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Aviation History
2004
2004-09 - 0728.PDF
Flight test: Learjet 40 at high AoAs, alleviating the need for a pusher. Relaxing stick back pressure and advancing the throttles allowed the aircraft to fly out of the stall. Before returning to Wichita, I had the opportunity to explore the Learjet 40's roll capabilities. At 15,000ft and airspeeds from 150kt to 250kt indicated, 1 did a series of half- and full-deflection aileron rolls. Assisted by the spoilers, roll rates exceeded 45a/s at half deflection. Full-deflection roll rates approached 802/s. Control feel and aircraft response in all cases was excep tional. Even at the limits of their deflection there was no aerodynamic buzz or adverse feedback from the ailerons. At 250kt Lundy held the yoke full right to demonstrate the aircraft's ability to recover from a jammed aileron condition. A single click of the captain's roll disconnect switch allowed me to use the spoilers to overpower the "jammed" ailerons. At this condition the spoilers had enough authority to reverse the roll and establish a left-hand turn. Wichita touchdown Recovery to Wichita was via radar vectors to an instrument landing system approach to runway 19R. With 1,501kg of fuel and flaps at 40s, approach speed was 116kt. Lundy computed a required field length of 820m. I used the flight director (FD) to maintain assigned heading and altitude before localiser intercept. The FD's split-cue format was easy to follow, allowing me to precisely control the aircraft. Once established on the localiser, pitch changes caused by gear and flap extension were easily trimmed out with the yoke-mounted two-axis trim switch. Once established on the 32 glideslope, around 60% Nj on the engines held 116kt. The FD continued to provide excellent guid ance during the final approach phase, sig nificantly easing my task of staying on the localiser and glideslope. Passing 30ft radar altitude (RA) I slowly started to retard the throttles to idle. At 15ft RA I initiated the flare manoeuvre. The trailing-link landing gear allowed the aircraft to touch down softly, despite my having rounded out a foot or two higher than desired. Once on the runway Lundy set the flaps to 20a for the touch and go. The next approach was a visual one to a full stop. Again flaps were set to 40Q. I found control harmony in the pattern and on final to be good, allowing me to easily fly a wing-low approach with the 18kt left quartering headwind. After touchdown the spoilers, armed on downwind, deployed. Moderate toe braking slowed the aircraft for runway turn-off and allowed us to taxi back for another take-off. For the final take-off, Lundy again set the flaps to 8Q. Take-off speeds remained unchanged from our initial departure. As the aircraft accelerated down the runway, Lundy rapidly pulled the right throttle to idle passing 115kt to simulate an engine failure at Vr The resultant yawing motion was easily detected, and about 35kg of force on the left rudder was needed to keep the aircraft tracking down runway centreline. Slightly after 120kt I initiated a slow rota tion to establish the climb-out attitude. Once airborne the gear was retracted and an additional 5kg of pedal force needed to maintain wings-level co-ordinated flight. Next Lundy turned off the Learjet 40's rudder boost system. Unlike some aircraft that use differential engine pressures to bias the rudder into the good engine, the Learjet uses a yaw damper. If more than a 13.6kg force differential is detected between the rudder pedals, the rudder boost system will use the damper to augment the pilot's pedal input. The boost system can apply a maximum of 40.8kg of force, but by itself is not sufficient to counteract the yaw caused by an engine failure. With the boost system off, an additional 20kg of force was required to maintain co-ordinated flight. Keep on rolling From my short experience with the Learjet 40, it seemed to have a fairly low ratio of roll angle to sideslide angle - the phi to beta ratio. The higher this ratio, the more a sideslip angle (beta) will generate a roll angle (phi). The conventional slip ball mounted above the primary flight display was useful for maintaining co-ordinated engine-out flight. The flaps were set to the single-engine approach deflection of 202 on final. Once established on the glideslope, 64% Nt on the good engine maintained a final approach speed of 121kt. As was the case with the previous two landings, I found the aircraft very forgiving and easy to con trol during the roundout manoeuvre. Once on the runway I centred the rudder trim and advanced the power on both engines for a touch and go. Ventral "delta fins" enhance stability and stall behaviour The fourth and final approach was a visual one, with flaps set to 40s for a short field landing. On this approach I established an aimpoint close to the approach end, ducking underneath the ILS glideslope. At 30ft RA I retarded the throttles to idle. Just before touchdown I brought the nose up slightly from the approach attitude to atten uate the sink rate. The aircraft touched down firmly and I applied maximum force on the toe brakes. The spoilers automati cally deployed, and by the time I got around to pulling the engines out of reverse idle, the aircraft had slowed to below 60kt. The carbon brakes brought the aircraft to a stop after a very short ground roll of 400m. After taxiing back to Bombardier's ramp, I found the post flight and shut down checks were easy to accomplish. Faster and cheaper Bombardier's decision to base its newest light-jet offering on the successful Learjet 45 lets it bring to market a capable aircraft for a reasonable price. Like the Learjet 45, from which it is derived, the Learjet 40 has advanced avionics and robust fault-tolerant systems. Its ability to climb directly to FL450 gets it above most other traffic. Once there its long-range cruise speed of 421kt is 45kt faster than the Citation Encore's. And should time be even more critical, the air craft can dash quite comfortably at M0.8. The six-seat standard cabin provides a com fortable environment that is larger than the Encore's. The Learjet 40's outstanding car bon brakes and wing spoilers give it remarkable landing performance, on a par with or better than the Encore's. According to Bombardier the Learjet 40 is less expensive than the Encore, while direct operating costs for the Learjet are within 1% of those of the Citation on a per km basis. Not only does the Learjet 40 make sense on a rational basis, it is a delight to fly and, given the choice, operators may well want to accept marginally higher DOCs so they can enjoy the need for speed. • Ji Like the Learjet 45, from which it is derived, the Learjet 40 has advanced and robust fault- tolerant systems rr www.flightinternational.com FLIGHT INTERNATIONAL 18-24 MAY 2004 61
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