A suite of leading-edge engine technologies is in transition as fierce competition drives the quest for innovation
GUY NORRIS / LOS ANGELES
In 1998 CFM International launched Project Tech56 with a three-year charter to improve every major component of the CFM56, validate new technology and identify applications. The initiative was CFMI's response to several threats, the most serious viewed as the imminent emergence of a new generation challenger from Pratt & Whitney: the PW8000.
As it transpired, P&W later redirected the PW8000 at higher thrust targets, while pledging to continue support and development of the International Aero Engines (IAE) V2500 along with major shareholder Rolls-Royce. The immediate threat which had sparked Tech56 therefore vanished, but longer-term reasons for the programme remained and were more valid than ever. They were a stronger IAE and the need to protect against the sort of complacency that had dried up the JT8D market.
"We cannot afford to be lulled into a false sense of security like Pratt was with the JT8D," says outgoing CFMI president Gerard Laviec. With 11,300 CFM56 engines in service by the end of 2000, he adds, CFMI is in a similar position to P&W 20 years ago.
Yet, such massive market success can be a double-edged sword when it comes to selling the Tech56 package. "The operators want to maintain commonality with their current fleets. We have also talked to both Boeing and Airbus and looked at the orderbooks, and today these say the airlines are satisfied with the products we have.
"We continue to evaluate with Boeing and Airbus what is the appropriate time to introduce new technology to the fleet. These decisions are happening again this year, and so we are using this time to validate the technology," says Laviec.
Since resources have already been committed to the Tech56 effort, validating the results of the first three years of design and testing is valuable to the CFM56's future - both in terms of upgrades and the possible emergence of a "next-generation" engine.
"We'd pretty much emptied the technology cupboard when we did the -5 and-7 engines," says CFMI executive vice president Bill Clapper, who adds that Tech56 is therefore a vital insurance policy for the programme's longevity. "It means that whenever the landscape changes we will be ready."
New applications
Despite the slowdown in finding applications for Tech56 in the CFM56 family, the original targets of the effort remain. These include: a 15 to 20% cut in the cost of ownership, up to 20% lower maintenance costs, a 4 to 7% lower fuel consumption rate relative to the -7B, and as much as a 20dB cumulative noise reduction compared to the higher-thrust members of the current generation CFM56 family. The plan also included cutting emissions by up to 50% below current International Civil Aviation Organisation rules.
First to test after the programme's launch was a new low-speed compressor design along with development of a new high-pressure compressor (HPC) rig design, a HP turbine (HPT) vane test and the start of evaluations of a novel twin annular, pre-swirl (TAPS) combustor incorporating micro-laminate technology. In 1999 tests began of a new swept blade shape and fan and fan structure.
The same year also saw the new HPC test rig's construction as well as the start of HPT rig work. By 2000, CFMI began brush seal tests, fan tests and started up work using the HPC rig itself. First full-up engine tests with the TAPS combustor also started last year. This year, the focus has moved up to larger scale validation tests using the turbine and HPC rigs, as well as fan tests on full-up engines.
Swept fan
Snecma is due to start tests of a 1.7m (68in) diameter, wide-chord, hollow, swept fan blade design at Villaroche by the end of June. It will be mounted on a CFM56-5C for performance and aeromechanics evaluation, and will be followed in August by a series of crosswind and acoustic tests.
This phase, which will also includeinitial performance check-out, will be conducted at General Electric's test site in Peebles, Ohio. The tests follow similar evaluations last year of a solid 1.5m-diameter titanium wide-chord fan on a CFM56-7B, which generated 6% more thrust than the identically sized -7B fan. It also had 2.5% greater airflow and 1.5% lower fuel consumption. "The 1.5m test results were very close to our pre-test predictions and have given us high confidence that the 1.7m fan will be just as good, or perhaps evenbetter," says Tech56 engineering manager Ron Klapproth.
CFM is accumulating hardware to begin tests by year-end of a modified booster (low pressure compressor) that can operate without variable bypass valves (VBVs). The development, which offers weight and cost savings, is to be tested on a -5 engine and is centred on casing treatments.
These are designed to raise the stall line with VBV doors closed at low speeds, "which is where the pinch point is", he adds. Three different casing treatment configurations will be tested, all of which replace the conventionally smooth casing of current designs, with "features that will strengthen the flow [over the tips of the LPC blades]", Klapproth says.
Tests are due to begin on "build 2" of an advanced six-stage HPC by the end of July. Tests of build 1, completed last year, showed a 24% high-speed stall margin, some 4% greater than the original target. "We want to hold on to that with the new tests," Klapproth says. The revised design has been "tweaked", he adds, to improve component efficiencies that were actually lower than the current production HPC design. "First we took care of operability and then we worked the blading to increase efficiency," he says.
The HPC has forward-swept rotors, bowed and swept stators, stronger blade tips and blisks in rotor stages one and two. Compared to the nine-stage current HPC which has a pressure ratio of 11:1, the new six-stage design has a pressure ratio of over 14:1, or a 29% improvement.
Increased pressure ratio
Stage loading is therefore higher, with pressure ratio per stage increased by 19%. The corresponding aerofoil number count is down from 1,518 to 968 in the new unit. Variable stage count is also reduced from four to three.
Hail ingestion tests of a complete CFM56-7B fitted with the TAPS combustor were wrapped up in May at Peebles after 26t of ice had been shot through it. "It handled the hail well with no problem. We intentionally flamed the engine out to map the operability boundaries and it came out very close to our predictions," says Klapproth. He adds: "We found control schedules that we can make it happen without penalties."
A further phase of endurance tests on the TAPS is under way, and the results will be fed into design of a "phase two" nozzle which will go on test at the end of the year.
Results to date indicate that the TAPS cuts nitrous oxide emissions to 65% of levels specified by ICAO, compared to initial targets of 50%.
Further tests of wire-based brush seals continue to give "encouraging" results with 40% reduction in leakage indicated to date. A new phase of tests begins in October with the new seals in place in a-5B test engine, replacing conventional labyrinth seals at the compressor discharge and turbine inlet.
Tests have included rapid transients, fan imbalance and other "abusive" evaluations, Klapproth says. Possible upgrades include revisions to the structure holding the wires in place.
Turbine tests
Tests of the latest iterations of new HPT and LPT designs on the purpose-built dual-spool test rig at Evendale, Ohio, have stalled after it developed vibration problems, requiring replacement of bearings.
The new HPT has 10% fewer blades and vanes than the baseline -5B/P unit (110 versus 122), a 15% higher pressure ratio and 9% higher stage loading.
The four-stage LPT also has a 15% higher pressure ratio and a 4% higher stage loading, while blade parts count reduces from 1,072 to 970.
The latest HPT design is optimised for further reductions in trailing-edge shocks (cut by 50% in the first two generations), while the "design 3" LPT will have 35% fewer airfoils compared to a -5B/P, versus a 20% reduction in "design 2".
Earlier Tech56 developments, such as the low-noise "chevron" exhaust design, are being incorporated into the CF34 family and considered for CF6 and GE90 derivatives. TAPS-based combustor concepts are also being studied for GE90 tests, while this and other Tech56 spin-offs such as the low-noise nozzles and advanced, counter-rotating LPT design are being proposed for GE's GENX sonic cruiser engine studies.
The question CFMI now asks itself is when will it be allowed to introduce the evident benefits of its Tech56 project into its own product line.
Source: Flight International
