Civil rotorcraft special: Rotorcraft makers strive to break the civil rotorcraft 'speed limit'

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A renewed demand for faster helicopters has spurred research programmes on both sides of the Atlantic

With a squadron of Bell Boeing V-22 Ospreys flying routine combat operations with the US Marines in Iraq, optimism is growing in the civilian sector that vertical lift aircraft unencumbered by the helicopter's traditional speed limit of 160-180kt (300-330km/h) could similarly transition from oddity to norm.

Along with the tiltrotor technologies being offered by Bell/Agusta Aerospace with the BA609, now in flight testing, Sikorsky and Piasecki are pursuing clean-sheet designs or augmentations aimed at boosting helicopter cruise speeds into the 250kt range, similar to that of the BA609. In July, Piasecki began flight testing a vectored-thrust ducted-propeller compound helicopter known as the X-49A "ring tail", and Sikorsky has been preparing to fly its X2 dual counter-rotating advancing blade concept demonstrator for some time.

"Three or four years ago, nobody wanted to have that speed," says Rhett Flater, president of the American Helicopter Society. "Now it's quite different."

For the military, the change of heart is partly because the US Marines Corp now needs an armed escort fast enough to accompany theV-22 on its missions. On the civil side, which is benefiting from military-funded technologies, the offshore oil and gas industry is now able to afford to cut down on travel time to platforms located farther and farther offshore. Flater says that one-way trip times of 45min are not uncommon.

"Speed seems to be becoming more and more a key characteristic that operators are looking for in helicopters," says Ray Jaworowski, senior aerospace analyst at Forecast International: "The further the distances you want to fly, the more of an attractive feature it becomes."

x-49

Speed Vs safety

Counterbalancing the need for speed will be economics and safety. "The commercial industry wants to go faster, but they also want to balance safety and price," says Marc Poland, vice president of commercial programmes at Sikorsky. "Seat mile cost has to be down, particularly for offshore."

First to market with a high-speed vertical lift aircraft with these attributes will be Bell/Agusta's 275kt BA609 tiltrotor, with first deliveries expected in 2011. The company has orders for 77 of the 6- to 9-seat aircraft - representing three years of Italian and US prod­uction - with 35% from fly-for-hire operators who may use the aircraft for shuttling people offshore, inter-city or inter-island hops 40% for private operators, including corporate clients, and the remainder for civil government for purposes that include border patrol and search and rescue.

"Any operator that has a mixed fleet of fixed wing and rotorwing aircraft wants to look at the BA609 as a consolidating idea," says Bell/Agusta Aerospace managing director Mike Cuppernull. "Think of it as a King Air that hovers."

Two of four test aircraft are flying, one at Bell's XworX facility in Arlington, Texas, and one at Agusta's test site in Cameri, Italy. As of early February, the fleet had logged more than 300 flight hours in envelope expansion tests, attaining speeds as high as 310kt and maximum operating altitudes of 25,000ft (7,600m) in preparation for the official start of US Federal Aviation Administration "powered lift" certification tests in early 2011 when the fourth prototype comes on-line. Bell/Agusta is seeking concurrent European Aviation Safety Agency (EASA) validation in conjunction with Italian civil aviation authorities.

Powered by twin Pratt & Whitney Canada PT6C-67A turboshaft engines, the BA609 will be certified with two crew. Pilots have a centre stick similar to a helicopter's cyclic control, floor pedals for sideslip control and a power lever on the pilot's left similar to a collective control. The aircraft has 8m-diameter three-bladed composite rotors that spin at 569RPM in helicopter mode and slow to 478RPM in aeroplane mode. The tiltrotor's triple-redundant fly-by-wire system is built by BAE Systems.

Aircraft three is being built in Italy and will be used for icing tests. Aircraft 4 is being built in the USA and will be used to certify the Rockwell Collins Pro Line 21 avionics suite and the corporate interior.

Sikorsky is researching similar if not larger-cabin designs for its advancing blade concept (ABC) technologies, looking potentially at a light twin with 6-9 passengers or an intermediate twin carrying 10-15 passengers. With ABC, counter-rotating rigid four-blade rotors remove the need for a tail rotor and allow for the retreating blade on each side to be feathered as speed increases, avoiding the tip stall that limits forward speed on conventional helicopters. The design also eliminates the need for a lift-providing wing on the structure, cutting down on downwash drag.

A propulsor linked to the single LHTEC T800-801 turboshaft engine (previously employed on the Comanche helicopter) on the tail-end of the helicopter should provide forward speeds of around 250kt. Key challenges include handling the weight and drag of the dual rotor design as well as the severe vibration caused by high speed. Early demonstrators of the technology - Sikorsky flew XH-59 prototypes from 1973-1977, reaching 240kt - shook so violently at speed that pilots were said not be able to read their instruments.

The company's two-seat X2 ABC demonstrator, to appear at Heli Expo, was originally set to fly in the 4th quarter last year. "It hasn't flown yet and we're not in a big rush," says Steve Estill, Sikorsky vice-president of sales. "We want to do it when it's safe and when it's aligned with our resources." Resources will be key in bringing forward a clean-sheet design, particularly since Sikorsky is sole investor.

Retrofittable

Piasecki has a compromise position with its army-funded VDTP technology - it can be retrofitted into an existing helicopter design. The company last year began test flights of the technology, reaching speeds of almost 180kt. Modifications to a US Navy Sikorsky YSH-60F include a flaperon-equip­ped wing and shell-like thrust vectoring device around the propulsor tail that is used partly to provide main rotor counter-torque at low speeds.

"There is a huge amount of non-recurring costs to penetrate a new market," says John Piasecki, vice president, contracts and administration for Piasecki Aircraft. "The compound helicopter, if it can be retrofitted, reduces a lot of the risks - financial and time - that is required to deploy a new technology. Several companies have expressed interest in this for existing product lines."