The number of crimes committed in Loudoun County increased by 4.7 percent in 1990, with crimes such as burglary, vandalism and assault leading the way, according to police statistics released last week.

Although the number of rapes and robberies decreased and the county recorded only one homicide in 1990, sheriff's deputies responded to 25 percent more calls, according to a report issued by the county Sheriff's Office. It was the fourth year in a row that the county had an increase in crime.

Sheriff's officials said the increases were a result of residential growth, an influx of crack cocaine and a sagging economy. Most crimes occurred in the more populated eastern end, said Deputy Carol Showalter, who prepared the report.

Sheriff John R. Isom, who faces an election this year, said that the number of crimes is likely to increase again next year as the county grows.

"We don't feel we're losing any ground," he said. "If we have an increase in the number of people next year, you're going to have more calls."

Loudoun Commonwealth's Attorney William T. Burch said his office saw the time between arraignments and trials grow as more cases came to court, crowding the dockets. He said his office used to wait about three months between arraignment and trial; now it is four to 4 1/2 months. Civil cases are taking longer too, he said.

Burch predicted that the slowing economy this year would lead to more burglaries, larcenies and domestic cases as financial pressure on some residents grows. "We've been lucky in many respects because Loudoun hasn't had many major crimes," he said.

According to the Sheriff's Office report, burglaries jumped from 175 in 1989 to 238 in 1990, assaults increased from 462 to 496 and incidents of vandalism rose from 480 to 506.

The total number of calls to the county sheriff's department for help climbed from 38,791 to 48,334, the report said. Leesburg police responded to 11,000 calls, a 5.8 percent increase, and the Middleburg and Purcellville police departments responded to a total of 1,769 calls, an 18 percent increase, according to the report.

The number of drug arrests by the Sheriff's Office nearly doubled, from 109 in 1989 to 199 in 1990, in large part because the narcotics division focused on arresting users, said Lt. Jeff Brown, supervisor of the narcotics division. Department officials want to eliminate the market for large dealers, he said.

Isom said the conviction rate was nearly 100 percent.

"I want them to be scared to death to buy," Brown said after a recent drug raid. "Frankly, I want them to go somewhere else."

Loudoun had 11 rapes in 1990, a decrease of three from 1989; one homicide (there were none in 1989); and eight robberies, a decrease of seven, according to the report.

Other crime statistics show:The value of stolen property in 1990 was $811,257.

There were 107 reports of stolen automobiles. The reported value of the vehicles was $176,614. There was a 55.8 percent increase since 1987 in the number of calls to county and town police departments for help, from 47,601 in 1987 to 74,199 in 1990.