Turboshaft Technology

 - April 30, 2009, 12:00 PM

Helicopter turboshaft manufacturers are incorporating new technologies in their engines to lower fuel burn, enhance capabilities and reduce operating costs. The major manufacturers are developing engines to meet these demands, along with the need for more power and lower emissions.

Pratt & Whitney Canada’s new 1,000-shp-class PW210S turbo-shaft, developed to power the Sikorsky S-76D and on offer to other manufacturers for new-build and retrofit applications, is “on track and meeting performance [specifications].” The Montreal manufacturer told AIN that key performance tests have been “successfully completed” as it continues to “optimize” the engine. Certification, scheduled at one time for the middle of last year, now “remains on schedule for the fourth quarter of 2009,” according to the company.

Seven PW210Ss have been allocated to the engine development program. The PW210S flew for the first time on a Sikorsky S-76D in February. The company expects to complete all mechanical certification tests by the fourth quarter.

Product improvements introduced on the PW207D1, now in production to power the Bell 429, will benefit the entire PW200 family in the long term, said P&WC. “What makes these latest engines stand out is the mechanical takeoff power. The PW207D1 and D2 show about nine percent more takeoff power [than] their predecessor.”

The company said the increased power is evident at all ratings, including single-engine operations. “For example, OEI [one engine inoperative] ratings are higher as well.” The two new variants differ in that the D2 introduces a fuel heater to meet specific customer power-to-weight ratio requirements. P&WC received PW207D1/D2 Canadian type certification last November.

Now more than 45 years old, P&WC’s ubiquitous PT6C turbo-shaft continues to be enhanced, the latest -67E variant having been earmarked for the new Eurocopter EC 175 medium twin. “Development is on track, with engine certification scheduled for 2011.” The new model sports dual-channel full-authority digital engine control (fadec). For environmental friendliness, P&WC has eliminated materials such as cadmium and chromium from the manufacturing and design processes.

Turboshaft engine manufacturer Turbomeca is considering diesel engines up to 500 shp. The Bordes, southwest France-based company is studying the possibility of using a diesel demonstrator as part of Clean Sky, a European research program.

According to Charles Claveau, senior vice president for product and market strategy, the main benefit would be a 30- to 40-percent reduction in fuel burn. The main challenge is weight. “A diesel engine is four times heavier than a turbine engine for a given power,” he said.

However, since the power output of a diesel engine is almost unaffected by outside air temperature, unlike a turbine engine, “you do not need to oversize the engine,” Claveau pointed out. Moreover, recent technology improvements are yielding “very interesting” power-to-weight ratios, he said.

A key issue for a diesel will be torque oscillation. “The main gearbox of a helicopter does not cope well with such oscillations,” Claveau said. A turboshaft provides much more constant torque than a diesel.

In the field of turboshaft technology, Turbomeca is moving ahead with demonstrators in four different power ranges. Tech 600 is a demonstrator in the 500- to 1,000-shp range. Tech 800 is for 1,000 to 1,500 shp. Focuses are fuel burn (and hence CO2 emissions), NOx emissions and noise. Notably, Turbomeca engineers are working on further improving centrifugal compressors. This is done through increased airflow and optimized surge margin. The company is also planning another two demonstrators: Tech 1500, for the 1,500 to 2,000 shp range (no launch date scheduled) and Tech 3000 for more than 2,000 shp (to be launched within two years).

Turbomeca engineers are also looking at reducing engine noise, with the goal of shaving 5 dB by 2015. For example, the firm’s Teeni research program focuses on combustor noise modeling. In engine control, Turbomeca is in the early stages of designing a demonstrator for a low-cost engine electronic control unit, which will probably be less complex than fadec. Separately, equipment manufacturer Goodrich has been working on reducing the cost of fadec to make the technology affordable for small engines.

Reducing Fuel Burn
Doug Kult, director of sales for helicopter and surface systems for Honeywell, noted that recent DoD turboshaft engine programs “fund us to push the limits of technology, reducing weight and fuel consumption. We’re able to use those improvements in our commercial engines as we develop them.”

As an example he cited the newly FAA-certified HTS900. “It has a specific fuel consumption of about .52. We got there by taking the twin centrifugal compressor from the Small Heavy Fuel Engine program, and combining it with new means of controlling surge. Most engines have a mechanical device in front of the compressor to regulate

the airflow. The HTS900 has no inlet guide vanes or flow fence; instead it has a portage shroud that guides the airflow through a series of holes to control surge in all conditions. It’s a passive device with no moving parts and requires no repair or overhaul. We also use cooled turbine vanes to allow higher operating temperatures for greater efficiency. Tip leakage control is by a segmented shroud that fits around the turbine and seals the blades. It grows and shrinks in concert with temperature changes to maintain a constant seal over a wide range of temperatures.”

The HTS900, with a 1,000-shp takeoff rating, is Honeywell’s newest turboshaft engine, originally built for the Bell 407 ARH (armed reconnaissance helicopter). “So we’ve got an engine and hope to announce a new application soon,” Kult told AIN. Originally, a 925-shp version of the HTS900 was also slated for the now-cancelled Bell 417.

Another new Honeywell turboshaft engine technology “in the pipeline,” according to Kult, is a dual-alloy turbine with a higher cycle life and lower operating costs. It will be a one-piece turbine with two different alloys, one for the hub and another for the blade section.

At the Heli-Expo convention earlier this year, Honeywell presented the latest and most powerful version of its LTS101 commercial engine, the 850-shp LTS101-850, being developed for certification on the Eurocopter BK 117. It will be offered to upgrade the BK117 “B” model to “C” performance specifications.

Rolls-Royce in February launched the RR500 turboshaft, a 475-shp derivative of the RR300 that powers the in-development Robinson R66. Certification is scheduled for late 2011.