The first batch of aircraft to fly with the Griffon 60 series engines were six converted Mk VIIIs JF316 to JF321 which were designated Mk VIIIG. The first one of these was flown by Jeffrey Quill on 20 January 1943:
"Changes to the aircraft were restricted to those essential to enable it to accept the new engine...I found that it had a spectacular performance doing 445 mph at 25,000 ft, with a sea-level rate of climb of over 5,000 ft per minute.... I remember being greatly delighted with it; it seemed to me that from this relatively simple conversion, carried out with a minimum of fuss and bother, had come up with something quite outstanding...The MK VIIIG, with virtually the same tail surfaces both vertical and horizontal as the Merlin MK VIII, was very much over-powered and the handling in the air was unacceptable for an operational type...I soon realised that a new throttle box would be needed giving a much greater angular travel for the hand lever...The next essential...was an improvement in the directional stability and control and a new fin was drawn out with a substantial increase in area (7.42 sq. ft) and a much larger rudder and fitted to the second aircraft JF317. This, though not ideal, produced a very marked improvement in directional characteristics and we were able to introduce minor changes thereafter and by various degrees of trimmer tab and balance tab to reach an acceptable degree of directional stability and control. The enlarged fin of JF317 had a straight leading edge but for production a more elegant curved line was introduced."...
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