The

experimental An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when a ...

Hirth Hi-25 Kria was only the second

glider Glider may refer to: Aircraft and transport Aircraft * Glider (aircraft), heavier-than-air aircraft primarily intended for unpowered flight ** Glider (sailplane), a rigid-winged glider aircraft with an undercarriage, used in the sport of glidin ...

constructed from

glass reinforced plastic Fiberglass (American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass clot ...

(GRP). Built in

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

in 1958, it was a single seat, span aircraft. Only one was built; it later flew for several years with

Akaflieg Stuttgart Akaflieg is an abbreviation for ''Akademische Fliegergruppe'', groups of aeronautical engineering students from individual German Technical Universities, pre and postwar, who design aircraft, often gliders. History Otto Lilienthal published his bo ...

Design and development

The first glider with a GRP structure was the Akaflieg Stuttgart Phönix which flew in November 1957.

Wolf Hirth Wolfram Kurt Erhard Hirth (28 February 1900 – 25 July 1959) was a German gliding pioneer and sailplane designer. He was a co-founder of Schempp-Hirth, still a renowned glider manufacturer.Segelflugbildkalendar 2011 Hirth was born in Stuttgart ...

had followed its construction with interest and had hoped to manufacture it. Instead, the Phönix was produced by Bölkow and Hirth built his own design, the Hi-25 Kria, using the construction methods developed for the Phönix. As well as their structural similarity, the Kria and the Phönix shared some aerodynamic features, particularly Eppler airfoils and fuselages which conformed to the upward flowing air ahead of the wings and downwash behind. The Kria, though, was a

high wing A monoplane is a fixed-wing aircraft configuration with a single mainplane, in contrast to a biplane or other types of multiplanes, which have multiple planes. A monoplane has inherently the highest efficiency and lowest drag of any wing confi ...

rather than mid-wing monoplane, was smaller overall (the Phönix had a 16 m span) and had a butterfly tail. Kría is the

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...

ic name for the Arctic tern. The Kria's

cantilever A cantilever is a rigid structural element that extends horizontally and is supported at only one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cant ...

wing was straight tapered over all the span except for elliptical tips, with an unswept

leading edge The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...

and a forward swept

trailing edge The trailing edge of an aerodynamic surface such as a wing is its rear edge, where the airflow separated by the leading edge meets.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 521. Aviation Supplies & Academics, 199 ...

. There was no dihedral. The wings carried short span ailerons at the tips and undersurface airbrakes, which could be extended at 90° to the airflow, inboard. The tail surfaces, separated by 110°, had swept leading edges and rounded tips. The airflow-conforming aerodynamics of the fuselage gave it a rather humped back profile, with the dorsal line dropping away aft of the wing to a slender tail. Ahead of the wing the combined

canopy Canopy may refer to: Plants * Canopy (biology), aboveground portion of plant community or crop (including forests) * Canopy (grape), aboveground portion of grapes Religion and ceremonies * Baldachin or canopy of state, typically placed over an ...

-nose line was almost straight. A single piece canopy covered a cockpit which fitted only the smallest pilots. The underside of the forward fuselage was bulged and carried a close-fitting landing skid, which was assisted by an integral tail bumper. The Kria was first flown on 31 December 1958, piloted by Rudi Lindner. It was pleasant to fly though rather lacking aileron response. Despite not being designed for serious competition, it performed better than any other span sailplane of its time.

Operational history

After Hirth died in a flying accident in 1959 the Kria was donated to the Akaflieg Stuttgart. It flew from their Hahnweide base for several years, also participating in glider evaluation courses under OSTIV auspices. Stored after retirement, the sole Kria was lost in a hangar fire in 2001.

Specifications

References

{{reflist, refs= {{cite book , title=Sailplanes 1945-1965 , last=Simons , first=Martin , edition=2nd revised , year=2006, publisher= EQIP Werbung & Verlag GmbH, location=Königswinter , isbn=3 9807977 4 0, page=256 {{cite book , title=Segelflugzeuge vom Wolf zum Mini-Nimbus , last=Selinger , first=Peter , year=1978, publisher= Motorbuch Verlag, location=Stuttgart, isbn=3-87943-448-4 1950s German sailplanes Glider aircraft Aircraft first flown in 1958