Nobody got excited.

A man in Chantilly had been stabbed so badly he had almost no blood pressure. The Fairfax County fire department paramedic, fighting to save his life, coolly requested helicopter transport.

Fairfax One, half the county police department's air force, was cruising near the area of the knife fight. Quickly, the chopper wheeled in a gentle arc and churned to the scene.

Firefighters on the ground already were marking out a landing spot with brilliant red flares in the parking lot of Chantilly High School. Fairfax One swooped down, medics loaded the bleeding man on board, and within four minutes the crew and passenger were airborne.

Four minutes later, the stabbing victim was at Fairfax Hospital. The victim was in an operating room before he would have even arrived at the hospital had he gone by ambulance.

To the medics and crew members, Saturday night's drama was routine. Making it routine is one of the major achievements of Fairfax County's unusual, 18-month-old, $1.5 million helicopter unit.

By now, the novelty of having an aviation unit has worn off, and the two choppers have become the police department's workhorses, despite the 13-member unit's short existence.

For instance, last weekend officers learned that a car they believed was involved in a hit and run accident was parked behind a house.

The owner of the house refused to allow police to look at the car. So, a helicopter fly-over provided them with pictures of the car and showed it fit the description of the hit-and-run vehicle.

"It is more successful than even we thought it would be," said Maj. John Rob, who has been promoted from head of the helicopter unit to head of the department's patrol division.

The police are so happy with the unit's performance that the department is now studying the acquisition of larger, more powerful helicopters because there is room for only one paramedic in the present models.

It was the politics of the traffic jam that got Police Chief Carroll D. Buracker his aviation wing. In 1983, he told the Fairfax Board of Supervisors he could help untangle the county's knotted traffic if his men could get a bird's eye view.

The board, overwhelmed with traffic problems, took 10 minutes to approve his request.

With the purchase of the two $500,000 Bell Jet Ranger helicopters, the department became one of the few suburban jurisdictions in the nation to go airborne.

Many state police departments, including Maryland and Virginia, and many large cities, including the District, New York, Los Angeles and Dallas, have helicopters, but Fairfax is pioneering the use of the helicopter as a tool of local law enforcement.

In particular, officials say, Fairfax's helicopters spend their time doing things urban helicopters rarely do: traffic control, rescues and medical evacuation. Urban jurisdictions cannot easily land for medical evacuations, do not face the sort of rescue situations posed by the Potomac River and do not have the traffic problems of a major commuting suburb.

Further, Fairfax is just beginning to feel out the effectiveness of the choppers for fighting the most common urban crime: burglary.

Tracking burglars using the sophisticated night vision and heat seeking equipment is far more successful than simply using dogs, Rob said.

The backbone work of the helicopter unit is the traffic beat. Twice every workday during rush hours, both helicopters fly over the major traffic arteries in the county.

The traffic beat takes up almost half the choppers' flying time, at a cost of $140 an hour.

The ability of helicopter crew members to spot accidents and tie-ups almost as they happen, police maintain, means officers on the ground can be on the scene far faster than otherwise.

Between the first takeoff in August 1983 and the end of October 1984, the helicopters have reported 385 accidents, 2,333 disabled vehicles, 2,984 abandoned vehicles and 204 "unusual traffic backups."

The choppers fly routine patrol when not on traffic duty, responding to calls about burglaries, bank robberies, prison escapes, lost children and medical evacuations.

Each helicopter is equipped with a heat-sensing device that "illuminates" a dark field so show trees, bushes, animals and suspects trying to hide from police.

In its first 18 months, the helicopter crews have been responsible for 140 arrests and the discovery of 27 missing children.

The helicopters have flown 71 missions for jurisdictions other than Fairfax, resulting in 21 arrests.

But the helicopters operations are not without their problems. They are not only expensive, they are noisy, and the county's desire to land them regularly at headquarters in Fairfax City aggravated the city and its residents to the verge of lawsuit.

But the dispute was settled with the county's decision to build itself a $261,000 heliport on West Ox Road, complete with a hanger, lighted pad and a 10,000-gallon fuel tank, which should be ready sometime next spring.

The county's helicopter crews, which include one of the few flying female police members in the country, presently is based out of a cramped trailer near a runway at Dulles Airport.

According to information from the Airborne Law Enforcement Association, there are currently 335 law enforcement agencies of various types with aviation units, using 1,100 aircraft.

Regis R. Raffensberger, commander of the tactical squad of the Baltimore City police and vice president of the association, said the "vast majority of those agencies come from states," except in the west, where sheriffs departments tend to be equipped with aircraft because of the enormous areas they cover.