National Aerospace Laboratories prepares to fly much improved second prototype
India’s National Aerospace Laboratories (NAL) in Bangalore is preparing a second prototype of the indigenously developed Saras light transport aircraft for its first flight, following tests and modification work to overcome design issues uncovered during tests of the first aircraft.
The 14-seat twin-pusher utility aircraft is powered by Pratt & Whitney Canada PT6A turboprops, and is the first Indian civil aircraft programme aimed at certification to US FAR Part 25 standards. Development was delayed due to sanctions imposed by the USA in the wake of Pokhran-2 weapon tests, and the first flight finally took place in May 2004 having originally been expected in 2001.
Designed to cruise at a maximum speed of 280kt (520km/h) with a ferry range of almost 2,000km and an endurance of 6h, the Saras has a span of 14.7m (48.2ft), and an overall length of 15m. It has a maximum take-off weight of 6,100kg (13,400lb), and a maximum payload of over 1,230kg.
Evaluation is being undertaken, through an agreement with NAL, by the Indian air force’s aircraft and systems testing establishment at Yemlur, Bangalore, and has “so far been going smoothly”, says trials team chief and senior prototype pilot, Wg Cdr R S Makker. “We have had to deal with issues to do with nose-wheel steering, engine oil temperature and rudder blowback,” he adds. Speaking at the Society of Experimental Test Pilots meeting in California, Makker said the planned test effort, including the second prototype, covers a further 500 sorties “in 13 phases to get certification”.
The oil temperature problem was so acute that the engine could not be operated at take-off rating for more than 3min at ISA+ 10°C (75°F), or could not operate at ground idle for more than 30min at ambient conditions. The issue, which was caused by restricted flow through the inlet and oil cooler, was cured by adding a splitter plate to the inlet and diffuser to improve the pressure drop.
Nose-wheel steering was discovered to be difficult due to limit cycle oscillations and was cured through changes to the command controller in the steering system. A third issue, related to a rudder feedback, or “blowback”, phenomenon, was also uncovered by tests to be most severe at ground speeds between 35kt and 55kt. It was found to be directly proportional to applied reverse torque beyond 15%, and is presently being tackled by limiting reverse torque to 10% between the worst case speeds.
GUY NORRIS/LOS ANGELES
Source: Flight International