Customer:Advanced Technology Aircraft Co., Ltd. (ATAC)
Type: joined-wing advanced agricultural airplane
Powerplant: 1 x Pratt & Whitney Canada PT6-34
Significant date: 1979
The concept of joining tandem wings is not a new one. It was originated in the 20's by Norman Warren and Rex Young of England. Wind tunnel tests were later done by Darroll Stinton and Warren using the concepts earlier proposed. Dr. Julian Wolkovitch, an aerodynamist from California, had crash resistance in mind as a primary design objective when he first pursued the joined wing as a sport glider prototype in 1974. He proposed several configurations of wing joining, to make maximum use of the structural benefits of the bracing achieved with large dihedral angles.
In 1979, Wolkovitch approached Joseph L. Johnson, Assistant Head of the Dynamic Stability Branch, with a request for a cooperative wind-tunnel test of an advanced joined-wing general aviation airplane designed for aerial applications, which Burt Rutan had designed. Known as the Model 58 Predator, the design had come out of a feasibility study done by RAF for David Record of Advanced Technology Aircraft Co., Ltd. (or ATAC). The requirement was for an efficient, high-capacity aircraft using the PT6-34 turboprop engine. The span had to be long for maximum swath width. Stall/spin resistance, visibility and pilot crash protection were also prime considerations.
After evaluating several configurations (including the alternative Model 59 alternative design study, of which nothing is known), a tractor-propeller-driven, connected-tandem wing arrangement was judged best for the requirement. The rear wing joined at the mid-span location of the forward wing. The elevators, on the inboard forward wing provided pitch and 'direct-lift' control for improved ground-proximity flying qualities. Differential ailerons on the outboard wing provided roll and proverse yaw without a pitch trim change. There were no controls on the aft wing, which had winglets. The inclined bracing of the wings allowed an extreme span without excessive structural weight.
The pilot was located in the 18-percent thick vertical tail of the vehicle, which allowed improved protection and visibility. This general arrangement allowed a long span and provided low induced drag. The chemical hopper at the cg did not interfere with wing structure. The RAF Model 58's estimated climb performance at its gross weight of 12,300 lbs. (6,700 lbs. in the hopper) in a 1.5-g turn was promising. Contemporary ag aircraft using the same engine had marginal turn performance using only 4,200 lb. payloads. Maximum L/D for the Predator was to be over twice that of current ag aircraft. Combining advantages of payload, speed, swath width, turn performance, ferry speed and fuel flow, resulted in a productivity of greater than twice that of current ag aircraft.
Wolkovitch and Rutan believed that the proposed agricultural plane design would offer significant safety improvement over conventional designs. Because of its interest in providing data for advanced configurations, NASA fabricated a scale model of the design and conducted a cooperative test in a 12-foot low-speed subsonic tunnel at Langley. The Model 58 Predator underwent extensive wind-tunnel tests conducted by NASA's dynamic stability branch at Langley, VA. Lead engineer for Langley during the exploratory tests was E. Richard White. The tests were directed by Joe Chambers and Joe Johnson who have had the foresight to keep alive an aging 12-foot wind tunnel to investigate many interesting aerodynamic theories and configurations. The wind-tunnel test program solved several stability deficiencies and confirmed the performance of the Predator configuration.
Still, the tests were regarded as exploratory and limited because of the low Reynolds number of the test conditions, and all participants had expected premature flow separation on the wings and junctures due to lack of simulated flight conditions. Nonetheless, it was felt that any aerodynamic data on stability and control characteristics of this remarkable configuration would be of great interest to the engineering community. Eventually, Rutan designed a completely different Predator agplane in the form of the Model 120, but since this early research, NASA and several major manufacturers never ceased exploring the joined-wing configuration, coming up with aircraft that often owe a lot to Rutan's early Predator.
Population: 0 (one wind tunnel scale model only)