NTSB report reveals modifications made by Sino Swearingen to improve lateral stability after 2003 fatal crash
Sino Swearingen Aircraft (SSAC) appears on track for certification of the SJ30-2 business jet in the third quarter of this year after making design changes to tackle a lateral stability issue that may have caused the 2003 fatal crash of a prototype during high-speed flutter testing.
A recent US National Transportation Safety Board factual report on the April 2003 crash reveals the company modified the SJ30-2 to improve high-speed stability and control after transonic windtunnel tests conducted in January 2004 showed that lateral stability deteriorated with increasing Mach number and angle of attack.
Aircraft 002 crashed after it began rolling to the right at around 30,000ft (9,000m) and M0.88, during a flutter test, and the pilot was unable to stop the roll. The crash aircraft's history, as detailed in the NTSB report, shows a tendency to roll right wing down, possibly due to deviations in wing and aileron twist from the design.
The transonic windtunnel tests showed the SJ30-2 became laterally unstable above M0.83, and roll authority deteriorated above M0.86. Flow visualisation revealed that wing upper-surface flow separated between M0.84 and 0.88, and lower-surface flow between M0.86 and 0.88, the shock-induced separation reducing lateral stability.
The tests also showed that rudder input to raise a low wing could instead aggravate the condition, while an elevator input that increased angle of attack also reduced lateral stability. Telemetry from the accident flight indicates increasing rudder and elevator inputs as the aircraft began to roll.
Following the accident, vortex generators were added to the wing to delay the onset of shock-induced separation, and ailerons with thicker trailing edges were fitted to improve their effectiveness at high Mach numbers. In addition, the single speedbrake panel on each wing was relocated further outboard to minimise the large pitch-down effects caused by tail lift interference, and made operational at all airspeeds, says the NTSB report.
Flight tests with aircraft 004, with the modifications, demonstrated improved high-speed stability and control, maintaining positive lateral stability to M0.83, and neutral stability at M0.85. SSAC successfully completed flutter testing in August last year.
San Antonio, Texas-based SSAC declines to comment of the NTSB report, as the accident is still under investigation, but says it is on track for certification in the second half of the year. Three SJ30-2 prototypes are now flying.
GRAHAM WARWICK WASHINGTON DC
Source: Flight International
