Guy Norris/PHOENIX
AlliedSignal's RE220 auxiliary power unit is setting new standards in corporate aviation and is poised for big business in the regional jet market In a little over two months, the RE220 auxiliary power unit (APU) will make its first flight on Bombardier's Canadair CRJ-700 regional jet. It cannot come soon enough for the AlliedSignal-led international team that launched the APU programme in 1993.
Although the APU is flying successfully on two long-range corporate jets, the Gulfstream V and Bombardier's Global Express, the regional jet market is the reason for its existence. The RE220 was designed from the outset as a regional jet APU - hence the "RE" in its designation - and the corporate jet applications have proved a providential launch pad for the airliner version. The roots of the little engine go back to 1991, when a few crystal ball gazing sessions at AlliedSignal convinced the company that the regional jet business was worth watching.
"By 1992, we were asking original equipment manufacturers what features they would value most. Would they trade weight for fuel, and generally how they would rank different features?" asks Steve Newell, team leader for the CRJ-700 programme and an original member of the RE220 design team.
At the time, the effort was aimed at prospective 100/130-seaters, although the regional jet revolution since the mid-1990s has predominantly been in smaller seat categories. "The real regional jet market has kind of grown up from below," says Newell.
While the embryonic regional jets of the future were still little more than a twinkle in the eye of design engineers, the RE220 got the go-ahead on the back of some largely unexpected corporate jet developments. "As luck would have it, these business jet programmes came up with high altitude start requirements and big electrical needs," says Barry Gillespie, director for business aircraft and regional airliner APU programmes.
The outline specification for the RE220, although aimed at a substantially different end user, happened to provide almost exactly the performance needed for the new breed of long haul business jets.
The timing of the RE220 development co-incided with other major changes within AlliedSignal. A new broom was sweeping out the complacency that had lost it the valuable Boeing 747-400 contract to Pratt & Whitney Canada, and promoting the emergence of strong competition from Auxiliary Power International (APIC). Above all, AlliedSignal recognised the value of international partnerships and, for the first time, embarked on a risk and revenue co-operative development effort for its latest APU.
"It was a first for us and we have done all of them like that since then," says Gillespie. Some of the partners responded to a request for quotes, while others were approached by AlliedSignal "-because we wanted to team up with key strategic partners", he adds. The final team consists of Alfa Romeo Avio, BMW Rolls-Royce, Kawasaki Heavy Industries and Singapore Technologies Precision.
While the team was being formed, AlliedSignal focused on the design itself, using three basic guidelines:
the RE220 was to use proven technology rather than require the development of new technology during the design process;
secondly the only LRUs (line replaceable units) used would be those that had already proved they would last for the entire life of the engine;
lastly the entire design would be based around a cycle durability test of more than 6,000h.
"Using these targets to match what the customer wanted, we came up with the basic path to go down. It helped us decide whether to go for a shaft or load compressor versus an integral bleed design, a radial versus an axial turbine or a digital versus an analogue control system," says Newell. "We had to talk to engine makers and airframers to see how the market was moving in terms of main engine developments, environmental control systems and electrical load requirements. We basically talked to them about everything you'd need for a 100-seater".
While part of the team scoured industry for input, another looked internally at the other AlliedSignal APU projects. The RE220 therefore inherited design features from the recently developed 331-500 APU for the Boeing 777 and the 131-9 for the Airbus A320 family and Next Generation Boeing 737.
The calls for low-maintenance and high reliability goals that guided the development of both these commercial APUs were repeated with the RE220. "The message we were getting from the airlines, other than low cost, low noise, low weight and fewer emissions, was: 'Don't use us as your development arena, we don't even want to know there is an APU back there'," recalls RE220 engineering manager Bill Tesch. "It led us to a much heavier investment in up-front testing".
The RE220 programme officially started in mid-1993 with the first contract from Gulfstream, and design freeze was achieved by the end of the year. The first engine run occurred in July 1994 and technical standard order (TSO) was obtained in March 1996 for the RE220(GV) Gulfstream aircraft. "It took us 30.5 months from kick-off to TSO, which was pretty good," says Newell. The TSO for the Global Express version, the RE220(GX), was obtained the following September.
The first production RE220 was delivered to Gulfstream in February 1996 and, today, 33 are in service, with more than 70 on order. "Entry into service has been basically flawless," says Gillespie, who adds that more than 10,050h have been accumulated to date.
Integration capability
Deliveries to Bombardier began in September 1996, but involved more direct involvement from AlliedSignal, which is responsible for the entire installation in the Global Express. "It was a first for us," says Gillespie, who adds that the system integration concept has grown in popularity with the company's selection by Bombardier for the Learjet 45 and CRJ-700, Raytheon for the Hawker Horizon and Fairchild Aerospace for the 728JET and related 528/928JET family.
The APU installation kit for the Global Express includes the exhaust system, consisting of tailpipe, sound-proofing muffler and rear support, as well as the inlet door, inlet duct, bleed system and surge system. It also includes the eductor, oil cooler duct, mounting struts and, of course, the RE220 APU itself. First deliveries of the Global Express are expected to begin around June.
The ramp-up to what AlliedSignal hopes will mark the real start of the RE220's long-awaited regional business is well under way, with two flight test units already delivered to Bombardier for the CRJ-700. "We're doing the APU system and we are looking at ways to combine this with the environmental control system [another area of AlliedSignal specialisation], and combine it as a single product," comments Gillespie.
As with the Global Express, the kit for the CRJ-700 includes ducts, inlet doors, struts and even drain plugs. "It is basically everything you need to put an APU into a tailcone. The first have been delivered and they fit," says Gillespie. The first flight is anticipated around May. The APU, meanwhile, is 90% through qualification tests which are scheduled to finish at the end of March. AlliedSignal plans to apply to the US Federal Aviation Administration for TSO approval by 1 April.
With work on the CRJ-700 well under way, AlliedSignal is moving resources to support the 728JET programme for which it was selected last September. The programme, which is due to be extended to cover the proposed 528JET and 928JET variants, is in the preliminary design review phase. "There has been lots of operator input from airlines like Lufthansa, which wants us to provide the whole package," says Tesch. "In previous aircraft, the airframe manufacturer and actuator maker controlled things like inlet door scheduling to coincide with the demands of the APU. Now we're doing the whole thing as a single package," he adds.
The first RE220 for the 728JET is due to run in October, with initial delivery of a flight test unit to Oberpfaffenhofen, near Munich, planned for March next year. The aircraft is earmarked to enter service in the fourth quarter of 2001, although discussions with ATR over a regional jet alliance may affect this schedule.
The APU is also in the running for two other potential regional jet programmes, Bombardier's BRJ-90 (widely known as the BRJ-X), and Embraer's recently announced ERJ-170 and -190. AlliedSignal's work with Bombardier on the CRJ-700 and Global Express puts it in a key position, says Gillespie. A request for proposals (rfp) on the APU and associated systems is expected mid-1999.
Securing the Embraer project, on the other hand, is less of a certainty because of the strong presence of Sundstrand, the incumbent APU supplier to the ERJ-135/145 programmes. An rfp is widely expected "within weeks" to co-incide with the timetable outlined for the Brazilian manufacturer's proposed 36-month development and flight test effort. The first delivery of the ERJ-170 is targeted for mid-2002, followed by the ERJ-190 in 2004. The programme's future still depends, however, on the outcome of partnership discussions which Embraer hopes to finalise by mid-1999.
Inside the RE220
From the compressor to the exhaust, the RE220 is strongly reminiscent of the 131-9. One of the most obvious shared design features with the 131-9, and the much larger 331-500, is the eductor cooling system.
This is a passive cooling system that uses the pressure differential between the exhaust duct and ambient pressure to create suction. This draws air in through the oil cooler, via the eductor nozzle and through the compartment before passing it out through the exhaust. The eductor nozzle is mounted on the support that holds the turbine bearing section in place.
The eductor eliminates the need for a dedicated forced air cooling fan which, in earlier APUs, was driven off the gearbox. Historically, this has been one of the most troublesome and maintenance intensive parts of the engine. The eductor, by contrast, has no moving parts to go wrong. The system is therefore easier and cheaper to maintain, produces lower oil temperatures and improves starter life by eliminating 'fan drag'. The absence of a fan also cuts noise emissions and eliminates the danger of an oil leak from the fan seal which, in the past, has caused the oil cooler to clog.
AlliedSignal adds that the eductor system dramatically improves compartment and LRU cooling. In some applications, the system lowers average compartment temperatures by up to 14°C, while LRU temperatures are reduced on average by around -2°C. The temperature reduction has other benefits, such as longer life expectancy for the electrical LRUs in particular.
The turbine backing on to the exhaust section is of a two-stage, axial, design. The first stage turbine nozzle is internally cooled with bleed air because of the higher operating temperatures of the RE220. AlliedSignal studied alternative methods, such as bi-metal materials, but eventually opted for the more conventional, low-risk, design.
The first-stage stator is made from Marem 247, a high-strength super alloy, while the first stage blade is made from the directionally solidified version of the same alloy. The second stage stator is made from Inconel 713, a high nickel content, heat-resistant alloy, while the second stage rotor blades, which are formed into a blisk - a combined blade and disc stage formed in a ring - are made from the powdered metal, Astroloy.
Close to the turbine section, the back end oil sump differs from some previous designs in this power category in having both a carbon ring seal and a knife edge seal for double-redundancy. This feature is taken from the -131-9. The reverse flow, effusion-cooled, combustor, on the other hand, borrows its origins from the 331-500 design. The combustor lining, which is drilled with thousands of holes to permit the "effusion" cooling process to work, is made from a nickel-based alloy, HA188.
Immediately forward of the combustor, and above the impeller, is the bleed scroll - a concept pushed by BMW Rolls-Royce - which also had design responsibility for the compressor section. In an engine with a radial impeller, like the RE220, the bleed air has to be removed between the diffuser and the de-swirl, rather than at the plenum as with an integral bleed design. The bleed scroll runs in a continuous loop and produces better recovery, which makes it "much better than just a bleed port", says Newell. The temperature of the bleed air is also lower because the scroll is further removed from the combustor area than the conventional port location.
The BMW R-R designed compressor is made up of titanium blades and splitters. Hub containment for the impeller and the turbine is built into the structure of the compressor section itself. Any debris from a rotor burst would be funnelled into the gap between the forward and aft shrouds that segment the powerplant at this point. The outer casing of the engine is strengthened in this area.
The curved face of the forward shroud supports the front bearing housing and forms the aft half of the gearbox compressor. The front end sealing is also designed with redundancy in mind. The oil is held at bay by a carbon face seal, a knife edge seal and is also "buffered" with compressed air. Sealing is being given top priority as problems encountered in the past with other APUs have led to smoke and odours in the cabin. A "witness drain" leads off from the sealing section.
The starter and gearbox were also given priority because of the high altitude start requirements of the initial business jet applications. "This APU starts higher than any production APU, and high altitude starts up to 43,000ft [13,000m] present a challenge," says Gillespie.
Although the starter itself is a standard DC motor, the gearbox features a resistance coil heater with a thermostat which heats oil in the sump just enough to keep it to a minimum viscosity level.
The slow heating prevents the oil from becoming too sludgy, or even freezing, during lengthy flights at altitude during which the airframe is subjected to "cold soak". The aluminium gearbox also houses spur gears made from low carbon, 4140, steel alloy.
With up to 14,000h of test running, plus over 10,000 h in real-life corporate jet operation, the RE220 team hopes that the APU will prove more than a match for the coming rigours of the regional jet business.
Source: Flight International
