Almost any of the familiar, upbeat cliches will do nicely for the final communique after next week's three-day meeting in Ottawa of the presidents and prime ministers of the seven largest industrial democracies.

World economics is supposed to be the subject. The participants will be the United States, Britain, Germany, France, Italy, Canada, and Japan. And right now you can bet that -- for the record -- their private discussions in their secluded rendezvous at Le Chateau Montebello will turn out to have been "free, frank and friendly . . . a useful tour d'horizon . . . a constructive exchange of views" -- or something of that general tenor.

That's how it's supposed to be at these annual affairs (this is the seventh). No decision-making or collective action is intended. As group therapy -- or sensitivity training -- it will be the more valuable since four of the leaders (President Reagan, France's Prime Minister Francois Mitterrand, Japan's Prime Minister Zenko Suzuki and Italy's Giovanni Spadolini) are first-time participants.

Given the profound economic disagreements among the seven, there's a lot to be said for talking them out. Nobody's expecting Mitterrand to warm to Reagan's supply-side economics, or Reagan to Mitterrand's socialism. Europe is inconsolably distressed by America's high interest rates. The Europeans and the Reagan administration disagree on the best approach (collective government help through the World Bank and other multilateral institutions or primary reliance on private investment) to the developing world.

A little understanding here, some reassurances there -- that's as much as you could expect. Which brings us to what may well be the fundamental flaw in economic summitry. The flaw lies in the essential inseparability of the economic issues tearing away at the Western alliance and the political and military issues that confound concerted action for Free World security.

How do you talk about trade, for example, without talking not only of protectionist practices within the Western nations but of East-West trade? Such trade is bread and butter for Europeans, and a strategic weapon for the Reagan crowd. Energy, as an economic agenda item, takes you swiftly to nuclear non-proliferation and arms control -- or to the military security of the Persian Gulf. It actually did take last year's Venice meeting to an urgent joint declaration of Afghanistan.

Can you talk of inflation, fiscal discipline and budgetary priorities and not wind up discussing military spending and, eventually, burden-sharing for the common defense?

In short, what the Ottawa summiteers should really be asking themselves is how long they can go on meeting this way. More precisely, the question is: iHow long can the Western allies make do with the various strictly limited and demonstrably inadequate ways they now employ to reconcile their appraisals of what it is they're up against around the world and what, together, should they do about it?

It's hardly a secret that the Europeans generally do not share the Reagan administration's perception of the Soviet-Communist threat -- to Europe, or to the Third World. They do not like the overheated revival of Cold War emotions. They would test disarmament prospects before they would re-arm. The Japanese don't even like to be in the room when increased defense spending is discussed.

It is also no secret that the institutions and mechanisms to deal with these differences don't work. The London Economist recently warned: "The alliance has been in trouble plenty of times before, but this time is the worst yet."

Not content to carp, the magazine offered an ambitious cure: first, a study group, composed of "a small group of intelligent men" trusted by the NATO governments, to do a 10,000-word study laying down firm recommendations of how to set things right.

The next step would be for President Reagan, with his "wise men's" report in hand, to call a summit of key NATO members and ask for a short list of "radical reforms." These would include recognition of the need for the West to re-arm, coupled (for Europe's sake) with a more precise sense of the limits to it. In exchange for this, and some effective mechanism for a larger voice in Alliance affairs, Europe would agree to shoulder a larger military load.

Other proposals abound. The New York Council on Foreign Relations and its British, French and German counterparts recently published a study ("Western Security: What Has Changed? What Should Be Done?"), which includes unusually precise recommendations for handling "political security problems outside the limited geographic area prescribed" by NATO. Among them: "principal nations" groups, with membership drawn from NATO but including outsiders directly involved. Such a group for the Persian Gulf, for example, would include Japan.

The study finds the seven-nation economic summit to be a "useful forum." But it adds a few well-chosen words of advice: "From now on [they] should be devoted at least as much to political and security concerns as to economic ones." In the quiet of their hideaway outside Ottawa, that's something the summiteers might want to talk about.