air force vs forces

air force

noun
  • A unit in the U.S. Air Force larger than a division but smaller than a command. 

  • A branch of the military mainly devoted to air warfare, as opposed to (land) army and navy. 

forces

noun
  • Troops (plural only). 

  • The orchestral instrumentation (and voices) used in a musical production (chiefly plural). 

How often have the words air force and forces occurred in a corpus of books? (source: Google Ngram Viewer )