Semi-rigid airships are airships with a partial framework. These often consist of a rigid, occasionally flexible, keel frame along the long axis under the aerodynamic hull envelope. The partial framework can also be inside the hull. Semi-rigids were built in quantity in the early 1900s but since around 1938 only one type is actively flying, and there have been a few abandoned developments.

Principle

More or less integrally attached to the hull are the gondola, engines and sometimes the empennage. The framework has the task of distributing the suspension loads of these attachments and the lifting gas loads evenly throughout the whole hull's surface and may also partially relieve stresses on the hull during manoeuvres. The boundary between semi-rigid and non-rigid airship is vague. Especially with small types, it is unclear whether the structure is merely a stretched gondola or a proper keel.

As in non-rigid airships, the hull's aerodynamic shape is maintained by an overpressure of the gas inside. Changes in volume of the lifting gas is balanced using ballonets (air filled bags). Ballonets also may serve to provide pitch control. For small types the lifting gas is sometimes held in the hull itself, while larger types tend to use separate gas cells which mitigates the consequences of a single gas cell failure and helps reduce the amount of overpressure needed.

History

An early successful example is the Groß-Basenach design made by Major Hans Groß from the Luftschiffer-Bataillon Nr. 1 in Berlin, the experimental first ship flying in 1907. It had a rigid keel under the envelope. Four more military airships of this design were built, and often rebuilt, designated M I to M IV, up to 1914.

The most advanced construction of semi-rigid airships between the two world wars took place in Italy. There, the state-factory Stabilimento di Costruzioni Aeronautiche (SCA) constructed several. Umberto Nobile, later General and director, was its most well-known member, and he designed and flew several semi-rigid airships, including the Norge, for his overflights of the North Pole, and the W6 OSOAVIAKhIM, for the Soviet Union.

Under Nobile's leadership such ships as the following were built:

• T 34 Roma, 33,810 m³, start of the 1920s
• N 1 Norge, 19,000 m³, reached the North Pole in 1926
• N 2 a 7000 m³-airship built in hangars at Augusta (Italy)
• N 3
• N 4 Italia crashed 1928 in the Arctic
• N 5 was a project for a 55,000 cubic metre keel airship, many times interrupted, eventually abandoned 1928
• The marine airship No. 6 was built in 1925 for Japan, and flew first on 1927-04-06. It was lost in 1927 after encountering a typhoon in the Pacific.

Up to 1938, other builders produced these notable semi-rigid airships:

• "Pax" from Severo in France in 1902, caught fire at its first ascent
• Forlanini F.1 Leonardo da Vinci, Italy, 3265 m³, 40 PS, first ascent: 1909; 1910-02-01 damaged beyond repair
• Forlanini F.2 Città del Milano, Italy, 11,500 m³, 2×85 PS, first flight: 1913-04-09, destroyed 1914-04-09 at Como
• M.1, Italian, first flight 1912, 83 metre long, 17 metre diameter, 2x 250 PS Fiat SA.76-4 engines each with one airscrew, payload: 3800 kg, first with the Army then the Navy, 164 flights, decommisioned 1924
• M.2, Città di Ferrara, Italian, first flight 1913, hull identical to the M.1, 83 metre long, 17 metre diameter, 4×125 PS driving two airscrews, payload 3000 kg, speed: 85 km/h, a Navy airship, stationed in Jesi, on 1915-06-08 shot down by an Austrian flying boat
• Siemens-Schuckert I (1911)
• The Luftschiff von Veeh, (also Veeh1 or Stahlluftschiff) built by Albert Paul Veeh from Apolda in Düsseldorf in the 1910s
• several airships from August von Parseval:
• PL 26 and PL 27
• Parseval-Naatz
• The Groß-Basenach-type airship (5 built for the Prussian army)
• Zodiac V10 was built 1930 for the French Navy
• some airships of the Russian airship program, such as the *the Soviet W6 OSOAVIAKhIM (1934-1938) designed by Nobile
• Le Jaune - first flight: 1902-11-13, French
• SR.1 (M-class) built by Italy for England 1918, 12,500 m³, 83 m long, 17 m Diameter, 9 man crew, internal keel of triangular steel components
• O-1 (airship) built by SCDA, Italy, and the only true semi-rigid airship to serve with United States Navy.
• RS-1 was the only semi-rigid American military airship (used by the United States Army) built in the USA. Manufacturer: Goodyear, maiden flight: 1926.
• Raab-Katzenstein 27 - maiden flight: 1929-05-04

Since 1938

As of 2008 the only flying manned semi-rigid airship is the Zeppelin NT. It comprises a single gas cell kept at a slight over-pressure, ballonets to maintain constant volume, and a triangular keel structure internal to the cell.

• CL160 "Cargolifter" was an unrealised design of the now liqudated German Cargolifter AG (1996-2003).
• Cargolifter Joey was a small semi-rigid experimental airship of Cargolifter AG

See also

• Airship hangar * Rigid airship * Non-rigid airship

Notes