Beneath a brilliant autumn sky, the Airlie Foundation stands in tranquil seclusion amid its rolling green fields, glittering lakes, luxurious Georgian manor houses and carefully manicured gardens in the foothills of the Blue Ridge Mountains.

Only an occasional hint -- an offhanded remark or wry joke by an Airlie official -- betrays the sense of uncertainty and foreboding that hangs over the sprawling, 19-year-old institution near Warrenton, almost 50 miles west of Washington.

"Overall, you'd have to say we're hurtin'," says one top Airlie official.

Dr. Murdock Head, the foundation's executive director and founder, was not at Airlie yesterday.He was meeting with his lawyers in Alexandria -- in preparation for what may be a key day in his bribery-conspiracy trial and a fateful day for Airlie.

Today, Head's lawyers are expected to start presenting the principal evidence in Head's defense against federal criminal charges that he schemed to bribe congressmen and other government officials in an attempt to secure federal funds and favorable tax treatment for his foundation and other ventures.

Witnesses called by the prosecution have already told the U.S. District Court jury that Head, 55, arranged to pay $49,000 in bribes to Rep. Daniel J. Flood (D-Pa.), former Flood aide Stephen B. Elko and former congressman Otto E. Passman in addition to an allegedly illegal $11,000 loan to a former Internal Revenue Service agent.

Head's lawyers have previously indicated that Head would likely take the witness stand to defend himself against the conspiracy and tax-evasion charges. It was not known yesterday, however, whether that strategy remained unaltered.

Amid the splendor of Airlie -- with its waterfalls, air strip, swimming pool and game preserve -- are scattered reminders of the trial taking place 50 miles away.

There is a glistening, blue 20-acre lake that prosecutors allege was illegally leased from Airlie to Raven's Hollow to escape U.S. income taxes. There are offices and a small film screening room, where Elko says he received and negotiated payoffs from Head. There are rooms where former employes say they saw $100 bills and plain white envelopes.

Despite it all, Head -- who lives on the Airlie grounds -- seems amiable and relaxed. Though he has declined to comment publicly, aides say he exudes his normal confidence. "He's still jogging -- six miles a day," an aide noted. "He doesn't always make it but he's fairly religious about it. Three miles in the morning and three in the evening.

Meanwhile, Head's trial and the protracted investigation that preceded it have taken a toll at Airlie. "I think it hurts -- even if he's acquitted," Frank Kavanaugh, an associate Airlie director, said yesterday during a tour of the institution's lavish grounds, spread over more than 1,700 acres.

The federal government has awarded no new contracts or grants to Airlie since the start of the investigation in early 1978, Kavanaugh said, although he noted that two previously approved government contracts will remain in effect until the end of this year. The institution's efforts to obtain similar financing from private industry have yielded only relatively meager results so far, Kavanaugh said.

Much of Airlie's work has involved motion picture films and other educational projects dealing with environmental issues, family planning, cancer, narcotics abuse and other social problmes. The federal investigation and trial of Head have already left a cloud over Airlie that may jeopardize its prospects of carrying out similar government projects in the future, Kavanaugh said.

Airlie -- regarded as the largest private employer in Fauquier County -- has already suffered a slight job loss since 1978 and the total number of Airlie employes -- now about 125 -- may decline further in the future, according to Kavanaugh.

A few organizations also have canceled plans to hold conferences at Airlie because of the federal investigation, Kavanaugh noted. Arranging conferences is one of the main aims of the foundation, which likes to bill itself as an "island of thought."

Head's trial has proved unsettling for a number of Airlie employes, who have been summoned before a grand jury and called to the witness stand in court.

"That's kind of like the dentist -- you hope you'll never have to go back," said Viola Westlake, a longtime Airlie official who testified in court on Friday.

"I don't want to get involved in that anymore -- I just want a job to feed my kids," remarked Raymond Garcia, president of Raven's Hollow Ltd., a filmmaking firm linked with Arilie. Garcia also was a court witness.