Just when you thought it was safe to go back on the Mall, the National Air and Space Museum announced yesterday that it will go ahead with plans to send a flying dinosaur flapping up and down the monumental greensward, shadowing the sleep of anxious 4-year-olds with reptilian dreams.

For those who couldn't handle the flying monkeys in the "Wizard of Oz," the appearance of Quetzalcoatlus northropi, a pterodactylian giant with bat-like wing claws and the wingspan of a Piper Comanche airplane, should mean a quick trip back to the pacifier.

Museum officials, however, expect the robotic reptile to intrigue more than it alarms, luring the aeronautically apathetic to the wonders of history and flight.

"What we're doing in this project is really symptomatic of the National Air and Space Museum," NASM Director Walter J. Boyne told a thoroughly captivated news conference. "We wish to educate, of course, we wish to diffuse knowledge. But we wish to do it in a way attractive not only to our natural clientele . . . but to other segments of society . . . . "

Quetzalcoatlus will be part of that outreach program -- a $350,000 interdisciplinary venture designed to use the scientific wonders of the modern world to recreate those that occurred naturally near the dawn of time.

The battery-powered, radio-controlled robot, 36 feet from wingtip to wingtip and weighing about 120 pounds, will be built by Paul B. MacCready of Pasadena, Calif.

It was MacCready whose 55-pound Gossamer Albatross aircraft crossed the English Channel in 1979 under pedal-powered flight. Yesterday he told the news conference that a six-month, $50,000 feasibility study had convinced him the dinosaur project would fly.

As every paleontologist knows, the last Quetzalcoatlus crash-landed some 60 million years ago in the sunset of the Age of Reptiles, the last and largest such beast ever to take wing. The only known remains turned up in 1971 in a badlands arroyo in west Texas, scattered over an acre of land.

Wann Langston Jr., director of the vertebrate paleontology laboratory of the University of Texas at Austin, said no one knows what the beast was doing there, 200 miles inland from the nearest sea where such toothless pterosaurs presumably fished.

He said it may have been blown inland. Remains of a number of smaller but similar pterosaurs were found about 20 miles away -- either immature Quetzalcoatli or diminutive relatives. "They may have even been a flock," mused Langston happily, resplendent in a pterodactyl-print necktie.

By pooling and comparing the remains, Langston said, scientists have a pretty good idea how the beast operated, though nagging mysteries remain.

They have unearthed no chest portions ("In an animal this light that was probably the only part worth eating . . . This was evidently somebody's dinner"), and the neckbones give no clue how the reptile carried its beaked head.

Most nagging for MacCready has been the absence of any tail, or any other indication how Quetzalcoatlus controlled the pitch of its flight. The robot will probably do so by moving its wings forward or backward, much as a modern hang-glider operator shifts his center of gravity in the air to control his flight. The robot will be able to flap its wings to gain speed and altitude but will not be able to run on its legs to take off.

Such robotics, MacCready said, would be too "elegant" for the budget, and Quetzalcoatlus will probably be catapulted into the air inelegantly with bungee cord.

Also in question is its projected landing mode, which will probably be crash. Parts may be designed to snap off harmlessly on impact, MacCready said, but presumably not across an acre of land.

Though the air and space museum funded MacCready's study, Boyne said, a private sponsor will have to be found to finance the rest of the project. He said he expected no difficulty finding such backing, but declined to suggest possible donors.

Langston, however, pointed out that in addition to being named for Quetzalcoatl, the feathery serpent of Aztec deities, Quetzalcoatlus northropi is also named for the Northrop Corp., which turned out delta-winged aircraft in the 1950s.

If the money flies as well as the model, the reptilian robot will be launched in time to star in a new NASM film, "On the Wing," in the spring of 1986.