FLUG REVUE November 2005: Grob SPn testing makes progress

November 2005

GROB SPn UTILITY JET TESTS ADVANCE

By Bernd Gaubatz

What could be nicer than sharing good news with other people? That was what Grob Aerospace and the Zurich-based ExecuJet Aviation Group (EAG) thought as well. In the middle of September they invited the trade press to an SPn briefing at the Grob plant near Mindelheim.

Grob SPn flying

The ExecuJet company, which has a workforce of 420, is to market the ten-seater SPn internationally and arrange support for it. The Swiss company's role on the programme is much more than just a distributor: it has made a critical contribution to the design of the jet and also to its financing. When it comes to the provision of services for Business Aviation, EAG is very much at home. It provides support for and operates 80 business aircraft. Moreover, the corporate group has its own maintenance facilities and FBOs around the world.

In the Swiss company, Grob has found a partner which has intimate knowledge of the needs of the Business Aviation clientele and has access to the world markets. Only one market, albeit a very important one, remains to be opened up from the point of view of sales and support: the USA. Whether ExecuJet will set up its own representative offices there or, as is more likely, seek out competent partners, remains to be seen.

The history of the origins of the SPn Utility Jet is typical of the entrepreneurial spirit of Grob. This is a company which does not bother with wordy pre-announcements on planned projects. Instead, complete, or rather, virtually complete facts are presented to the specialist public when no one is expecting them.

Grob is a genuine “can-do company”, which in the course of three decades has pushed back the frontiers of what is achievable in aircraft construction in a number of directions and does not just think about what is feasible but what is physically possible as well.

At this year's Paris Air Show, the Mindelheimers actually succeeded in amazing trade visitors. They were exhibiting for the first time a prototype close to production standard of a ten-seater jet which they had almost succeeded in flying for the first time before the start of the air show.

The twin-engined jet was developed in an astonishingly short time. Initial thoughts about whether it might be feasible occurred in the autumn 2003. The programme was then launched in January 2004, and only nine months later the design was frozen. Work began on building the first airframe in October 2004. Four months later, in February 2005, the static tests were completed and final assembly of the first prototype followed. On 20 July this prototype had its maiden flight.

By mid-September, chief test pilot Gérard Guillaumaud had clocked up 23 flying hours on it in the course of 24 flights. Flight testing started off, as his customary, with the general flight characteristics. Guillaumaud was visibly impressed with the handling of the SPn.

Extension of the velocity band was completed without any problems either. Up to 8,000ft the calculated maximum operating speed (VMO) is 260 KCAS, and up to 29,500ft it is 272 KCAS. The maximum permissible Mach number is 0.70.

The Utility Jet can not only fly fast, but it should come up with excellent slow flight figures as well. Although the stall speed with extended flaps had not yet been precisely ascertained, the French test pilot is already sure that this will be below the calculated 77 KCAS.

For take-off, the electrically powered Fowler flaps are lowered to 20 degrees. The powerful Williams turbines accelerate the SPn to take-off speed in next to no time. Rotation occurs at 95 KIAS. Ten knots later the main wheels are off the ground. The climb rate is well over 4,000ft/min.

In the course of the visit to the Grob factory, the visitors were naturally allowed a glimpse into the cockpit of the first Utility Jet. Understandably, the Honeywell Apex avionics suite which is planned for production aircraft is not yet integrated into the panel. The visible portion comprises two 38cm primary displays, two multi-function displays of 25cm diagonal and the associated control and data entry panels.

A second aircraft will soon be available for further testing. This is scheduled to fly in March. In parallel to this, fatigue tests are to commence on the airframe that was used for the initial strength tests.

There are no indications at present that anything could delay EASA certification, which is planned for the first quarter of 2007. US certification should follow three months later. First deliveries will commence at this time. ExecuJet believes the prospects are good for the sale of 400 aircraft over a period of ten years.

Both Grob and ExecuJet are agreed that the SPn will convince prospective customers through advantages that no other light jet offers. According to Grob, the composite construction offers unparalleled durability. Tolerance to damage is higher than with metal aircraft and minor repairs can be carried out on-site without any problems. The defective area is simply removed and replaced with new composite material.

Moreover, composite materials are not susceptible to corrosion, and components made from this material normally weigh less than comparable metal parts. On top of this, composite aircraft have significantly fewer parts. All of this enables the weight to be kept down and the kilograms saved in this way are then available for the much more important useful load and payload. Finally, one should not overlook the fact that with composites it is possible to create shapes that are not feasible in metal. In this way, advanced aerodynamic concepts can find their way into the construction of “everyday aircraft”.

The SPn airframe has a minimum useful life of 28,000 flying hours. Its safe maximum g loading is +3.1/-1.24g. In the course of the static tests it became apparent that the structure could endure 1.725 times those values without any problems.

The Grob jet is fitted with two Williams FJ44-3A turbofans, the same engine that is used on the Citation CJ3. Power output is 2 x 2,800lb (2 x 12.44kN). For every kilonewton (kN) of thrust there are 255kg of aircraft mass. Given such favourable figures, it is no surprise to hear that the SPn, when fully loaded, only needs about 900m of runway to take off. This also explains the impressive climb rate.

The FJ44-3A is fitted with a FADEC digital engine control unit. This not only reduces the pilot's workload, but also conserves the powerplant. Due to the low idle thrust, the “Dash-3A” does not have any thrust reversers.

The wing, which is constructed from a single part and is attached beneath the fuselage with four titanium bearings, has plenty of room for fuel. Almost 2,500 litres of avgas can be carried, and there is even an optional single point fuel opening. When flown by a single pilot, the aircraft can carry six passengers 3,300km (1,800nm) under IFR conditions with this fuel load. Its maximum cruise speed is likely to be around 410 KTAS (755km/h) at FL 330. The operational ceiling is 41,000ft (12,497m).

Luggage can be carried either in the cabin or externally. There is a 0.06sqm compartment in the nose and another one 0.09 sqm big in the rear fuselage.

If required, the passenger compartment can be converted to a cargo/pax or a pure cargo configuration in a thrice. The 84cm wide cabin door is large enough for 80cm x 120cm europallets to pass through. The robust landing gear, which has been designed for towing, opens up the possibility of even using the jet from unpaved airstrips. This capability puts the SPn in terrain which up to now, at least in the civilian sector, has been the sole preserve of turboprop aircraft.

As the mock-up shows, the over five metre long cabin is extremely spacious. The available height is 1.64m. There is no wing spar running through the cabin. The cabin width is 1.52m. Eight seats in a Club configuration offer passengers a spacious, stylish atmosphere. After sitting down, even a person two metres tall does not have to duck. There is plenty of space between one's head and the ceiling. Four folding tables are provided so that passengers can work or take their lunch. 14 cabin windows ensure that they do not lose contact with the outside.

A powerful pressurisation system ensures that passengers enjoy a pleasantly low cabin altitude. At FL 300 (9,300m) the cabin altitude is just under 1,400m, and at FL 410 (12,710m) it is still under 2,500m.

Altogether, the first SPn leaves an excellent impression. But then one would not expect anything less than that from Grob.

From page 36 of FLUG REVUE 11/2005

Last updated 12 October 2005

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