In 1976, Hughes Aircraft rolled out a new product called the Defender - an export version of the US Army's OH-6 Cayuse. It quickly attracted orders from a variety of countries - including Iraq, Israel, South Korea and the Philippines - in the market for a gunship helicopter in a class below the Bell Helicopter AH-1 Cobra and AH-64 Apache.
Some 35 years later the Defender concept has been reborn - and significantly enhanced - in the Boeing AH-6i.
Hughes and later McDonnell Douglas sold 471 MD500 Defenders around the world. Those aircraft are now in need of replacement - and still the AH-6i stands alone in the class for a light, single-engine gunship.
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| © Boeing Following in the footsteps of the Defender, the AH-6i is ready for action |
After spending two years marketing the aircraft around the world, Boeing is now on the edge of receiving the first order, and it is likely this will come from a country in the Middle East. The rebuilding Iraqi Army has already passed on the opportunity to become the launch customer, selecting the armed Bell 407 instead, but interest remains high around the region.
Both Jordan and Saudi Arabia have announced plans to sign contracts for the AH-6i, among other interested buyers in the region.
There is no mistaking the design heritage of the helicopter. It is the 10th major iteration of the 50-year-old light helicopter first unveiled as the Hughes 300, which is now rebranded as the Sikorsky S-300C.
In 1960, Hughes offered an evolved version of the original design - the Model 369 - in response to a US Army request for a light observation helicopter. The Model 369 - redesignated by the army as the OH-6A - bore the defining characteristics of this aircraft type as it progressed for more than half a century.
The most distinctive feature is the shape of the fuselage. It is variously described as tear-drop shaped or as a "flying egg".
Despite the gross weight of the aircraft increasing from 1,224kg (2,700lb) in 1960 to 2,132kg today, its silhouette has changed only to become sharper and more pointed at the nose. This space now houses the wiring and boxes necessary to support a modern cockpit and the addition of a sophisticated targeting sensor for the type.
Pointing aft from the cockpit of the AH-6i is another unmistakable evolution of the original design of the OH-6A, and even some modern antecedents. While the OH-6A featured a conventional, semi-boomerang shaped vertical fin at the end of a long, narrow boom, the AH-6i configuration has exchanged the boomerang for a T-tail, which provides greater aerodynamic stability at increased speeds.
Like many new features of the AH-6i, the T-tail is borrowed from preceding designs, including the A/MH-6M-series delivered in the last decade to the US Army's special operations forces.
But the structural changes offer no hints of the performance improvements of the AH-6i, as compared to the Defender 500. Maximum take-off weight has improved from the Defender's 1,361kg to 2,132kg, and the service ceiling has increased from nearly 14,000ft (4,267m) to 20,000ft. Meanwhile, maximum speed has risen from about 113kt (209km/h) to 126kt at sea level.
The AH-6i's increased power is the result of two key changes - the rotor system and the engine. For the AH-6i, Boeing integrated the Rolls-Royce Model 250-C30R/3 turboshaft engine. As the military variant of the commercial Model 250-C47 engine, the C30R/3 is the same engine that powers the US special forces' MH-6 mission enhanced little bird (MELB) and the US Army's Bell Helicopter OH-58D Kiowa Warrior.
Like the AH-6-series, the Model 250 engine has evolved dramatically over the last half-century. The original powerplant was certificated at 317shp (236kW), while the latest version generates nearly 710shp - although the AH-6i drive system is limited to supplying 600shp. An improved fan draws 3kg (6lb)/s of air into the compressor.
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| © Boeing A varied weapons loadout can be fitted on the stub-wings |
Before the airflow enters the combustor it has been compressed by a factor of 9.2 compared to when it entered the inlet, according to Rolls-Royce.
The addition of full authority digital electronic control (FADEC) is perhaps even more important than the thrust improvement. The digital controller starts the engine, governs the fuel flow into the combustor and limits torque and gas temperatures.
However, while the Model 250-C30R/3 adds power, this means nothing unless the rotor system can carry a heavier load.
The A/MH-6 MELB addressed this requirement by adding a sixth blade to the rotor system, which was carried forward to the AH-6i. It is a feature normally reserved for the world's largest helicopters, including the Sikorsky S-64 Skycrane and CH-53A Sea Stallion, but the sixth blade is a necessary addition for the AH-6i, adding the capacity to lift 272kg more payload.
"It's got a very, very powerful engine in it," says Boeing experimental test pilot Todd Brown. "With a six-bladed main rotor there's lots of control power, so we can generate very rapid pitch and roll rates with the aircraft as well. With an aircraft of this size it makes it very well suited for close fights."
Although the maximum gross weight is more than 2,100kg, the AH-6i still lacks flight controls boosted by a hydraulics system. This makes the control load heavier on the pilot, but the AH-6i is still nimble and stable enough for close air support missions where high agility is critical.
"The aircraft, I guess you can say, has a reputation of being maybe more difficult to fly than other aircraft because it doesn't have a boosted flight control system," Brown says. "There's not augmentation to the flight control system. But in terms of being easy to fly and being a stable enough platform to do the mission, it doesn't really need a boosted flight control system. I can roll into a Toyota pick up truck and I can put rounds into a Toyota pick-up truck."
The AH-6i's designers had plenty of ideas for taking advantage of that extra power. Billed as a "light gunship", the AH-6i offers a full suite of weapons on two racks extending from both sides of the fuselage on stub-wings.
In Boeing's representative configuration, one rack carries a M-134 7.62mm minigun on the right inboard station and a .50cal GAU-19 machine gun on the left inboard station. On the outboard stations, the type carries two semi-active laser-guided Lockheed Martin AGM-114 Hellfire missiles on the left side and a seven-shot M260 launcher for 70mm folding fin aerial rockets on the right. The unguided rockets could be exchanged for precision guided rockets, such as the Lockheed direct attack guided rocket.
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No helicopter gunship is relevant today without help from the world's most sophisticated sensors. Special forces fly with the Raytheon ZSQ-2 electro-optical/infrared sensor and target designator. The ZSQ-2 is not available for export, so Boeing selected the L-3 Communications Wescam.
Customers can also choose between the MX-10D or MX-15D imaging sensors, with laser desgnators. Installed in a turret attached on the fuselage centreline - directly beneath the cockpit - both mid-wave infrared sensor options offer megapixel-class video in high-definition colour.
Another feature that distinguishes the AH-6i from the special forces variant is a new twist on the aircraft's integrated digital cockpit. The AH-6i is not the first Little Bird variant to be equipped with an advanced avionics suite, but it is the first to leverage the software suite designed for the Boeing AH-64D Block III Apache.
Source: Flight International
