Design and development
In April 1992, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt e.V - DLR) commenced a programme to develop an aircraft to carry out atmospheric, stratospheric and climatic research. It chose Grob Aerospace to design and build an aircraft to meet these requirements, based both on its experience in use of composite material in aircraft structures together with its successful development of the Egrett surveillance aircraft, with the aircraft expected to be operational by 1996.
To meet the requirement to operate at an altitude of 24,000 m (78,700 ft) for 48 hours, Grob designed a twin-engined aircraft with a straight, very high aspect ratio wing of 56.5 m (185 ft 4½ in) span. The wings featured winglets, and were mounted across the top of the fuselage which terminated in a T-tail configuration. The aircraft was designed to be crewed by two pilots, and could accommodate two scientists and associated mission equipment in a pressurised cabin. A galley, rest facilities and a toilet were provided.
Unlike the Egrett, which was powered by a single turboprop engine, the Strato was powered by two wing-mounted pusher compound engines consisting of a turbocharged piston engine supplied by the low- and intermediate-pressure compressors and associated turbine machinery derived from a PW127 turboprop engine. Together with the piston engine's own turbocharger, this formed a three-stage turbocharging system that provided pressurised intake air at high altitude. This had the advantage of maintaining power at high altitudes. Each engine drove a 6 m (19 ft 8 in) diameter five-bladed propeller.
The propulsion system was developed under a programme of less than three years and a total development cost of less than DM30 million. To reduce cost and development risk, existing components were used wherever possible. The two-spool gas generator of the PW127 was selected for the low-pressure charger because it was a proven component incorporating advanced compressor and turbine technology, although its adaptation resulted in penalties in weight and efficiency and required a separate oil-supply and scavenge system.
The PW127-derived unit formed a two-spool low-pressure turbocharger incorporating low- and intermediate-pressure compressor stages and their associated turbine machinery. Exhaust leaving the piston engine's high-pressure turbocharger was supplied to the unit through a hot-gas distributor and twelve supply tubes. A new compressor-inlet case without the original accessory gearbox was fitted, while the low-pressure charger used its own lubrication and control systems. It could be disconnected at lower altitudes by an electrically operated hot-gas bypass valve, with intake air continuing to pass through the slowly windmilling compressors.
After each of the three compressor stages, the charge air was cooled to about 0 °C (32 °F) by air-to-air coolers. Cooling airflow was regulated by a movable inlet flap in the forward part of the nacelle according to altitude, ambient temperature and engine power. A digital control system matched the compressor stages during steady and transient operation, operated the low-pressure-charger bypass and bleed valves, regulated cooling airflow and limited rotor speed and manifold pressure.
Construction of the airframe moulds started mid-November 1992, with airframe construction beginning in April the following year, starting with the tailplane. The airframe was completed in 1994, and engine installation commenced.
Operational history
The prototype first flew on 31 March 1995. Costs overran, however, and the prototype, which was intended as a proof-of-concept aircraft with off-the-shelf equipment and a heavier wing structure than planned for the production aircraft, was late and did not deliver the expected performance.
On 4 August 1995, during its 29th and ultimately final flight, the aircraft set a world altitude record for a crewed piston-engined aircraft of 18,552 m (60,867 ft). The DLR cancelled the programme in 1996.
The sole prototype is on static display outside Grob Aircraft's headquarters at Mindelheim-Mattsies Airport.
