copilot vs flying buttress

copilot

noun

  • A backup or assistant pilot of an aircraft. 

verb

  • To serve as relief or assistant pilot. 

flying buttress

noun

  • A buttress that stands apart from the structure that it supports, and is connected to it by an arch (flyer). 

How often have the words copilot and flying buttress occurred in a corpus of books? (source: Google Ngram Viewer )