Spirit Wing modification enhances Learjet design

 - January 26, 2007, 10:47 AM

Calvin Burgess is a man who likes Learjets…always has and always will, to hear him tell the story.

Burgess began flying in 1986 for a very pragmatic reason. Then a resident of Vancouver Island, he found that a set of wings was the easiest way to get off the island regularly. He’s owned a Cessna 421, a Piper Cheyenne 400 and a P-51 before purchasing a Learjet 24. Burgess gained considerable business-aircraft operations experience over the past 18 years as the president of Spirit Wing Aviation in Guthrie, Okla. Spirit Wing was the flight department that served the transportation needs of Burgess’s first construction company.

Burgess also possesses the soul of an inventor, a problem solver and an entrepreneur. Like Lear, he’s a man in search of opportunities. “And aviation is my passion,” he said. “I never see solutions as an ‘if’ proposition…only as a ‘how.’”

“Although I love my Model 24,” Burgess says, “I quickly realized the fuel burn and range with the current engines are very limiting.” A self-taught engineer, Burgess has spent nearly four years marrying the Learjet 25 airframe to the new Williams FJ44-2C engines to give the proven Learjet airframe a new lease on life. The end result is the SpiritLear.

With approximately 270 Learjet 25s still around, Burgess sees plenty of conversion possibilities. He chose the 25 over the 24 simply because the aircraft appeared to offer more initial interest to buyers. The 25, some 42 inches longer than the 24, was also certified to more stringent Part 25 airliner standards while the earlier airplanes were certified under Part 23. The Spirit-Lear can climb to 45,000 feet and has an Mmo of 0.81. The aircraft has already flown smoothly at Mach 0.827 during testing, so that limit might increase.

But Spirit Wing’s project is more complex than simply hanging new engines on the airplane. The company will deliver the completed SpiritLear with a fully automatic pressurization system that will operate normally with one engine shut down, as well as full-time wing and nacelle de-icing, automatic emergency pressurization, a more robust tail design, removal of all hydraulic pumps, accumulator and reservoir from nearby sources of engine heat, electricity and the rotor burst zone area. The Spirit-Lear includes considerable redesign work to the engine air intake area to reduce drag. The RVSM-certified SpiritLear will also use a more reliable starter-generator and offer reduced stopping distances.

Burgess said the aircraft offers “more speed, range and payload than many new designs that cost millions of dollars more.” In SpiritWing-provided data, the company says the SpiritLear will carry four people 1,705 nm while burning 128 gallons/858 pounds per hour at speeds up to 440 knots. A Bombardier Learjet 40, which costs nearly four times as much as the Spirit Wing conversion, will cruise slightly faster at 457 knots, but travel only 1,618 nm with the same load and burn 183 gallons/1,226 pounds per hour in the process. A Cessna Citation CJ2 will cost a new owner about $5.2 million and fall about 150 nm short of the SpiritLear’s range while cooking along at only 407 knots on 159 gallons/1,065 pounds of fuel per hour. Burgess considers the Raytheon Premier I, Hawker 400A, Cessna Citation Encore and even the Learjet 35A to be competitive with the SpiritLear.

The Work Begins
During early development in 2000 Spirit Wing engineers gathered data from a number of sources that included not only their own baseline testing, but data from the entire series of Learjet 25 service and technical manuals available. During the course of eight months, they learned that engine intake air was entering the GE powerplants at supersonic speeds. The company cleaned up enough egregious drag on the wings alone to gain 20 knots of airspeed. Said Burgess, “The improved airflow around the new engine nacelles adds the equivalent of about 150 pounds of thrust to the aircraft.” Maximum pressure differential on the SpiritLear has been increased to 9.4 psi, in line with the limits on later Learjets.

Because the Williams turbofans are shorter than the GE engines, the new engines are mounted 16 inches farther back along the fuselage and four inches farther outboard. This work required modifying the front engine mount while constructing an entirely new rear engine support structure. The new engine location also improved lift in the critical inboard portion of the wing, resulting in a significant reduction in approach and landing speeds.

Since the new powerplants are located farther back on the fuselage, the original rear engine mount actually becomes the new forward engine mount. Before the installation, however, Spirit Wing performs a thorough inspection of the aircraft and specifically an eddy-current examination of the main engine support beam and the critical fastener holes to satisfy FAA damage-tolerance regulations for engine support structures.

The airframe side of the fuel supply system is also modified to replace the main engine boost pumps and the tiptank transfer jet pumps. Spirit Wing also removed from the SpiritLear the rear fuselage fuel bladder from the original Learjet 25. That tank held about 1,308 pounds, and the pumps added another 30.

The Williams engines run hotter than some others and do not require Prist fuel treatment, so engineers were concerned about where to dump the excess bleed air. The answer usually is just overboard. Spirit Wing engineers, however, developed a system to use some of that excess air to continually heat the wings and engine intakes at essentially no performance cost. This means everything stays hot, even during a descent with the throttles at idle.

During testing, those higher Part 25 standards required removing some equipment, such as pressure regulator valves and the hydraulic lines that were originally routed over the top of the GE engines, from the engine rotor burst zone in the tail. Engineers placed the new hydraulic lines under the Williams powerplant.

Burgess said that during flight-testing the airplane also encountered some unexpected 28-Hz tail flutter that “cost me many nights of lost sleep.” The testing necessary to cure the problem took an additional eight months and more than $1 million. “The original Lear certification flutter test was not very sophisticated,” Burgess said. “Of course, that testing was conducted 40 years ago.” Burgess said he is convinced that the tail flutter and the illegal addition of the infamous “Go Fast” switches that silenced some Learjets’ overspeed warnings were responsible for a number of fatal accidents in the early airplanes.

Interestingly, the original Learjet 25’s CJ610s produced 2,950 pounds of thrust each at takeoff and at altitude propelled the 15,000-pound aircraft at nearly 465 knots. The SpiritLear is expected to have an mtow of about 14,600 pounds when the STC is complete. The SpiritLear’s Williams powerplants however, produce only 2,400 pounds of thrust each at takeoff and maintain nearly the same speed at altitude on less fuel.
Spirit Wing engineers also considered Honeywell engines for the project but ultimately rejected them because of their higher weight. More powerful Williams powerplants would have required more expensive FADECs and precise electrical control, something the Learjet 25 wiring harnesses will not handle. “Quite honestly,” Burgess added, “we are already right at the barber pole most of the time anyway in cruise. The larger engines might offer a bit of extra takeoff performance, but not much else.”

Burgess said the “Power by the Hour” maintenance charges on the Williams engines are also substantially lower than those for the Honeywell engines, at $64 per engine versus $159, respectively. Mean time between overhauls is expected to fall somewhere between 3,500 and 4,000 hours. The SpiritLear carries 400-amp starter-generators for added backup capabilities but Burgess says the aircraft could be certified with only 200-amp versions. One generator is sufficient to provide power for all onboard systems.

The SpiritLear is not equipped with thrust reversers, which add another 150 pounds to the old airplane. To make the landing roll as short as possible with only spoilers and brakes, the FJ44s are set to produce only 100 pounds of residual thrust each upon touchdown. Less thrust means less forward movement and more effective braking.

SpiritLear customers can bring their own airframe to Spirit Wing for revitalization for a cost of $1.8 million or purchase a completed SpiritLear for approximately $2.2 million. The price does not include a new interior, cockpit panel or RVSM approval. Depending upon when it is performed, RVSM approval costs between $125,000 and $150,000, according to Spirit Wing.

Burgess says he’s researching a new autopilot and avionics package that would add approximately $275,000 to the price of the aircraft. To balance the extra cost, the new panel is expected to save another 150 pounds in weight. While Spirit Wing will manufacture almost all conversion components and retain ownership of the STC, the actual labor will be carried out by Banyan Air Service of Fort Lauderdale, Fla.; General Dynamics in Buffalo, N.Y.; and West Star Aviation in Grand Junction, Colo.

Airborne in the SpiritLear
Spirit Wing invited AIN to ride jump seat in the prototype SpiritLear, N17AH, serial number 316. Burgess occupied the left seat, with company test pilot Neil Brown in the right. The aircraft was stuffed with critical flight-test equipment and lacked a standard interior and soundproofing, so no evaluation of the aircraft’s creature comforts was possible. Spirit Wing PR guy and former Learjet communications veteran Dave Franson occupied the fourth seat.

Since the proof of the SpiritLear claims would really be in the pudding, Burgess and Brown fired up the new Williams engines and taxied out to 4,102-foot-long Runway 16 at Guthrie, Okla., elevation 1,068 feet msl. During start, the left engine peaked at 670 degrees C, while the right one went to 675 degrees C. A hot start would have displayed temperatures closer to 1,000 degrees C. The engines settled down at about 100 pounds per hour per side at idle as we taxied out.

The runway was still slightly wet from a recent rain and the temperature was 21 degrees C. Brown calculated the balanced field length at 3,800 feet for takeoff with 4,750 pounds of fuel and two passengers, and a takeoff weight of approximately 13,800 pounds, or about 800 pounds less than the projected mtow of 14,600 pounds. V1 was calculated at 116 kias, while Vr/V2 came in at 125 kias using eight degrees of flap. The aircraft’s basic operating weight was 8,718 pounds.
The plan was to head south out of Guthrie toward Houston and climb to FL370, then turn west to Del Rio, Texas, where we planned to add fuel and have lunch while Burgess checked into some of his real estate projects. After takeoff from Del Rio, we’d head direct to Wichita for a full-stop landing. Burgess mentioned that if the winds were not severe anywhere along the route, we could very well overfly Del Rio, turn northeast to Wichita and land with plenty of reserve fuel.

The takeoff was brisk and the airplane used about 3,000 feet of pavement before we became airborne. Initial climb speed was set at 190 kias and the vertical rate settled at about 3,000 fpm. Burgess showed us a 170-kias climb for a short while that netted a rate of 3,500 fpm, but the deck angle was too steep to safely scan for traffic. We shot to 14,000 feet in less than four minutes.

Unfortunately, Oklahoma City departure kept the airplane level there for some three minutes before Burgess pushed the throttles ahead for the next segment of the climb, which took us unrestricted to FL370. The temperatures everywhere this day were warmer than ISA by varying degrees, but the SpiritLear still performed well. Twenty-four minutes after brake release, the airplane leveled at FL370 and accelerated to what became Mach 0.79 at ISA +9, for a true airspeed of 424 knots on 460 pounds of fuel per side per hour. Burgess said that under similar conditions, his Learjet 24 would show a slightly higher speed but burn almost twice as much fuel per hour.

Making the turn over Houston, we climbed to FL410, where fuel flows dropped to about 400 pounds per side. Two hours and one minute after takeoff from Guthrie, we were sitting on the ramp at Del Rio after a 40-degree-flap landing that consumed about half of the 5,000 feet of pavement available. Total fuel burn for the trip was 2,200 pounds. At the end of the trip 2,550 pounds of fuel remained in the tanks.
An hour-and-a-half later we climbed out of Del Rio toward Wichita Mid-Continent with a final requested altitude of FL410. The climb took 27 minutes and consumed 682 pounds of fuel. With the outside air temperature nearly 10 degrees above ISA, the speed still settled in at 429 ktas on 400 pounds per side. Burgess said the flow would have dropped to about 380 pounds per side at FL430.

When we landed in Wichita, Burgess demonstrated a maximum braking effort. Cleared for a straight-in on 19 Right, he crossed the end at ref speed and used only 2,000 feet to stop before turning off toward the Lear factory ramp. As we taxied in, one of the linemen held his ears in anticipation of the noise and then suddenly removed them with a somewhat puzzled look on his face. He later said he’d never heard a Learjet 25 that was so quiet.

“While other companies have been selling the concept and introducing the idea of a very light, economical business jet, Spirit Wing has built and flown an airplane that offers better range, speed and performance with a passenger load of up to eight people,” Burgess noted.

The company has not yet accepted any orders for the Spirit- Lear, but Burgess says he’s had plenty of phone calls, especially since the NBAA Convention in October. “I think our work proves you don’t need to go slow in order to go far in a Lear.”

The company said it is on track to receive FAA approval for the STC by the second quarter of this year. Spirit Wing is also considering hanging the Williams engines on some of the 230 Learjet 24 airframes.

Although the companies are competitors of sorts, a Bombardier business aircraft spokesman said, “This Spirit Wing effort is a testament to the well respected Learjet 25 airframe 40 years after it was built.”