GRAHAM WARWICK / WASHINGTON DC
Package aimed at 700 aircraft in service, and will later be applied to other models
Raisbeck Engineering has revealed the Learjet 35ZR, an upgrade of the Learjet 35/36 series with modifications designed to reduce drag at high cruise Mach numbers by 10%. Changes include inboard wing gloves, recontoured flaps and "horizontal winglets", which combine to increase cruise speed and range and reduce stall speed. Certification is scheduled for early next year.
Spiritwing Aviation's Williams FJ44-2C-powered Learjet 25 made its NBAA debut alongside the ZR. Certification is also scheduled for the first quarter. Re-engining with the FJ44 doubles range, allows a direct climb to cruise altitude and makes the aircraft Stage 3 compliant, says Spiritwing. The modification will cost $1.7-2 million.
Raisbeck will price the ZR package after gauging interest at NBAA. The company says the technology is likely to be applied later to the Learjet 31, 55 and 60, which share the same basic wing.
The double-delta supercritical inboard wing is fitted with a 45°-swept glove extending out to 30% span and shaped to minimise supersonic airflow over the inboard wing and reduce the entrance Mach numbers at the venturi formed by the wing/pylon/nacelle/fuselage junction. The glove is composite, with a bleed air anti-iced aluminium leading-edge. Raisbeck says the gloves could house a combined 380 litres (100USgal) of extra fuel, adding 45min cruise flight.
Composite extended reflexed trailing-edge flaps replace the existing aluminium flaps and are contoured to minimise local Mach numbers in the venturi area where nacelle and wing overlap, delaying the onset of high-Mach drag rise. The new horizontal winglets are 45° aft-swept, upward-curving composite fillets that fill the 0.6m (2ft)-span gap between the ailerons and the tip tanks, blocking the development of wingtip vortices, and reducing lift-induced drag
Docile stall characteristics have been achieved by removing all existing stall control devices and fitting the wing glove, new smaller stall strip and three vortilons under the leading edge. The glove generates a vortex where it meets the 16°-swept wing, which controls flow, and the aircraft stalls softly with "virtually no wing roll-off" through the flight envelope, Raisbeck says. The slightly longer flap chord reduces stall speeds and take-off distances and improves engine-out climb performance.
The test aircraft has flown more than 300h and achieved a 10% drag reduction at M0.81 high-speed cruise, says Raisbeck. Galvin Flying Service will begin installations at Seattle's Boeing Field in the first quarter of next year.
Around 700Learjet 35/36s are still in service.
Source: Flight International
