By Guy Norris at the Farnborough air show
Boeing is studying a 777-style electronic tailstrike protection system for the 747-8, which would enable steeper touchdown and take-off angles and help reduce noise during critical approach and departure phases. The electronic tailskid trade study forms a key element of Boeing's attempts to close the gap on QC1 noise limits at London, and the outcome is largely dependent on windtunnel tests being performed at Qinetiq's facility at Farnborough in the UK.
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| Windtunnel tests at Farnborough are a vital element in the tailskid study |
The tests are to assess low-speed aerodynamic characteristics to help finalise aircraft lines, design loads and control laws. Tailstrike protection system technology, developed for the 777, is embedded in the flight-control computer. It uses aircraft performance data and sensor input to fix the location of the aft fuselage relative to the runway, as well as the closure rate at which the fuselage approaches the runway during take-off and landing.
If the system senses an impending tailstrike, it automatically commands the elevators to avoid it. The idea would be to approach the runway with a higher fuselage attitude, slowing the approach without the use of maximum flap and requiring a steeper flare angle. The reduction in flap deployment, already cut to a simpler double/single slotted configuration on the -8 versus a triple-slotted design on the -400, would be one way of further reducing noise. The higher take-off angle similarly reduces noise by reducing the "footprint" around the departure path, adds Boeing.
Roy Eggink, 747-8 product development chief engineer, says: "It could be helpful for both take-off and landing noise performance." The overall approach would remain unchanged at the standard 3° glideslope angle, but "are you going to do that with flap or by deflecting the entire aircraft?" asks Eggink. Although the 747-8 will have fly-by-wire controlled spoilers and drooped ailerons, the current plan calls for the elevator to be conventionally controlled.
"However, we can still put in a device that activates a part of the elevator," he says, adding that the windtunnel work will help show if "we are stall limited or attitude limited. If we are attitude limited then the electronic tailskid could pay off." The -8 is quieter than its predecessors, with a 30% lower noise footprint than the -400 (based on 85dBA noise at take-off with cutback at maximum take-off weight), and Boeing is working on more reductions. It guarantees QC2 departures for passenger and freight models, but is "in the middle" of the 3dB gap between QC1 and 2.
Source: Flight International
