Hi,
This is a problem that FliegerJGI and I have both been interested in for quite a period. The term is actually quite rare in contemporary British pilot's letters and diaries. They generally referred to him simply as Richthofen (certainly not as the 'Red Knight", which I believed was coined by Gibbons - I don't think he ever used the term Red Baron). Certainly the term was never used, in English or German, by the Germans themselves during the war. At least that's my belief. For a long time, I believed that "Red Baron" was a term coined by Charles M. Schultz in 1965.
However...
The log book of
Roderic Stanley Dallas of No. 1 Naval Sqdn (as found in our own "Breguet's" wonderful book, "Australian Hawk over the Western Front") contained this entry on 5 May 1917:"Had a scrap and destroyed a Hun. Perhaps the one with the red tail who would insist on shooting at me from below was the 'Baron'" ?" So that's kind of close.
Squadron Leader C P O Bartlett's book (first published as "Bomber Pilot", then re-published in 1994 by Naval Institute Press as "In the Teeth of the Wind") is made up mostly of Bartlett's diary entries. Bartlett was flying DH4's in No 5 Naval Sqdn in March 1918, and on the 18th of March his flight participated in a huge fight with JG I, led by MvR himself. After their return home, Bartlett wrote that "The Colonel came over after lunch and discussed tomorrow's show, his idea being to finally smash up these big bands of Huns in one huge aerial battle. He proposes putting up some 60 fighters, we ourselves being used as bait once more...(we will be) crossing 3 or 4 miles over up north, entice the enemy after us and draw him south near St Quentin, where all our fighters will be hanging in the sun waiting for him..."
On the next day - 19 March 1918 - Bartlett wrote:
"Rain at last which provides a welcome rest, but it is a pity that the show can't come off. The enemy and our side were teed up for it and it would have been a thrilling spectacle if 60 of our fighters could have fallen on the 'Red Baron' and all his crowd, but we of course might not have succeeded in drawing them into our territory and could have suffered heavy casualties in the attempt."
If that's actually an accurate transcription of Bartlett's diary (and not edited in much later), that's the earliest actual documented use of the term "Red Baron" that I've ever found. I've been looking a long time.
The same goes for the term "Flying Circus" for Richthofen's "Squadron" or squadrons. Generally the RFC chaps called Richthofen's group simply "The Circus" or "The Travelling Circus". The "Flying Circus" seems to have been used mostly by Americans and was popularized soon after the war by Rickenbacker's book.
Greg