NavList:

Not my specialist subject, but I'm interested in why RAF preferred Air almanac tables to Bygrave at beginning of the war. 

Chichester, great Bygrave user in 1931, was teaching RAF navigators in 1940 only using the new "secret" Air Almanac tables. No mention of the Bygrave in his little book"The observers book of astro-navigation part one". On page 86, suggests they were based on 1918 Davis tables from US and later US HO 214. apparently Weems acted as a consultant to UK RAF during late 30's and was very influencial, pro tables. (214). On page 100 Chichester also recommends the Haversine method over the cosine formula because of LHA >90^o is negative with cosines but always + with haversines.He says neither suitable for air work during flights because likely to make mistakes with 5 figure logs etc.(He said similar during the1931 Tasman sea flights.Here he used the Bygrave. the Air almanac not then available. Would he have used them if they were?)

Gary LaPook and I  recently "re-enacted" (me from the comfort of my arm chair though) his famous flight using his original chart and our Bygraves. remarkably , we both got perfectly similar results to Chichester! Beached this year waiting surgery (next week now!) , this was the best fun I've had with my Bygrave for years! thanks Gary, who showed me how!) I suspect Gary could have done it in an open byplane!

Having used both the tables and the Bygrave, I'm still not sure why the Bygrave was not mass produced (but wiith a simple lock to prevent slippage, the only fundamental design flaw). The scales could have been mass produced easily photographically and the rest is basic simple craft skills. 

Best wishes

Francis