FLIGHT TEST: Grob Aircraft G120TP - Pocket rocket

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Problems with the development of the Grob180 SPn business jet led Grob Aerospace to declare insolvency in late 2008. But early 2009 brought the emergence of Grob Aircraft, a new private company that inherited Grob Aerospace's trainer aircraft and support businesses.

Owned by five investors, Grob Aircraft operates under the corporate umbrella of H3 Aerospace. It is based at the Grob Aerospace production facility and airfield at Tussenhausen-Mattsies, around 65km (40 miles) west of Munich in Germany.

Since 1971, more than 3,500 Grob aircraft and gliders have been delivered. These include 420 G115 and G120 trainers. The G120TP is a development of the piston-powered G120A and retains the latter's massive bubble canopy, roomy side-by-side seating configuration, retractable gear and carbonfibre fuselage, wings and tail. However, it incorporates a new nose section to house the Rolls-Royce turboprop engine and a completely new digital cockpit.

grob g120tp, grob aircraft
 © Grob Aircraft

All-carbonfibre construction endows the aircraft with light weight, exceptional strength (>26g crashworthiness), zero risk of corrosion, exceptionally smooth aerodynamic surfaces, easy repair and a quoted structural service life of 15,000 flying hours.

Maximum take-off weight is 1,590kg (3,500lb) as a utility limit and 1,550kg as a full aerobatic limit. Basic empty weight is 1,095kg. Full fuel capacity is 290kg, which, combined with an assumed combined instructor-plus-student weight of 200kg, means that the G120TP should not be weight restricted when readied at the start of an instructional day.

Aerobatic limits are +6g/-4g, while the maximum operating altitude is 25,000ft (7,620m) (crew oxygen/unpressurised) with an initial quoted rate of climb of 2,780ft/min (14.1m/s) at MTOW, mean sea level and ISA. The aircraft will be cleared for visual flight rules and instrument flight rules operation, day or night, in non-icing conditions.

The new G120TP engine is a Rolls-Royce 250-B17F rated at 380shp (285kW) at maximum continuous power or 456shp with a 5min limit on maximum power. The 250-B17F is a derivative of an engine widely used on helicopters. It is powered by jet fuel, eliminating the complexity and expense of using avgas.

The engine drives a short-span, five-bladed MT propeller capable of reverse thrust, with the propeller in beta range. VMO has been boosted to a calibrated air speed of 245kt (455km/h) in the 0-10,800ft altitude range, while MMO is Mach 0.45 above 10,800ft. Maximum range at 10,000ft is 735nm (1,360km). The take-off distance to 50ft is just less than 400m and the landing distance from 50ft just less than 500m. These are outstanding performance figures for this class of aircraft.

Additionally, Grob is upgrading and digitising the G120TP cockpit. The revamped flightdeck will include an instrument console featuring three Elbit 8 x 6in (205 x 150mm) electronic flight information system (EFIS) multifunctional displays, a digital combined standby flight display and a dedicated digital engine parameter display, giving the G120TP a fully glass cockpit.

grob gp120tp, peter collins
 © Peter Collins

The left and centre EFIS displays will be multifunctional and programmable, and therefore able to display air-to-air radar symbology, threat radar warnings, ground moving maps, weapons stores management information, aircraft systems synoptic pages, electronic checklists, and instructor set-up and training event debriefing pages. The right-hand EFIS screen will always act as the pilot flying display (PFD), cockpit-configurable to analogue or tape data side format, and will feature separate digital architecture from the mission-configurable displays to ensure protection of its flight data.

Embedded simulation within the aircraft will supply virtual air-to-air radar targets to the EFIS screen selected as the radar display, or virtual ground-to-air radar threats to the EFIS screen selected as the radar warning receiver display, and project threat information on to a ground moving map acting as a tactical situation display.

The cockpit will also deploy a computerised mission planning and mission debrief system that incorporates on-board "flash card" reader/recorder and off-board laptops. The simplified fighter-type hands-on throttle and stick will feature nine multifunction switches on the stick top and six on the throttle. A throttle on the left side of each seat will be operable in single-lever, jet-like fashion.

HELMET DISPLAYS

The cockpit's helmet-mounted display and synthetic head-up display will project flight and simplified navigation/target information on to the visor of an Elbit Systems Targo trainer helmet. Dual Martin-Baker 15B lightweight seats - the lightest seat that the UK manufacturer has ever produced - will offer zero/60kt escape capability with the canopy shattered by a seat-top spike. Optional extras will include a traffic collision avoidance system, terrain warning, weather detection and cockpit voice recorder.

The quoted performance envelope and the various cockpit upgrades will grant the G120TP, when certificated, a level of tactical sophistication and training potential that is little short of revolutionary in this type of elementary/basic training aircraft.

Grob plans to gain the initial European Aviation Safety Agency/US FAR 23 certification of the G120TP at the end of 2011 and target certification of the three-screen EFIS in 2012. The projected purchase price (dependent upon options) is around €2 million ($2.7 million).

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.

grob gp120tp, peter collins
 © Peter Collins

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.

grob g120tp, grob aircaft
 © Grob Aircraft

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.

grob g120tp general arrangement
 

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.

On the second sortie, with oxygen masks fitted, the aircraft was climbed to 25,000ft in 14min. A 60e_SDgr bank turn at M0.30 showed no sign of wing buffet. A descent at M0.45 (MMO), idle TQ, 15° nose down, generated a descent rate of 5,000ft/min.

At MMO, aileron forces were high, but the aircraft was perfectly controllable. The second sortie was concluded with a practice forced landing, with a high key of around 2,500ft above the ground, a low key of 1,250ft and an initial glide of 100kt, with the final landing point being simple to target once full flap was selected.

Both sorties were exactly 1h each in duration and each sortie used just 81.7kg of jet fuel. This fuel use indicates that a fully fuelled G120TP could support three 1h instructional sorties - with proper reserves for the final sortie - from just one full internal tank of jet fuel (289kg). This is unbelievable economy, even for a turboprop.

In my opinion, the G120TP will render all of its current single-engined propeller competitors in the elementary/basic category virtually obsolete, at a stroke, when certificated. The exquisite matching of the R-R 250-B17F turboprop to its five-bladed MT propeller and to the G120 airframe is, in itself, an undeniable success and one that is clearly evident to me even well before certification.

However, much more than this, I believe the G120TP represents a "systems of systems" training concept that will revolutionise future military training standards beginning from a student pilot's first day. The G120TP concept offers a level of "embedded frontline simulation" training capability that many frontline air forces currently do not possess, even at the present advanced/tactical stages. The advantages of introducing initial student pilots to a 21st century configured trainer, when they are selected to fly 21st century front line types in a 21st century air force, are, to my mind, immense.

The first challenge for any air force will be where to set the training boundaries for this aircraft, such is the incredible training potential that its systems will incorporate. The second will be to reassess the role of the flight instructor at this first stage of training given the vast amount of frontline weapon systems simulation and tactical awareness that can be introduced to a student once basic flight techniques have been mastered at the elementary level.

I feel that the G120TP is a natural complement to the higher-performance - but more expensive - Pilatus PC-21/Aermacchi M-311 flight, attack and systems trainer types and the Aermacchi M-346/KAI T-50/Hawk 128 lead-in fighter trainer types.

The G120TP comes with such a low purchase price and minimal direct operating costs, but with such high performance and massive training potential for its size, that the complete package seems to me to be amazing value for money within present, cash-constrained defence budgets. Grob still has a busy period as it works towards certification, but it has placed this aircraft at exactly the right spot within the future trainer aircraft marketplace.

My predication is that, in the next 12-24 months, many military test pilots and military instructor pilots will follow in my tracks along the small country road leading to Tussenhausen and evaluate this aircraft as it nears certification and then enters production.