A combined flight-test team from Northrop Grumman, the US Air Force and Calspan has revealed new details of the flight characteristics of the Tacit Blue technology demonstrator. The formerly classified USAF/Defense Advanced Research Agency project was initially used as a proof-of-concept stealthy surveillance radar platform. The 17m-long low-observable aircraft defeated radar detection with curved linear (Gaussian) surfaces, rather than using the multi-faceted approach taken by Lockheed Martin with the Have Blue prototype, flown three years before.
The control laws used in the General Electric quadruple-redundant digital flight-control system (FCS) were tested initially in Northrop Grumman's Large Amplitude Flying Simulator, before being transferred to Calspan's Convair 580 reconfigurable in-flight simulator. Pitch and roll problems were mostly unearthed using the Calspan aircraft, which was then used to test software solutions. High-speed taxi tests were conducted on 2 February, 1982, before the aircraft had its first flight three days later. The aircraft required a long take-off run of 2,620m (8,600ft) at a maximum take-off weight of 13,390kg. This entailed roughly eight orbits at the forward edge of the battle area, allowing the Tacit Blue to "peer" with its Hughes Aircraft low-probability-of-intercept surveillance radar into the "second echelon". The data were fed into the Pave Mover programme, which led to the Northrop Grumman E-8 Joint Surveillance Target Attack Radar System. The test team also encountered problems starting the second of Tacit Blue's AlliedSignal ATF3-6 high-bypass turbofans. The left engine started and, because both shared a single gaping inlet in the upper surface of the fuselage, would start a reverse flow through the neighbouring engine. Various solutions were tried before the solution was found to be a change in the starting procedure in which one would start within a second or two of the other.
Source: Flight International