McLean Chamber Orchestra, 3 p.m. June 7 at the McLean Community Center, 1236 Ingleside Ave. Admission is $4 adults, $2 students and senior citizens.

When the last concert of the 1980-81 season starts this Sunday afternoon, members of the McLean Chamber Orchestra will be reveling in their newfound professionalism: They'll be sitting on chairs someone else arranged, and playing for an audience someone else drummed up.

The "someone else" is the orchestra's nine-member board which, according to musical director Dingwall Fleary, has "been wonderful about taking over marketing and arranging chores, so we could really concentrate on our music."

The difference has been "astonishing" to a group long accustomed to being performers, promoters and audience for each concert, says Fleary. In fact, Board Chairman Charlotte Sedam refers to the members as "50 families from Rockville to Springfield, whose lives revolve around those Tuesday night rehearsals."

The rehearsal date was the only certainty at the orchestra's inception in 1972 when, after returning from Europe, Fleary -- ex-Vassar professor, ex-St. Louis street kid -- was asked by the former Academy of Musical Art in McLean to start a small orchestra.

"We were aware through students and parents at the academy of many musicians in the area who wanted to make music, but didn't want to get involved in a large symphony orchestra," says Fleary. So Academy owners Robert and June Trayhern agreed to make it a community endeavor, and the McLean Chamber Orchestra was launched with "four cellos, a pair of bassoons, one violin and me on the piano," Fleary recalls.

The next four years were a study in the kind of persistence and perspiration it takes to keep a community arts group afloat, with rehearsals and concerts in homes and halls, and members who came and went, "especially in the string section. I never have trouble keeping wind players, but strings are fickle," says Fleary.

Then in 1975, St. John's Episcopal Church "came to our rescue and gave us a place to perform," says the conductor. "Then at least our small audience knew where our concerts would be held."

"If I had to do it over again, I probably wouldn't," Fleary admits. "But I could sense from the musicians that what we were doing was unique to the area, and valuable to them -- all the orchestra members chipped in money, and helped drum up business."

The next big step came with the orchestra's first board, which "grandfathered us in as a resident company at the McLean community center." That, says Fleary, was the "beginning of our success. It changed our audience and prompted the interest of musicians, since we quickly took on a more successful image than we probably deserved."

Part of that success comes from Fleary's ability to develop good friends in high places -- former Virginia governor Linwood Holton and his wife are loyal followers, and network newsman Roger Mudd has narrated more than one children's concert for the group.

The other part comes through sheer hard work, or as Sedam describes it: "speaking to local citizens' groups, Rotary clubs; sending out fliers to private schools, putting them on cars in parking lots and distributing them with trash trucks on Saturday morning; and going after grants."

Last year, the board received a $3,000 challenge grant from the Virginia Commission for the Arts which it "matched quickly." Sedam says. The funds pay Fleary's salary and other expenses, but the "musicians are all volunteers," Sedam. The rules they adhere to, however, are far from amateur -- missed rehearsals are grounds for dismissal, and orchestra members are chosen only after "intensive auditions."

The orchestra's repertoire reflect this professional attitude, with one piece in nearly every concert chosen from a "new or American composer," says Fleary. "I feel that I have to be rabbi and teacher to the audience. How will they learn to enjoy contemporary music unless they get a chance to hear it?"

The concert this Sunday includes a piece by one of the orchestra's own, violinist Alan Newhouse. Called "Creation Myths," it's intended to balance the more traditional offerings by Mozart, Strauss and Mendelssohn.