Raisbeck adds zip to the Learjet 35/36

 - November 2, 2007, 11:07 AM

“Welcome to Lear Jet Country,” a marketing slogan that attached itself to the early-20-series Lear Jet, is likely to be remembered only by industry old-timers who recall the airplane’s ability to take off and climb to 41,000 feet without effort. It is a capability that disappeared with the advent of the Learjet 35/36.

Last month at the NBAA show Raisbeck Engineering introduced the Learjet 35ZR. It is a performance-enhancement package for the original Learjet 35 that the company will offer to the first five buyers for $200,000, including direct installation supervision by Raisbeck personnel.

James Raisbeck, CEO of the Seattle-based company, told AIN, “I’m 67, so the Learjet 35ZR is probably my last shot at breathing new technology into an already amortized airframe, and a good one at that.”

Z, the last letter of the alphabet, represents the ultimate, and R is for Raisbeck. “The Learjet design is almost 50 years old, but they have been fruitful ground for me,” he said. “I’ve been involved hands on, in one way or another, for 32 years.”

“Lear Jet Country” was the result of 20-series Learjets having turbojet engines, which don’t lose a lot of thrust as they gain altitude. Adding to the mystique, the aircraft was very sleek, and compared with the four-engine JetStar and other jet aircraft of the day it didn’t burn a lot of fuel.

“Because it looked fast, it was thought to be a low-drag airplane,” Raisbeck said, “but in fact it wasn’t. It was just highly powered and had outstanding performance. Then in 1974 the Learjet 35 entered the market and everything changed.”

When the Learjet 35 was introduced it stood out because it was a quieter aircraft with a longer fuselage than its early-20-series predecessors, but it also had high-bypass- ratio turbofans. The problem was that high-bypass turbofans lose thrust with altitude much faster than turbojets do. The end result was that climbing directly to FL410 was no longer an option, and suddenly “Lear Jet Country” was inaccessible. A Learjet 35 crew is likely to request an initial altitude of FL330 or maybe FL370 and then still have to step climb to FL410. On hot days performance is further eroded. “It’s been so long that people have almost forgotten what Lear Jet Country is,” Raisbeck said. “Well, we say welcome back to Lear Jet Country.”

Raisbeck said from the beginning of the project he wanted a 10-percent reduction in high-cruise Mach drag. This goal was accomplished with a modified leading edge; a twisted, extended-chord flap; and horizontal winglets called batwings.

A Kinder, Gentler Stall
“One of the goals was to get rid of the adverse stall characteristics,” he said. “They’re so marginal on the 35/36 that if you put the leading edge on a little crooked, the aircraft will flip on its back when stalled.” A check of the Learjet Maintenance Manual 27-31-00, page 203, supports this claim.

The manual states: “Due to the possible effects on stall characteristic, whenever maintenance as noted (maintenance requiring removal and installation, repair or installation of a new wing leading edge is performed; this includes loosening and retightening any of the leading-edge screws), the aircraft must be flight-tested to verify aerodynamic acceptability and stall speeds. This flight test must be conducted by a pilot approved by Learjet for stall test flights.”

Raisbeck said once the stall speed was lowered they were able to reduce takeoff speed, resulting in a shorter takeoff distance and thus allowing the Learjet to operate at more airports. Pilots knew, according to the performance charts, they could technically get out of an airport they couldn’t get into. That situation is changed by the modification because it reduces the landing distance and drag in all flap configurations. Now the aircraft can carry 500 to 600 more pounds out of the same airport with a failed engine, according to Raisbeck. “So we’re improving the safety margins, too,” he said.

The reduced drag also allows the pilot to choose a higher initial altitude–between 2,000 and 4,000 feet higher. Raisbeck estimates that for a given Mach number the conversion saves fuel at about 1 or 2 percent per 1,000 feet of altitude.

When comparing a typical mission at FL390 with one at FL410 with the ZR conversion, all other things being equal, the ZR picks up 26 knots. At FL420, a 31-knot increase is realized.

Also a result of reduced drag, all things being equal, more fuel remains on landing or longer trips can be made.

The two-year-old program has cost Raisbeck almost $3 million and 300 hours of flight testing so far without a single sale. Raisbeck has yet to determine a final price for the conversion beyond that for the first five he proposes performing under his direct supervision.

“Look,” Raisbeck said, “I don’t need the money; we’re doing just fine. The Boeing 727 Stage 3 hush-kit project for American Airlines gave me enough money to be able to fund this one. The thing is that there are enough people out there who will appreciate it when they know it’s available and it will sell itself, whatever the final price is. The people who can afford it and value it will buy it, and the ones who can’t, won’t. Additionally, we believe the ZR technology will survive longer than the 35/36 program.”

The company is looking at initiating a similar program for the Learjet 31, as well as the 55 and 60.