M.Simons The World's Vintage Sailplanes 1908-45
THE AUSTRIA
The Austria, designed by Dr Kuepper of the Munich Akaflieg to the order of Robert Kronfeld, was the biggest sailplane ever built. It weighed nearly half a tonne in flying trim and on its first launch defeated the tug aircraft ДальшеMore>>>
and had to be assisted into the air by a curious double tow arrangement, a powerful truck helping the aeroplane to get the monster moving and off the ground, then the rope to the truck was released and the aero-tow proceeded normally.
The best contemporary sailplanes had wingspans about twenty metres. Other things being equal, the glider with a high aspect ratio, the relation of span to total area of wing, will always be more efficient than one with shorter wings of the same area. Kronfeld hoped to gain at least 15% improvement in the glide ratio by having an aspect ratio of about 25. It was still thought important to keep the wing loading down to ensure a low minimum sinking speed. The only way of achieving both was to increase the span.
To control such a large wing, Kuepper designed full-span, metal-framed ailerons, divided into six sections so that they would not bind as the wing flexed under load. The wing itself was wooden. A very deep 'I'-beam mainspar took the bending stresses, and the whole was skinned with plywood, instead of the usual fabric covering over the rear of the wing, to ensure adequate torsional stiffness across the wide span. The aerofoil section was the thick, strongly cambered Goettingen 652 but although the slow speed performance of this profile was very good, to penetrate downcurrents Kuepper arranged for the broad ailerons to act as camber flaps as well. They could be drooped for climbing and raised for gliding. With their aid Kronfeld hoped it would be possible to fly across country without having to waste time circling in thermals. He had discovered that clouds and thermals sometimes align in ‘streets’, and thought that by flying slowly through the upcurrents with flaps down, and faster with flaps up through the downcurrents, he would be able to go a long way, rarely being forced to circle. Thus the technique now known as ‘dolphin soaring’ was envisaged already in 1931.
The Austria also had airbrakes of a kind. Twin fins and rudders were mounted at the ends of the tailplane, these surfaces having a slight outward camber. In normal flight the rudders were operated one at a time, pressure on the right pedal deflecting the right rudder outward to yaw that way, and left pedal giving deflection on the left surface only. If the pilot pushed forward hard with both feet, both rudders would move outward, opposing one another and increasing the drag without yawing the sailplane. Probably this was the first sailplane to be equipped with airbrakes of any sort. The force needed to make them effective was very great and would increase considerably at high airspeeds, as Kronfeld was soon to discover.
The extraordinary fuselage was the result of careful thought. The great spread of wing necessitated a tall pylon to keep the tips clear of the ground. The outer panels of the wing were given slight anhedral when the glider was at rest, because in flight the wing would bend upward and the dihedral would then increase, making for difficulty in controlling the aircraft. Under load the drooped outer wing would flex up to an approximately level position. The pilot was housed in a perfectly streamlined pod ahead of and suspended well below the wing. The tail boom was then attached, conveniently, to the wing. It was a tube of plywood about 30 cm in diameter at the rear end and thus larger than the rear fuselage of most more orthodox sailplanes but it looked very slender in proportion to the rest of the Austria.
Great attention was paid to sealing off all gaps in the wing, and the whole aircraft was smoothed, painted and polished to reduce skin friction. The wing was built in four pieces. On the ground the Austria required a large crew and was named 'The Elephant’. In the air, the few times it got off, it was impressive, sailing like a great galleon, calm and stately, while the smaller ‘ships’ bobbed and wavered in choppy conditions. When the end came it was fittingly spectacular.
On 22nd July 1932, at the Rhoen meeting, Kronfeld was towed to 500 metres and after release worked lift up to the cloud base, disappearing into the cumulus about 6 km from the Wasserkuppe. For blind flying he had a turn and slip indicator, airspeed indicator, compass and variometer. He had more experience of cloud flying than any other glider pilot but it still proved insufficient. After a while, he found he was unable to interpret the gyro instrument. The airspeed began to build up and he could not reduce it, he saw the needle go round past the 200 km per hour mark and feared that if he moved the controls the aircraft would be damaged. The rudder brakes had become immovable under the increasing pressure of the airflow.
Spectators saw the giant emerge from the base of the cloud in a steep spiral dive. One half of a wing twisted clean off, the Austria executed a great half roll and Kronfeld jumped as the other wing also broke. His parachute opened but watchers gasped as the dismembered 'Elephant', relieved of its load, reared up in a stall, swinging dangerously close to the tiny figure dangling beneath his canopy. Then the Austria fell away and plunged straight down into the valley to smash into small pieces. It is now believed that Kronfeld entered the classic, high speed spiral dive in cloud, a consequence which is almost inevitable if the pilot disbelieves or misinterprets his turn indicator; the glider takes up a steeply banked attitude and if the pilot pulls back on the control stick, the effect is only to force the aircraft into a tighter turn which instead of reducing the airspeed causes it to build up until the wing fails.
Kuepper continued with the design of a smaller sailplane of very similar layout and appearance to the Austria, but with a span of only 20 metres. It was destroyed in an accident early in its career.
Technical data:
Austria: Span, 30.00 m. Wing area, 35 sq m. Aspect ratio, 25.7. Empty weight, 392 kg. Flying weight, 482.4 kg. Wing loading, 13.8 kg/sq m. Aerofoil, Goettingen 652.
The "Austria" which was built for Herr Kronfeld in the Segelflugzeugwerke at Kassel. The small wheels have recently been added to facilitate towing-off by an aircraft.
The Austria once visited England. It was aero-towed across the Channel for a gliding display at Hanworth in June 1931. The Segelflugzeugbau Kassel emblem appeared on the fuselage. Unlike most sailplanes of its time, the Austria was painted overall cream.
The Austria. A new 103-ft. span cantilever glider which has been built for Herr Kronfeld. The wings are completely plywood covered, and the rudders can fold inwards together for use as air brakes.
The Austria amid the hay at Hanworth.
LOW HEAD RESISTANCE - STYLE TWO: Herr Robert Kronfeld believes in a blunter nosed fuselage than the "Meiningen" as can be seen from this unusual view of his "Austria" taken shortly before his accident at the Wasserkuppe.
The unorthodox tail unit of Austria. The new glider which has been built for Herr Kronfeld to create a few more records on.
The Austria de-rigged. Some idea of the difficulties encountered on the ground with a sailplane of such size and proportions, is given by this photograph. For such a huge span, the wing root fittings seem very flimsy, but in fact it was the wing itself that failed in flight, about half way along.
The Austria wing under construction. The factory roof supporting pillars were used cleverly as a jig to hold the mainspar straight as the ribs were glued to it, front and rear. Only the centre sections of the four-piece wing are shown here.
The wreckage of the mighty Austria in the valley below the Wasserkuppe.
