IPTN's N250

IPTN's N250 will be a winner, if performance figures match the aircraft's characteristics

IF THERE IS any lingering cynicism, over the destiny of IPTN's N250 programme, a visit to the company's design, manufacturing and flight-testing site at Bandung, Indonesia, would be likely to put it to rest. The site lacks no area of modern technology, and IPTN has accumulated and retained a strong core of talent in all related disciplines, as its achievements to date amply demonstrate.
Science and technology minister Dr Bacharuddin Habibie recalls with satisfaction that, when he announced the 50- to 70-seat-N250 programme in 1987, he set 10 August, 1995, for the first flight, to coincide with the 50th anniversary of Indonesian independence: "At exactly eight minutes past ten on 10 August, 1995, attended by the president, members of cabinet, foreign dignitaries and the press, we made the first flight of the N250 on schedule. To meet a schedule set eight years beforehand is a task completed by few aerospace companies in the world. Normally, companies prefer to conduct the first flight in secret, to be sure that everything is working correctly, but we had announced the flight in advance and, if we are to establish ourselves in the global aerospace market, we need to prove ourselves on the world stage," Habibie says.
FIRST PROTOTYPE
In the subsequent six months, the first (development) prototype has been flown for 230 flight-test hours. The flight envelope has been opened up to the dive speed for flutter clearance in clean configuration. Still outstanding is the flutter clearance with flaps deployed, but IPTN expects no problems. Single-engine performance in all configurations was defined by the end of May. Stall characteristics, including dynamic stalls, have been explored; engine and auxiliary-power-unit (APU) air-start envelopes expanded and to date, there have been no major surprises.
Stall testing has identified the need for an artificial stall warning (stick shaker) and Danuwinata expects that the aircraft will probably also need a stick pusher for stall prevention. The first prototype's performance, stability and control trials will be finished before the Indonesian air show, followed by systems testing, putting the flight-test programme on schedule. Aircraft numbers two, three and four, all -100 70-passenger versions, will be the certification airframes. Number two, to be flown in September, is destined to develop flutter clearance for the full flight envelope, thence for certification of performance and stability/control throughout the low- to mid-speed and high-speed flight envelopes to certification limits. Systems-certification work will start on number three, and number four will follow for certification of the cockpit and interior, and functionality and reliability flying. Three and four will then be configured as series-production aircraft. The entire 4,500h certification programme is scheduled to be completed by the end of 1997.
FLY-BY-WIRE CONTROLS
The world's first regional turboprop airliner to be wholly controlled by a fly-by-wire (FBW) flight-control system, IPTN stresses that the N250 is not a CN235 derivative and has no structural or component commonality with that type.
Compared to the original 50-seat planned prototype, the 70-seat N250-100 incorporates a 1.5m fuselage length-extension plug, a lowered wing with reduced drag, and an extended constant-section fuselage with a circular cross-section in the aft fuselage. The first prototype has an elliptical cross-section, and the change will reduce structural weight and improve producibility. Production of the 50- and 70-passenger variants is likely to proceed concurrently, but IPTN's US production and marketing partner, American Regional Aircraft Industry is already pressing for a 72-seat version at increased seat pitch, which would call for a further 3m plug in addition to the current serial-production model. The N250's unusually large CG range, from 17% forward to 40% aft, is explained by the anticipation of these stretch derivatives.
The twin Allison AE2100C engines, de-rated to a normal take-off power of 2,400kW (3,270shp) equivalent, drive six-bladed Dowty Rotol R384/6-123-F/8 propellers and offer plenty of growth potential. Two dual-channel full-authority digital engine-controls (FADECs) manage engine and propeller operation in all starting, ground and flight modes. The FADEC provides engine and propeller digital-signal data transmission, status information, monitoring and fault indications to the data concentrator unit for pilot display and for maintenance monitoring. A gearbox-mounted accessory drive provides mounting and power for an AC generator and a hydraulic pump on both engines, and for gearbox-oil pump, propeller pitch-control unit (mounting only) and propeller oil-pump and overspeed governor. Engine build-up is identical for left and right installations. Pneumatic starters on each engine are connected to the pilot-activated pneumatic manifold, and the starting sequence is controlled from initialisation to stabilisation by the FADEC.
The engine-indicating system, consists of torquemeter pick-ups, thermo-couples for measured gas temperature, expressed as inter-turbine temperature, propeller speed and engine-monitoring system.
The fully feathering and reversing composite-construction propellers are 12.5ft (3,810mm) in diameter. The advanced aerodynamic blade design is optimised for the high cruising speeds for which the N250 is intended, and IPTN anticipates that the high wing configuration will provide ample protection from propeller and engine foreign-object damage. The blades are counterweighted, to deliver a safe coarse pitch and rotational speed in the event of loss of system oil pressure or electrical supply. Blades are spray-coated with polyurethane and have a nickel leading-edge guard against ice and erosion damage. An electrically driven feather pump and motor unit provides an emergency source for the primary and secondary feathering system, and a propeller over-speed governor begins to restrict oil flow to the propeller control unit if an over-speed condition (of about 105%) occurs.
The Sunstrand APS-1000 APU supplies pneumatic power for the environmental-control system and engine start. Between firewalls in the extreme aft fuselage, it is mounted on the aft pressure bulkhead. APU start is by an autonomous electric starter-motor powered by the aircraft's batteries.
CARBONFIBRE COMPOSITES
Carbonfibre composites are also used for the radome, nose-gear and main landing-gear doors and fairings, wingtip, elevator tip, vertical-stabiliser tip and wing/fuselage fairings; wing fixed-trailing-edge surface panels; dorsal fin; aft rudder; and tail cone.
The main entrance door will be a 1750 x 780mm type 1 plug at the forward left position, and will be provided with a folding airstair. The 1,397 x 610mm type 1 service doors opposite and at the rear, along with a left side type 3 915 x 610mm rear emergency door, will meet all evacuation requirements. Flight-attendant stations will be forward and aft, and the 8.8m3 main (pressurised) baggage compartment will be behind the passenger cabin, accessible through a 1,114 x 1,150mm external door on the left side. The aft baggage door is a structural, non-plug, type with C-latch locking, and is electrically operated from an external access.
Baggage-door sill height is 1,200mm. Cabin and toilet servicing are provided for on the right fuselage, with fuel truck aft of the right wing for single-point fuelling at up to 450litres/min aft of the right main-landing-gear fairing. There is also, provision for over-wing fuelling. Maximum useable fuel capacity is 4.2t, contained in inboard and outboard tanks in each wing.
All primary and secondary flight-control surfaces are controlled by the full-authority FBW system. Hydraulic actuation of the surfaces is electrically signalled by electrical control units (ECUs) from the pilot's dual-channel control column, control wheel and rudder pedals, which are of conventional layout (provided with artificial feel), and from the flap levers. ECUs are scheduled, with flight parameters for airframe-overstress protection from excessive control loads. For back up, ailerons and elevators have separate mechanical signal paths through a single cable to mechanically controlled hydraulic actuators.
The double-hinged rudder has two FBW-controlled hydraulic actuators, with the upper actuator normally active. Both actuators are powered by the centre hydraulic system, which is independent of the left or right engine-driven hydraulic pumps and, therefore, of engine failure. Failure of an FBW channel to any aileron or elevator surface prompts automatic switching to the manual channel for that surface, without discernible change to control feel. Capt Danuwinata says: "Once, in flight testing, we had a reversion of the aileron system from FBW to mechanical backup due to a power interrupt, and we didn't even notice it from the cockpit. 'Mission control' advised us we were in backup mode."
Although the test aircraft was fitted with a special shut-off valve to allow test-flight de-activation of the FBW system, there is no plan to make this available in production aircraft, as it can be adequately practised in a simulator. Several failure cases have already been evaluated in simulation, and predicted control forces are almost identical with the FBW case, the only difference being that speed scheduling and several other FBW system functions are de-activated.
Flaps and spoiler/ground spoilers are also FBW. The four spoiler panels, which can also be armed to act as landing-activated ground spoilers, operate when airborne in aileron-assist mode at large wheel deflections to tailor roll capability with speed. Nosewheel steering is also FBW-activated, without mechanical reversion. The tiller provides +/- 65° steering either side of neutral, and limited authority of +/- 7¡ from neutral is provided through the rudder pedals for take-off and landing.
COCKPIT LAYOUT
In terms of modern cockpit layout, there is nothing unusual about the N250, which closely follows modern convention. IPTN selected a horizontally aligned five-screen configuration for its Rockwell-Collins Proline electronic-flight-instrumentation (EFIS) system, with primary flight displays outboard, adjacent multi-function displays (MFDs) inboard, and engine-indicating and crew-alerting system (EICAS) centrally mounted. Lighting controls are at the outboard extremities of the main panel, with clock and MFD mode switches inboard and adjacent to the MFDs. Standby attitude indicator, altimeter and airspeed indicators are to the right of the captain's MFD, symmetrically opposite landing-gear selector and trim indicators. The main panel slopes 25¡ up from the vertical. On the glare shield are the automatic flight-control system and EFIS control panels, with master warning and caution lights outboard above the MFDs.
A single power-lever module, within comfortable reach of both pilots, is centrally mounted on the centre console, which also contains all avionics controls, and the control panels for the EICAS, ground-spoiler/anti-skid system, internal lighting, flap and elevator trim and aileron/rudder trim, as well as the EFIS source panel, and power-management unit. There is ample space on the centre console for dual flight-management systems.
Considerable thought has been applied to limiting clutter on the compact overhead panel, with all items well forward to obviate reaching back. System design is managed to minimise the need to push buttons, and largely automated, so that lighted captions on the push button will indicate the abnormal condition, an unlit button indicating normality.
Control-column-mounted switches for aileron and elevator trim will be standard on production models, but were absent on the certification aircraft. The conventional trim switches at the aft extent of the centre console were difficult for a first-timer to locate.
The pneumatic system, routed entirely outside the pressure hull, controls and distributes bleed air to the environmental-control system; engine-starting system; aerofoil de-icing system; and cabin-pressurisation control system. IPTN believes that it is well covered against any tightened US Federal Aviation Administration requirement, with 15% boot coverage and a growth capability, if required, because of the aft location of the front spar.
Some autopilot testing has been carried out and, with Rockwell-Collins, IPTN has optimised the gain settings on the autopilot logic. Danuwinata says: "We came up with a satisfactory solution, but this has been done on a special test rack installed in the aeroplane, where we could change the gains according to the test results. Now, the new gains will be implemented directly in the flight-control computer software, and we expect to have the new computer in within a month."
Extended-range twin-engined operations, has not been a consideration in the design, because of the medium- and short-haul nature of the typical mission. The aerodynamic concept of the N250 is based on the concept that the aircraft should be powered by high thrust for high-speed flight, without surrendering short take-off and landing capability. The same requirements dictated the use of the double-hinged rudder for engine-out capability at high thrust and low speed. Span of the double-slotted flaps is 80%. A noticeable external feature is a visibly nose-down thrust line, which locates it close to the vertical centre of gravity, the outcome being relatively moderate elevator-trim changes, with thrust variations.
IPTN has unquestionably produced a "pilots'" aeroplane. The challenge during the certification programme will now to be to marry those excellent characteristics with performance figures, which deliver operating economics of equal quality.

Source: Flight International, 19/06/96