstly the elevator worked very well, which ironically proved to be a major problem. Even with a slight movement of the command control stick by the pilots, the aircraft could prove very sensitive to elevator inputs. On the other hand, the rudder controls were quite stiff,  as a consequence the pilot had to use considerable force in order to use it effectively.  Analyzing this problem showed that the rudder’s large surface area was to blame for its stiff control. But besides the two problems, the aircraft was reported to be easy to fly when being used in a gliding flight. These defects  were of a more or less technical nature, which were not necessarily irremediable through further development of the overall design.

The final results of evaluation flights showed that the Stipa-Caproni does not have any particularly great advantages compared to other more standard aircraft designs.  In addition, Stipa-Caproni’s overall aircraft shape offered limited space within the fuselage for passengers or payload.

As Stipa predicted from the start, his principles would not offer any major advantage over a standard smaller-dimension aircraft. The real application of the Stipa-Caproni design was only feasible on larger aircraft. Stipa hoped that his further research would enable him to construct large aircraft powered by two to three tube-shaped engine mounts. Unfortunately for him, after a series of test flights during 1932 and 1933 the interest in his work died out. It was briefly used in various Italian aviation propaganda publications before being scrapped in 1939.

Despite being in general an unimpressive design, the French showed interest in it. Particularly the company ANF Lex Maureaux, which went so far as to acquire a license for the design in 1935. According to initial plans, a two-engine variant was to be built for testing and evaluation. The project did not go beyond basic work was later canceled.

Lastly, an interesting fact is that many people considered Stipa-Caproni to design some sort of proto-jet engine. Whether this was the case or not, Stipa felt his work was overlooked, and according to some sources, he remained bitter throughout his life until he died in the early 1990s.

Replica

In 1996, aviation enthusiast Guido Zuccoli began working on a smaller replica of this aircraft.  However, the death of Zuccoli in a landing accident caused a delay in the replica’s final delivery. It was finally completed in 2001 when numerous small flights were achieved. The aircraft, powered by a 72 hp Simonini racing engine, managed to achieve a flight distance of 600 m (1,968 ft). After that, the aircraft replica was stored as an exhibit at the Zuccoli Collection at Toowoomba, in Australia.

Conclusion

The Stipa-Caproni represented an intended for the purpose of testing his new concepts in practice. While surely an interesting and unusual concept, Stipa-Caproni’s overall design was not that practical in reality, offering little improvement over a standard aircraft design of similar dimensions.

 

Stipa-Caproni  Specifications
Wingspans	14.3 m / 46 ft 10 in
Length	6.04 m / 19 ft 10  in
Height	3.2 m / 10  ft 7  in
Wing Area	19 m² / 204 ft²
Engine	One 120 hp (89.5 kW) De Havilland Gipsy III
Empty Weight	595  kg / lbs
Maximum Take-off Weight	850 kg / 1,874 lbs
Maximum Speed	133 km/h / 83 mph
Landing Speed	68 km/h / 42 mph
Climbing speed to 3,000 m	40 min
Maximum Service Ceiling	3,700 m / ft
Crew	1 to 2 pilots
Armament	None

Illustration

 

Credits

• Written by Marko P.
• Edited by Henry H. & Ed J.
• Illustration by Godzilla

Source:

• J. Thompson (1963) Italian Civil and Military Aircraft 1930-1945, Aero Publisher
• R. Giacomelli, (1933) The Stipa-Caproni Monoplane, Aircraft Engineering and Aerospace Technology, Vol. 5
• D. Nesic  (2008)  Naoružanje Drugog Svetsko Rata-Italija
• L. Salari, Caproni Storia della nascitadell’ industria aeronautica
• M Taylor, The Wolrd Strangest Aircraft, Metro Books
• O. E. Lancaster (1959) Jet Propulsion Engines,  Princeton University Press
• L. Stipa (1933) Stipa Monoplane with Venturi Fuselage, Technical Memorandums Nation Advisory Committee For Aeronautics No.753
• https://resources.system-analysis.cadence.com/blog/msa2022-the-role-of-the-venturi-effect-in-airplane-design
• https://www.historynet.com/caproni-flying-barrel/
• https://aeropedia.com.au/content/stipa-caproni/