Guy Norris/Fort Worth
LOCKHEED Martin is claiming the edge over rivals for the Joint Advanced Strike Technology (JAST) programme after successful tests of its large-scale powered model at the NASA Ames, California, wind tunnel site at the end of February.
Speaking on the eve of the release of the request for JAST proposals, expected around 29 February, Lockheed Martin JAST programme manager Dave Wheaton said: "We are the only one in the wind tunnel with a working propulsion system. We see this as a major advantage from a risk-reduction standpoint."
Boeing built its own hover-test rig and elected not to use the large (24 x 37m) NASA wind tunnel. The late configuration change by the McDonnell Douglas/Northrop Grumman/British Aerospace team prevented it from being able to field a large-scale model in time for the NASA tunnel.
Lockheed Martin places "...high value on what we've done with the large-scale powered model", says Wheaton, because "...we will have eventually demonstrated half a lifetime of our integrated propulsion system". Testing moved into the wind tunnel from the adjacent outside aerodynamic-reference site in November 1995.
Lockheed Martin large-scale-test project engineer Jim Eshleman says that more than 170h of tests have been conducted on the propulsion system in all aspects of low-speed flight. "We have taken the model to 18¡ AoA [angle-of-attack] and 24¡ sideslip, and from idle to maximum power at 95kt. We're happy with the data, and from hover testing we got outstanding agreement with small-scale-modeling predictions. Preliminary data from the wind tunnel say it will be the same," he says. The model, is powered by a Pratt & Whitney F100 and a shaft-driven lift engine, with a P&W F119 fan.
Lockheed Martin has revealed major changes to the model configuration, the most obvious of which is the adoption of F-22-like horizontal tails in place of the original foreplanes. The move away from the delta canard to an aft tail configuration was driven "...mainly by carrier requirements", says Eshleman. The need for lower conventional landing speeds and hence high-lift systems and large trailing-edge flaps led to "...lots of loading on the tail, which became a significant load on the canard".
To reduce the pitch moment on the flaps and increase the flexibility to carry a wide variety of external stores, the wing is also expected to be moved slightly further forward relative to the new horizontal tail surfaces. The US Navy version will have additional span and chord compared with the other versions, but shares an identical main wing-box. The Navy wing will also fold. Another change yet to come will be the movement of the roll-control ducts out and aft from their present location by the wing root, "...mainly because of suck-down effects, caused by the sloping side-wall and the proximity of the jet", says Eshleman.
The company has also opted to replace the two-dimensional vectoring nozzle with a three-bearing swivel nozzle (3BSN), similar to that used on the Yakovlev Yak-141. The simpler system is an improved pressure vessel and helps reduce yaw, which would have had to be countered with a sophisticated flight-control system. The 3BSN will be made by Rolls Royce.
The moveable vanes on the lift-fan nozzle, which were intended to augment yaw control, have now been fixed. Instead, the company is studying a moveable hood, or cowl, over the nozzle exit.
Source: Flight International
