Flight-testing Europe's huge new turboprop
Before the long-delayed first flight of the A400M, the new airlifter’s TP400 turboprop was flown 18 times on a C-130 flight test bed (FTB) modified and flown by Marshall Aerospace. During a presentation to the UK’s Royal Aeronautical Society last October, Marshall’s chief test pilot Iain Young and flight test engineer Rob Boyle described the challenging task.
The TP400 produces 2.5 times the thrust of a standard T56 engine and weighs twice as much, they explained. “The flight-idle thrust of the TP400 is greater than a T56 at full power on the ground,” noted Boyle. Marshalls had to build a “pilot-in-the-loop” simulator from the cockpit of a retired C-130K and do extensive aerodynamic modeling and computational fluid dynamics analysis before the aerodynamic differences of the C-130 FTB versus a standard C-130K could be estimated.
After the contract was awarded in December 2004, Marshalls purchased a low-time C-130K (XV208) that the UK Ministry of Defence had used for meteorological research. To accept the huge turboprop on the inner left-wing position, the center wing had to be reinforced and a three-part pylon designed. It added damping struts linking the pylon to the fuselage, mainly because of the prop flutter concerns. There was less than 11 inches clearance between the C-130 fuselage and the TP400’s eight-bladed, 17.5 feet diameter propeller. The left fuel pylon of the C-130 was removed, but the right one was retained to help balance the aircraft by means of water glycol.
Inside the FTB, the flight deck was rebuilt to add new caution and warning lights, an additional throttle box for the TP400 and two panels for an engine indication and crew alerting system. Boyle explained that the 11,065-shp TP400 has four propeller settings (655, 730, 842 and 860 rpm), which together with the propeller angle are controlled by the Fadec.
Since the C-130K has no digital data bus, Marshall had to install its own to interface to the modern Fadec system and to replicate the A400M aircraft engine controls. Three sets of console were installed for monitoring the aircraft and systems supplied by the engine maker EPI and the propeller maker Ratier-Figeac.
Boyle said it installed “a huge package of instrumentation” to measure more than 800 dedicated instrumented parameters, along with over 2000 data bus parameters. The propellers on the A400M turn “down between engines.” This feature also had to be tested on the FTB, which meant instrumentation also had to be added to the outer left T56 propeller to ensure it stayed within stress limits.
Up to eight flight-test engineers could be carried, plus the standard C-130K flight-deck crew of four. But as Young noted, “We were never able to carry additional technical observers, because the ‘risk’ level remained high throughout the flight tests.“
The structural mods were completed in March 2007 and a dummy engine was fitted pending the arrival of the real thing late that year. The propeller arrived in early 2008 and all four engines were ground run for the first time that June. Even this was a major technical achievement. “In case of a runaway propeller, we built the world‘s biggest set of chocks. They weighed more than the aircraft!” joked Young.
The ground runs revealed some problems: Hot gas ingestion from the TP400 into the rear flap shroud and the consequent potential heating of the rear spar “got our engineers excited,” said Young, so a “boat-tail” shroud was designed to alleviate this problem. “We also found that the left wing dropped 3.5 degrees because of changes in aerodynamics, so we compressed the left gear oleos [by making a tight turn prior to lining up for takeoff] and transferred fuel to compensate,” he added.
Vortices that were generated during the ground runs induced additional vibration that was measured as plus or minus 0.7g in the cockpit. Because of concerns over noise and vibration, the test crew wore fast-jet standard helmets with active-noise-reduction systems and earplugs, however, during tests it was found that the latter were not required.
After 24 hours of ground runs, followed by low- and high-speed taxi trials, the C-130 FTB finally made its first flight on Dec. 17, 2008. A matched thrust technique was used, with the TP400 running at flight idle at the start of the takeoff run.
The maximum speed was limited to M0.5Vd (Mach 0.32 at 15,000 feet) until the flutter clearance could be expanded. There were concerns about aircraft response, particularly in roll, should the big engine fail. “We spent many hours testing every possible contingency in the simulator. I doubted whether we could make it faithful enough, but we did,” said Young.
After exploring the basic handling with only gentle throttle modulation on the first flight, Young said, they progressed quickly toward the major test milestones on subsequent flights. The TP400 was run to takeoff go-around (TOGA) thrust and feathered and de-feathered on the second flight. The third flight reached M0.45 and 32,000 feet; a full shutdown and relight was accomplished. Starter-assisted relights were fully explored on the sixth flight, together with throttle slams and intake distortion testing. The first windmill relight was done on the next flight, plus high-speed feathering and de-feathering.
The entire flight envelope was achieved on the 12th flight, which took off with the TP400 advancing to 70 percent power as the takeoff run accelerated, and the Number 1 engine set to a reduced thrust to minimize directional asymmetry. This flight also measured the high angle-of-attack performance of the engine. “We had some worries about the slow-speed tests: Would there be an asymmetric stall? But we did go down to 113 knots, with the TP400 at TOGA power,” commented Boyle. The C-130 FTB could test the entire low-speed envelope required of the TP400, but its maximum speed was M0.64 versus M0.72 for the A400M.
After 18 flights and 55 hours, plus another 61 hours ground running, the TP400 flight test program on the C-130 was wrapped up on September 30 last year. According to Boyle and Young, the primary objectives set by Airbus Military were to identify any Fadec issues; to measure propeller stress in the different rpm modes; to check the nacelle ventilation; to explore engine failure modes, feathering and the relight capability; and to measure noise and vibration. These objectives were all met, they added.
Airbus Military has subsequently told AIN that a propeller measurement scheduled for the C-130 FTB was deferred to the A400M flight test program. This caused a holdup in mid-January, when wet weather in Seville meant that the moisture-sensitive prop sensors could not be flown. Marshall told AIN that some testing was intentionally deferred due to the differences in the flight envelope between the A400M and the FTB; however, all scheduled testing on the FTB was completed.