Sometime this month, when the winds around the cheese-making town of Tillamook, Ore., seem right, Vera Simons of McLean will set off in a balloon larger than the Goodyear blimp to attempt the first nonstop balloon crossing of the United States.

The flight plan of her craft, named the DaVinci TransAmerica, calls for a 2,295-mile, six-day and nin-hour journey that will end in Norfolk, Va. Simons and her crew plan to break the endurance record, set last August by three Americans in their unprecedented transatlantic balloon crossing, for most hours aloft in a balloon.

But while they are drifting eastward in America's late summer sky, they intend to do more than count the hours. This is to be an extravaganza of science and hoopla and helium.

One crewman, a federal environmental scientist, will examine the relative filth of the nation's air. Another crewman, a radio expert and eye doctor, will be taking calls from smalltown America.

Simons, who is described by her friends as a top-notch balloonist, an artist and a "very temperamental gal," will be piloting the balloon and indulging her interest in light shows and periodic showers of leaflets, which Americans are supposed to fill and and mail to McLean.

"I am very interested in having a dialogue with the people along the flight path," said Simons who has been planning the transcontinental journey since 1971.

Sponsored by the National Oceanic and Atmospheric Administration, along with more than half a dozen major aerospace companies and the Seven-Up soft drink company, the planned flight of the DaVinci Trans-American differs radically in style and objective from the hazardous attempts to cross the Atlantic that have preoccupied the minds of the world's great aeronauts.

Seven persons have died trying to cross the Atlantic. Ben Abruzzo, one of the three Albuquerque, N.M., men who made the first successful crossing last year, describes that journey across the Atlantic as "an awful physical and mental strain" in which failure to succeed means "that you could very well get killed."

Abruzzo, who failed earlier this year in an attempt to cross the country in the same balloon that made it to France, called the chances of Simons and her crew succeeding "very slight."

"On the basis of overland attempts that have been made in the past, I would bet against them," agreed Roger Pineau, a balloon historian who helped design the balloon exhibit at the National Air and Space Museum. "I would say anything over 1,500 miles is gravy."

But the route of the DaVinci, although tricky in its crossing of the Rocky Mountains is unlikely to endanger the lives of the four baloonists aboard, some balloon experts say.

"If they have to plop down, they can plop down safely just about anywhere," said Pineau.

Simons, a blonde-haired German immigrant who has been involved in making and flying balloons nearly all her life (she refuses to tell her age, saying only she's in her 40s), claims the experts are too skeptical. She says they don't understand the sophisticated weather information that she will have to keep the balloon aloft and headed east.

"I expect to make it. Our chances are way up there at 80 or 90 percent. If we take off in the right weather, at the right moment and if we do what the weather people tell us, we'll make it."

Balloons, which are notoriously hard to steer, can be guided to some extent by moving up and down to take advantage of winds blowing in different directions at various elevations.

Simons' radio expert, Dr. Fred Hyde, of Prairie View, Kan., will be in continuous contact with federal private weather services, which, in theory, can tell Simons the best elevation for the winds she needs.

The clear plastic, tear-shaped balloon, which has the capacity to rise to 18,000 feet without dumping any of its 8,000-pound load, will be forced to climb to at least 14,000 feet to cross the Cascade and Rocky mountains.

Simons said she plans to drift over the dangerous mountain passages at night, when the air is calmer and there is less chance of a sudden downdraft that could crash the balloon into a mountaintop. During night hours, she said, the balloon could stay at lower altitudes and the crew would not be forced to use oxygen masks.

In an interview yesterday, Simons spoke of flying over the Rocky Mountains in a balloon as though it were comparable to grocery shopping.

The daugheter of an Austrian mountain climber, Simons has been married three times, once to a man with whom she helped found a balloom company that was instrumental in space research and once to a man who briefly held the world's altitude record in a balloon.

She is now married to a General Electric executive, a man who she says has given her "complete support" in her dream of a transcontinental voyage.

Simons, an artist and sculptor of balloon-related art, said she had not flown for years in a balloon until 1971 in Amsterdam. At that point she decided to attempt to cross the United States.

"I was not content to paint. I started to fly much more. I spent a couple of years looking for a scientist to put the concept of my art together with scientific experiments on air quality," she said.

She finally found Dr. Rudolf J. Engleman, an environmental scientist with the National Oceanic and Atmospheric Administration (NOAA), who was looking for a way to gather information on air pollution and its effect on weather across the country.

"Because a balloon is slow moving, it makes an excellent platform for such studies," Engleman said.

The floor of the gondola in which the four balloonists plan to spend nearly a week is a cramped, 10-foot square. The gondola has two levels, connected by an access hatch. On the upper level, the crew will work with flight control and scientific equipment. The lower level contains food, storage batteries and sleeping room.

As historian Pineau said of manned balloon flights, "The crews members have to be very good friends."

With federal and corporate contributions totaling about $250,000, the Davinci TransAmerica has a marked advantage over past attempts to cross the United States by balloon.

The great ballonists of the past were preoccupied with crossing the Atlantic, and cross-co-ntry flights have been low-budget operations for the most part.

Magazine publisher Malcolm Forbes, financing himself, crossed the nation in a hot-air balloon in 1973, but he made frequent stops for rest and balloon repairs. That same year, a young adventurer named Bob Willigunda, using his balloon as a movie prop, traversed the country in a truck, boarding the balloon only in photogenic areas.

This spring, the well-financed Alburquerque balloonists flew their Double Eagle II 1,000 miles from the Pacific to Colorado, only to be stopped by a storm.

Simons said yesterday that the successful crossing of the Atlantic has nothing to do with her planned aventure. She said she wants to fly her balloon over people, sharing its beauty and doing something useful for science.

Simons who is described by one friend as someone who "makes a production out of everything in her life," said the main reason she plans to spend a week floating in the sky is that "once you fly balloons, you always want to fly." CAPTION: Picture, Balloonist Vera Simons with one of her crew, meteorologist Rudolf Engleman. By Craig Herndon - The Washington Post; Map, Probable Route of Transcontinental Balloon Crossing, By Dave Cook - The Washington Post