"Feel Like Makin" Love!" Too hot, you might think, besides the air's too dirty, and it's too risky - you might have a heart attack. But when Roberta Flack belted out "Feel Like Makin" Love" - a song which has become her trademark - to a huge crowd on the lawn, the steps and the parapets of the west front of the U.S. Capitol last night, it was as Joe Casella of Washington said: "She sure knows how to cool off a hot evening."

Roberta Flack was the guest star at the third of a series of free concerts given by the National Symphony this summer at the Capitol. By all accounts, her appearance was the most successful concert of the three - at least as far as drawing the biggest, most enthusiastic, and most diverse audience. Crowd estimates ranged from an official's 55,000 to a policeman's 150,000 to Flack's 175,000.

Flack is not merely popular with her Washington fans: She is in fact revered. And of course, she is a native.

All these people came specifically to see and hear Roberta Flack sing. But it's also true they came to hear the National Symphony - playing for free.

People like Vera Davis of Alexandria, who said, "I like music - all music. I love the symphony. And I adore Roberta. And it's friendlier here - you can talk to people. You don't have to sit quiet like you do in the Kennedy Center." Or like 11-year-old Aaron Brown of Maryland, who came just to hear the orchestra. He said he liked the "Star Wars" part.

The concert was to have been conducted by Arthur Fiedler, who died last month. Harry Ellis Dickson, for years Fiedler's associate conductor of the Boston Pops, stepped in.

Dickson is no Herbert von Karajan as far as conductors go - but then the crowd was in no mood to hear Bruckner's Seventh Symphony either.

With the indomitable Willard! of WRC-TV as master of ceremonies - this crowd was in good humor and moderately sedate behavior. Oh, there was beer and an occasional illegal substance to be seen or sniffed, and there was dancing, people hanging from lamp posts and lovers hugging and kissing. And there was also rapt attention to the orchestra and the music they were playing: Rossini, Offenbach, Stephen Foster, John Williams ("Star Wars") and so forth.

Willard promised that there would be no rain - though the crowd doubted his judgment as threatening, vigorous winds blew across the lawn. And he punctuated his effervescent remarks with such standard jokes as "Would the owner of a blue 1976 Chevy please come to the stage - you've just won the building behind you."

And when disaster struck as all the lights went out on stage during the high point of Flack's singing, and then stayed out for the longest time, a few people did begin to leave, but most just quietly chatted and waited for things to crank up again, which they eventually did.

Then, with the stage bathed in blue, and backed up by the NSO, Flack gave mer mammoth audience exactly what they wanted when she seductively slid through "Killing Me Softly."

And for the Washington songstress, former ingenue of Mr. Henry's on Capitol Hill, Roberta Flack, it was the warmest of homecomings. CAPTION: Picture, At last night's Roberta Flack concert, by Ken Feil - The Washington Post