Machine Design

Airbus builds a Military Airlifter

A new, multirole transporter will replace aging heavylift aircraft in military fleets.

The A400 military transport plane from Airbus will have four 11,000-shp turboprops, giving it enough power to take off from short, rough landing fields (under 3,000 ft), and still carry a 20-ton payload. Propellers on each wing turn in opposite directions to improve aerodynamics and to reduce noise.

The TP400-D6 turboprop engine uses an offset gearbox, keeping the total engine length down and improving the engine's stiffness. The 11,000-shp engine uses a three-shaft design, which also helps stiffness, says Airbus.

Built with airborne refueling in mind, the A400 can transfer fuel to two aircraft at a time. In this image, both wing-pod drogues are extended, but only one is being used.

Twelve wheels on the main landing gear spread out the weight of the A400, letting it land on soft, unprepared fields.

The Airbus airlifter will be able to fly low and avoid radar, then drop armored units.

The cockpit of the A400 features eight interchangeable large screens, head-up displays for both pilot and copilot, lighting compatible with night-vision goggles, and sidestick controllers rather than yokes for unrestricted views of the various displays. For more complex missions, a third crewmember can sit behind the pedestal.

Acritical task of any military is to get troops and supplies where they are needed as quickly as possible, and that usually means large transport aircraft. Currently there are only 2,600 tactical airlifters and about 350 strategic airlifters worldwide. (Strategic airlifters like Lockheed's C-5 Galaxy are much larger and have greater ranges than tactical ones.) But most strategic transporters are operated by the U.S. and the Commonwealth of Independent States (what remains of the U.S.S.R.). And the average age of the world's tactical transport aircraft is 26 years. So they lack the range, power, and reliability of modern aircraft, their cargo holds aren't sized for today's loads, and interoperability between fleets is abysmal.

To the managers at Airbus back in 1999, this meant there would be a market for an all-new aircraft aimed at the logistic needs of European and NATO militaries. They began a program to develop just such a plane, consulting with future buyers on what they wanted. If all goes well, the first A400M will fly in 2008 and customers will begin taking delivery the following year.

At the heart of the A400 will be its four TP-400-D6 engines, each weighing about 4,000 lb and putting out 11,000 shp, making it the largest turboprop engine ever built in the West. (The Russians built the largest, the 13,800-shp D27 used on the Antonov 70.) The A400 will need all that power to handle anticipated loads of 81,000 lb.

Turboprops have several advantages over straight jet engines. They are more fuel efficient, giving the A400 a 2,000-mile range (loaded). Turboprops can generate lots of thrust using variable-pitch props even when moving slowly for good takeoff and climb performance. The A400, for example, can take off from a 3,000-ft field. The engine can also be slowed while the props provide the drag needed for quick descents. The speed, climb and descend performance, ability to fly at up to 40,000 ft, and quick engine response will let the A400 keep up with airliners operating in crowded, traffic-controlled airspace. They also make it less risky to go in and out of airfields on or near a battlefront.

The A400's eight-bladed props will send a significant amount of air over the wings, regardless of airspeed, giving the plane better low-speed agility, an important factor for parachute drops. The props also let the plane taxi in reverse and up slight slopes (2% gradient) on less than ideal airfields.

The TP400 is being developed by Europrop International (EPI), a partnership between U.K.'s Rolls-Royce (28%), France's Snecma (28%), Germany's MTU (28%), and Spain's ITU (16%). They will rely heavily on Rolls-Royce's experience with multishaft Trent engines to build a three-shaft configuration for the TP400. The 3.5-m-long engines will have low, medium, and high-pressure sections, each with its own compressor and turbine stages on independent shafts spinning at different rpm. For comparison, the three-shaft Trent 800 engine used on 44% of Boeing's 777 has a low-pressure section spinning at 3,000 rpm, the medium-pressure section spins at 7,500 rpm, and the highpressure section turns at 10,000 rpm. (Exact figures for the A400 are confidential.)

Three-shaft engines are typically shorter and more rigid compared to the more conventional two-shaft designs. So the TP400 will flex less in flight, leading to less wear and letting it maintain its performance over time. It is also less costly in that only components in the high-pressure section need specialized coatings to withstand the extreme temperatures (perhaps over 1,600°C).

The three-shaft design is inherently modular, so it can be easily updated and upgraded. The Trent family of engines, for example, can be scaled from 55,000 lb of thrust up to 100,000 lb by using different combinations of compressors, combustors, and turbines. Three-shaft engines are also lighter than comparable two-shaft engines. A Boeing 777 with two Trent engines, for example, weighs 7,500 lb less than one with the competing engine. To further reduce weight and get the part count down, the engine will use blisks, monolithic components that combine the disc and blades.

Each engine turns an eight-bladed variable-pitch, fully reversing prop at 840 rpm (max). This gives them a 951-ft/sec tip speed at Mach 0.68. The props have a 17.5-ft-meter diameter and are designed and made by Ratier-Figeac, a French subsidiary of Hamilton Sunstrand. Each blade has a carbon spar surrounded by a composite shell with a polyurethane anticorrosion coating. The leading edge has electric deicing while the outer edge has a nickel guard to protect against foreignobject damage (FOD) and erosion. The props will be geared to turn "down between the engines." So if you were looking at an A400 nose-on, the inboard engine on the wing to the right spins clockwise while the outboard wing spins counterclockwise. This makes for more symmetric airflow, reduces wing loading and noise, and improves lift.

If the heart of the A400 is its engines, its brains must be the Loadmaster Workstation. It's loaded with data on practically every piece of military equipment listed in the European Staff Requirements for the seven countries involved in developing the A400. The data includes weights, volumes, centers of gravity, and tie-down points. The station is located on the left side of the cargo deck just below the cockpit, giving the Loadmaster a commanding view of the cargo hold, doors, and ramps. From there, a single loadmaster controls all freight-handling operations, such as loading, unloading, low and high-altitude deliveries, and parachute drops, and other cargo-related tasks. The loadmaster also uses the computer for ancillary tasks including lighting, heating, and operating the plane's two electric winches.

The plane can carry 116 fully equipped troops in four long rows of seats in its cargo hold, which is 58-ft long (with a 17.5-ft ramp), 13-ft wide, and 12.5-ft tall. Seats along the wall are mounted permanently but can be folded up and out of the way if needed. The two center rows are fully removable. Paratroopers can jump from two rear doors or the ramp, and four static-line anchor cables can be used by troops or cargo.

For medical evacuations, the plane carries 66 standard NATO stretchers along with 25 medical personnel. Stretchers attach directly to tie-down rings in the floor and there are provisions for curtaining off an intensive-care section. And the plane's oxygen supply has provisions for handling the extra demand patients might have.

For cargo, it carries nine standard military pallets (108 88 in.), with two on the ramp. The floor is outfitted with a roller system for moving and securing the pallets, but it can be easily stowed below the floor, leaving a flat surface for holding vehicles. The hold is sized to carry a variety of vehicles such as six light trucks and trailers, two medium-sized trucks, two attack helicopters, or one transport helicopter.

To simplify loading and unloading, the ramp has three extendable hydraulic "toes." These mini-ramps adjust up and down so cargo can be rolled on or off to trucks with different cargobed heights. They can also be used to extend the ramp when needed. To further ease cargo handling, the A400's main landing gear has hydraulic chambers that let the gear kneel, lowering the back of the plane.

The retractable landing gear has also been optimized for short, unprepared airfields. It uses two, six-wheeled main landing gear with long-stroke shock absorbers and three independent lever-type struts per gear. Up front is a twinwheeled nose gear. Using a dozen wheels on the main gear spreads out the load, letting it land on fields with California Bearing Ratios (CBR) of 6 or above, which translates to a soggy football field. (CBR is a standard measure of the soil's shearing resistance. A CBR of 1 equates to loose sand while a CBR of 100 is crushed rock.)

Airbus promises the A400 will come off the assembly line with the necessary software, plumbing, and mounting hardpoints on the wings for two fueling pods to convert it from a cargo hauler to an airborne gas station in 2 hr. The refueling pods carry trailing drogues that transfer fuel to other aircraft at 2,640 lb/min. The pods draw fuel from the A400's own tanks, which hold 50 tons. If the mission calls for it, two pallet-mounted 6-ton fuel tanks install on the cargo deck, and another-fuel drogue pumping 3,960 lb/min can be sent through the rear cargo door. Up to two aircraft can refuel at a time. And thanks to the high-lift wing and powerful engines, the A400 has a generous refueling envelope. It can off-load fuel to jets flying 420 knots at 24,000 feet or to helicopters down at 4,000 feet flying at 110 knots. On a typical tanker mission, the A400 can stay on station 400 miles from base for 2 hr and give away just over 40 tons of fuel. And if needed, the A400 itself can be refueled through its nose probe.

C-130 Hercules


  A400M C-130 HERCULES
First flight 2008 (planned) 1955 (C-130A)
Wingspan 139 ft, 1 in. 132 ft, 7 in.
Length 138 ft, 6 in. 97 ft, 9 in.
Turning radius 94 ft 122 ft
Weight (empty) 147,000 lb 73,000 lb
Max take-off weight 287,000 lb 175,000 lb
Payload 82,000 lb 45,000 lb
Max speed 350 mph (Mach 0.77@ 37K ft) 386 mph (Mach 0.58@ 22k ft)
Service ceiling 40,000 ft 33,000 ft
Crew Three Three to five
Range (combat) 4,000 miles 2,490 miles
Range (ferry) 5,600 miles 4,605 miles
Cost $100,000,000 $67,000,000 (C-130J)

The fuselage is a conventional monocoque, but to maximize cargo volume, the cross section is not a true circle. Primary support structures, such as the skin, stringers, and frames, are aluminum. But titanium alloys are used in highly loaded areas such as around the windscreen, the wing-to-fuselage attachment, and around the landing gear. Lightly loaded areas, such as the fairings, use glass or carbon-reinforced composites. The fuselage maintains 8,000-ft cabin pressure when flying at 37,000 ft, and for high-altitude missions, it will keep 9,000-ft pressure at altitudes up to 40,000 ft. For medical evacuations or other sensitive cargo, the plane can maintain sea-level pressure up to 19,400 ft.

The wing makes extensive use of composites, technology Airbus learned making civil airliners. The wings use a front and rear composite spar, giving them a wide wing box and a significant amount of volume for carrying fuel. The left and right sides of the wing have an outboard aileron, five spoilers used for roll control, speedbraking, and lift dumping, and inner and outer fixed-vane flaps. All are made of carbon-fiber composites for a high strength-to-weight ratio.

Composites are also used for the wing skin, stringers, and spars, but metal is needed for the ribs, leasing edges, engine mountings, and fuselage pick-ups.

The aircraft uses a T-tail, which improves aerodynamic efficiency by keeping the tail's control surfaces out of the wing's wake. It also cuts the risk of FOD to the tail.

The A400 will be a military plane, flying missions in and out of hotspots. To protect the crew, interior fixtures will hold armor plating, and windows will be bulletproof against 12.7-mm rounds. Engineers will duct and muffle the exhaust to minimize the plane's four-engine-IR signature and vulnerability to shoulder-fired missiles.

To detect radar-homing missiles, the plane will carry missile-launch and approach warning systems that rely on radar and visual cues (engine firing). The avionics will detect ranging lasers bouncing off the aircraft. The pilot can then launch chaff and flares from up to 24 dispensers to decoy incoming missiles away from the plane. The A400 can also upset incoming heat-seeking missiles with lasers and confuse radarhoming missiles with a towed decoy. There will be hardpoints on the wing for mounting jamming and other electronic warfare pods. And if the plane is hit, the fuel system is pressurized with inert gas to prevent explosions and slow down fires. A Defensive Aids Computer coordinates all these systems, connecting the various detection systems to appropriate countermeasures. It can also be programmed to look specifically for anticipated threats.

Airbus has already booked 180 sales of the $100 million plane to eight NATO nations, and they've guaranteed delivery, performance, and final weights, much like a civilian purchase agreement.

Capacity to carry either:

  • Six land rovers and trailers
  • Two lightarmored vehicles
  • Dump truck and excavator
  • Patriot-missile system
  • Puma or Cougar helicopter
  • Truck and 25-ton trailer
  • Nine cargo pallets: or 66 stretchers and 25 medical personnel
Cargo and the A400M

The A400M has twice the payload and range as the C-130.

TAGS: Defense
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