The skies were so crowded over College Park Airport yesterday that its single runway looked like a miniature version of National Airport on the eve of a busy holiday weekend.

Brightly painted biplanes dipped their wings in a tandem dance. Helicopters whirred and circled overhead. And the star attraction of the airport's 75th birthday bash, a 164-foot-long blimp made of space-age plastic, hovered over the crowds like a giant silver cloud.

College Park Airport, which claims to be the world's oldest continuously operating airfield, is hosting its 10th annual open house this weekend. The free-admission "air fair" features an air circus, fireworks, plane rides and a display of about 40 antique and experimental aircraft to mark the anniversary, 75 years after the Wright Brothers started flying there for the military.

Organizers expect as many as 60,000 visitors by the fair's finish at 5 p.m. today.

Owned by Prince George's County, the airport is home to a mere 76 private planes. But it boasts an illustrious history of firsts that could stump the most avid pursuer of trivia, including: the first woman airplane passenger (Mrs. Ralph Van Deman, Oct. 27, 1909), the first flight over one mile in altitude (June 1, 1912) and the first successful helicopter flight (Feb. 3, 1924).

Yesterday, the airport was a flying enthusiast's dream, housing an array of flying machines to melt the Red Baron's heart.

Jim Dick of Riverdale exhibited his ultralight, a motorized hang glider with rayon wings and a 30-horsepower snowmobile engine.

George Economos of Silver Spring dropped in aboard his home-built Breezy, an open-air metal chassis with a seat and wings that cruises at 75 mph.

Jerry Michaels flew in for the day from Berkeley Springs, W. Va., in his 1946 Luscombe two-seater. Michaels, who restored the plane to its original gleaming aluminum, winced only slightly as 2-year-old Kristen Brewster of Burtonsville sat in the cockpit, pulling on the controls with the zeal of a future Sally Ride.

But the blimp, whose twin flew over the recently completed Olympic Games, was the focus of attention. The $2-million craft was built by Airship Industries Ltd., a British company trying to convince the Navy to commission a model twice its size to accompany America's battleships.

Yesterday's mission was much less hazardous. The blimp floated 1,000 feet above College Park, soaring over the University of Maryland marching band as it practiced formations for the opening game of the football season, and swooping low over the gaping crowds at the airshow.

The public was not invited aboard the lighter-than-air craft. The rides were reserved for reporters, Federal Aviation Administration officials, politicians and a select few others.