Boeing is considering the use of a third, tail-mounted engine in the 777-200X/300X that would also double up as an auxiliary power unit. The virtually unprecedented use of a thrusting APU is one of a wide range of changes being studied to improve the field performance of the aircraft as the Seattle-based airframe builder attempts to find a launch customer for the aircraft after being beaten to several key orders by the latest versions of the Airbus A340.

The study of what is dubbed an "auxiliary power and thrust unit" (APTU) is considering dual-role powerplants covering the 31-66.7kN (7,000-15,000lb) thrust range, including the Allison AE3007, BMW Rolls-Royce BR710 and General Electric CF34-8. GE has also proposed a civil version of the F414 fighter engine, but Boeing says noise considerations will make it "more likely to go with an engine of a higher bypass ratio".

The added thrust of the tail engine would augment the main engines for take-off as well as for initial climb, but would otherwise "provide the capabilities of today's APU" says 777 preliminary design chief engineer Mike Burtle.

Tail-mounted actuated inlets would "open and close like a hinged door" and be closed flush with the skin during the cruise. "It seems a little less conventional at first but in reality, when traded against more thrust under wing, it could trade favourably well," according to Burtle.

John Monroe, the 777X derivatives deputy programme manager adds :"We also have to look at it from an airline's point of view from an operability standpoint. It will use more fuel and we just have to balance all those things out. It could have higher operating costs, more weight and mean more drag. It's a humungous business decision" he adds. Boeing plans to hold initial discussions on the APTU concept with airworthiness regulators in the next month.

To increase take-off thrust Boeing is working with GE, Pratt & Whitney and Rolls-Royce to improve engine flat rating temperatures so that higher thrust levels will be available at a wider range of ambient temperatures. It also continues to "trade against the potential for more thrust on the main engines" says Burtle.

Outside the possible powerplant changes, the -200X/300Xstudies are concentrated on three areas: lift/drag improvements for better take-off, improved stopping and added thrust.

For enhanced take-off, Boeing is studying gapped outer ailerons which would improve outboard span loading and reduce induced drag. The aileron would be slightly drooped with a gap along its leading edge to help produce lift at low speeds. Boeing is also looking at rescheduling the inboard double slotted Fowler flaps to reduce the take-off angle of attack slightly. The change, in concert with the aileron changes, would produce a more even span loading, thereby reducing drag, says Burtle.

Boeing is also studying changing the inboard leading edge slat, from the engine to the fuselage, to a Krueger flap such as those used on the 737 and 747. The change could lead to an improvement in the parasitic drag of the leading edge, says the company.

To improve aerodynamic braking, it is also looking at changing the sequencing of surface movements to "get more drag, more quickly" for rejected take-offs. This would include deflecting the outboard ailerons and flaperons up sooner. "It's essentially tuning of the fly-by-wire system" adds Burtle.

Thanks in part to the gapped outboard aileron study, Boeing is also revisiting the use of a larger raked tip for the wing, such as that designed for the 767-400. "We ruled out such a device last year," says Burtle, who adds that a hybrid constant-chord extension and raked tip was adopted as a result. The improved wing tip aerodynamic flow produced by the revised aileron design means that a larger raked tip is now a potential option though span will still be restricted to a maximum of 65m (213ft).

Boeing says it is also "going back and looking at winglets and comparing them to extensions" following the recent in-flight experience gained by the 737-800 test team working on a possible winglet configuration for the Boeing Business Jet (BBJ). "They have really picked up good knowledge with the BBJ" says Monroe.

Another potential improvement "still on the list" is the semi-levered, or articulated, gear for the longer -300X. This would make the aircraft stand higher at rotation, thereby enabling the take-off speed and runway length to be reduced. "It has the potential to increase the maximum take-off weight [MTOW] by 10,000-15,000lb [4,540-6,800kg] out of a given field length," says Burtle.

The 777-200X as now configured has an MTOW of 333,700kg and 324,600kg for the -300X, although the gear change could raise this to "almost the same level" says Boeing.

To improve braking, Boeing is studying an increase in the torque of the existing main gear brakes. "We could increase the area of the pistons or increase the heat in the [brake] stack itself," says Burtle.

"These are performance improvement studies that we believe will make the 777X a better aircraft. It is our plan to begin going through the individual business cases on these studies, though we are not committed to any of them," cautions Monroe.

Most of the improvements are aimed at the -200X and -300X, whilst some are also being considered for the baseline -200ER. "We think the -200X has the right range, but we think it needs better take-off performance" says Burtle.

Meanwhile, Boeing has briefed several potential -200X/-300X users on the study package, including Air France, American Airlines, British Airways and Emirates. "We're also getting ready to talk to Malaysia Airlines," says Monroe who adds: "We're really at a point in the programme where we will soon be signing up definitive agreements with customers." The studies will be completed by the end of the year to meet the first quarter 2000 firm configuration target date.

Source: Flight International