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Flight International's G120TP evaluation was conducted with D-ETPG, a company proto­type fully representative of the production aircraft/engine combination that Grob will aim to certificate in 2011, but with a development G120A hybrid test cockpit that had conventional analogue instruments on the left-hand side, a single Elbit EFIS screen acting as the PFD (set to analogue format) on the right-hand side and ancillary engine/navigation/communications displays in the centre.
The cockpit was fitted only with individual Martin-Baker 15B ejection seat "outer shells", so backpack type parachutes were worn for escape. The canopy featured a single lever manual jettison handle.
Since most of the planned G120TP embedded simulation was not available for me to evaluate in this prototype, my role was to be as an elementary/basic student graded for fighters and flying his/her final handling test at the end of the elementary flight training phase. My objective was to answer a simple question: could the aircraft challenge a young student while at the same time delivering that challenge in a safe and docile manner?
My safety pilot was Grob test and demonstrator pilot Uli Schell, who would sit in the left-hand seat and handle the radio/navigation. I would fly two complete sorties from the right-hand seat (since the prototype did not have a second left-hand throttle), the first sortie at medium and low levels and the second at high level. Both sorties would be conducted from Tussenhausen airfield (elevation 1,857ft) with CAVOK conditions, wind 240/5-10kt, QNH 1017, outside air temperature -6°C (21°F) and a snow-cleared Runway 15/33 (1,149 x 20m) - but with all the tarmac surfaces completely ice covered.
On strap-in, the cockpit immediately felt spacious with the canopy offering an almost 360° field of view including rearwards over the horizontal stabiliser and fin. The G120TP cockpit instrument console glareshield line will be cut back from that of the G120 to further improve the forwards and downwards field of view even though the short, sharply tapering nose of the G120TP prototype offered little obscuration in this sector on the ground on the seated side.
With any side-by-side seating, Grob and Martin-Baker will need to work closely together to ensure that seat arm/disarm mechanisms of the 15B ejection seats are unambiguous and that individual seat straps and umbilical cables to each pilot - linking to oxygen, mic-tel, helmet-mounted display, the dinghy and so on - are as simple, neat and as robustly connected via the seat as possible.
Engine start was effected using an external battery supply, although the normal method will be the aircraft internal battery. With the internal battery now in the nose for balance, Grob is considering a repositioning of the external battery connection point to the wing root. With 15% N1, the condition lever was placed into the idle gate with the engine stable after 30s. Thereafter all power was controlled by throttle primarily using percentage torque (TQ), with approximately 90% TQ equating to maximum continuous power.
SMOOTH RESPONSE
With the PFD erect, take-off flap selected and the simple checklist complete, we were ready to taxi gingerly to the runway. My first distinct impression as I left the chocks was that, as I increased engine power, the engine/propeller response was akin to that of a sewing machine: beautifully smooth, instantly available and precise to control.
Taxi speed could be governed exactly on the icy surface, without wheel brakes, by graduating the propeller in and out of the beta range. The throttle-mounted, finger-operated trigger protecting the beta range was easy to manipulate.
I elected for a rolling take off from Runway 15 to prevent any slip developing on the icy runway surface due to the light crosswind. Approximately 95% TQ gave a take-off roll of around 400m for a rotate at an indicated airspeed of 75kt. Power response on acceleration was jet-like and I noticed virtually no ground swing.
Gear (140kt limit) and take-off flap (160kt limit) were raised immediately with just a hint of nose-up trim change as the flaps came fully up. With 120kt achieved by the end of the short runway, I banked the aircraft over sharply in a 70-80° wingover on to the reciprocal heading while maintaining 120kt and climbing at around 3,000ft/min.
My second distinct impression during the wingover was that this aircraft immediately felt like a mini-fighter. I had to remind myself that this was a prop and not a jet because it was so powerful, and yet the power was so linear in its delivery.
In the climb to flight level 80 the aircraft showed that it was highly responsive but very well-damped to pitch (elevator) inputs at all airspeeds, and the sustained maximum roll rate with full aileron at 160kt was 75-80°/s.
When at FL80, I conducted two constant-speed wind-up turns, one at 160kt and one at 200kt. With 90% TQ set - at 10° nose down and 160kt - the aircraft was pulled progressively up to +5g showing that the back stick displacement/g gradient was linear and that the manoeuvre boundary at this g level was indicated by stick force and very light wing buffet, but with no wing rock.
At 200kt and 15° nose down, passing 5g, the manoeuvre boundary was not the aircraft itself but my own body needing a g suit.
The next test point showed clean stall indications starting at 82kt, with light buffet acting as a natural stall warning but augmented by an unmistakable stall warning audio horn and a red warning light in the cockpit. The actual clean stall at 72kt showed no sign of wing drop.
With full flap, the stall occurred at around 60kt, with a rate of descent of 1,000ft/min and some wing drop that was still controllable within the fully developed stall, but with the same unmistakable audio and visual cockpit stall warning.
Several three-turn spins were then conducted from FL90 using the standard spin entry of pull back stick, full rudder and full opposite aileron, starting from 80kt. The spin was mildly oscillatory in pitch during the first turn, and yaw rate was high throughout, especially during the second full turn. But by the third turn the spin was stable and the spin characteristics completely acceptable. Standard spin recovery (full opposite rudder, aileron central and stick forward to neural) showed the aircraft taking one turn to recovery.
With recovery from the dive to level flight, a complete three-turn spin event used just 2,500ft of altitude. A further recovery with full anti-spin rudder but stick released showed the spin taking just a little longer to stop, but doing so in almost exactly the same manner and with only a minimally increased height loss.
My recommendation would be to fit the cockpits with a large, fluid-type slip ball so that students and instructors can instantly identify the spin turn direction. The split yaw/bank indicator "pyramid" in the PFD was not designed for this sort of manoeuvring and is not easy to find or interpret if a pilot gets disorientated.
A quick aerobatics sequence showed what my earlier tests indicated: that the aircraft was a delight to fly and with no vices that could catch out an aspiring student pilot. I marvelled at the power and smoothness of the engine/propeller combination. If height was needed to be regained at medium level for a training event, with power applied the aircraft simply zoomed effortlessly back to altitude like a jet.
WEAPONS ATTACK
Next I descended to low level at an altitude of 500ft above the ground for a short low-level navex and simulated "pop-up" weapons attack. A 90% TQ gave a cruise indicated airspeed of 210kt with ease, a large power margin in hand and a fuel flow rate of just 1.82kg/min. Low-level ride was a little bumpy but still perfectly acceptable for the navex or IP-Target map reading. Forward field of view downwards through the front canopy on the seated side was surprisingly good and I could track objects on the ground down to around 250-300m in front of the aircraft without obscuration.
Aileron forces had increased but not markedly so, and they did not detract from manoeuvring the aircraft hard to simulate defensive missile breaks, pop-up/roll-in attacks or tactical low-level turns. The low-level performance of this aircraft was nothing short of amazing.
Returning to the airfield to run and break, several different tight visual circuits were flown to both overshoot and to roll. Typical downwind was at 120kt with take-off flap and gear selected, turning finals at 100kt with full flap (110kt limit) selected for an 80kt final approach. The large bubble canopy makes "cross cockpit" circuit flying easy. The aircraft exhibits excellent speed stability on finals so that maintaining flightpath approach angle/airspeed - very accurately - seemed almost effortless.
Going deliberately low on approach could be instantly corrected with a direct 1-2s burst of power, without other attendant problems of yaw or power lag or TQ overswing. Grob is considering fitting the throttle with a mechanical detent to separate the maximum continuous power (MCP) range from the maximum power range, as up to 90% TQ (approximately MCP) is completely adequate power for circuit work, roller landings and low overshoots.
On the final landing, the aircraft was slowed to a walking pace on the icy runway using just propeller reverse thrust, and the stopping action was predictable and easy to control.