Lockheed Martin is heading towards windtunnel tests next spring of a cruise-efficient STOL transport designed under the US Air Force Research Laboratory's Speed Agile concept demonstration. Boeing completed tunnel tests of its design in late 2009 and is preparing for piloted simulations in spring 2011.
Concept: Lockheed Martin
Lockheed's concept will be familar to those who have tracked the Air Force's long-running efforts to define a C-130 replacement, from AMC-X to AJACS to JFTL. But the latest design incorporates a number of refinements, as revealed in a presentation given by AFRL Air Vehicles Directorate engineer Cale Zeune at last week's International Powered Lift Conference in Philadelphia
The goal of Speed Agile is to demonstrate, well, "speed agility" - the ability to fly efficiently at the slow speeds required for short take-off and landing (around 70kt) and the transonic cruise speeds (Mach 0.8-plus) needed to share airspace with commercial airliners. The target field length is less than 2,000ft.
Traditional STOL aircraft cannot fly that fast, and the Speed Agile concepts use integrated lift systems to reduce drag. Boeing's design uses upper-surface blowing from embedded engines on the inboard wing and blown flaps for circulation control on the outboard wing. Lockheed's design also uses blown flaps outboard, but inboard uses patented reversing ejector nozzles.
As the illustration above shows, this nozzle can allow the engine exhaust to go either straight aft to produce thrust (Fig 4a); vector it downwards in an ejector/augmenter arrangement that boosts low-speed lift and thrust (Fig 4b); or turn it around to provide reverse thrust for short landings and ground maneuvering (Fig 4c). Vectoring the engine exhaust also provides pitch control, which avoids the oversized tail typical of other STOL designs, says Zeune.
In a separate presentation at IPLC, Zeune briefed on progress with Boeing's Speed Agile design (above), which has completed more than 2,000h of low- and high-speed windtunnel testing. Initial tests used a 5%-scale model of a "narrowbody" design with 289,000lb take-off gross weight and 128in-wide cargo box.
AFRL then increased the STOL payload requirement to 65,000lb from 55,000lb to reflect the increasing size and weight of Army armoured vehicles like Stryker. Later tunnel tests used a 5% model of a "widebody" design with 303,000lb gross weight and "A400M-size" 158in-wide cargo box. Power comes from four IAE V2533 turbofans.
Zeune says early tests showed upper-surface blowing was not providing the additional lift required because the flow over the wing and flap was not turning through the required angle. A redesign of the nozzle and flaps doubled the turning angle to 60deg, with the flaps deflected 90deg, and achieved the lift goal, he says. Boeing is now preparing for pilot-in-the-loop flight simulations in NASA Ames' Vertical Motion Simulator to evaluate the handling qualities of its Speed Agile design.
Concept: Lockheed Martin
Lockheed's design (above), meanwhile, will undergo low-speed testing in the National Full-scale Aerodynamics Facility at Ames and high-speed testing in the National Transonic Facility at NASA Langley. The low-speed model will include reversing ejector nozzles and circulation-control wing; high-speed tests will investigate the use of blown flaps for transonic drag reduction.
What happens next? Well Zeune says AFRL's research into cruise-efficient STOL transports will end next year, with some $20 million having been spent since 2005 to bring technologies to a readiness level of around TRL 5/6. It will then be up to the customer - likely the USAF's Air Mobility Command - to decide whether to fund a flight demonstrator program.
With the budget pressures on DoD and the uncertainty surrounding the Air Force/Army Joint Future Theater Lift effort to begin work on a C-130 replacement, a demonstrator does not look likely in the near term.
Concept: Northrop Grumman