New York Air, whose bagels have given the Eastern Air Lines shuttle a stomachache, will announce plans today to open a major hub in a $3.6 million terminal it has built at Washington Dulles International Airport.

The airline has been building its schedule at Dulles since February and now has about 20 flights a day there. In September, it will coordinate those flights into a hub-type operation -- where passengers can easily connect from one flight to another. By then, New York Air will operate 35 flights a day at Dulles, primarily to cities along the Eastern Seaboard.

The new facility at Dulles has seven gates, so New York Air could run five banks a day of seven flights each. Continental Airlines, like New York Air a subsidiary of Texas Air Corp., will use some of the gates. Its flights to Denver and Houston, and possibly other locations, will be tied into the New York Air hub.

New York Air is the second major airline in recent weeks to announce that long-underused Dulles has become a center of operations. Pan American World Airways started to connect domestic and international flights there June 1.

The New York Air terminal is temporary, constructed of manufactured panels. It runs west from the base of the Dulles tower and is being fitted with "sky bridges" so that passengers can walk directly from the terminal to the airplane without having to walk outside or ride a mobile lounge from the main terminal.

The interior is done in New York Air grays and reds. A restaurant area, which New York Air calls the Apple Club, is located in the middle of the terminal and will be available to all its customers.

Other airlines are talking to Dulles officials about constructing a similar terminal east of the tower. Several airlines now park their planes at the tower base and walk their passengers outdoors to climb aboard flights. When it is raining, they are likely to notice the New York Air facility.

Dulles officials plan to remove the temporary terminal after a midfield terminal is constructed. Plans for that terminal are waiting, in part, on Congress to approve Transportation Secretary Elizabeth Hanford Dole's plan to transfer control of Dulles and Washington National airports to a regional authority.

The biggest drawback of the New York Air facility is that two mainstays of the domestic airline fleet -- the Boeing 727 and the McDonnell Douglas MD80 -- cannot use it because they are too heavy for the concrete in the area and too long to safely clear a nearby taxiway. New York Air will use McDonnell Douglas DC9-30s and Boeing 737-300s, both of which meet the Dulles requirements.

New York Air is a child of airline deregulation. It was organized specifically to attack Eastern's virtual monopoly on Washington-New York-Boston service. New York Air was unheard of in 1977; it is now the third-largest carrier at Washington National Airport, with hourly flights to New York's LaGuardia Airport.

Passengers on New York Air get a free bagel, which has been featured in the airline's advertising.

Traffic at Dulles has been growing steadily since 1980. In May, despite the United Airlines strike, Dulles had 16.9 percent more passengers than it had in the same month a year earlier.

The airport has become attractive for expansion because, as one airline official said, "it's the last major unconstrained airport in the East." Air traffic nationally is setting records every month.