WASHINGTON (AP) _ Two of the nation's leading aviation museums issued a challenge to would-be aeronautical engineers on Tuesday: build the world's best paper airplane and win the Second Great International Paper Airplane Contest.

''Imagination will be the key,'' said Walter J. Boyne, director of the National Air and Space Museum, where the new event was announced.

Boyne's museum is joining forces with the Museum of Flight in Seattle, Wash., and Science 85 magazine to renew the challenge first issued 18 years ago by Scientific American magazine.

Allen L. Hammond, editor of Science 85, said the organizers hope to receive as many as 20,000 entries from around the world, nearly double the 11,851 entered from 28 countries in the first such contest.

The contest is not just something being done for fun, Boyne emphasized. ''It has genuine scientific aspects to it.''

The first contect resulted in a book of paper airplanes that remained in print for several years and spawned a variety of local contests, Boyne said, although no major advances in aeronautical engineering came out of the event.

This new contest will vary somewhat from the first to reflect changes in the design and construction of real aircraft, said Howard Lovering, director of the Seattle museum.

Composite materials, in which fibers are layered to produce lightweight but strong materials to replace metals in airplanes, have become popular, he said.

Thus, Hammond said, the contest rules will now permit entrants to glue layers of paper together, laminating them into rigid structures for sturdier and more interesting airplanes.

The contest will have categories for amateur and professional designs as well as for youngsters. And in addition to time aloft, distance and aerobatic categories there will also be a contest for beauty of design.

Boyne said, however, than even the planes entered in the contest purely for looks must be able to fly at least 15 feet to be considered.

The records from the first contest are 58 feet of flight for an amateur design and 10 seconds aloft. For a professionally built plane the record was 90 feet, but that craft hit a wall at that point so likely could have done better, Hammon said. All the records have since been bettered in local contests.

The planes will be tested in a Boeing Aircraft hanger in Seattle, launched with one-manpower throws, Lovering said.

Winners of the first three places in each division will be awarded medallions. The first place winners will be flown to Seattle in June for an awards ceremony.

Entries should be sent by May 1 to the International Paper Airplane Contest, Museum of Flight, 9404 E. Marginal Way South, Seattle, Wash. 98108.

Contest judges will be Michael Collins, a former astronaut; Dennis Flanagan, editor emeritus of Scientific American; Ilan Kroo of the National Aviation and Space Administration; Yasuaki Ninomiya, a designer of toy airplanes; and Sheila Widnall of the Massachusetts Institute of Technology.