hang out to dry vs let down

hang out to dry

verb
  • Used other than figuratively or idiomatically: see hang, out, dry. 

  • To abandon someone who is in need or in danger, especially a colleague or one dependent. 

let down

verb
  • Used other than figuratively or idiomatically: see let, down. 

  • To disappoint; to betray or fail somebody 

  • To reduce one's level of effort. 

  • To soften in tempering. 

  • To allow to descend. 

  • To lengthen by undoing and resewing a hem. 

How often have the words hang out to dry and let down occurred in a corpus of books? (source: Google Ngram Viewer )