Grumman Corp. plans to lay off about 10 percent of its workforce or 3,100 employees by the end of 1989, including 2,500 from its main operations in Long Island, the military contractor announced today. The layoffs will begin in early January, with notices sent to 1,800 workers from various managerial and personnel departments, said Grumman spokesman Bob Harwood. According to Harwood, the company will eliminate any unnecessary jobs not directly involved in production. He said some production people will be affected but most of the layoffs will be in overhead and managerial positions. ``We are going to eliminate jobs we (Grumman) don't need in areas such as security, personnel, public relations, business operations,'' Harwood said. Harwood said all layoffs within the past two years have been caused by declining defense spending, which has increased pressures to cut costs. ``For us to stay competitive we have to cut cost and cutting cost translates to cutting people,'' he said. ``Our whole industry is facing flat or declining budgets,'' said Grumman Chairman John O'Brien. In the past two years Grumman has been bumped from three major military aircraft contracts. According to Harwood, in 1987 a contract to produce the Navy's Advanced Tactical Aircraft was awarded to McDonnell Douglas and General Dynamics in St. Louis. Grumman also was shut out from producing the Air Force's Advanced Tactical Fighter. During that time, the company fired 1,400 Long Island employees. According to Harwood, the ATA would eventually replace the A-6E, a Navy bomber which is one of Grumman's most-produced aircraft. This year, the government stopped development plans for an upgraded version bomber, the A-6G. That plan would have generated 1,000 new engineer positions and brought in a quarter-of-a billion dollars annually. Out of the company's 32,000 workers, 21,000 are employed on Long Island.