The expression "One test is worth 1,000 expert opinions" is alive and well in the ready rooms at three helicopter manufacturers that are either flying or preparing to take off in rotorcraft hybrids designed to upset the vertical lift paradigm for ever.
Bell/Agusta Aerospace, Piasecki Aircraft and Sikorsky have become the frontrunners in a race to place 250kt-plus (460km/h) "helicopters" on the product shelf, putting their money and corporate reputations on the line to prove the point.
Close behind is the Russian rotorcraft industry, with several new or previously proposed designs under way to keep up with the West or even take the lead in the 2015 timeframe (see P86).
Fastest of the three European and US contenders so far is Bell/Agusta's BA609 six-passenger tiltrotor. With two of an eventual complement of four powered lift vehicles taking part in test-flight programmes in Texas and Italy to date, Bell/Agusta has reached velocities as high as 310kt and altitudes up to 25,000ft (7,620m).
The BA609 is designed to take off and hover in helicopter mode and cruise at 275kt after transitioning to fixed-wing mode. Initial customer deliveries are expected in 2011.
If new US Army funding arrives in the autumn, Piasecki is prepared to break the 180kt Naval Aviation Training and Operations Procedures and Standards (Natops) limits imposed on the company during the first phase of the military-funded X-49A "SpeedHawk" compound helicopter development and test programme, now nearing completion.
Piasecki's military-funded X-49A features an aircraft wing and five blade propeller placed in a controllable ring-tail
The X-49A features an aircraft wing and five-blade propeller placed in a controllable ring-tail in lieu of the H-60 Black Hawk's standard anti-torque rotor.
By offloading and slowing the main rotor with the flaperon-equipped wings and thrusting forward with the variable-pitch rear propeller, Piasecki envisages a family of compound helicopters that will offer vertical take-off and hover performance as well as a 200kt cruise speed, 45kt above the 155kt cruise speed of the standard H-60 helicopter.
Piasecki first tested the vectored thrust ducted propeller (VTDP) concept in the 1960s, exceeding 195kt with its Pathfinder II demonstrator in 1966.
The SpeedHawk programme was originally launched by the US Navy in 2000, but the army became the sponsor in 2004, using congressional "plus-ups" to fund a small development team at Piasecki.
"We're doing all this for the unheard-of price of $3-6 million a year," says Piasecki Aircraft president and chief executive John Piasecki.
In addition to military funding, the company has also received $3 million of "in-kind" support from Boeing, given in part by allowing Piasecki to use Boeing's test facilities at New Castle airport in Wilmington, Delaware, where Piasecki had accumulated more than 66h of flight time on 50 flights, as of 29 May, with aircraft N40VT.
Boeing's help will ideally take the form of helping Piasecki mechanise a production process for the helicopter if a large order arrives. First flight of N40VT took place on 30 June 2007.
Chuck Jarnot, head of army requirements for Piasecki, says the company is lobbying army leaders to include the technology as a candidate for UH-60 upgrades.
"We are competing as a technology to be a principal part of that aircraft's planned Block 2 improvement programme to start in the next four to six years," says Jarnot. "Block 2" refers to the army's UH-60(X), also known as the Future Utility Rotorcraft, a programme that may include upgrading or replacing the UH-60.
Interest also remains high from the US Marine Corps, in part because of the service's need for an armed escort that can keep up with the 240kt Bell Boeing V-22 Osprey.
Piasecki officials say the Marines had earlier shown interest in a VDTP version of the Bell AH-1 Cobra, but withdrew support for the programme so as to not compete with its AH-1Y and AH-1Z upgrade programmes.
Piasecki says performance parameters for the compound Cobra included 245kt cruise speed, 1,330km (720nm) range and 3,605kg (7,950lb) useful load at a cost of $12 million per copy.
The US Coast Guard also has a need for the advertised speed potential of the X-49A for search-and-rescue operations, although officials admit that funding issues mean the army and USMC would have to pay for the development.
So far, that development is proceeding nicely. "We've reached the point in the programme where we have hard data to show the army," says Piasecki, "and the results are significantly better than expected."
As an example, he says that at the "test metric" of 10,000ft altitude and 10,000kg aircraft weight, the X-49A flies 47% faster than the H-60 at the same power setting - 9.5% better than Piasecki engineers had predicted.
Along with the extra speed comes less vibration, says Piasecki, at some points down by 50%.
Chief drawbacks of the current design include the open loop controls for the propeller pitch, flaps and elevator control and hundreds of kilos of added weight for the tail and wings.
Included in the Phase 2 proposal being evaluated by the army for launch in the autumn is a fly-by-wire system that will greatly simplify pilot workload.
To address the decreased payload caused by a 9% greater empty weight, Piasecki also plans to change the H-60's auxiliary power unit for a 600shp (450kW) Rolls-Royce C-250-30 "Little Bird" engine in Phase 2.
With the new supplementary power unit (SPU) configuration, which allows either the SpeedHawk's dual General Electric T700 engines or the SPU to power the pusher propeller through a new Piasecki-designed and tested gear reduction box, the X-49A should have a capability for 455kg more payload by removing the demand for 500hp for the tail rotor from the 3,400hp available from the main engines.
Also planned for Phase 2 are aerodynamic enhancements, including hub drag reductions and a retractable landing gear.
A possible third phase would include an optimum speed rotor transmission that slows the rotor at higher speeds, avoiding the "retreating blade stall" problem and therefore allowing for yet higher cruise speeds.
Piasecki says the company is in an excellent position to offer the military optimal value for its budget. "The military will spend $40 billion over the next 20 years recapitalising its fleet," says Piasecki. "The taxpayers get much more for their investment with us."
Sikorsky's X2 advancing blade concept helicopter promises a 250kt or greater cruise speed.
Several hundred kilometres to the north of Piasecki's Pennsylvania headquarters, taxpayers are getting their money's worth with Sikorsky's self-funded X2 advancing blade concept (ABC) demonstrator, now being prepared for first flight in Elmira, New York.
Using counter-rotating, four-blade rigid composite main rotors, the ABC gets its 250kt or greater cruise speed by having advancing blades always on opposite sides of the rotor disc and offloading retreating blades.
Counter-rotating main rotors also remove the need for an anti-torque rotor, leaving the tail-mounted propeller for thrust. The company tested the concept with its XH-59 in the 1970s, achieving 240kt along with problematic vibration levels.
That Sikorsky is funding the entire project also explains the deliberately slow march toward first flight, originally said to be on tap as early as 2006 by Sikorsky after a successful demonstration of the X2's fly-by-wire technology in a surrogate Schweizer 333 in late 2005.
"It hasn't flown yet and we're not in a big rush," said Sikorsky vice-president of sales, Steve Estill, in February. "We want to do it when it's safe and when it's aligned with our resources."
Although first flight is now not likely to take place until July or later, partly because of limited resources caused by a crush of military and civilian helicopter orders and nine first flights planned for this year, Sikorsky officials confirm X2 progress is being made.
The company performed a series of successful test runs at its Elmira "Hawk Works" in April and May.
Programme manager Jim Kagdis says "bare-head" testing came first - a check of the entire drive system, without main rotors, with a focus on electrical, cooling system and cockpit displays spanning 28 runs.
After bare-head testing, the main rotors were installed and runs were made exercising the X2's full rotor speed range, up to 650ft/s (198m/s) tip speed. The pusher-propeller (propulsor) is not installed for ground runs, but will be present for first flight.
"It was gratefully boring," says Steve Weiner, X2 chief engineer, of the test programme.
Transmission blades and blade dynamics were "right on prediction", he adds, with track and balance resulting in "S-76-like" low vibration levels.
Weiner says engineers have not found any counterintuitive surprises.
"It's been remarkably clean from a test point of view. The things we are working on are things we knew we'd have to be working on," he says. "Right now we're in the process of getting our control system working."
To validate the FBW flight controls, Sikorsky is running the powered-up aircraft in the hangar using a hydraulic test rig to exercise the controls.
Next will be a series of checkouts in "full-up" configuration, followed by a 25h pre-flight acceptance test with full control motions and "significant" thrust while tied down to the pad, says Weiner.
First flight will follow "if everything looks good", he adds. "With our ground testing with blades-on testing behind us, we are within arm's reach of first flight."
After first flights at Elmira, achieving forward speeds to 40kt, Kagdis says Sikorsky will move the aircraft to West Palm Beach, Florida, for the next three phases, culminating in a 250kt cruise ability late next year.