The price of Navy planes has climbed so high that the service can no longer afford to replace all those being lost or worn out, according to new congressional studies.

Rep. Jack Edwards of Alabama, ranking Republican on the House Defense appropriations subcommittee, said yesterday that the problem has provoked such concern in Congress that his panel will soon conduct special hearings on the Navy's "aircraft gap."

More bad news is due to be released today by the Pentagon in a report that shows that the Navy plane developed to bring down soaring aircraft costs, the F18 fighter, has gotten more expensive.

Instead of the Pentagon's 1975 prediction that, after allowing for inflation, it would cost $15.8 million a copy for 700 F18s, the Pentagon's new estimate is $17.4 million even after doubling the order to 1,366 planes.

"Some big cost overruns are hidden in there," said one government procurement specialist. "Doubling the order should have brought the unit price way down, not up."

The F18, being built by McDonnell-Douglas and Northrop Corp., was billed by Pentagon leaders as the bargain-basement alternative to the Navy's sophisticated F14 fighter, running about $25 million a plane.

Edwards said the committee commissioned Library of Congress studies which show that the Air Force is buying 10 times as many warplanes as the Navy with 2 1/2 times as much money.

"Here is a problem that cries out for some kind of solution," Edwards said. He added that Defense Secretary Harold Brown has acknowledged in recent hearings that an aircraft gap is opening up in the Navy.

Edwards said the prospect of aircraft carriers sailing around the world short of aircraft "indicates that nobody in the Pentagon, White House or up here knows what the future mission of the Navy should be."

Last year, Brown said the Navy would have to buy about 180 fighters and attack planes each year to keep 12 aircraft carriers and three Marine air wings at full strength. However, President Carter's new defense budget calls for buying only 39 Navy warplanes in fiscal year 1980.

One Library of Congress study, after documenting that an aircraft gap is opening up, said the Navy "might" be better off shunting some of its shipbuilding money into aircraft to keep the 12 carriers well armed in the 1980s.

Bert H. Cooper, defense analyst for the Library of Congress, made these other findings in two reports to the House Defense appropriations subcommittee on Navy and Marine aircraft trends:

Navy purchases of aircraft declined from 339 in fiscal 1970 to 131 in fiscal 1979. The average cost of those planes rose in that period from $5.9 million a copy to $33.3 million.

The Air Force paid about $14 million for its fighter and attack planes in fiscal 1970 and $26 million in fiscal 1977. But the Air Force's average perplane cost dropped to $17.7 million by fiscal 1979, largely due to savings from volume production.

The Navy already has had to reduce the size of its A6 attack aircraft squadrons from 12 to 10 planes and will run short of "several types of fighter and attack aircraft during the 1980s."

"Smaller annual buys of Navy and Marine Corps fighter and attack aircraft in recent years have increased the contractors' overhead charges and driven up" the cost of each plane purchased.

Edwards said such competing Navy hardware as the $1.5 billion Trident submarine is keeping the Navy strapped for airplane money, forcing the service to settle for relatively small orders.