Embraer’s original idea for a “C-390” airlifter once seemed as straightforward as the designation implies. Engineers could borrow the wing and engine of the proven, passenger-carrying E-190, and modify the cabin as a cargo hold with a rear ramp.
With the structures and power based on the mature E-Jet, Embraer could afford to be ambitious with the task of systems integration and in-source the integration of several key systems – such as synthetic vision and closed-loop fly-by-wire – for the first time.
That was the original plan, as described in a series of leaks and presentations about the project from 2005 to 2007.
Then, the plan changed. By the time the “KC-390” programme was launched in 2009, Embraer had dropped any trace of the E-190 from the design, adopting a new fuselage cross-section, wing, engine and flight deck.
“There’s no commonality at all [with the E-190] – it’s a totally new aircraft,” says Paulo Gastão, Embraer’s vice-president for the KC-390 programme.
The company’s concept began to evolve from straightforward derivative to all-new aircraft when the programme’s most significant customer got involved.
The Brazilian air force, which created Embraer in 1969, expects much of its primary supplier – and the idea of developing a new airlifter caught its attention.
A Brazilian-made C-390, by default, would be the natural replacement for the air force’s 17 Lockheed Martin C-130Es and two KC-130Hs. Moreover, the air force was obliged to cover Embraer’s cost of development.
Rumours of the programme originally surfaced in 2005
Embraer
Therefore, the air force was in a unique position to set terms, and had no intention of settling for anything less than the capability offered by the venerable C-130.
The Embraer engineering team trudged back to the drawing board. Any hope of reusing the relatively benign wing and the General Electric CF34-10E turbofan engine of the E-190 regional jet was gone.
“We were able to do a nice aircraft with that [original] approach, but at a lower [performance] class,” Gastão says. The E-190 has a “limited wing surface, and limited thrust engines”.
By contrast, the Brazilian air force requires the KC-390 to replace nearly the entire operating envelope of the C-130J, but at a higher speed, altitude and weight class.
As the performance requirements were raised, Embraer found itself taking on an entirely new challenge. The C-130J has dominated the tactical airlift fleet since entering service in 2004. It now faced a new rival as a market emerged to replace hundreds of tactical airlifters worldwide – of which at least half are C-130s, Gastão says.
For such an audacious move, Embraer selected a potentially revolutionary configuration.
Turboprop engines have dominated the class of military airlifters below the Boeing C-17. Whether it’s the C-130, the Airbus A400M or the Antonov An-26, the most popular tactical airlifters in the market have always been powered by turboprop engines.
Embraer, however, decided to equip the KC-390 with two modern turbofans. The International Aero Engines (IAE) V2500-A5 was selected after a competition with CFM International. It is the same engine that hangs on the wing of the Airbus A320 family, with no modifications to the turbomachinery. The full authority digital engine control (FADEC) – a system that monitors and controls ignition for maximum efficiency – is new and tailored to the unique mission profile of the KC-390.
The selection of a turbofan engine for a tactical airlift mission raised eyebrows. Gastão acknowledges it is the most frequent question he receives about the aircraft. For Embraer, however, the choice was an engineering no-brainer, he says.
The most common concern raised about the use of a turbofan engine in the tactical airlift role is the threat of damage by foreign objects while taking off, landing or taxiing.
Gastão, not surprisingly, has an extended and well-rehearsed reply. It starts with a brief history of the design of turboprop blades. In the early 1950s, when the C-130 was designed, propeller blades were small and metallic, so their tips sat higher off the ground where they were less prone to damage.
As turbojet and turbofan engines appeared, turboprop blades evolved significantly to stay competitive. “The way of improving propeller efficiency is increasing the diameter and having complex geometries,” Gastão says. “Complex geometries today mean composite material. So when you have big propellers running near the ground made from composite materials, that makes propellers not so robust.”
As propeller blades have become less resistant to damage, the internal core of turbofan engines has improved, he says.
Another concern about foreign object damage is the thrust reverser mechanism on a turbofan engine, which is not necessary on a turboprop engine. But Gastão says the threat of debris ingestion by the thrust reverser is largely a myth. Ingestion occurs only at speeds below 40-50kt, a range at which using the thrust reverser is unnecessary.
Gastão acknowledges there will need to be an exception to this when the KC-390 uses the icy landing strips in Antarctica, where the Brazilian air force supports scientific missions from a Chilean air base. The base has a short runway covered in ice and snow, which requires deployment of the thrust reverser until the KC-390 comes to a stop, he says.
“But in this situation no [foreign object debris] is ingested. You just may ingest some water.”
Some concessions to performance, however, are inevitable when comparing a twin-engined aircraft with a four-engined aircraft like the C-130. An engine failure on take-off is one area where a twin-engined aircraft is always inferior to a four-engined alternative.
In the design process, Embraer took note of the Antonov An-32, a re-engined variant of the An-26. The An-32’s launch customer, the Indian air force, needed more power than the An-26 offered to take off from runways in the lower air density of the Himalayan mountains. As a result, the An-32 remains a twin-engined aircraft, but the increased size of the engine means it is over-powered for the majority of its mission, Gastão says.
Embraer faced the same decision with a twin-engined KC-390, but took the opposite approach.
“We decided not to do that because we would jeopardise the efficiency of the aircraft on more than 95% of the operational life,” Gastão says.
As a result, twin-engined aircraft like the An-32 will perform better than the KC-390 in the “high-and-hot” corner of the flight envelope. A four-engined aircraft, such as the C-130 or A400M, also has an advantage in certain conditions, such as engine failure. It is the difference between losing 25% or 50% of the aircraft’s engine power.
Gastão, however, points out that Lockheed made similar design decisions in the 1950s, when jet engine reliability was very low. The decision to use turboprop engines on the A400M came later, but it was still at least a full generation of engine technology ago, in the early 1980s.
“I’m not sure if [Airbus] had to restart today they would keep that decision and develop a better propulsion system for the aircraft,” he says.
The wing of the KC-390 is another departure from the design philosophy of the E-Jet. The differences here are as fundamental as how the wing is joined to the fuselage. The E-Jet fuselage features a stub on each side, to which two separate wings are attached. There is no wing stub on the KC-390. A single-piece wing is attached to the top of the fuselage section.
The wing itself was another design challenge. Embraer designed the wing to meet the Brazilian air force’s requirement for a cruise speed of Mach 0.8. At the same time, it also had to be reconfigurable in-flight, to achieve the same approach speeds as the C-130 without stalling.
Embraer addressed this challenge by drawing on the company’s background in designing advanced high-lift systems, with slats, flaps and multiple spoiler panels. The thrust reverser on a turbofan engine may not be as efficient as a turboprop, but Embraer compensates by deploying spoilers.
The airframer briefly considered the idea of deploying thrust reversers in-flight, mimicking how the C-17 performs a spiralling tactical approach but, in the end, the spoiler design on the KC-390 wing was deemed sufficient for the most demanding tactical landing situations.
The systems architecture of the KC-390 presented an opportunity for the company to develop its skills in integration. Airbus and Boeing usually integrate the flight deck and flight control systems of their own aircraft, but companies such as Bombardier and Embraer have often outsourced integration tasks to Tier 1 suppliers, such as Parker Aerospace and BAE Systems.
Embraer performed systems integration for the first time more than two decades ago with the AMX attack fighter/trainer, which was developed jointly with Aermacchi. But the company took a different approach with commercial aircraft programmes.
The flight control systems of the KC-390 feature a diverse selection of suppliers. Goodrich, a member of United Technologies Aerospace Systems (UTAS), supplies the primary controls. Hamilton Sundstrand, another UTAS company, delivers the secondary flight controls and slats. Sagem provides the horizontal stabilisers. All of the hardware, including BAE electronics and UTAS and Sagem actuators, is integrated into a functioning system by Embraer, Gastão says.
“It’s considered a strategy to have full capability in house,” he says. “It’s very important because with this kind of airplane you may need to tune the software for different parts of the envelope, different missions. So it’s nice to have all that in our hands.”
Tim Hall
EMBRAER'S 'AGGRESSIVE' FLIGHT TEST SCHEDULE
The Brazilian air force launched the Embraer KC-390 with the award of a $1.6 billion development contract five years ago. First flight is scheduled by the end of this year, followed by a scheduled delivery of a complete tanker and airlift system in 2016.
If that schedule seems aggressive, bear in mind that Embraer is building only two flight test aircraft, compared with an average commercial aircraft development programme that usually has five or more pre-production prototypes.
Paulo Gastão, Embraer’s vice-president for the KC-390, acknowledges the scheduling challenge posed by entering a year-long flight test programme with only two prototypes on the flightline.
“We have a very aggressive approach for our test campaign,” Gastão says. “We have just two flying prototypes. We have a very aggressive schedule to do that.”
As a manufacturer with multiple commercial and military aircraft already in service, Embraer was aware that the flight test schedule would be more “comfortable” with one or two additional prototypes, Gastão says. However, flight test prototypes cost a lot of money upfront to build, and the Brazilian air force contract limited construction to only two examples.
“So we have been working very, very hard to have an efficient test programme to work with those two prototypes in the needed timeframe, and being able to certify the aircraft to civilian and military standards,” he says.
The challenge of having only two aircraft lies in scheduling. By definition, the aim of a flight test campaign is to reveal design deficiencies that must be changed before the aircraft can be delivered to a customer. As changes are rolled in, the prototypes must be grounded temporarily to integrate new components or software.
“We have been working since the beginning of this programme to mitigate to the extent we can the probability of having to modify and do lay-ups on the prototypes,” Gastão says. “We have a lot of mitigating strategies for that purpose.
“But we do expect to have to make changes and layups on the prototypes,” he adds. “Only the test campaign will show if all the precautions we took were enough or adequate. But we do feel comfortable with all the mitigation strategies we implemented.”
THE LONG ROAD TO A WORKABLE PROTOTYPE
Sometimes, the features of an aircraft that did not make it into the final design can be the most revealing.
In 2011, Embraer and the Brazilian air force offered a rare glimpse to the public of a wooden mock-up showing an early version the KC-390’s cargo compartment.
Inside the full-scale mock-up was a moveable aft pressure bulkhead. It was shown in the retracted position, rolled up into the aft ceiling like a garage door.
Within two years, this novel design feature had been removed, but not before offering a revealing insight into how Embraer was learning to design a feature – a rear cargo door that opens and closes in flight.
The movable pressure bulkhead was necessary because Embraer was not yet comfortable with a design for a pressurised ramp door. The wooden mock-up shown in 2011 featured clamshell doors that were not designed to pressurise the cabin upon closing. Instead of using pressurised doors, Embraer proposed the moveable bulkhead, which would retract to offload cargo or paratroopers in flight.
But Embraer continued searching for a better solution to the problem.
“It was a very complex mechanism and we finally got a way to avoid that,” says Paulo Gastão, Embraer’s vice-president for the KC-390.
The design of the clamshell doors was driven by geometric restrictions for the aft fuselage with a Mach 0.8 cruise speed. At that time, Embraer’s design studies had not found a way to provide for the use of a conventional door with enough clearance for dropping cargo without raising the height of the aft fuselage, Gastão says. The extra height would cause an increase in transonic drag.
But Embraer’s engineers kept working the problem until they found a way to eliminate the clamshell doors and install a conventional ramp and pressurised rear door.
Source: FlightGlobal.com
