The Navy and Air Force have contracted to buy up to $100 million worth of 1981-vintage personal computers in a deal that appears, according to market analysts, to lock them into either technological obsolescence or a losing proposition.

The contract was awarded May 1 to Federal Data Corp. of Chevy Chase, which outbid five competitors by offering to sell IBM-compatible portable computers at steadily dropping prices that will hit bargain-basement rates at the end of three years.

The services can buy up to 1,000 computers per month at any point, leading contract officers to anticipate a big purchase as the agreement winds down.

But computer marketing analysts say the services will be forced by the pressures of technological change to concentrate their purchases in the early period of the contract when Federal Data's prices are higher than those quoted by other bidders.

Waiting for prices to drop in 1987 and 1988 would mean buying computers that are already at the end of their life cycle, according to experts.

"Nobody will want that computer in three years," said David Wilson, senior analyst at Future Computing Inc., a research and consulting firm based in Dallas. "With the technology advancing so rapidly, two years is a long time in this industry. A year or 18 months is the longest contract I'd get into for microcomputers."

Another industry analyst said the Navy is "buying microcomputers the way it buys ships, which don't change significantly in the short run."

Wilson said Federal Data outbid its competitors by offering rock-bottom prices in the last 18 months of the contract, knowing it would sell most of the computers at the start, when prices were higher.

Federal Data's prices in the first 18 months exceed those offered by all but one of its competitors, according to an analysis of the bids. When the costs of accessories, software, spare parts and maintenance are figured into the total, Federal Data is still higher than one other firm -- Zenith Data Systems.

Capt. Ray Chalupsky, commanding officer of the Navy's Automation Data Processing Selection Office, which negotiated the contract for both services, said there is no obligation to buy computers in the early, more expensive phase.

"It would be advantageous for us to get them in the third year," Chalupsky said. "We're not looking for scientific computers that can crunch numbers in a laboratory. We had a low-price shootout and Federal Data won with the lowest bid."

Asked if the computer would be bought if it becomes obsolete, the captain replied, "I have absolutely no idea. I don't state the requirements."

The Navy solicited bids in February for portable computers of up to 25 pounds with the capacity to communicate with a host computer at various sites. Contract specifications called for a maximum of 36,000 machines to be sold in the following proportions over a three-year period: one-sixth in the first six months, two-thirds in the next two years and one-sixth in the final six months.

Federal Data won the contract, offering to sell the Chameleon XL computer, software and spare parts and to provide maintenance at the rate of $23.6 million per 10,000 machines.

It structured its pricing on a sliding scale, with computer costs falling from $1,435 apiece in the first six months to $450 in the final six months of the three-year contract. Similar reductions were offered for accessories and software.

Precipitous price cuts in the final 18 months helped the firm outbid Zenith, which offered to sell its computers for $999 in all three years and to keep prices constant for software and accessories. Zenith's overall prices were lower than Federal Data's in the first 18 months but reached $27.4 million over the life of the contract.

"I would have gone for the lowest bid in the short term before the computer becomes obsolete," Wilson said. The analyst said that Federal Data's bid appears to have been "artificially contrived" to secure the award. He said the firm is "counting on that computer being so obsolete that no one will buy it at $450 or any price."

"Most new software is written for the current wave of machines and that wave will have passed," he said of the Chameleon XL.

Wilson said the $450 price tag should have "raised a warning flag" to Navy contract officers. The price is "unrealistically low" even for 1988, he said, which indicates that the firm simply bid low to win the award without intending to deliver.

Federal Data president Dan Young said his company is "prepared and very happy" to supply the computer for $450, a price that he believes will reflect lower manufacturing costs by 1988.

Young said he knows of no technological advance in the portable computer industry that will soon overtake the Chameleon XL -- a point disputed by independent marketing analysts and computer salesmen who say a lighter, faster machine is on its way.

"I'd give the Navy 18 months before it looks for another machine," an executive of a competing computer firm said.

Chalupsky said the Chameleon XL should be suitable for Navy and Air Force data processing. His superiors set the three-year time frame for the contract, he said, adding that "they must have some idea this computer will be around by then."

"I don't know what the future will hold," he said.