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Aviation History
2004
2004-09 - 0726.PDF
Flight test: Learjet 40 complete the post-start checks, an elec tronic one is available as an option. A slight advance of the throttles started the aircraft rolling out of the chocks. Once moving, idle power was enough to taxi at a comfortable pace on level surfaces. The steer-by-wire electronic nosewheel steering is controlled by the rudder pedals and pro vides ±60Q of deflection. The speed-sensi tive system allowed me to negotiate slow, tight 90Q turns precisely, while not being overly sensitive at higher speeds. Lundy set the flaps to 8Q in preparation for take-off. With a 20kt (37km/h) left quartering headwind and an ambient tem perature of 28^, the take-off speeds for the 8,727kg aircraft were: Vj 115kt indicated, VR 117kt and V2 126kt. Computed bal anced field length for these conditions was 1,481m. Once cleared for take-off, I advanced both throttles to the take-off detent. The DEECs stabilised Nt at the 90.1% take-offsetting. As the aircraft accel erated down the runway, the nosewheel steering allowed me to track centreline accurately. At roughly 117kt, 15kg of aft- stick force was needed to establish the lift off attitude, the aircraft breaking ground after a run of only 940m. Gear and flap retraction caused negligible changes in pitch forces as the aircraft accelerated to the initial climb speed of 250kt. Passing 3,000ft above mean sea level the throttles were retarded to the maximum continuous detent for climb to our initial cruise altitude of FL450 (45,000ft). Even at maximum take-off weight the Learjet 40 can climb directly to FL450, above most tur bulence and almost all other traffic. Passing FL325 the autopilot, engaged for the climb, tracked Mach 0.7 until we levelled off. Despite hotter than standard temperatures and ATC-directed turns, the climb took just 26min from brake release, burning 375kg of fuel. This compares quite favourably with the book values of 24min and 336kg. The Encore can also climb directly to FL450 at maximum weight, gets there lmin earlier and at an equal distance downrange has burned about 45kg less fuel. Cruise altitude Once level at cruise altitude I left the throt tles in the maximum continuous detent and allowed the aircraft to accelerate to a high-speed cruise Mach of 0.8. A throttle setting just below maximum cruise held the Mach steady. At 214kt indicated total fuel flow was 500kg/h (l,1001b/h) and the aircraft maintained 452kt true airspeed. Slowing to a long-range cruise speed of 429kt true showed a Mach of 0.757. Total fuel flow was identical to the predicted book value of 445kg/h at 202kt indicated. At altitude the relative merits of the Learjet versus the Encore shift to a ques- Supercritical winglets reduce drag by around 20% at altitude and increase range tion of time. According to Bombardier, with over 455kg of payload the Learjet offers superior payload-range capability for all missions up to 3,340km in length. While the two aircraft offer nearly identical direct operating costs (DOC) on a per-kilo- metre basis, the Learjet is 45kt faster at long-range cruise speeds. While level at FL450,1 unstrapped and went back into the passenger cabin. Ambient noise was fairly low and allowed for easy conversation at normal voice lev els. Even near the cabin entry door, which employs a single passive seal, noise levels were remarkably low. Further contributing to the placid cabin environment were the 13 large windows. In addition to providing a light and airy feeling, they are mounted relatively high for easy viewing while seated. While the Learjet 40 is not an ultra- long-range aircraft, its pressurisation sys tem maintains a sea-level cabin until pass ing 25,700ft. At the ceiling of 51,000ft, a pressure differential of 0.65 bar (9.411b/in2) yields a cabin altitude of 8,000ft. The lower the cabin altitude, the less dehydrated and fatigued the passengers will be on arrival at their destination. I returned to the cockpit and Lundy requested a descent to 15,000ft to investi gate the airaaft's slow-speed handling qual ities. Before initiating the descent, I slowed the aircraft in idle power until the red slow- speed marker came into view at M0.56 in the primary flight display's airspeed tape. The Learjet 40 has relatively light wing load ing, 294kg/m2 for the current fuel load, and even at this high altitude the aircraft had a safe operation range of M0.25. Next I advanced the throttles and low ered the nose to accelerate to the maximum operating Mach number of 0.81. Passing FL310 an aural "overspeed" warning sounded as the aircraft sped past M0.81. I stabilised the aircraft at M0.82, where a series of sharp control inputs in each axis elicited a well-damped response. To slow the aircraft, I retarded the throttles to idle and extended the spoilers. Spoiler extension caused the nose to pitch up slightly, while the ensuing airframe buffet provided a valu able tactile cue that lift was being dumped. T-tail stalls The first stall was in the clean configura tion. An idle-power level-altitude decelera tion of the 8,396kg aircraft approximated the certification-standard lkt/s entry rate. In a 13Q nose-high attitude at 121kt indi cated, the stick-shaker activated. Control response in all three axes was good, as I jammed the throttles up to the take-off detent. The DEEC rapidly established take off power and I relaxed aft-stick pressure to initiate a level flight recovery from the stall. Pleased by the aircraft's ability to power out of the stall, I again retarded the throttles to idle and reapplied aft stick pressure to slow the aircraft below the shaker speed. Slowing through HOkt, moderate airframe buffet announced the approach of the full aft-stick stall at 105kt. At the aft limit of the stick, continual small deflections of the ailerons were needed to keep the wings level in the nose-high descent. Relaxing aft stick pressure let the nose drop and the aircraft to accelerate out of its stalled condition. The final stall was in a landing configura tion, flaps set to 40e. The power was set to 60% N; to attain the desired deceleration rate. The shaker activated at 105kt. As with the clean stall, control effectiveness was good at shaker speed. Pulling the stick fully aft slowed the aircraft to 91kt and was again accompanied by moderate airframe buffet. While some T-tail aircraft have stick pushers to avoid a deep stall, the Learjet 40's ventral fins provide a nose-down pitching moment www.fliqhtinternational.com FLIGHT INTERNATIONAL 18-24 MAY 2004 59
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