Setting the Foundation for Flight Test - The AC-1 30U Gunship Drag Reduction Program

An experimental wind tunnel investigation and computational performance evaluation was conducted which focused on reducing the incremental drag resulting from external modifications to the fuselage of the AC- 130U Gunship. The objective of this effort was to establish a solid foundation via experimental and computational ground efforts to support flight test of drag reduction modifications to an aircraft. Currently, additive drag resulting from AC- 13 0U modifications decreases the range of the aircraft, increases the infrared heat signature of the engine exhaust due to the need for higher power settings, and has an adverse impact on takeoff and engine out performance. Specifically, the program consisted of six major thrusts:

1. 1.
   Wind tunnel testing of a 1/48 scale model of the AC-130U fuselage to determine the drag on eight specific protuberances.
2. 2.
   Design and testing of fairing modifications in 1/48 scale on three of the high drag protuberances.
3. 3.
   Optimization and testing of the fairing designs for the Forward Looking Infrared (FLIR) Pod in 1/3 scale and for the Left Sensor Array in 1/2 scale.
4. 4.
   Wind tunnel evaluation of the drag on the 105 mm Howitzer blast suppresser and 40 mm Bofors cannon in near full scale; design and evaluation of fairings to reduce the drag on each of these weapons.
5. 5.
   Projection of the drag reduction potential of all fairings and modifications to the full scale aircraft in terms of drag counts.
6. 6.
   Evaluation of the mission performance impact of the modifications using a computational performance prediction code.

High drag contributors are identified and methods of reducing the drag are presented. As a result of the investigation, recommendations were made to remove the spoilers in front of the 25 mm Gatling gun and the 40 mm Bofors cannon, to replace the spoiler in front of the 105 mm Howitzer, and to add fairings on the FLIR pod, Left Sensor Array, 40 mm cannon barrel, and 105 mm Howitzer blast suppresser. With these modifications, a drag reduction potential of 29 drag counts was projected.

Operational impact⁹ of an approximate drag reduction of 29 drag counts on the AC- 130U was projected to include:

• Lower mission required power settings resulting in real IR signature reductions improving aircraft survivability.
• Combat loiter time increasing by a minimum of 15 minutes.
• Two engine cruise ceiling increasing by approximately 1,000 feet and improved takeoff performance.
• Fuel savings of over $150,000 per year for the AC-130U fleet.