the U.S. Army's force of 8,240 choppers -- and the military doctrine justifying its existence are hovering between a stall and a climb.

Why this is so and the dilemma it presents illustrate what the armed services will be facing for the rest of President Reagan's term as they scale down five-year plans to fit tighter budgets and reassess strategy to match weaponry they will have in the 1990s.

When Reagan came to office in 1981, there was little doubt in Congress or the Pentagon that the Army needed to do something about its helicopter fleet. Most of the aircraft were built for the Vietnam war and would be too old to fly in the 1990s, unless -- like the Air Force B52 bombers -- they were continually rebuilt.

The helicopter had come into its own as a war machine, everybody seemed to agree, between the Korean and Vietnam wars. From just an ambulance to transport wounded to MASH-type field hospitals in Korea in the 1950s, the helicopter had become a vehicle to bring men and firepower to bear at a critical spot at a critical moment -- as well as to evacuate wounded during battle.

In Vietnam, Army, Navy, Air Force and Marine helicopters flew 36 million sorties -- one aircraft to the target and back -- to fire machine guns and rockets at the Viet Cong, to hopscotch infantry around the countryside and to perform reconnaissance and rescue. With comparatively primitive weapons -- mostly rifles -- the enemy shot down more than 2,000 helicopters during the conflict. A total of 1,069 helicopter pilots were killed by hostile fire and accidents.

Inspired by the mobility the helicopter offered and confronted with superior numbers of Warsaw Pact tanks and other armored vehicles, the Army wrote a strategy called AirLand Battle. For NATO forces to win a land war in Europe, AirLand Battle calls for finding the enemy's soft spots, particularly behind the front lines; hitting them quickly with troops and firepower, and then making a hasty exit to fight someplace else.

"The helicopter is essential to maximize the potential of AirLand Battle," said Maj. Gen. Ellis D. Parker, commander of the U.S. Army Aviation Center at Fort Rucker, Ala. "With the helicopter, the terrain stops being your enemy and becomes your friend." In addition to scouting and delivering soldiers and projectiles to weak points in the enemy's force, he said, it enables commanders to "synchronize" men, tanks and firepower in the air and on the ground.

In a contention some specialists dispute, Parker said Army helicopters will be able to survive on the modern battlefield with new tactics that feature staying hidden by sticking close to earth and popping up for only a few seconds to fire antitank missiles. Undersecretary of the Army James R. Ambrose, an industrialist, agreed at the outset of the Reagan buildup that the Army should earmark $40 billion to $60 billon for a new fleet of helicopters but did not want more of the old aluminum-and-rivets types still being manufactured. He sold the Army on trying a space-age helicopter -- one made out of plastic and so automated and computerized that one pilot instead of the usual two could take it to war in any kind of weather, day or night. The new helicopter, known as the LHX, would be a do-everything machine -- firing weapons over the battlefield, transporting troops and weapons, scouting forward areas and serving as an ambulance.

But to buy the LHX and other weapons, such as the M1 tank, the Army had to shelve plans to put more soldiers in uniform. Throughout the five years of the Reagan defense buildup, the Army has remained at 781,000 men and women. It was a choice of hardware over manpower -- one that, because of the lean budget years ahead, does not look as glittering in 1986 as it did in 1981. The LHX is a prime example of a program started when hardware was being stressed but whose future is in doubt because of Congress' change in attitude toward defense spending.

The Army estimates it would cost $60 billion to buy the 5,000 LHXs needed to support the AirLand Battle strategy.

Sen. Lowell P. Weicker Jr. (R-Conn.) said there is no way to get that much money under the austere budgets ahead. The concerns of Weicker, a member of the Senate Appropriations Committee, typify those of LHX skeptics.

He recently wrote Ambrose that a General Accounting Office study of the Army's helicopter plans "raises some very fundamental questions regarding the Army's ability to fulfill the goals established for the LHX program. And Mr. Secretary," Weicker continued, "you are on record as saying that if those goals are not met, then the program would be terminated and alternative helicopter programs would be pursued. Is that still your position?"

Ambrose, when asked in an interview whether he thinks that the Army will get enough money to build the LHX and what he will do if it does not, sat silently for a moment in his Pentagon office, smiled and replied, "I'm like the guy in 'Fiddler on the Roof' who asked himself where tradition came from and said, 'I don't know.' "

The GAO, in providing ammuntion to shoot down the LHX, focused on costs, not battlefield potential. Two of the Army's most questionable assumptions, GAO said, are that its procurement account will grow every year for the next 12 to 15 years and that other hardware programs would not eat into money earmarked for the LHX. The Army timetable calls for the LHX to go into full-scale development in 1987 and reach peak production in 1997.

Also challengeable, the GAO said, are the Army's assumptions that the LHX would be cheaper to fly and maintain than today's choppers and that the service could do the same job with fewer aircraft.

Ambrose stressed that the Army is studying alternatives to the LHX. He said it will make its decision on what to do about its aging helicopter fleet in time to shape the defense budget sent to Congress next year.

Ambrose is enthusiastic about the potential of building a new generation of helicopters. He said they could communicate with satellites for precise navigation and gunnery, fly and fight at night and kill tanks with missiles guided by laser beams.

If critics do end up downing the LHX, the Army most likely will extend production of its most modern attack and troops helicopters, the AH64 Apache and UH60 Blackhawk. The Army intends to buy only 675 Apaches, partly because it wants a less-vulnerable helicopter to carry out the AirLand Battle doctrine.

If the Army does settle for building more of today's helicopters for the 1990s, Army aviation enthusiasts contend that the Soviets will open a gap in attack helicopters. Army pilots interviewed predicted that both superpowers will be forced into building not only new generations of tank-killing helicopters but a new breed of chopper to protect them. The age of the antihelicopter helicopter is fast approaching, they said, complete with the kind of air-to-air missiles now found on fighter planes.

The vision of helicopters fighting each other on the battlefield is so real to Rep. William L. Dickinson (R-Ala.), ranking Republican on the House Armed Services Committee, that he has warned the Army that Congress will not provide money for any new helicopters that are not armed to shoot others down.

In its first hasty response to this new dimension of helicopter warfare, the Army this summer began jury-rigging helicopters with Stinger heat-seeking missiles designed to be shot from the ground but adaptable to helicopters for air-to-air combat. The Army also is training helicopter pilots to fight at night and employ maneuvers to foil new antiaircraft missiles.

The Soviets have been using attack helicopters effectively in Nicaragua and Afghanistan, although their losses in Afghanistan have been high. Army leaders insist that tests show that their attack helicopters are deadly enough against tanks to tilt the adverse NATO-Warsaw Pact armor balance toward the West.

The Soviets, Army tests and the lobbying pressure of the defense contractors in position to cash in on the $60 billion will give the LHX program lift while its cost threatens to cause it to stall or crash before Reagan leaves office.

Ambrose said, "We're going to have to make an economic-payoff case. We can't just say the Russians are 11 1/2 feet tall and growing."