A proposal to convert Davison Airfield at Ft. Belvoir into an airport for small planes was rejected yesterday by the Fairfax County Board of Supervisors.

The Fairfax Economic Development Authority had asked the Board to endorse the idea of a general aviation airfield at Davison because there is a "pressing need" for such a facility and the field was "virtually unused."

But the Board decided by a 6-to-0 vote to tell the Army that the county was not in favor of converting the field, located a few miles north of Mason Neck, home of a few of the rare bald eagles.

In an interview after yesterday's vote, David A. Edwards, executive director of the authority, said he didn't think flight patterns would be as disruptive of the environement as airfield foes have indicated.Present Army helicopter flights, he said, are much more disruptive than fixed-wing craft, the kind that would use a civilian facility.

The Board's vote marked the second time it and the authority - an agency of the county government - have taken opposing sides on industry-related aviations issues. While the supervisors launched a legal fight - eventually unsuccessfully - against the supersonic Concorde, the authority said it wasn't opposed to the 16-month trial flights and from Dulles International Airport.

The Board also unanimously passed a resolution relating to the Virginia Supreme Court decision Friday that local governments can do longer negotiate with labor organizations as representatives of public employees. All current agreements between the unions and local governments are "null and void," the court ruled.

The Board said the county would continue to pay salaries and offer fringe benefits to its employees at the level called for in the now-void contracts, but it would no longer recognize the three unions that had agreements with the county.