THE MOST coherent thing about the new action thriller "Blue Thunder," which opens today at area theaters, is its eagerness to succeed and its rabble-rousing spectacle of stunt flying and aerial combat.

"Blue Thunder," a chase melodrama with police helicopter pilots as the good guys, "transposes the salty tone of "The French Connection" and "Dirty Harry" to a chopper squadron in Los Angeles.

Nothing less than a super-hero, Roy Scheider plays the resident maverick hot shot of the Astro division, Frank Murphy. A Vietnam veteran, haunted by a teasingly inserted traumatic episode from the war, Murphy must steady his frayed nerves and keep his wits about him long enough to foil a preposterous military conspiracy.

The object of this strange scheme is to create incendiary conditions among the city's ethnic minorities in order to promote a new piece of flying hardware, an advanced snoop-and-attack helicopter nicknamed Blue Thunder, someone's misconception of the ultimate prowl car.

Although the misconception is ascribed to the military, it originates in the addled pates of screenwriters Dan O'Bannon and Don Jakoby, who seem to think like unreconstructed 1960s cranks. The script reflects a permanent fear of impending urban disaster on one hand and the fear of military usurpation on the other. It wouldn't surprise me to learn that the original screenplay was actually written in 1968 and set 15 years in the future.

Stealing from his own bag of tricks, O'Bannon has revived a number of double-cross and reversal devices he used a few years ago in "Alien," where the good guys were also imperiled by shadowy military plotters.

The sheer feebleness of the conspiracy from every social and political perspective makes it essential that the nature of the scheme be concealed as long as possible.

On one night patrol Murphy and his amiable copilot, Buddy Lymangood (Daniel Stern, the closest thing to a satisfying hero in the movie), climax a routine coralling of armed robbers by loitering outside the picture window of a nude contortionist, thus arriving too late at another call to prevent a fatal assault on a city official moments after she pulls into her driveway.

The assailants are killed by officers who rush to the scene. However, Murphy earns a reprimand from his crusty superior, Captain Braddock (the late Warren Oates, back from eternity in an enjoyable cliche' role, since it's taken Columbia two years to get "Blue Thunder" into release), and Murphy feels pretty punk about larking around at such a tragically untimely moment. He takes it upon himself to do some snooping at the murder scene and discovers a piece of paper with a possible clue conveniently scrawled on it by one of the killers.

Murphy's stay in the doghouse proves a short one. Braddock selects him as the Astro division pilot best qualified to fulfill a request, apparently from the Air Force, to test a new miracle chopper, a high-tech boondoggle evidently designed to combine outrageously sophisticated electronic surveillance with the combat potential of a Death Star. The pilot controls the ship's cannons with the motions of his helmet, and the little toys on board include a computer mysteriously free to poke into every security file in the country.

When Murphy learns that an old Vietnam nemesis, Malcolm McDowell (as an obvious rotter called Colonel Cochrane), is associated with Blue Thunder, he suspects the worst. While testing the ship without authorization, Murphy confirms his suspicions.

Cochrane and his coconspirators are recorded redhanded by Blue Thunder's spying lenses and microphones discussing measures they've taken, murder included, to force their wills on an unsuspecting public.

From the straight military point of view, of course, Cochrane and his cronies have wandered so far off the reservation they could hope to return only to life sentences at hard labor. However, there's no regular military in "Blue Thunder," only a villainous conspiratorial military.

Director John Badham and the stunt coordinators must struggle to finesse the fantasy they're stuck with. Murphy can't rely on the assistance of any local, state or federal law enforcement authorities, since these are imagined to be automatically helpless against Cochrane. The hero must go it alone, operating as a lone wolf of the smog corridor to restore justice and tranquility to greater Los Angeles. He is allowed timely minor assists from Buddy Lymangood and a token girlfriend played by Candy Clark (a frazzled edition of the traditional adorable scatterbrained helpmate).

When Blue Thunder is originally tested, we're supposed to share the cynical reaction of Murphy and Braddock to Air Force claims of its "surgical" efficiency at combating urban terrorists without harming innocent bystanders. It only seems logical that a stray civilian or two might get regrettably blown away while being protected by such a high-caliber, high-performance flying fortress.

Nevertheless, once Scheider has hijacked this wonder chopper and begun standing off numerically superior but underpowered foes, as they play hide and seek over the streets of Los Angeles, we're suddenly obliged to have absolute faith in the hero's surgical precision. Although he's disabling rivaling aircraft with the greatest of ease and contributing to spectacular demolitions all around town, one is expected to go along with the pretense that Scheider's maneuvers and shots will harm only the antagonists.

The filmmakers wouldn't have a movie unless they made a point of getting audiences charged up about the fighting prowess of Blue Thunder, yet they insist on consigning it to the junkheap once Scheider has demonstrated that prowess. How cavalier can you get? This is Hollywood liberal doublethink at its wooziest. BLUE THUNDER

Directed by John Badham; written by Dan O'Bannon and Don Jakoby; director of photography, John A. Alonzo; art director, Sydney Z. Litwack; edited by Frank Morriss and Edward Abroms; music by Arthur B. Rubinstein; produced by Gordon Carroll for Rastar-Gordon Carroll Productions. A Columbia Pictures presentation. Rated R; 110 minutes. THE CAST Murphy....Roy Scheider Braddock....Warren Oates Kate....Candy Clark Lymangood....Daniel Stern Cochrane....Malcolm McDowell