After five years of calculations, engineering professor George Trevino and his family have mastered the mechanics of moving.

Their secret: a little-known but increasingly popular concept called the corporate apartment rental.

Targeted toward the mass of executives, employees and consultants who live away from home for several weeks or months of the year, such apartments are promoted as cheaper, homier alternatives to hotels.

Unlike residential apartments, leases are shorter -- some as brief as a week -- and the apartments are fully furnished down to the knives, forks, towels and vacuum cleaners.

For Trevino, his wife, Norma, and their three children, these apartments mean the family can stay together each summer while Trevino hires out as a consultant during vacation time at Michigan Technological University.

This summer, the family is living in a $1,600-a-month unit at the Oakwood Apartments in Falls Church while he conducts research for the Navy at the David Taylor Research Center in Carder Rock on how noise travels through fluids. Trevino's wife is interning at the Resolution Trust Corp., the agency created to dispose of property held by failed savings and loan institutions.

As Washington area landlords struggle to fill vacant apartments in the midst of a glut, apartments like the one the Trevinos are renting are proliferating throughout the region.

Ironically, landlords are moving into this somewhat risky but highly profitable market niche at the exact time that the slowing national and local economies are reducing their customer base.

The concept of corporate rentals was pioneered by Los Angeles-based R&B Realty Group in 1963. "Companies were coming to us and asking us for short-term leases," said Louise Messano, regional sales manager in Washington for R&B. "We developed the idea of using furnished apartments to fill that need."

The idea caught on nationwide as the U.S. economy began shifting more toward services and away from manufacturing, said Robert Sheehan, a consultant and economist for the National Apartment Association. "Employers use them for employees who are in an area for a short period of time, say three to nine months. They also are used for contractors and consultants like me to stay in."

Not surprisingly, the idea is most popular in major cities with a strong business base. R&B, one of the industry's biggest players, has 30,000 apartments in cities like Philadelphia, Chicago, Los Angeles, Denver, Las Vegas, Dallas, Houston, Memphis and Orlando, Fla. About 70 percent, or 20,000, are corporate rentals. In Washington, about 2,700 of its 4,000 apartments are rented short term. Some also are in smaller cities like Annapolis.

Today, an estimated 1 percent to 2 percent of the nation's 33 million rental housing units are used for corporate rentals, Sheehan said. "Almost every major apartment owner has a relation with corporations."

Apartment owners are attracted to corporate rentals by their higher profitability. Because of the short leases and rapid turnover of residents, such apartments generally rent for 10 percent to 30 percent more than their residential counterparts, Washington apartment owners and managers said.

A typical corporate unit rents for anywhere from $50 to $100 a night, depending on size and amenities. In one month, for example, an apartment owner could realize $1,500 for a studio corporate rental compared with $500 for the same unit if it was rented to a regular tenant.

Corporate rentals also help landlords fill the apartments that have less desirable locations or floor plans, said Furniture Rental Association President Paul Broyhill, who touted the concept in an article last year for the apartment association's magazine.

Moreover, if a corporation rents a block of apartments, landlords "can fill units more quickly with less labor," said Jerry Anderson, marketing director of Lincoln Property Co., which fills 200 of its 1,000 Washington area units with corporate rentals. "With one phone call, you can fill 10 to 30 units."

As an alternative source of customers, corporate rentals also can help stabilize an apartment market. Such an option is particularly attractive to Washington area landlords wading through a softening rental market, said Tom Bozzuto of Bozzuto and Associates, which rents 5 percent to 10 percent of its luxury apartments short term.

Indeed, more and more of the region's landlords are taking advantage of this once-overlooked market segment, said Vickie Touchton, director of corporate business development at Long & Foster's relocation counseling center.

"Temporary housing has always been difficult to find in the Washington area," she said. "When there aren't many vacancies, there isn't much temporary housing."

But now, "with vacancy rates up, apartment complexes are becoming more liberal with three-to-six-month leases," Touchton said.

While more landlords are turning existing units into corporate rentals, new complexes also are saving as much as a third of their units for short-term leases. Lincoln Property, for example, plans to lease 30 percent of its new complex at Ballston on a short-term basis, Anderson said.

These rentals can be risky, however, particularly for landlords who rely on them for a big portion of their business. "When the economy turns down, companies cut back" on travel, Sheehan said.

Evidence of such cutbacks already is materializing in Washington, said Debbie Harvey, another of R&B's regional sales managers here. "Our corporate rentals are up a very small percentage this year," she said.

The company relies heavily on the federal government and its contractors for clients, and cuts in the defense budget means defense contractors are trimming their spending as well. "That affects those of us who work for them, too," she said.

Linda Kurtz, owner of Apartment Home Finders, which specializes in corporate relocations, also has seen customers disappearing. "We don't see as many companies renting as we used to," she said.

Moreover, some landlords are wary of the size of the corporate rental market, slowing economy or not. "We're not a large corporate town," said Ellen V. Sigal, principal of the Sigal/Zuckerman development firm, which plans to offer 20 percent to 30 percent of the units in its new Pennsylvania Avenue development for corporate rental. "If people are looking at that market to replace residential renters, I'm skeptical," she said.

Added to that are the first signs of penny-pinching by corporations, Kurtz said. For the first time, companies are exploring cheaper alternatives to the full-service, fully furnished corporate apartment, she said. One client asked her to investigate the costs of leasing an apartment unfurnished and then providing its own furniture.

Trevino doesn't spend as much as he used to, either. Although the Navy allows him a per diem living cost, it does not entirely cover his expenses. "We used to take the whole package, dishes, maid, towels, everything," he said last week. "Now we bring our own, all except the furniture."