ON THE AIR TONIGHT: Helen Hayes , starring in a different half-hour play each week, on CBS at 8:00, E.S.T., rebroadcast to the West Coast at 7:30, P.S.T, and sponsored by Lipton’s Tea. Unless you’re a fanatical devotee of Charlie McCarthy (or unless you live in the Pacific Time Zone, where Helen and Charlie aren’t on at the same time), you couldn’t do better than to tune Helen Hayes in tonight. Most radio acting is good, but hers is magnificent. You’d never guess than Helen was one of America’s greatest actresses if you watched her rehearsing her radio shows. She loves radio, but is quite willing to admit that other people know more about it than she does. No displays of temperament ever go on at a Hayes rehearsal, and afterwards, when the script has to be cut so it won’t run overtime (it always does have to be cut, too), Helen goes home and lets other people wield the blue pencils. “ I’d only get in their hair,” she explains. Besides acting on the air, Helen is starring