Peter Henley/NORTH WEALD
A MERE GLANCE at the Grob 200 reveals its designer's novel approach to his task. The airframe is constructed of composite materials, its engine is mounted behind the cabin (driving a three-bladed pusher propeller which lives on the end of a long tailcone), directional stability is afforded by a conventional dorsal fin (surmounted by a T-tailplane) and a large ventral fin (the secondary purpose of which is to protect the propeller from being ground into the runway on take-off or landing). The wing is of relatively high aspect ratio, has a cranked leading edge and is mounted sufficiently far aft on the fuselage to need blending into the diminishing fuselage cross-section by sweeping wing-root fillets, and the wing tips terminate in pert little up-sweeps.
The objectives behind this radical design concept for a business and sporting aircraft are obvious. The well-aired advantages of composite materials (lightness, strength, fatigue life, corrosion elimination and relatively easy damage repair) are obvious. The aerofoil planform, section and tip treatment of the laminar-flow wing are derived from Grob's extensive experience with gliders and are aimed at achieving an efficient, clean wing for a relatively high-altitude, fast, cruise performance. (Relatively high and fast, that is, for a piston engine: Grob is considering a family of aeroplanes derived from the GF200, which could include a turboprop - here the wing design would presumably show maximum advantage.) The rear-engine/pusher-propeller configuration is aimed at drag reduction by eliminating the disruptive effects of propeller slipstream over the airframe and the bulk of an engine nacelle inherent in a conventional layout and cabin-noise reduction.
LONGER LEGS
The disadvantages arising from these concepts are the loss of the shorter, lighter, undercarriage legs generally found on low-wing designs (not compatible with propeller ground-clearance in the case of the GF200); the loss of the advantages of propeller slipstream over the tail surfaces; the undoubted complications to engine cooling (particularly on the ground); and, arguably, more difficult access to the power plant for servicing and engine changes. Last, but crucially, the propeller cannot be seen by the pilot, but is fully exposed to any unwary soul abroad on a general-aviation ramp.
It has to be stressed, however, that Grob's GF200 prototype (D-EFKH) is very much a proof-of-concept and development aeroplane. I was able to fly it briefly (with Grob's chief test pilot, Uli Schell) while it was at the Flight International-sponsored North Weald '95 general-aviation exhibition. On this uncertificated prototype, I was obliged to fly the aeroplane from the right-hand seat, where I was deprived of foot brakes, pitch-trim switches and the speed brakes switch. None of these inconveniences proved insurmountable, and I was therefore able to form clear first impressions of the aircraft.
CLEAN SHAPE
From the ground, and accepting the novelty of its design, the general excellence of the build quality is obvious (despite its status as a development prototype). The clean shape and smooth surface finish, and details such as the fit of its split Fowler flaps when retracted are impressive. Its undercarriage legs are long and heron-like, but Schell says that extensive trials have shown that about 50mm could be lopped off them on subsequent aeroplanes without detriment to the welfare of the propeller. A development pilot tube is firmly planted in the elegant nose of this prototype.
The main wheels are fitted with hydraulically operated steel disc brakes, and the nose leg has a landing/taxi light. A small hydraulic jack translates commands from the rudder pedals to the nose-wheel. The lofty T-tail incorporates a rudder, which has been disconnected because the rudder in the ventral fin has been found in trials to be sufficiently effective. The tail-plane uses variable incidence for pitch trim, with conventional elevators for longitudinal control.
After much experimentation, the definitive three-bladed propeller is now fitted, and does its work sufficiently far behind the vertical tail surfaces to avoid interference. It is driven through a clutch device and composite propeller shaft from the engine. The engine and drive are, however, very much provisional: the current 200kW (270hp) Textron Lycoming TIO-540 is to be replaced by a liquid-cooled 230kW Teledyne Continental TSIOL-550 in the planned production aeroplane, while the clutch would be deleted and the propeller shaft beefed up. Overall, the appearance of the GF200 is more of a machine with which Grob is set to redefine the frontiers of general-aviation technology, than of a vehicle of timeless elegance.
INSIDE THE AIRCRAFT
Climbing aboard is easy enough, despite the high door-sills - a production aeroplane would have steps in the lower half of the split cabin door. Meanwhile, portable alloy steps help old test pilots to scramble into the cabin in the passenger area behind the pilots' seats.
The cabin is now comfortably trimmed with a passenger seat against the rear bulkhead. Production aircraft could have seating for three, in addition to the two pilots' seats and, as the 200 is to be certificated for single-pilot operation, its eventual capacity could be a pilot, plus four passengers. Access to the pilots' seats via the gap between them is uncomplicated: a standard flight bag can subsequently be placed in the gap. Stowage for charts is provided outboard of each seat. Schell and I were required by German regulations covering uncertificated aircraft to wear parachutes (the excellent, unencumbering GQ350) - although the chances of escaping from the cabin in a potentially terminal situation looked remote to me.
The current instruments are peculiar to this prototype, and production aircraft would have "glass cockpit" instrumentation. Suffice to say that the present fit is adequate for development work. Conventionally styled control yokes emerge from beneath the instrument panel on fore-and-aft horizontal shafts: the left-hand control has a press-to-transmit button, speed-brake switch and pitch trimmer on top of the left horn; the right-hand control has no speed-brake or pitch-trim switches. Each pilot's seat has fore/aft and up/down adjustment, an inflatable lumber pad and a foldaway armrest. Production aeroplanes would have rudder pedals adjustable for reach.
The field of view is good, allowing all-round views of both wing leading edges and tips, and in an arc forwards, sideways and downwards, but the upward view is abruptly curtailed, making look-out less than reassuring in even moderately banked turns. As already mentioned, the propeller cannot be seen from the cabin, begging questions about safety during engine start, particularly at night with single-pilot operation.
Engine starting is straightforward, as is taxiing using power, rudder-pedal-controlled nose-wheel steering and toe-operated brakes. The aeroplane is easily manoeuvred on the ground. The throttle, pitch and mixture (fuel- flow) controls are mounted on the centre console, and come naturally to hand for either pilot. The primary control surfaces are mechanically operated, with little static friction in the systems. The control yokes can be moved through their full range without coming into conflict with adjacent bits of aeroplane or the limbs and bellies of average-stature pilots. Full aileron deflection in either direction requires a control-wheel rotation of about 60¡ from the horizontal. The wing flaps are electrically driven and selected (via a conventional switch next to the flap gauge).
The undercarriage is hydraulically operated by an electric pump, and has a "three greens" and "red unlocked" indicator next to the selector. Both selectors and their respective indicators are mounted above the centre console, where they can easily be reached and read by either pilot.
Engine and propeller constant-speed-unit checks before take-off are normal piston- engine items. Full power is set, and compatible fuel-flow scheduled via the "mixture" control before the brakes are released, after which the aeroplane, with take-off flap set, accelerates well and is easy to keep straight using a little rudder to counter the slight crosswind. The GF200 unsticks quickly after rotation at 65kt (120km/h) indicated airspeed (IAS). Retraction of the undercarriage and flaps during the climb-out provokes little pitch-change.
CABIN-NOISE LEVEL
Subjectively, I rated the cabin noise-level during take-off as being probably quieter than that of a comparable front-engined aircraft - but not amazingly so. Although unpleasant vibration from the current propeller shaft is clearly discernible on the ground (but, says Grob, will be cured on production versions), I thought that the general vibration levels apparent in flight compared favourably with those of many front-engine/propeller aeroplanes.
After the climb (at about 110kt IAS) to required cruising altitude, the GF200 readily accelerates and settles into an unfussy and sprightly cruise at about 160kt IAS at 3,000ft (900m). With the current power unit, however, engine throttle, propeller RPM and fuel flow all have to be set, using the relevant controls, to parameters related to pressure altitude by reference to a simple graph. For a production aeroplane, the airframe's innovative concept will have to be complemented by simpler, easier and more sophisticated engine and propeller control - the ideal would be automatic control via a single lever, as in the average turbine aircraft.
The GF200 has no rudder trim and, from my brief acquaintance with it, the decision to do without it is justified. There is, of course, no propeller-slipstream effect, but there is a discernible propeller-torque effect requiring a little right rudder to maintain a balanced climbing turn to the right, for example. Otherwise, in the climb, cruise and descent, there is not much directional trim change, and that can comfortably be compensated for with slight rudder application requiring light forces.
Large power and speed changes produce longitudinal trim changes which are readily apparent, but easily contained until trimmed out. The electric pitch-trim control I found too low-geared for my liking (ie, I would have preferred quicker responses for small demands), but most such pitch-trim controls do take some getting used to, and my competence with the Grob was impaired by having to stretch across the long-suffering Schell to reach his switch, because I did not have one of my own.
Extending the electrically controlled and operated speed brakes (which extend vertically above the top surface of the wing) produces a fairly marked pitch-down, and there is a corresponding pitch-up on retraction. Pilots who regularly fly the 200, would learn to compensate subconsciously, for these speed brake trim changes, but lessening the effect at source would obviously be preferable.
The primary flying controls are well harmonised, and the aeroplane has an endearing rate of roll for its class - indeed, it seems to enjoy upward barrel rolls. The GF200 appears (from a snapshot look at a clean, mid-centre-of-gravity 150kt IAS, constant power configuration) to be statically stable longitudinally, directionally and laterally. The stall both clean and in the landing configuration with power off, was conventional with good aerodynamic stall warning from strong pre-stall burble and a clean nose-drop to define the stall. Aileron and elevator control remained effective up to the stall, and any tendency for a wing to drop was easily recoverable with rudder. The T-tail contributed no obvious characteristics to the stall, and Schell assured me that there have been no "deep-stall" recovery problems at any time during the development programme.
THE APPROACH
The return to a landing at North Weald presented no dramas. The undercarriage can be lowered at 140kt IAS or below, with no significant pitch change. Selecting full flap, not surprisingly, generates considerable drag, and a fairly late selection of landing flap on finals is therefore wise. The aeroplane flares pleasantly, leading to smooth landings enhanced by an apparently compliant undercarriage. Main-wheel braking brings the aircraft smoothly and reassuringly to taxiing speeds.
The GF200 has no airframe or propeller ice protection, but these omissions will be rectified on future examples. Similarly, the electrically switched and driven engine-cooling fans on D-EFKH have to be selected manually when the aircraft is on the ground, but production aeroplanes would have considerably modified cooling arrangements to be compatible with the new engine and to meet both in-flight and on-ground conditions.
The GF200 project is an intriguing one, and it has produced altogether a neat aircraft. Its evolution into a production aircraft will be fascinating to monitor, but only the final, definitive performance figures, however, will show whether there are enough gains over a conventional configuration to justify such an unconventional design concept.
Source: Flight International
