The Grob G109 is a light aircraft developed by Grob Aircraft AG of Mindelheim Mattsies in Germany. It first flew (G109 prototype and then production G109A form) in 1980. The G109B followed in 1984. It is a two-seat self-launching motor glider in which the pilot and passenger or student sit side by side, with good visibility provided by large windows.
As well as normal civilian use this aircraft was also used in the Royal Air Force Volunteer Gliding Squadrons (VGS) to train air cadets through the gliding induction and gliding scholarship courses up until 5 May 2018, when it was retired from service. The Grob 109B was known in RAF service as the Vigilant T1. The G 109 was the first motor glider built using composite construction to be granted Federal Aviation Administration approval.
Design and development
Technical description (G 109B / Vigilant T1) The G109B was developed after series production of the G109A which itself followed a number of early prototypes with differing wing spans, configurations etc. The G109B addressed several issues learnt from operation of the earlier G109A which included improvements to power (95Hp up from 80Hp on the G109A), performance of the wings when wet, crosswind behaviour and limits and cockpit space.
The G109B aircraft is a low-wing cantilever motor glider, with a T-tail, folding (and detachable) wings, and side-by-side seating with dual controls. It is mainly constructed from glass-reinforced plastic and has a taildragger undercarriage arrangement. Entry and exit from the cockpit is via two perspex doors which open upwards individually – a modification from the original one-piece G109A canopy. The cockpit can be heated, providing that the engine is running, and the seat backs can be adjusted and cushions of differing thickness inserted to accommodate a range of body sizes.
Total weight is around with a maximum load of though this depends on mod state and history of the airframe and is rarely above 200 kg on most examples. The RAF sought and obtained a number of military specific modifications; including a dispensation from Grob for operating the military variant aircraft to a Max AUW of 908 kg providing an extra 59 kg of useful load. The aircraft was not modified to allow this and no changes to the operating procedures were required as the centre of gravity constraints were not exceeded. Grob have applied to EASA for permission to have an exception to the design standard (CS22) for the Grob 109B which would allow it to be certified to a similar 903 kg Max AUW however while the two objections received were rejected by EASA the authority has not yet taken the necessary rulemaking action to allow this. Cruising speed is in the region of 60–100 knots (110–190 km/h) on the engine, which can give the aircraft a top speed of . The engine (based on a Porsche air-cooled car unit) can be shut down in flight with its propeller blades feathered. The aircraft then becomes a pure glider, with a best glide ratio of around 1:28. The Porsche-derived horizontally opposed air-cooled engine had the cylinder bore increase by Grob to make the engine 2.5-litre capacity and has had its Time Before Overhaul (TBO) increased by a technote to 1,600 hours after many examples run on condition reached up to or in excess of 4,000 hours in service.
Under the current CAA LASORS document, the G109B can be classed as either a touring motor glider (TMG), or a self-launching motor glider (SLMG).
Engine
The G 109B is powered by a Grob 2500 E1 horizontally opposed, four-cylinder, air-cooled petrol engine that develops approximately at 2,950 rpm. The propeller is a two-bladed, manually operated variable-pitch type driven directly from the engine. Three pitch settings can be used: Fine for take-off and general flying, Coarse for cruising, and Feathered for gliding with the engine off.
Carburettor heat can be supplied to the twin carburettors when there is a risk of icing. An electric fuel pump is used.
Controls
The G 109 uses conventional controls, duplicated for both seats, including the throttle (not available on the left in standard production G 109s). The rudder pedals, which also operate the wheel brakes, are adjustable forward and backward to suit individuals of differing leg length, and airbrakes are used to increase the rate of descent during the landing approach as well as limiting airspeed to Vne (velocity never exceed), if required.
Operational history (Vigilant T1)
