Northrop Grumman is beginning high-speed windtunnel testing of the oblique flying wing X-plane it is designing for the US Defense Advanced Research Projects Agency. If built, the OFW technology demonstrator will be the first supersonic, tailless, variable-sweep flying wing.
The high-fidelity model will be tested at speeds up to Mach 1.3 in a tunnel at Calspan in Buffalo, New York as a step toward possible flight tests of the unmanned demonstrator around 2011. Low-speed windtunnel testing was conducted in January, achieving "excellent correlation" with computational fluid dynamics analyses, says programme manager Joe Pawlowski.
Northrop is working under a 25-month contract, awarded in March last year, to complete preliminary design of the X-plane. The preliminary design review is planned for Mach 2008, leading to a decision whether to proceed into Phase 2 and construction and flight test of the subscale demonstrator.
Studied since the 1950s, the variable-sweep oblique flying wing promises aerodynamic efficiency at both low and high speeds, but poses challenges with aeroelasticity and controllability. The goal of the X-plane is to prove the OFW concept is feasible, but not to characterise its performance, says chief engineer Gary Tiebens.
Previous oblique wing aircraft have all been subsonic - and tailed - and the X-plane is focused on proving that a tailless OFW can transition to supersonic speed. The benefit of a variable-sweep oblique flying wing is that airflow normal to the leading edge stays subsonic, keeping the drag rise low as the aircraft goes supersonic.
The flying wing has a span of 17m (56ft) unswept and sweeps between 0º and 65º, measured at the trailing edge. The X-plane will take off and land with the wing at 0º, the sweep increasing linearly with speed until it reaches 65º before going transonic. "It will be fully swept before the transition, to reduce risk," says Tiebens.
The X-plane is powered by two General Electric J85-21 afterburning engines (from Northrop's F-5E). In a change to the original design the previously paired engines are now in separate pods, one moving forward and one aft as the wing sweeps, to improve area ruling and reduce wave drag at the design maximum speed of M1.2.
The demonstrator has "taildragger" landing gear, with the main gear mounted forward between the engine nacelles and a castoring tailwheel, says Tiebens. The nacelles and flight control surfaces embedded in the wing and along the trailing edge are all electrically driven.
Sweep is varied aerodynamically by yawing the wing using the outermost control surfaces - "like a rudder kick to produce sideslip", says Tiebens - then retrimming in all axes to maintain the yawed condition. The engine nacelles rotate to face into the airflow. "They do not rotate fast enough to use as a control device," he says.
Aeroservoelasticity - the interaction of aerodynamic, structural and control responses - is the "Achilles heel" of the OFW, says Tiebens. All three axes - pitch, roll and yaw - are coupled and wing bending caused by manoeuvres or gusts effects stability in all three. The flight control system must be able to decouple the responses.
The configuration's assymetry, its sensitivity to roll moment and the coupling of roll and pitch instability at high oblique sweep angles are among the factors the control system has to alleviate. But the demonstrator's instability is within the range of Northrop's experience with the B-2, Tiebens says.
"The X-plane is not performance-driven. The big push is the control algorithms," says Pawlowski. "The objective is to open up the design space for future aircraft. We want to prove we can fly a tailless, supersonic oblique flying wing with subsonic leading edge and address the aeroservoelastics."
A future operational OFW, envisioned for beyond 2025, could combine efficient subsonic loiter and supersonic dash capability to perform military reconnaissance and strike missions requiring rapid deployment, long range and long endurance, DARPA believes.
Source: Flight International
