A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brian Walton
Date: 2015 Nov 12, 15:32 -0800
Chichester found that contemporary reduction methods using almanacs and log tables would not work whilst flying a biplane solo, and elected to use the newly marketed Bygrave slide rule.
Some time ago I was able to handle a real old Bygrave at the RIN in London, and subsequently built my own same size version using cardboard tubes (Pringles and Saxa) and, with his permission, stuck on downloaded tables published here by Gary LaPook.
After much practice, adjustment and alteration my slide rule was sufficiently robust and smooth to produce satisfactory results, and I added areas on its surface to accommodate various correction tables, and a worksheet, attached as suggested in the original manual, by rubber bands. This made the rule autonomous.
About a year ago, I took it on its first flight in a Stearman, together with a sextant and watch. The evening before, I had calculated, as Chichester should have been able to do, meridian angle, dec and co-lat for the place I intended to overfly, for the 2 whole hours spanning my likely passage, and noted these on the slide rule. On the day, the forecast was CAVOK, and I took off within 30 minutes of my planned time to arrive at the planned position. The slide rule would furnish azimuth and sextant altitude to be preset, to find an intercept.
When airborne, I found the horizon unsuitable for a sight, but was able to do the slide rule work. I should emphasise now that flying straight and level means continuously monitoring the horizon and various dashboard instruments, or the aircraft will deviate from course, and need manual correction. It is also essential to avoid cloud, look out for other aircraft, and keep an eye open for landing areas in the event of engine failure. This is called scanning, requires flicking the eyes and head around every few seconds, and is a full time job. Navigation and its calculation is further down the list. Neither Chichester nor I had a turn needle to enable safe flight in cloud.
I found it best to hold the Bygrave in my left hand, with my little finger around the stick top, whilst my right hand manoeuvred the slide rule and held a ballpoint to record findings. The Bygrave requires 12 movements, and 2 math additions. It was immediately obvious that I could only complete one of these movements needing about 5 seconds, to any precision, before I was driven to check something else. If the train of thought is broken, it is easy to forget the reading one just took. I therefore deliberately noted the result of each action in writing, as it was quicker than going back and restarting.
Whilst I was able to complete the pre- calculation process, it took perhaps 10 to 15 minutes. If the result is essential to survival, a re-run is advisable, and that should be a 5 minute job because the previous results are replicated.
I question some statements that only 2or3 minutes are needed for a calculation. Flying a biplane is a bit like riding a motorbike at 90 mph, with the windscreen blacked out. Chichester often found himself in dangerous unusual positions.
i shall next treat problems with taking a sextant sight.