FLUG REVUE January 2005: The latest helicopter engines detailed

ROTARY WONDERS: SMALL TURBOSHAFTS

By Patrick Hoeveler

Time is pressing. The pilot of the rescue helicopter starts up the engines. Within next to no time the two engines have been brought up to take-off power. The turbines are rotating at up to 40,000rpm. Compared with this, the 12,000rpm achieved by the high-pressure turbine of the GP7200 that is to power the gigantic Airbus

A380, seems almost slow. On the other hand, without the gearbox the compact powerplant fits comfortably into a large sports bag. “The challenge with turboshaft engines for helicopters is to deliver the maximum power with the minimum weight,” explains Charles Claveau, director of helicopter engine programs at Turbomeca during a visit to southern France by FLUG REVUE.

“The main parameter for helicopters is the ratio between power and weight.” Fuel consumption he views as more of a secondary factor. “The power required on helicopters is growing every year. For example, in the 1960s Bell had the JetRanger certificated at 313kW. Today it produces 485kW with the same weight.” Above all, with today's twin-engined helicopters, one engine has to be capable of generating the output of both units for a short time in an emergency situation. Although the growth potential of a turboshaft is limited to 25%, the engine builders are essentially concentrating on enhanced versions of existing models. After all, as Claveau points out, a new engine will cost about 80 million euro through to certification, while each new version can be had for around 10 million euro. “On top of this, the market is spread over an incredible number of helicopter models, over 60.” Whereas Turbomeca supplies the entire bandwidth, other manufacturers concentrate on particular segments.

“Our core business in turboshaft engines for helicopters lies in the range below 900kW,” says Jean-Luc Couderc, marketing manager turboshaft engines at Pratt & Whitney Canada. Here the Canadians are concentrating primarily on the PW200 family, which last summer passed the one million operating hours mark. Since certification of the first production model PW206A for the MD Explorer in December 1991, the company has produced over 1,140 units. Production for the year 2004 was expected to total around 200 units. As regards to new versions, Couderc is not giving anything away. “We are in discussions with helicopter manufacturers regarding new applications and also retrofits.” He was not prepared to discuss any details. P&W Canada has supplied six PW207K's for the Kazan Ansat, which are now being used for flight testing of the new helicopter. Couderc expects the engine to achieve certification with the new platform in 2007.

Meanwhile, Honeywell is working on improvements to the LTS101 family, of which there are around 1,700 currently in service. The LTS101-850 earmarked for an upgrade to the HH-65 helicopters operated by the US Coast Guard has already been certified. Thanks to a cooled gas generator turbine, it produces 14 percent more power at sea level than its predecessor, the LTS101-750. The service life of the turbine disc is now 15,000 flight cycles. However, the US Coast Guard's order for 225 engines went to Honeywell's rival, Turbomeca, which was offering the Arriel 2C2 CG. At present the main focus of work on the latest version, the LTS101-850B-2F, which is scheduled for certification in mid-2005, is on a new compressor design and two-channel Full-Authority Digital Engine Control (FADEC). Another variant known under the present Honeywell designation system as the HTS900 (Honeywell turboshaft power class 900 shp) is planned. This will have a new compressor and generate 30 percent more power. The manufacturer expects this engine to be certified this year. As at Pratt & Whitney Canada and Turbomeca, plans are also being developed for the future Modular Affordable Product Line (MAPL) family of helicopters from Bell, which should be available at the end of the decade.

The most widely used engines continue to be the model 250 family from Rolls-Royce (formerly Allison), the first of which dates from the 1960s. Today some 16,000 of these engines are in service. The 29,000th unit is expected to be delivered during Heli-Expo in February. The engineers are working on improvements to the series, with the emphasis above all on service life, reliability and reduction of manufacturing costs. They are also studying plans that were begun back in the 1990s for enhanced performance versions, which amongst other things could have a single-stage gas generator turbine.

With 12,320 shaft engines in service, Turbomeca also has plenty of plans. Some 5,500 units of the Arriel, which was certified for the first time in June 1977 and which Charles Claveau describes as “the best-seller of our range”, have been delivered to date. Up to 500 of them are built every year. An enhanced version 2S2 for the Sikorsky S-76 is currently undergoing testing. It generates six percent more power than the 2S1, its blades are constructed from a new single-crystal alloy and it also has a two-channel FADEC. Altogether there are around 30 versions in the Arriel family. “Each time we changed one element.” As Claveau explains, as a result of this continual evolutionary process, fewer than ten parts in the latest variants are built the same way as they were on the first. The main changes between Arriel 1 and Arriel 2 are the single-stage, rather than two-stage, turbine and the introduction of a FADEC system. Turbomeca's other engines below 900kW are the Arrius (first certification February 1988, 1,300 engines produced) and the TM 333. Turbomeca has supplied 82 of the latter to HAL for the Dhruv. On top of this, there is a potential in India of 150 units of the TM 333 2M2 for the Cheetal, a modified Cheetah (Lama). It was with one of these helicopters that the Indian Air Force set a new world record on 2 November 2004 by landing at an altitude of 7,670 metres.

But just how many new records advances in technology will bring in the future remains to be seen. Fuel consumption is one of the foremost issues, as Jean-Luc Couderc of Pratt & Whitney Canada explains. “The operators want greater range. That means lower fuel consumption with an engine that is lighter at the same time.” According to Charles Claveau, due to the difference in operational profile of the helicopter compared with commercial aircraft, the savings in this area are on a quite different scale. “A two percent reduction doesn't interest anyone. Ten percent is the minimum for helicopters.”

Another area of focus are the maintenance costs, which essentially is determined by the time between overhauls (TBO). The standard market value is around 3,500 hours. Here the engineers have to do a balancing act between performance and the associated high temperatures and the durability of the parts in the hot section. To give an example, on the Arriel, the air temperature at the intake to the gas generator turbine is currently 1100ºC. It would be technically possible to cool down the turbines, “but below 750kW this does not make any sense due to the high associated cost.” Didier Desnoyer, vice president operator support corporate at Turbomeca, concurs. “With cooling systems, you have to be very careful in respect to reliability and robustness.” Present engine models are generally overhauled a maximum of three to four times. Although there is no technical limit, often this is the upper limit for practical purposes, as after 12,000 hours an overhaul gets “really expensive”.

The interest in more electrical engines, discernible also in the area of helicopters, is likely to constitute a challenge for maintenance as well. “Customers are asking for more and more electrical power for anti-icing, air-conditioning, and so on,” says Gérard Paty, research program manager at Turbomeca. In future, the generator could be integrated the main shaft instead on the accessory gearbox as it is now. A core engine that required no oil is even conceivable. However, this would necessitate resolving problems caused by vibration, and safeguarding reliability. Already now there are difficulties with the digital technology, as Claveau points out. “Today there are more parts in the FADEC than in the rest of the engine. The problem of electrical components becoming obsolescent is terrible. We can't produce FADEC systems for two years without modifying them.” The control units have to be replaced at least every ten years.

On the other hand, there is a change of generation in the powerplant families every 30 years. In the class of the Arrius 2, the aim is to reduce the life-cycle costs over the next five years by 5 percent, and by 2014 by 10 percent. Whereas a possible Arrius 3 with a rating of 600kW would not be available until 2010 at the earliest, according to Claveau, it is “time to work on a successor to the Arriel. We possess the technology, but we are still looking for an application.” He estimates the development time at between four and five years. With the same rating, it should consume 15 percent less fuel and it could have a radial compressor and a gas generator turbine with two stages each. Dynamic simulations should predict vibrations and loadings for components like gear teeth and turbine blades as well.

In the 800 to 900kW range, Paty expects to see a new Turbomeca platform in 2006/07 which for the first time will be equipped with a single-part, combined axial and radial compressor and a single-stage turbine. Here the airflow will follow an S-shaped route through the axial component and after the second pair of bends pass through the radial component. The first full engine run of a compressor design sized in the class of the MTR390 is expected at the end of 2005. The new design promises a pressure ratio of 15.8:1, 20 percent more airflow and up to 30 percent more power. For a possible Arriel 3, the engineers would then scale down the compressor accordingly. The research departments are also looking into new materials such as non-metallic alloys in the cold section and noise reduction generally. Using ceramic or metallic liners in the exhaust outlet, Turbomeca has already demonstrated a reduction of four decibels. The EU research programme, Friendcopter, for environmentally friendly helicopters envisages a reduction of up to ten decibels. Therefore the development departments will have their hands full over the next few years.

From page 92 of FLUG REVUE 1/2005

Last updated 8 December 2004

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