Vietnam War movies are rife with cliches, but if one stands out as being the most overused, it is probably a sound effect - the whoosh, whoosh, whoosh of helicopter blades. Usually, it begins as a soft whirring flutter, then gradually builds to a loud, dramatic crescendo.

The most powerful use of this device can be found in one of the earliest films on the war, Francis Ford Coppola's "Apocalypse Now" (1979). An overhead camera peers directly down at Martin Sheen, who is lying on a cot, staring up at the whipping blade of a ceiling fan. Whoosh, whoosh, whoosh.Ironically, the second most powerful use of this device may be in "The Scent of Green Papaya," a movie that came out of Vietnam and which was nominated earlier this year for an Oscar as best foreign-language picture.

"Scent" is an eloquent, low-key look at life in Saigon during the '50s and '60s before the war. The moment in question is subtle and simple. During an indoor scene, we hear background noise from outside, the gentle, familiar hum of helicopter rotary blades, quietly signaling the U.S. military's arrival. It is "Scent's" only acknowledgment of the horrors of war on the horizon. Whoosh, whoosh, whoosh.

A significant film on several counts, "The Scent of Green Papaya" is the first Vietnamese motion picture to be nominated for an Academy Award, as well as the first to receive wide distribution in this country. It is also timely, a look at what Vietnam once was to help us understand what Vietnam may yet become. There is also a gentle dignity and a sweetness that are lacking in American-made movies about Vietnam.

The film's writer/director, Tran Anh Hung, has said of "Scent," "I wanted to show the regular life of the Vietnamese people. I wanted to show their humanity, which hasn't been seen yet at the movies."

This simple statement also sums up the reason so many movies fail in their attempts to explore the Vietnam War. Too often they lack humanity, and not just in their treatment of Vietnamese characters.

Of course, there are built-in problems for filmmakers who wish to say something profound on the subject. The war in Vietnam was complicated, filled with culture shock and conflicting emotions. It was resolved unhappily - if "resolved" is the word - and the feelings that linger and continue to fester some 20 years later are not easy to sort out on film.

That may partially explain why, unlike World War I and World War II, the Vietnam War was not given big-screen treatment while the battles were in progress . . . with two exceptions.

The first was a low-budget 1964 programmer, "A Yank in Viet-Nam," starring B-movie leading man Marshall Thompson as a U.S. Marine shot down by the Viet Cong. The plot has him rescuing a doctor, then falling for the physician's daughter. But the film was merely a by-the-numbers action picture, using the Vietnam War as a timely backdrop.

The second, and most famous was the 1968 fiasco "The Green Berets," a pro-Vietnam War propaganda effort, directed by and starring John Wayne. The Duke cast himself as a gung-ho Green Beret colonel, shaping up his men, then heading for Nam. David Janssen co-starred as a cynical newspaperman who ultimately sees the error of his ways.

Wayne's most obvious folly was his attempt to juxtapose a World War II sensibility on the Vietnam War, as if he was doing a Southeast Asian follow-up to "The Sands of Iwo Jima." But the film was fraught with problems and became one of Wayne's biggest flops, both financially and artistically.

Unofficially, Robert Altman's "M*A*S*H" also qualifies. Though the setting for this dark, anti-war satire is the Korean War, there was no question at the time that the film was meant as a direct take on the Vietnam conflict (as was the subsequent hit television series).

Released in 1970, at the height of protests against the war, the "M*A*S*H"-Vietnam metaphor was acknowledged by screenwriter Ring Lardner Jr., who said he and Altman were commenting on "a special kind of war, an American one on the Asian mainland, and our habit of taking our culture along with us and ignoring the local variety."

Though that sums up the brief oeuvre of theatrical movies that dealt with the conflict as it raged, either directly and indirectly, Hollywood did not ignore the subject elsewhere. There were other movies . . . but they were made for television and dealt primarily with veterans returning from this unpopular war and attempting to reassimilate into civilian life.

The first of these was "The Ballad of Andy Crocker," a 1969 ABC-TV movie, with Lee Majors in the title role, as a vet who comes home to find his girl (Joey Heatherton) has been forced into an unhappy marriage by her shrewish mother (Agnes Moorehead). If that's not enough, his business has been taken over by a crooked partner.

In 1971, Dennis Weaver took the lead role in a similar yarn for another ABC movie-of-the-week, but with a more directly political agenda. In "The Forgotten Man," Weaver plays a former POW thought to have been killed in action. When he gets home, he finds that his wife (Anne Francis) has remarried, his daughter has been adopted by her stepfather and his business has been sold off.

Both were serious films, and both benefited from strong lead performances - Weaver was particularly good in the latter picture. But both were also rather ordinary productions, fairly typical TV-style melodramas. And there is little question that they provided the foundation for The Stereotype.

You've seen The Stereotype, of course - the burned-out Vietnam veteran who returns home to find that his world has been pulled out from under him. Not only is he misunderstood by his friends and family back home, he has trouble getting back in step with his loved ones and often suffers flashbacks relating to his war experiences.

After the war was over, Hollywood's theatrical filmmakers began a string of uncountable movies that portrayed Vietnam vets as crazed lunatics, whether for purely exploitation purposes (vengeance-minded William Devane in the 1977 over-the-top thriller "Rolling Thunder") or more serious and acclaimed efforts (wacked-out Robert De Niro in Martin Scorsese's very violent 1976 drama "Taxi Driver").

Even heartfelt and well-intentioned films like the 1978 melodrama "Coming Home," which won Oscars for Jon Voight and Jane Fonda, perpetuated The Stereotype. In this much-lauded effort, Voight is a paralyzed vet who meets volunteer worker Fonda in a VA Hospital. They begin an affair as Voight helps alter Fonda's view of the war, despite the fact that she is married to a gung-ho Marine (Bruce Dern) who is on active duty in Nam.

Voight's character is an anti-war hippie in military clothing, intended to be the voice of reason. He has been cut down by that perpetual enemy, the war. And Dern is as crazed a nutcase as any of these films has ever produced, ultimately taking the coward's way out, apparently after watching James Mason in "A Star Is Born."

Today, "Coming Home" seems oddly dated - much more so than, say, "The Best Years of Our Lives," a 1946 Oscar-winner about World War II returnees adjusting to civilian life. In that film, there is an abundance of the aforementioned commodity that so many Vietnam War movies lack, "humanity."

"Best Years" has the benefit of three-dimensional characters, real people attempting to come to terms with the changes in their personal worlds and in the world at large. "Coming Home," however, is built more on "types," which seems more obvious now than it did in 1978.

But it was the era of "Coming Home," 1978-79, that marked the emergence of the first "serious" cinematic examinations of the war in Vietnam - "The Boys in Company C," "Go Tell the Spartans" and more importantly, in terms of effect, "The Deer Hunter" and the aforementioned "Apocalypse Now."

"The Boys in Company C" (1978) was the first Vietnam War movie produced by a major studio (Columbia Pictures), but it was apparently intended as merely a low-budget action picture. In retrospect, however, the film holds up as a cynical, angry look at the war, at least when it gets past the necessary combat-training sequences and finally thrusts its young characters into the thick of battle. The anchor for the picture is unquestionably the lead performance by Stan Shaw, an underrated actor who truly shines in this underrated movie.

"Go Tell the Spartans" (1978) is another unrecognized gem, another cynical view, yet also darkly humorous. Anchored by one of Burt Lancaster's most riveting performances - he plays an over-the-hill, undervalued major - the film is set in the early days of the war and is quite intricate in its attention to detail. (Trivia note: Craig Wasson, best-known for his starring role in Brian De Palma's "Body Double," co-stars in both "Spartans" and "Company C.")

But later the same year, it was "The Deer Hunter" - with Christopher Walken's Oscar-winning supporting performance as a drug-crazed, Russian roulette-playing version of The Stereotype - that really opened the floodgates. A huge financial and artistic success, which director Michael Cimino has never come close to duplicating in his subsequent floundering career, "The Deer Hunter" spends the first of its three hours stateside, as we get to know the blue-collar cast of characters, led by Robert De Niro. The Russian roulette device has been rightly criticized, but the film as a whole maintains its enormous power, and the actors are all at the top of their form, including Meryl Streep as one of "the girls back home."

The next year, "Apocalypse Now" proved "The Deer Hunter" was not a box-office fluke, with its Vietnam variation on Joseph Conrad's "Heart of Darkness." Martin Sheen has the lead role, as a drunken officer ordered to head upriver for an unsettling assignment: He is to assassinate a crazed colonel, a renegade in the jungles of Cambodia. Despite the film's rambling finale, during which Sheen meets his prey, a bloated and incomprehensible Marlon Brando, the film has much to say about the chaos of war in general and this war in particular.

Of the many Vietnam War films that have followed, there are two notable players who have helped keep serious cinematic explorations of the war alive - actor Robert De Niro and filmmaker Oliver Stone. Each has participated in a trilogy of films on the subject, each one notable for very different reasons.

De Niro has given us three different shadings of the troubled Vietnam veteran, and two of his characters have managed to transcended The Stereotype, even when the films have not.

Notwithstanding its contribution to the damaging view of Vietnam veterans as drug-crazed weirdos, "Taxi Driver" offers a horrifying, yet fascinating observation of its central character, drawing parallels between the jungles of Vietnam and the jungle that is Manhattan. This doesn't happen directly - the Vietnam experience is never shown, or overtly identified as a significant aspect of the film. But it hovers over De Niro's character, Travis Bickle, like a cloud of Agent Orange.

Love it or hate it - and critics have been evenly divided for nearly 20 years - it's hard not to recognize "Taxi Driver's" symbolic take on our torn allegiances regarding the Vietnam War. The film's core is built around Bickle's conflicted sense of values, especially as he tries to "rescue" a child prostitute (Jodie Foster) from her sordid profession. And it is implied that his war experiences have seriously contributed to pushing him over the edge.

And just as Vietnam is often portrayed as hell on Earth, with faces hidden in the bushes and trees, waiting and watching, so does that seem to be Scorsese's visual take on the city - right down to grates and manholes constantly spewing steam, covering the wet, slick city streets and obscuring our view of their inhabitants.

As if in answer to The Stereotype in "Taxi Driver," De Niro's role in "The Deer Hunter" has him playing a returning vet who is the voice of sanity among his peers. This small group of friends is comprised of young hard-hat steelworkers who have lived all their lives in a small Ohio town, working together, playing together and, eventually, going to war together.

De Niro's rooted, civilized portrait of a moral man who is changed by the war, but who manages to come to terms with his place in the world, is a subtly rich and deeply affecting performance that is perhaps an underappreciated aspect of this still-powerful movie. Sadly, "The Deer Hunter" would seem to be more readily remembered for its more flamboyant elements, including those harrowing Russian roulette scenes.

And finally, there is "Jacknife," a little 1989 release that has been largely overlooked since it flopped at the box office. "Jacknife" is not a wholly successful film. There are awkward moments that seem artificial, and the story's narrative structure is uneven. But De Niro's role, as yet another Vietnam veteran, is rich and his character is relatively "normal."

"Jacknife" is set in real time, about 15 years after the war, as De Niro returns to his home town to reacquaint himself with a Vietnam buddy (Ed Harris). Harris plays The Stereotype, and De Niro's character spends half of his time trying to help him leave behind his Vietnam War angst. The other half has De Niro wooing Harris' reluctant spinster sister (Kathy Baker). The three lead players are all quite good, though De Niro is able to lend some extra depth to his role, which almost plays as a sequel to "The Deer Hunter."

This trio of film portrayals firmly establishes De Niro as Hollywood's premiere Vietnam veteran character player, one who manages to lend some dignity and decency to the kind of role that too often takes on negative imagery.

As for Oliver Stone, his trilogy is closer together, in chronological terms, and certainly the most personal of any major movies to portray the American experience in Vietnam. His first effort on the subject was "Platoon" (1986), which reflected his own experiences as a combat soldier in the war, and which was certainly the most successful of the three. (And to this critic's mind, it is by far his best work to date.)

"Platoon" stars Charlie Sheen, who is quite good as a green 19-year-old soldier, through whose eyes the story unfolds from the moment he arrives in this hot, humid foreign land, and the first thing he sees is body bags being loaded on a plane. But it is the brilliant casting against type of the film's two good-vs.-evil sergeants - Willem Dafoe as the good guy and Tom Berenger as the bad guy - that sets the film's acting pace. Both are brilliant.

"Platoon" is largely comprised of set-pieces, as the platoon slogs through the jungle by day and escapes in alcohol and drugs by night. Their routine is interrupted periodically by unexpected firefights with the Viet Cong, as well as the constant panic of not knowing whether local villagers are in collaboration with the enemy. A confrontation between Dafoe and Berenger is inevitable, and when it comes the loyalties of the platoon members are sharply divided. The metaphor is clear, representing the split between those back home who supported and protested the war.

The film was a resounding success on every level but should perhaps be most heartily con-grat-u-lated for acting as an artistic counterpoint to the ridiculous heroics of Sylvester Stallone's "Rambo" series and the "Missing in Action" movies with Chuck Norris, cartoony views of the Vietnam conflict that were popular at the time, as were their many clones.

"Platoon" remains, in the eyes of most critics and Vietnam veterans, the most honest and affecting movie about the war. It is also perhaps the only movie that is very specifically about the Vietnam War, told from the viewpoint of "grunts," those who spent their year of combat service in the thick of the day-to-day conflict.

Stone's next look at Vietnam was "Born on the Fourth of July" (1989), another "true" story, though the film was sharply criticized for its manipulation of the facts, as well as the heroic portrait Stone painted of the film's protagonist, Ron Kovic. Despite its flaws, which include Stone's penchant for flamboyant overstatement, the central performance by Tom Cruise holds this one together, as he vividly demonstrates Kovic's transformation from innocent young civilian to hardened combat recruit to angry, wheel-chair-bound paraplegic to foaming-at-the-mouth anti-war protester. Cruise is so good, in fact, that he makes the movie better than it has any right to be, a tour-de-force of acting that should have erased his "pretty-boy" screen image forever (though it didn't). Still, in the end, one can't help but feel that we have been through yet another variation on The Stereotype.

Stone's most recent examination of the Vietnam War was last year's "Heaven and Earth," yet another "true" story, this time from the viewpoint of a Vietnamese woman, Le Ly Hayslip (played by Hiep Thi Le) who married The Stereotype (played by Tommy Lee Jones). As her life is ravaged by the war, and as she changes, becoming Americanized - and Ugly Americanized, at that - it becomes apparent that Stone intends for Le to represent the entire Vietnamse people, its culture and perhaps its very identity. Unfortunately, a story like this cries out for a gentle hand, and Stone is at his most bombastic. The result is a political tract that is so heavy-handed and cinematically clumsy, it's hard to believe "Heaven and Earth" was created by the same talent that so carefully shaped "Platoon."

Stone's histrionic ravings can be seen as a symbol of the still divisive feelings that make up our collective memory of the Vietnam War. That any former political hot potato can remain this charged and stinging 20 years later is rather remarkable, but that a single filmmaker like Stone still can't get it off his chest would seem to represent the conflict in all of us.

During the '80s there were other worthy Vietnam War movies, from Stanley Kubrick's anthological epic "Full Metal Jacket" (1987) to the low-budget, independent "84 Charlie Mopic" (1989).

There were also important documentaries about the conflict, from "Hearts and Minds" (1974) to "Dear America" (1987) (perhaps the most highly charged and emotionally wrenching of them all).

And yet another explosive look at the Vietnam War may be out there on the horizon, waiting to be made. (Let's hope so - we'll need an antidote to "Rambo IV," which Sylvester Stallone recently announced as an upcoming project.)

For good or ill, it seems clear that wherever movies may take us in the future, trips back to the Vietnam War will be on the agenda.