The Army plans to spend $61 billion to modernize its helicopter force, but not enough money is in sight to complete such a program in the coming era of austere defense budgets, the General Accounting Office said yesterday.

In a report on the Army's buying plans for the new generation of helicopters known as LHX, for light helicopter experimental, the watchdog agency for Congress said the plan to buy 480 of them each year in the 1990s does not appear "affordable."

Defense Department civilians included the LHX last month on their hit list of projects that might have to be canceled if Congress goes through with its plan to slash President Reagan's fiscal 1987 defense budget by as much as $35 billion. Canceling the LHX would save $155 million in fiscal 1987, the Pentagon said.

If the Army strips down the LHX to save money, the GAO said, the new helicopter might not be able to accomplish its mission of performing reconnaissance, carrying troops and supporting them with rocket and gun fire.

The Army intends to buy 5,000 LHXs in varying configurations to replace existing UH1, AH1, OH58 and OH6 helicopters in what would be the biggest helicopter-building program since the Vietnam war.

The GAO applauded the Army concept, saying:

"If the LHX meets all its goals, it would offer a new fleet of helicopters whose performance met the needs of the modern battlefield, whose operation and support costs would be nearly half those of the fleet being replaced and whose average flyaway costs in 1984 dollars would be $5.3 million per aircraft."

But the GAO termed as "soft" Army estimates that it could build 5,000 LHXs for $38 billion in fiscal 1984 dollars and $61 billion in actual outlays as the program progresses.

The GAO noted that the LHX has not been subjected to independent cost analysis, although the Army has promised to have one done.

Army plans call for the new helicopter to go into full-scale development next October. If all goes well technically and the money can be found, the LHX would reach peak production in 1997. "Projected available funding falls short of LHX requirements in 1997 and during several other peak production years," the GAO said.

Sen. Lowell P. Weicker Jr. (R-Conn.), a member of the Senate Appropriations Committee who asked the GAO to study the LHX program, wrote undersecretary of the Army James R. Ambrose that "an independent assessment of Army cost estimates for the LHX and alternative helicopters should be undertaken without delay."

The GAO said one alternative to building the LHX involves keeping the most modern of the current helicopters in production, something the Army does not want to do.