While others fiddle, Allison blazes ahead with a private turboprop. BY MARK M. LACAGNINA Timing is a critical element in the suc• for these applications have been either prop that were replaced. This is not cess or failure of any new concept or abandoned or put on the back burner directly translatable to increased pay• design. Powered flight, a concept pro• due to lack of adequate financing or load, however, because each gallon of posed centuries ago by visionaries such because those involved in the projects jet fuel in the Turbine Bonanza's fuel as Roger Bacon and Leonardo da Vinci, do not believe the time (that is, the tanks weighs about 0.7 pounds more remained little more than whimsy until market) is right. than the avgas previously required for society and technology were ready for There is at least one exception. The the piston engine. The lighter engine it. The first mechanism generally cred• Allison Gas Turbine Division of Gen• was installed in such a way as to re• ited with achieving powered flight eral Motors, which is working with quire no changes to the Bonanza's used wing-warping for flight control. It Soloy on the Allison-powered Turbine certificated center-of-gravity limits. As worked well enough 80 years ago but 206 and other applications, believes a result, the propeller arc is 22 inches would be only of passing interest to the time is, indeed, right for a farther forward. modern aerodynamic engineers. Con• nonutility turbine single. The company When I visited Allison's Indianapolis versely, such things as stall fences and currently is test-flying a Turbine Bo• headquarters in July, the Turbine Bo• boosted controls, designs that have nanza with the intent of obtaining a nanza had been flown only about 100 applications today, would have been of supplemental type certificate (STC) for hours. Specifications for the conversion little use to the Wrights. a conversion package by the middle of were far from frozen. R. Frederick One of the most intriguing concepts next year. Allison plans to certify tur• (Fritz) Harvey, Allison's director of in general aviation today is the turbine bine conversions for the Beech A36 Bo• small aircraft engine products, and F. single. Two years ago, there was a rush nanza models, first, and then to pro• Jack Schweibold, the company's chief of development for civil applications ceed with the 33- and 35-series test pilot, had a long list of tweaks and (turbine singles long have been used Bonanzas. things-to-try still to be accomplished. for military pilot training and other Allison's testbed is a 1979 Model Therefore, the final design of the Al• specialized operations). Several compa• A36 with a 420-shp 250-B17C engine lison Turbine Bonanza may look some• nies and private designers set off in installed by Soloy. The engine cur• what different from the aircraft I flew various directions to see if they could rently is used in several foreign-aircraft and photographed in July. The ram-air make the concept of a general aviation applications, including the Australian scoop on top of the cowl will .be re• turbine single a reality. So far, the re• GAF Nomad N22B, the Partenavia placed with a flush-mounted, NACA• sults have been mixed. Spartacus, the Pilatus Britten-Norman type duct. The cabin air inlets will be As noted in the August issue of Pilot Turbine Islander and the Siai-Marchetti relocated from the wing roots to a loca• (see "Turbine 206," by Edward G. SF.260TP and SF.600TP. The engine is tion isolated from the path of exhaust Tripp, p. 28), the timing, at least for being used in another turbine single gases. A forward baggage compartment utility applications, does appear to be project in this country-Jerry Dietrick's will be fabricated in the nose of the right. Soloy Conversions has received Windecker Eagle Allison Turboprop• aircraft. Engine controls and instru• 20 orders to date for its Turbine 206, but, as reported earlier, the project is ments will be rearranged. the first turbine single certificated; and experiencing little progress. The winglets and tip tanks that ap- : Cessna Aircraft has 50 orders from According to Allison, the Bonanza's pear in the photographs accompanying Federal Express for the Caravan. turbine engine and three-blade Hartzell this article mayor may not be a part of But what about a turbine single for propeller weigh about 200 pounds less the finished aircraft. Allison is con• private business or personal transporta• than the 285-hp Teledyne Continental cerned with the potential for direc• tion? Most of the development efforts 10-520 piston engine and McCauley tional controllability problems during PHOTOGRAPHY BY THE AUTHOR AOPA PILOT • 81 -•• - cOllti"urd takeoff and slow-speed, high-power The Allison 250-B17C is a free-tur• haust ducts. The first two stages are operations because of the high torque bine, turboprop engine. In the Turbine called the compressor turbine. They are produced by the engine. The company Bonanza installation, air enters the en• linked with a shaft to the compressor has found that the winglets improve gine's gas-generator section through an stages in the gas-generator section of controllability as well as increase cruise inlet below the propeller spinner. In• the engine. The second two stages, speed by four or five knots, and vari• take air is compressed by six axial com• called the free-power turbine, turn the ous winglet designs will be evaluated. pressor stages to about seven times propeller through a reduction gearbox. Also, rather than using winglets, the normal atmospheric pressure. The At 100-percent engine speed, the free• company may derate the engine below compressed air flows back through two power turbine stages rotate at 33,290 420 shp and establish a minimum con• tubes and enters the combustion cham• rpm, and the gas-generator stages spin trol airspeed. A McCauley prop, as ber at the rear of the engine, where it at 50,970 rpm. Maximum propeller well as the Hartzell, will be evaluated. is mixed with fuel and ignited. The hot speed is 2,030 rpm. In addition, because the turbine engine combustion gases expand and flow for• The powerplant is managed with burns more fuel, the Bonanza's stan• ward into the turbine section of the en• two controls: a power lever and a con• dard 74 gallons of usable-fuel capacity gine. The gases travel through, and dition lever. Both are connected to a will be supplemented to provide rea• power, a total of four turbine stages power coordinator, which manages the sonable endurance. Several methods, before leaving the engine through ex- fuel control unit and the propeller gov• including tip tanks, are being consid• ernor. The controls have three posi• ered for this. tions. In the center position, the power Keeping exhaust fumes out of the TURBINE control causes the engine to idle (either unpressurized cabin is a major concern. ground idle or flight idle, depending The exhaust ducts, which exit the na• BONANZA on which way the lever is twisted). celle above and ahead of the nose-gear With the power lever all the way for• doors, may be lengthened and routed Allison has a long ward, the engine and propeller pro• back through the nacelle to exit list of things to try. duce full forward thrust; all the way through the openings for the cowl back, full reverse (Beta) thrust. The aft flaps (which will be removed). Noise The winglets and position of the condition lever cuts off suppression is another concern: Allison tip tanks mayor may the supply of fuel to the combustion wants the cabin "whisper-quiet." Air chamber; the central position initiates conditioning will be part of the conver• not be part of the kit. fuel flow. As it is advanced from the sion package. central to the forward position, the 82. SEPTEMBER 1984 condition lever brings the propeller out B17C engine is 3,500 hours. The cost is of feather. well as pushed and pulled, I found the about $20,000. Hot-section inspections transition to the Turbine Bonanza to be Engine gauges are arranged in two must be performed every 1,750 hours relatively straightforward. I had been clusters. The first includes propeller or 3,000 cycles and cost between forewarned about the need for right rpm, turbine outlet temperature and $7,000 and $10,000, according to the rudder and aileron during takeoff, but torque (the force, in pounds per square company. the deck angle took me by surprise. At inch, being exerted by the engine to Weather conditions were marginal 96 knots, the best rate-of-c1imb speed, turn the prop shaft). The second cluster during my visit with Allison, and my all forward visibility vanished, the atti• includes compressor rpm (expressed as evaluation flight in the Turbine Bo• tude indicator showed about 17 de• a percentage), oil temperature and oil nanza, N250AT (nee N222BT), com• grees nose-up, and the VSI indicated pressure. prised a short hop from Indianapolis more than 2,000 fpm. I found 120 There are four large annunciators in International to Eagle Creek Airport for knots initial and 140 knots for cruise• the center of the panel. One annunci• a series of takeoffs and landings and a climb to provide adequate visibility ator indicates that the propeller is in quick excursion to 10,000 feet on vec• over the long nose and good perfor• the Beta mode (reverse). Another tors from Indianapolis Approach. mance. The Turbine Bonanza uses warns that metal particles have been After a bit of fumbling with the en• about half as much runway on takeoff detected in the engine oil or gearbox oil.