Complex sentence: Guynemer became influential enough to affect French fighter aircraft design; in December 1916, he wrote a letter to the chief designer at Spad, criticizing the Spad VII as inferior to the German Halberstadt that was its contemporary.
Split into simpler sentences: Guynemer became influential enough to affect French fighter aircraft design. In December 1916, he wrote a letter to the chief designer at Spad, criticizing the Spad VII as inferior to the German Halberstadt that was its contemporary.