Paul R. Boucher, the inspector general of the Small Business Administration, was fatally injured in a freak accident in Loudoun County yesterday when he was struck in the chest by a gasoline-powered model airplane.

Boucher, 40, a lawyer for several government agencies over the past 10 years, was one of 15 federal inspectors general who were ousted from their posts in a controversial purge by President Reagan on his first full day in office on Jan. 21, 1981.

A former official in the criminal division of the Justice Department, Boucher was also among five of the inspectors general who were quickly restored to their posts as sentries against federal fraud and waste.

According to preliminary information gathered by the county sheriff's department and a county medical examiner, yesterday's accident occurred at a flying field for radio-controlled model airplanes at the intersection of Rte. 50 and Rte. 606 in Arcola where Boucher had gone to fly his own model plane.

Boucher, a Vienna resident, reportedly had finished flying his plane, and at about 11:30 a.m. was standing in an area in which planes were being prepared for flight.

A propeller-driven plane weighing about seven or eight pounds that was being guided from another portion of the field, apparently out of sight of Boucher, suddenly went out of control and struck him in the right side of his chest, authorities said.

So far as could be determined yesterday, Boucher never saw the plane approach, officials said.

He was taken to Loudoun Memorial Hospital in Leesburg, where he died during surgery about 2:30 p.m. Death was attributed to liver damage and an internal hemorrhage.

It "can only be described as a freak accident," said county medical examiner Dr. David Caley. He said he knew of only one other such incident, which occurred several years ago in New York City.

The reason for the plane's loss of control could not be immediately determined. According to one report it may have lost a portion of its tail section in flight.

Boucher, married and the father of two children, obtained his law degree from Suffolk University law school in his native Massachusetts. Before joining SBA in 1979 he had served as a Navy Reserve officer, in naval intelligence, as a lawyer for the Government Printing Office, and with the Justice Department, where he was deputy chief of the general crimes section in the criminal division.

In 1977, while in the Justice Department, Boucher was named to head an eight-member task force to work on a stalled investigation of alleged break-ins, wiretaps and mail openings by the FBI in the early 1970s in its pursuit of radical fugitives in New York.

His dismissal from the SBA in 1981 and the dismissals of the 14 other Carter holdovers was criticized by various members of Congress--mostly Democratic--who contended that the post of inspector general, supposedly nonpartisan, was being politicized by the Reagan White House.

In statements at the time the inspectors general were removed, the new administration had placed particular emphasis on the importance of their posts and on the rigorous standards by which prospective occupants were to be measured. The rapid reappointment of Boucher and the four others was seen as a reaffirmation of their qualifications.