Aerodynamic Cruise Performance Flight Test Challenges of the Re-engined C-5M

Abstract

The C-5M RERP is a modernized and re-engined, widebody military transport aircraft under development by Lockheed Martin. The three flight test airplanes for this program were removed from the C-5A/B fleet and modified into the C-5M configuration. C-5M Ship 099 was designated as the flight test cruise performance vehicle. Because this aircraft was the first C-5M aircraft to fly, it incorporated necessary drag producing external instrumentation for flight envelope expansion, flutter testing, engine monitoring, and new C-5M RERP system testing. The aircraft was in service for 18 years as a C-5B and it contained rough paint, numerous dents, bumps, scratches, structural skin repairs, etc. While typical small to medium size re-engined aircraft flight test programs allow the aircraft to be remanufactured and repainted, the sheer size of the C-5 did not allow for such beneficial restoration. Using C-5M configuration wind tunnel testing and analysis, and computational fluid dynamics (CFD) modeling results, C-5M drag polars were estimated prior to flight test. Existing C-5A/B drag polars served as the starting point. However, over the years, the C-5A and C-5B aircraft did not remain in the condition or configuration that the test vehicles were in during C-5A flight testing. The Avionics Modernization Program (AMP) modification has added external antennas that did not exist during C-5A flight testing. Various updates and structural skin modifications, such as adding doublers, have occurred to the A and B models. Furthermore, the original flight tested wing of the C-5A has been replaced with the new stiffer wing of the C-5B and this stiffer wing is on all C-5 models. All of the changes to the C-5A/B over the years presented unique challenges to the C-5M cruise aerodynamic performance flight test program. It was imperative that all differences between the original C-5A and the new C-5M that affect the lift and drag of the vehicle, excluding the new engine or pylon, be accounted for. This paper addresses the philosophy employed and methodology used to account for these differences. The unique challenges of a large-sized flight test vehicle, borrowed from the fleet, with various external flight test instrumentation, wear-and-tear defects, and a drag polar database that was not fleet representative, are discussed. The solutions employed and resolution of the problems are presented along with lessons learned and recommendations.