May 19, 2022

Single-Engine Turboprop

Tracing The Single-Engine Turboprop: Novelty Aircraft Become Assets

The high-speed, single-engine turboprop business aircraft that has become a business aviation mainstay during the past three decades ironically has roots in the 1950s-vintage Morane-Saulnier MS.760 Paris jet, built in Tarbes, France.

Former Mooney Aircraft owner Alexandre Couvelaire flew the Paris jet while a pilot in the French Air Force and dreamed of creating a modern civil version, carrying 6-8 people, cruising at 350+ kt. and capable of flying 1,500 nm. Key to the upgrade was finding a pair of modern, small turbofan engines that would replace the 30+ year-old, fuel-thirsty and noisy Turbomeca Marboré turbojets on the new model.

Tracing The Single-Engine Turboprop: Novelty Aircraft Become Assets
Tracing The Single-Engine Turboprop: Novelty Aircraft Become Assets

But it would be years before light-weight, fuel-efficient 1,000-2,000-lb.-thrust-class turbofans from Williams and Pratt & Whitney Canada would make their debut. This steered Couvelaire toward alternate turbine engines. At the time, this turned out to be Pratt’s PT6A single-engine turboprop, which he regarded as an interim powerplant for his personal business aircraft concept.

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When Couvelaire and a group of investors bought Mooney in 1984, they saw potential for a single-engine turboprop in design chief Roy Lopresti’s six-seat Mooney 301, so named for its top speed in miles per hour. The 301 was developed in response to a new generation of turbocharged, pressurized, piston singles being designed by Beech, Cessna and Piper. But Couvelaire’s team determined that it was too heavy and 40 kt. too slow to vie with competitors’ new models.

The energetic French businessman had no interest in building a “me, too” Mooney single-engine piston that would offer performance equal to that of a Cessna P-210 or Piper Malibu. He was convinced that the decline in light general aviation aircraft in the 1980s was due to manufacturers’ failures to develop innovative new products, claiming that they “had rested on their laurels” too long. Typical business aircraft missions were 300-400 mi. Couvelaire wanted to create a completely new aircraft that was tailored to these trips, one that would leapfrog competitive piston aircraft, thus bridging the gap between 200-kt. piston twins, cruising in the teens, and 400-kt. jets and soar in the thirties and forties.

Couvelaire dreamed of creating a twin-turbofan version of the TBM 700. But, turboprop power proved a better choice.
Couvelaire dreamed of creating a twin-turbofan version of the TBM 700. But, Single-Engine Turboprop power proved a better choice.

Aérospatiale, the successor company to Morane-Saulnier, also saw the potential of this new class of executive aircraft. Couvelaire’s team started discussions with the French company about building a single-engine turboprop derivative of the Mooney M301 shortly after acquiring the iconic, Texas-based planemaker. Their goal was to create a new class of single-engine turboprops that would cruise considerably faster than twin turboprops of the era, while also offering markedly better fuel efficiency.

At the 1987 Paris Air Show, the two companies announced their intent to build a clean-sheet, owner-flown, pressurized, single-engine turboprop. Aérospatiale’s mock-up at Le Bourget showed visitors two pilot chairs up front and a four-seat club section in the middle. Power would come from a single 700-shp PT6A-40. And thus its name: TBM 700 — TB for Tarbes, M for Mooney and 700 for its power rating. The companies promised the aircraft would have a 300-KTAS cruise speed at FL 250 and a seats-full 1,150-nm range. Pressure differential was nearly as high as that of the Beech King Air 200, giving the TBM 700 a 6,400-ft. cabin altitude at FL 250. With a full fuel load of 2,000 lb., max range could be stretched to 2,000 nm. The initial price was $955,000.

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By the time aircraft design was frozen, Aérospatiale had upgraded to the PT6A-64, a 1,583-thp engine rated to 700 shp, which provided substantially better hot-and-high airport performance and higher cruise speeds at altitude.aza

Heavy design work and component manufacturing would be performed by Aérospatiale in Tarbes. Couvelaire intended to set up a production line at the Mooney factory in Kerrville, as it would be able to assemble the aircraft at low Texas non-union labor rates with parts supplied by Tarbes to keep costs in check. However, Couvelaire’s investors couldn’t stomach the investment, so Aérospatiale became the sole manufacturer. At that point the new aircraft rightly should have been renamed the TB-700, but the TBM 700 label remained

Single-Engine Turboprop: Morane-Saulnier MS.760 Paris Jet III was Couvelaire’s inspration for creating the TBM 700.

Even so, Couvelaire continued to dream of building a twin-turbofan variant. In November 1986, he wrote his friends at Aérospatiale, “Originally, it was foreseen that the TB 700 would be the starting point for a range of single- or twin-engine business aircraft, with different types of engines.” The twin-turbine design would cruise at least as fast as 350 kt., or about Mach 0.65. “The cost of developing a twin turbine at the beginning of the TB 700, will be reasonably reduced compared to a clean-sheet twin-turbine design. In fact, from the moment when one admits that the cylindrical fuselage section is identical for both of these airplanes, certain elements of the twin turbine could be derived from the TB 700, thanks to the addition of reinforcements and local modifications, design and manu-facturing costs can be reduced. He continued, “The development of the twin turbine will thus permit to draw the best part of the studies made for the TB 700.”

Deliveries of the TBM 700 began in August 1990, but Aérospatiale showed little interest in developing a twin-turbofan variant. And perennially cash-strapped Mooney did not have the resources for such an investment. This was even more evident during development of the 1988-89 Porsche Mooney M20L, equipped with the 3.2-liter Porsche Flugmotor. Funding primarily came from Stuttgart, Germany

Crossroad in the Marketplace

Time was not on Couvelaire’s side in launching his dream jet. The entry-level light jet market was heating up at a record pace. Cessna was proceeding with development of the CE525 CitationJet, the start of a new family of faster, higher flying, more efficient, entry-level turbofan aircraft. Adam Aircraft, ATG, Avocet, Century Jet, Eclipse, Epic, Piper, Safire and VisionAire, among others, would soon join the fray.

All this activity may have been a blessing in disguise for Couvelaire. As the light jet competition became more intense, the appeal of high-performance single-engine turboprops became stronger. While most of the new light jet entries boasted 350-400-kt. cruise speeds and promised at least 1,200-1,500 mi. of range, most owner-flown business jet trips were no longer than 300-500 mi. Such short mission lengths made them direct competitors to the TBM 700. And while some could save 15-20 min. on such trips, they consumed nearly twice as much fuel and had considerably higher direct operating costs. This gave the new TBM 700 quite an advantage.

Single-Engine Turboprop: Morane-Saulnier MS.760 Paris Jet III was Couvelaire’s inspration for creating the TBM 700.

Twin turbofan advocates fired back. Having but one engine, they lambasted the TBM 700 for its lack of powerplant redundancy. It just was not as safe as a twin turboprop or twin turbofan, they asserted. But the -64 Pratt proved impressively reliable in day-to-day operations and the popularity of the aircraft grew, paving the way for many more single-engine turboprops.

The Swiss were next to jump on development of a high-performance, pressurized, single-engine turboprop. At the NBAA’s annual convention in 1989, Pilatus took the wraps off of its PC-12, a secret project that had been under development in Stans for several months. The aircraft was so named because of its 1,200-shp PT6A-67B engine. Assembly of the first prototype was well underway, with certification originally planned for 1991. Subsequently, Pilatus redesigned the wings and added winglets to production aircraft to assure they met performance projections. Swiss certification of the first production units occurred in March 1994 and the FAA issued a type certificate in July of that year.

The Swiss airplane couldn’t match the TBM 700’s climb performance or cruise speed, but its large, left rear, 4.4-ft.-wide-by-4.3-ft.-high cargo door, along with its flat floor, gave it superb utility. And it also had a left-side forward pilot door for direct access to the cockpit.

The PC-12’s interior dimensions actually were larger than those of a King Air 200. Overall cabin volume was 360 cu. ft. Standard interior furnishings were work denims and farm gloves tough, but many buyers ordered the posh six-chair executive cabin. The wingspan was longer than that of the Cessna Citation II. One operator told BCA during a PC-12 Operators Survey he used the aircraft to move his daughter’s household goods, including refrigerator, washer, dryer, televisions and bed, from one city to another. Another owner told BCA that he intended to haul around Caterpillar turbo-diesel engines in the back of the airplane.

The PC-12 also earned plaudits from Australia’s Royal Flying Doctor Service as an air ambulance aircraft that could shuttle patients between short, unpaved runways in the outback and larger airports serving major medical facilities at coastal cities.

Not Resting on Laurels — In Tarbes or Stans

Even as early deliveries of the TBM 700 and PC-12 began, growth versions were in the works that offered more utility, better loading flexibility, more range and higher cruise speeds. In 1992, the TBM 700 was upgraded with EFIS, becoming the “A” model. The 3.9-ft.-high, 2.1-ft.-wide clamshell rear entry door was replaced by a 3.9-ft.-high, 3.5-ft.wide swing-up cargo door for the 1999 “B” model, providing easy access to the cabin for combination passenger/freight missions. And the “C2” in 2003 was structurally beefed up and fitted with 20-G chairs, plus 10-ply tires to make possible a much-needed 815-lb. MTOW increase.

Pilatus also had many opportunities to improve the original PC-12. Similar to the TBM 700, it needed a series of weight boosts to increase its useful load. First-generation aircraft had onerous roll control forces because the Pilatus design chief did not believe in aileron servo tabs to provide aerodynamic boost. Later models had the tabs, which reduced roll control force by two-thirds.

Daher’s TBM 900-series aircraft are the reigning speed champs of the single-engine turboprop segment.
Daher’s TBM 900-series aircraft are the reigning speed champs of the single-engine turboprop segment.

Key milestones were achieved in 2008 by both Socata, successor to Aérospatiale, and Pilatus. Both manufacturers upgraded their aircraft with more-powerful versions of the PT6A, boosting both climb and cruise performance. The TBM 850 was fitted with a 1,825-thp -66D in place of the 1,583-thp -64. While retaining the 700-shp rating for takeoff, power could be pushed up to as much as 850 shp for climb and cruise. Similarly, the PC-12NG was equipped with a 1,744-thp -67P in place of the original 1,605-thp -67B. In addition, the new engine was rated at 1,200 shp for all operations. The older engine was rated at 1,200 shp for takeoff, but power had to be reduced to 1,000 shp at all other times.

Both aircraft were upgraded with full glass cockpits. The Garmin G1000 was installed in the TBM 850 and Honeywell Primus Apex was fitted to the PC-12NG. In 2014, Socata upgraded its aircraft with two dozen improvements, including a more-efficient engine-inlet ram recovery duct, winglets, a five-blade Hartzell carbon-fiber prop that reduced interior sound levels and several aerodynamic refinements that allow the -66D’s 850 shp to be used for all phases of flight. The TBM 900 series aircraft officially made their debut. Pilatus subsequently upgraded the PC-12NG with a similar Hartzell five-blade composite prop that cuts cabin sound levels.

Two-plus decades of improvements added capabilities to both aircraft, but their prices climbed even as demand remained strong. Today’s TBM 900 series aircraft sell for more than $4 million and the PC-12NG retails for well over $5 million. TBM 900 series aircraft can cruise as fast as 330 KTAS and the PC-12NG now speeds along as fast as 285 KTAS. That’s tough competition for most twin turboprops and ample speed to be viable with twin-turbofan aircraft on shorter trips. With six occupants aboard, TBM 900 series aircraft actually can beat some twin turbofans on longer range missions because it can fly nonstop when the light jet has to stop en route for fuel.

The escalating prices of the European single-engine turboprop contenders opened the door for Piper to develop a more-affordable single-engine turboprop derivative of its pressurized, single-engine piston PA-46 Malibu/Mirage in the late 1990s. Piper’s PA-46-500TP Meridian, powered by a single 1,000-thp PT6A-42A flat rated to 500 shp, made its debut in September 2000. It offered two-thirds as much long-range cruise speed, two-thirds the cabin volume, two-thirds as much payload and two-thirds the price of the TBM 700. Buyers loved its price/performance value point.

The Vero Beach, Florida, manufacturer delivered more than 550 Meridians before it was succeeded by the Piper M500, available with a Hartzell five-blade composite prop, plusher interior furnishings and upgraded G1000 NXi avionics. The M500 is off to a strong start with more than 70 deliveries to date.

Many Meridian and M500 operators longed for greater range, more useful load and higher operating speeds. So, Piper upgraded the aircraft with a new wing holding more fuel and more robust construction and dialed up the -42A’s output to 600 shp. The new model, appropriately named the M600, can fly 70% farther. With a maximum range of more than 1,400 nm, it can fly coast-to-coast missions with one fuel stop, unless westbound into especially stiff winter jet-stream winds. Priced at $3 million, it’s a strong competitor to TBM 900 series aircraft, even though it’s up to 50 kt. slower. Piper has delivered more than 80 M600 turboprops since it was certified in 2016.

Textron Aviation’s upcoming Denali, its first high-performance, single-engine pressurized single-engine turboprop , takes dead aim at the Pilatus PC-12NG. It’s the first aircraft to be powered by General Electric Aviation’s Catalyst single-engine turboprop , a 2,000-shp-class engine that promises a 15% improvement in fuel efficiency owing to its 16:1 pressure ratio, single-crystal, internally cooled turbine blades and 3-D printed components. It’s the first general aviation single-engine turboprop to be equipped with a FADEC/computer-controlled prop governor for carefree handling and health trend monitoring and recording. For the Denali, the Catalyst is flat-rated to 1,240 shp up to FL 280, as much as 10% more power than its competitors, and it powers a Hartzell five-blade carbon-fiber prop. Initial TBO is 4,000 hr.

The Denali is slightly larger than the PC-12NG in all exterior dimensions and it appears to be a touch heavier in weight, although Textron has yet to release final specifications for the aircraft. It will have a 1,100-lb. tanks-full payload, a 285-KTAS top speed, an FL 310 ceiling and 1,600-nm, four-passenger range.

The aircraft will be certified for up to 10 occupants. Similar to the PC-12NG, there is a forward main entry door, providing easy access to the cockpit and cabin, plus a large, swing-up aft cargo door. The standard six-chair executive interior is available with an optional, fully enclosed, externally serviced lavatory. At 7.55 psi, the Denali will have the highest pressurization in class, promising low cabin sound levels and improved comfort for passengers. First flight is slated for later this year and certification is due in 2020.

Epic Aircraft, located in Bend, Oregon, at the former Cessna Corvallis production plant, has its sights on taking the top speed crown from TBM 900 series aircraft. Its all-composite Epic 1000, the production variant of the Epic LT kit airplane, again is due for certification “later this year,” according to sales director Mike Schrader, who has made this prediction in years past. The latest delay was caused by a redesign of the engine air inlet in late 2018. A single PT6A-67A turboprop provides propulsion.

Epic promises that the E1000 will be “the fastest, highest performing, most affordable and comfortable” pressurized single-engine turboprop. Similar to the TBM, the cabin is 15-ft. long, but it’s 9-in. higher and 6-in. wider. The company estimates the E1000 will have a 3,000-fpm initial rate of climb, a certified service ceiling of FL 340 and top speed in excess of 330 KTAS. Cabin pressurization is 6.6 psi, actually slightly higher than the Beech King Air 250. Max range is projected to be 1,650 nm at 265 KTAS and estimated tanks full payload is 1,100 lb. The flight deck features Garmin G1000 NXi avionics. The interior appears to be considerably larger than TBM series aircraft, but BCA has not yet evaluated the aircraft or measured its dimensions.

The future looks bright for single-engine turboprops. Aérospatiale, and successor companies Socata and Daher Aerospace, have delivered more than 780 TBM series aircraft. Pilatus has built more than 1,600 PC-12/PC-12NG single-engine turboprops. Piper has produced a total of more than 700 Meridians, M500s and M600s. Textron Aviation is on track to start deliveries of Denali next year and Epic arguably could enter service in the next several months.

This entire class of 3,000+ business aircraft traces its roots back to Alexandre Couvelaire and Aérospatiale’s TB 700 concept airplane of the mid-1980s. While Couvelaire’s twin-turbofan variant of the TB 700 never came to fruition, he can take a large part of the credit for pioneering the concept of today’s fuel-efficient, high-performance, single-engine turboprop. His visionary leadership has changed the business aircraft industry for decades to come

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