Thanks to brilliant technical innovation, a newly-established French company is poised to make the small aircraft turboprop a reality. Mario Boric reports…
To say aviation and turbines is a happy marriage is true?albeit mainly in the airline, business aircraft and helicopter world. Light aviation, especially the ultralight segment, remains essentially a turbine-free field?apart from noble exceptions, in the form of single-engine jets and ‘experimentals’. Now French newcomer Turbotech is on a mission to challenge the dominance of piston engines and?unheard of in this field?is promising turboprop fuel consumption comparable to piston engines. The company also has an interesting turbogenerator proposal for new e-VTOL and electric aircraft.
Currently small turboprops are practically non-existent, although Czech company PBS has in its portfolio the TP100, which should soon be ready for aircraft installation (a few units have apparently been delivered to interested aircraft manufacturers). The TP100 is not in focus of this article as it is way too powerful for typical four-seaters and ultralights, producing 241shp (shaft horsepower).
Today’s focus is on Turbotech’s brand new product, the TP-R90 turboprop, which is rated at 90kW (120shp)?neatly falling in the ultralight segment, where aircraft are typically powered today by 100-115hp Rotax piston engines. Turbotech is also offering?based on a similar, but smaller turbine core?another interesting powerplant, the TG-R55 turbogenerator?a ‘range extender’ solution for hybrid-electric powered aircraft.
Seeking to establish a toehold in the market, Turbotech showcased its products last year at the most important exhibition in the field, AERO Friedrichshafen in Germany. At their booth I met company CEO and founder Damien Fauvet, who described his products to me. It must be said that many exhibitors hype their wares and you find their miracles tend to take a long time till they prove to work, even if the company doesn’t simply disappear prior to them actually coming to the market. However, Damien’s credibility and the very professional look of his company’s products intrigued me?especially the almost unbelievable claim that Turbotech fuel burn would be comparable to existing piston engines?so I decided to take a closer look.
By the end of February this year, the company and the engine’s state of development were apparently at least half-way ready to host a visit. While there was the opportunity, I wanted to see the turboprop running and be able to figure out how they managed to dream up a turbine sipping so little fuel. So I dropped in on Turbotech, which is based at Toussus-le-Noble Airport in the western outskirts of Paris.
In Turbotech’s offices, Damien showed to me a video of the first tests of the proof-of-concept engine, run in 2016. That day, the turbine was run up to 41.3kW (56hp)?which approximates the cruise power setting for most ultralights?and the fuel flow was 28.7 litres per hour, a specific fuel consumption (SFC) of 557g/kW hr, to use the yardstick preferred by engineers. At a slightly lower power setting of 38.9kW (53hp) the fuel flow dropped to 25.2 lph (507g/kW hr). Discussing the figures with Turbotech technicians, I gathered that these were only the preliminary data, and that the fuel consumption of the second-generation prototype unit now on test would be further reduced with improvements to the igniters, fuel nozzles, EEC (Electronic Engine Control) and many other components, which were far from being developed production items.
Damien proved these predictions correct when, just prior to producing this article, I received the data of the second engine, tested in March this year, where the fuel flow figures were?in line with Turbotech’s promises?slashed dramatically. Running at 66.4kW (90.2shp?just over 70% power), the development engine was now burning only 29.4 litres per hour of Jet-A1, an SFC of 354g/kW hr. At 31kW (42hp), the fuel consumption was just 15.2 lph. According to Turbotech, the definitive version will reach its 90kW target rating and will demonstrate a specific fuel burn close to the target of 340g/kWhr?wow!
It shouldn’t be possible…
Turbines are generally taken to be compact and lightweight relative to the amount of power they deliver, only fuel-efficient when operating close to their maximum power output and most suitable as high-power units (2,000shp-plus) for aircraft flying at medium to high altitudes.
As benchmark for ‘the turbine advantage’, one of the latest arrivals on the turboprop market, the 7,000shp powerplant for the A400M military transporter, is actually more fuel-efficient than the typical diesel car engine. Thanks to its high pressure ratio and turbine operating temperature, it extracts something like forty per cent or more of the energy contained in the fuel.
The advantage over piston engines in airliner and commercial aviation applications is very difficult to match in small turbines. One design path?followed widely in the industry?is to scale down big turbines, but this is unfortunately accompanied by great loss in fuel efficiency. According to Daniel Fauvet, simply scaling down established designs would lead to a very low efficiency microturbine “perhaps extracting only around ten per cent of the energy contained in the fuel”. This is the fundamental reason why we do not have on the market small turbines which are as fuel efficient as modern piston engines.
The other stumbling block is that the usual business case for a microturbine?given the large financial investment needed and uncertain market?simply doesn’t make any economic sense.
… So how did Turbotech do it?
The key to the possible success of small turbines is solving the problem of their fuel efficiency?specific fuel consumption has to be lowered, and drastically so. Turbotech has done its homework and the configuration of its unit has been refined through CFD (computational fluid dynamics), CAD (computer-aided design) and CAE (computer-aided engineering) using Dassault product Catia and Ansys software, Jean-Michel Guimbard leading the mechanical and aerodynamic design aspects. The result is that the Turbotech microturbine operates at 26 to 30 per cent efficiency.
It’s not just the turbine wheel design; when I first saw the Turbotech’s TP-R90 and TG-R55 (TP for turboprop and TG for turbo generator) I thought they looked bulky and over-long, giving the impression of being overweight. Other small turbines are way sleeker and shorter. Aha, but within this extra volume lies Turbotech’s unique selling proposition and the main reason for their engine’s parsimonious fuel consumption. In technical terms, the company says its designs are ‘regenerative’ (i.e. heat recuperating) cycle turbines that re-use energy that is otherwise wasted. Using a proprietary, patented heat exchanger, Turbotech has engineered a breakthrough in small turbine design that will surely be a game changer.
To appreciate why, you need to understand how a turboprop engine works. In simple terms, the compressor blows air into the combustion chamber where fuel is introduced and the mixture burns continuously. Energy is taken from the combustion gas by passing it through the turbine, which acts rather like a wind, or water mill. The turbine in turn drives the propeller and compressor. All well and good, but as the turbine is an imperfect device, the exhaust gas, emerging at 700°C or more, still contains a lot of energy. Rather than allow this to go to waste, Turbotech circulates the exhaust gas through a heat exchanger (think of it as radiator) that heats?puts energy into?the air flow from the compressor. And the hotter the air going into the combustion chamber, the smaller the amount of fuel you need to sustain operation. To give an idea of how effective this is, in the absence of Turbotech’s heat exchanger, air emerges from the compressor to enter the combustion chamber at 200°C. With the heat exchanger, it is warmed to 530°C, representing a considerable proportion of energy recovered from the exhaust.
As they say, there is nothing new under the sun. What Turbotech is doing is novel in the field of small aircraft turbines but not for large ones. There have been several attempts since WWII by big names in the turbine field like Rolls-Royce, Pratt & Whitney and Allison to use heat recuperation in their designs but none of them was successful, as in practice the weight penalty was excessive and they were too bulky and complex (and therefore too expensive) to justify their use.
(Sadly forgotten now, British car maker Rover’s turbine powered Le Mans racer was in 1964 fitted with a rotary regenerator that halved its fuel consumption, and the company came close to putting a similarly equipped saloon car in to production?Ed.)
Heat exchangers have been used in stationary turbine powerplants, marine applications and even in the Abrams battle tank (1,500hp, 28% thermal efficiency)?all applications where the extra weight and complexity are acceptable and justified by the fuel saving.
Aside from the heat exchanger, the TP-R90 and TG-R55 have the same basic architecture as the typical large-aircraft APU (auxiliary power unit)?a single spool turbine in which a single-stage centrifugal compressor and a radial-flow power turbine are mounted on the same shaft. The clever bit is the way Turbotech makes the compressed air follow a considerably longer path through its heat exchanger before coming to the combustion chamber. To achieve a higher temperature exchange ratio, and to contain its total length, the heat exchanger has been ‘folded’, reducing its installed length by half. In this case he compressed air first travels toward the rear, around the outside perimeter of the annular combustor, and through a first stage of the heat exchanger, and is then turned through 180° to flow forward to combustor, making a second pass through the heat exchanger.
Closely guarded IP
The heat exchanger is Turbotech’s ‘secret weapon’ in slaying the fuel consumption dragon. I have seen it but unfortunately?and understandably?the company has not allowed me to take any images of it, fearing possible theft of intellectual property. What I am at liberty to say is that it comprises thousands of microtubes made of Inconel, approximately 300mm long which are grouped in two packs of cylindrical shape: one outer ring?running cooler and one inner ring?running hotter, being closer to the exhaust gas path. (Inconel is a registered trademark of Special Metals Corporation for a family of austenitic nickel-chromium-based high temperature superalloys that have a low coefficient of expansion)
“The key to a successful microturbine is to build the heat exchanger channels using careful design and the right kind of microtubes,” says Jean-Michel Guimbard. “They need to be as light as possible and they have to have a long life-cycle.” Accordingly, the heat exchanger was designed and manufactured to resist vibration and mechanical and thermal stress. The mechanical stress was of particular relevance as the exhaust gas velocity and temperature varies between the two banks of Inconel microtubes.
Probably the most important moment in the process of development of the Turbotech powerplant and of the heat exchanger was the contact with the aerospace supplier Le Guellec, which was asked to manufacture the heat exchanger and became a partner and investor in the Turbotech project after it received insight to the project. Le Guellec is manufacturing its own microtubes on a very cost-effective basis.
One important side effect of use of the heat exchanger is that the gases exit the turbine way cooler, at 350°C and at lower velocity compared to traditional turbine engines, so the noise and thermal ‘footprint’ are radically lower. The outer metal casing of the heat exchanger doesn’t exceed 250°C, which makes the Turbotech engine suitable for use in UAVs where a minimal heat signature is essential, and for aircraft made of carbon composites which are particularly sensitive to excessive heat.
Availability and certification
Turbotech is testing and refining the TP-R90 and TG-R55 units, which should be commercially available by mid/end of 2021. A more powerful version of the TG-R55, the 90kW TG-R90 is in the pipeline. Almost identical to the turboprop version, this will weigh 64kg dry/74 kg installed, and should be commercially available by mid 2022. The electric generating efficiency is expected to be 23 per cent (from fuel tank to inverter output). EASA certification for all units will be pursued and is expected to follow in two to three years.
Capitalising on the low noise and low thermal signature of the TP turboprop, Turbotech is targeting the UAV market and is a considering number of light aircraft applications including experimentals, ultralights and small helicopters.
The promised ‘fuel-burn on apar with the best piston engines on the market’ is almost there, as the latest tests confirm. Of course, Turbotech has yet to prove the reliability of their products in everyday use?so important in aviation world?but having the turbine experts they do in their team I’m confident they will succeed.
In my opinion, the emerging market of electrically driven aircraft, and especially the countless e-VTOL door-to-door air taxi designs, will be interested in the TG turbine generator as these applications are power-hungry and we’ve spent years forlornly waiting for promised but not yet delivered high density, lightweight batteries. According to Turbotech, multiple TG-R90 units can be coupled together for electric installations demanding 500kW and more?a heaven-sent power pack for new eVTOLs?
For my part, I’m already dreaming about a fast European composite ultralight with a TG-R90 under the cowling. Get ready for an exciting Pilot flight test!
Bâtiment 209, Aérodrome de Toussus-Le-Noble,
Toussus-Le-Noble, 78117, France
Bringing their skills with them
To understand how such a small group of people was able to bring the project to today’s status we have to look at the company’s origins. Turbotech is a French start-up founded by four members in 2009. Their secret is that they previously worked for the Safran Group which is today, together with Go-Capital, one of their principal investors. Ile-de-France (the Paris region) and DGAC (Direction Generale de l`aviation civile) have provided further aid in the form of grants. Damien Fauvet developed a proof-of-concept and then sought and found partners among his former colleagues at Safran. The small group consisting of Fauvet (founder), Jean-Michael Guimbard (Co-founder, CFO & CTO turbomachinery), Baptiste Guerin (co-founder, COO) and Marc Nguyen (co-founder, CTO mecatronic) formed the core team that embarked on fund raising. Success was ensured when Le Guellec (precision tubes and profiles) joined the project and started delivering its microtubes. Le Guellec co-founder and CEO Francois Korner is now part of Turbotech’s management team.
The product range
Turbotech has as now, two products in their portfolio, the TP-R90 turboprop and the TG-R55 turbogenerator (the R standing in either case for regenerative and the numbers indicating the turbine power output in kW). Both units feature dual fuel injectors and sparkplugs for ignition, are driven by a proprietary EEC (Electronic Engine Control) system. The EECis similar to the FADEC systems on larger turbines and piston engines, and beside turbine control is capable of the logging data from numerous turbine and gearbox/generator sensors as well as controlling a variable pitch propeller on TP units. The projected TBO is 3,000 hours.
A variety of fuels can be used, including Jet-A1, diesel fuel, UL91 avgas and biofuel. Stated fuel consumption (Jet A1, cruise power) is 18-25 lph for the TP-R90, and 15-22 lph for the TG-R55.
TP and TG units have only two ceramic bearings (1 ball, 1 roller) where ninety per cent of oil flow is used for cooling and only some ten per cent serves for lubrication.
The 90kW TP-R90 single-spool turbine drives a propeller trough the propeller gearbox at the turbine intake end, reducing the turbine’s 80,000 rpm to 2,272 prop rpm. The unit is capable of delivering additional 10kW boost power – in total 100kW – supplied by starter/generator mounted on the gearbox. Price: 65,000 Euro (net)
The TG-R55 the turbine drives an generator delivering 53kW electric continuous power at generator output (400 to 900V DC or to customers specs). Engine starting is by the generator running in starter mode. The TG-R55 weighs 55/65kg dry/total and has an electric efficiency of 26 percent. According to Turbotech, as the range extender, the 115kg weight of a package of TG-R55 plus 50kg of Jet A1 fuel offers 155kW hr of electric energy, equivalent to the output of 1,000kg of batteries. Price: 70,000 Euro (net)
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