* 2 stealth fighters open show with brief flypast
* 'Big step forward in China combat capability' - expert
* Too early to say if J-20 matches U.S. F-22 jet - analysts
* China will need 6,865 new planes over next 20 yrs - COMAC (Adds COMAC market outlook, details on wide-body plane)
By Tim Hepher and Brenda Goh
ZHUHAI, China, Nov 1 (Reuters) - China showed its Chengdu J-20 stealth fighter in public for the first time on Tuesday, opening the country's biggest meeting of aircraft makers and buyers with a show of its military clout.
Airshow China, in the southern city of Zhuhai, offers Beijing an opportunity to demonstrate its ambitions in civil aerospace and to underline its growing capability in defence. China is set to overtake the U.S. as the world's top aviation market in the next decade.
Two J-20 jets, Zhuhai's headline act, swept over dignitaries, hundreds of spectators and industry executives gathered at the show's opening ceremony in a flypast that barely exceeded a minute, generating a deafening roar that was met with gasps and applause and set off car alarms in a parking lot.
Experts say China has been refining designs for the J-20, first glimpsed by planespotters in 2010, in the hope of narrowing a military technology gap with the United States. President Xi Jinping has pushed to toughen the armed forces as China takes a more assertive stance in Asia, particularly in the South China and East China seas.
"It is clearly a big step forward in Chinese combat capability," said Bradley Perrett of Aviation Week, a veteran China watcher.
State-owned Commercial Aircraft Corporation of China (COMAC) was also bullish on China's appetite for new civilian planes, estimating the market would need 6,865 new aircraft worth $930 billion over the next 20 years.
The COMAC forecast - similar to long-term outlooks from well-established rivals Boeing Co and Airbus Group - said China would make up almost a fifth of global demand for close to 40,000 planes over the next two decades.
QUESTIONS UNANSWERED
After screeching onto the Zhuhai stage as a pair at low-level, one of the J-20s quickly disappeared over the horizon, leaving the other to perform a series of turns, revealing its delta wing shape against bright sub-tropical haze.
It was China's second successive display of stealth at the biennial show, following the 2014 debut of the J-31.
But analysts said the brief and relatively cautious J-20 routine - the pilots did not open weapon bay doors, or perform low-speed passes - answered few questions.
"I think we learned very little. We learned it is very loud. But we can't tell what type of engine it has, or very much about the mobility," said Greg Waldron, Asia Managing Editor of FlightGlobal. "Most importantly, we didn't learn much about its radar cross-section."