For the past week, the backwoods calm of this Balkan capital has been disrupted repeatedly by the deafening roar of columns of military helicopters, pursued by squadrons of Soviet-built MiG fighters screaming low over the center of the city in tight formation.

Together with the sea of red bunting that has suddenly risen throughout Sofia, and slogans praising Bulgarian-Soviet friendship, the repeated overflights reflect the determination of Communist Party leaders to ensure that everything goes right in their big parade today. It marks the 40th anniversary of Bulgaria's liberation from fascist rule by the Red Army on Sept. 9, 1944.

The elaborate preparations for the anniversary also provide an insight into why Bulgaria's veteran president, Todor Zhivkov, has been able to proceed with plans to visit West Germany while the sharp deterioration in East-West relations has forced his East German counterpart, Erich Honecker, to cancel a similar trip.

With emotional and historic ties to Russia that precede the birth of communist ideology, Bulgaria is in a unique position among Soviet Bloc states. It is the one East European country where most citizens actually like Russians and whose leaders are comfortable with a "little brother-big brother" relationship.

Bulgarian officials and western diplomats seem to agree that the absence of any controversy about Sofia's ties to Moscow has allowed Zhivkov a certain freedom to pursue what he sees as Bulgarian interests.

"The Soviets feel no qualms about what the Bulgarians are ultimately up to. The government here is riddled with people who have close personal or professional ties with the Soviet Union. The loyalty question simply doesn't arise in the same form as elsewhere," commented the political officer of a western embassy.

By taking every opportunity to stress "undying gratitude" to the Kremlin, Zhivkov has insulated Bulgaria to a certain extent from the breakdown in East-West detente. He has cultivated relations with most West European countries, notably West Germany, which is Bulgaria's largest capitalist trading partner.

Neighboring Romania, which has deliberately drawn attention to its foreign policy differences with the Soviet Union, and Bulgaria are the only Soviet Bloc countries without a permanent Red Army presence. Nor has Bulgaria been required to accept Soviet intermediate-range nuclear missiles in retaliation for NATO's deployment of U.S. cruise and Pershing II missiles in Western Europe.

Bulgarian Deputy Foreign Minister Ljubin Gotsev acknowledged in an interview that Bulgaria's ability to develop ties with the West, like that of other East European countries, was dependent on the overall international climate. But he insisted that Bulgaria's situation was "completely different" from East Germany's.

"They are much more exposed than we are to the political ambitions of the Federal Republic" of West Germany, he said, noting that over half of Bulgaria's trade was with the Soviet Union and a further 22 percent with other Soviet Bloc countries.

In addition to the political rhetoric and banners proclaiming Soviet-Bulgarian friendship, there are plenty of reminders in Sofia of this country's intimate links with Russia. In the city's central square, which leads off "Russian Boulevard," there is a statue of the Russian Czar Alexander II, whose armies liberated Bulgaria from Turkish rule in 1878.

The two countries share a common alphabet -- the Cyrillic script -- and a common religious heritage in the Orthodox Church. Many of Sofia's public monuments are miniatures of Moscow buildings, including the Lenin-type mausoleum for the founder of the Bulgarian Communist state, Georgi Dimitrov.

References to "Bulgarian-Soviet friendship" have become so ubiquitous that cynical western diplomats have abreviated the phrase to "BS," as in "BS boulevard" or "BS day."

By and large, western analysts agree, Bulgaria has prospered from the closeness of its Soviet ties. The Soviet Union has provided Bulgaria with plentiful supplies of subsidized oil and raw materials and guaranteed political stability in a part of the world that used to be known as "the tinderbox of Europe."

"Unlike other East Europeans, the Bulgarians have no reason to dislike the Russians," commented a western diplomat in Sofia. "They have been liberated by them twice in the last century and have benefited from the relationship both economically and politically."

Living standards in Bulgaria, whose population is nearly 9 million, are higher than in the Soviet Union. While shops look empty to western eyes, they are much better stocked than those of many other East European countries.

By concentrating on agriculture and being cautious about accumulating foreign debts, Bulgaria has managed to avoid the economic mess of other Soviet Bloc countries, notably Romania and Poland.

Bulgaria was careful to keep out of the debate that divided Soviet Bloc news media over the past few months concerning ties with the West. The two extreme positions were represented by Czechoslovakia, which insisted that at a time of East-West tensions it becomes all the more necessary for all Soviet Bloc countries to fall into line behind Moscow, and Hungary, which put the emphasis on the need to develop good relations across Europe's ideological divide.

Questioned about where Bulgaria stood in the debate, Gotsev insisted there was merit in both points of view. "I don't see any contradiction between these two lines. We all follow the same general policy as allies but at the same time we retain the opportunity of pursuing our own interests," he said.