A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2021 Mar 8, 01:40 -0800
Tony Oz you wrote: Now I wonder if that drift-corrected DR position needs a further correction for the sequence of the lefr-right or righht-left turns made for the drift determination. After the last turn we only can restore our initial course - but not the initial track line. Because of the drift - unless it was strictly 0° or 180° - we'll always be carried leeways.
Chichester was using the ‘air-plot’ method of air navigation, a technique some say he invented; others disagree. His DR position would have depended upon the accuracy of his air position and the accuracy of the wind he found. The factors affecting the accuracy of a manually plotted air position are many, which we needn’t go into here except to say that if he didn’t allow for the time spent off course, his plotted air position along course would have been further along course than he actually was. That’s why he only turned off by 30 degrees. 30 degrees was enough to give a reasonable estimate of w/v without seriously affecting his air-plot.
Looking at his chart, he does not appear to have plotted his wind finding course changes, so for every mile he flew at 30 degrees off course his actual air position would be 1-cos30=0.14miles short of his plotted air position. Brian Walton will be able to tell you how long three-drift wind finding takes in a biplane, but if we assume that in the very worst case it took the equivalent of ten miles off-course flying, his actual air position and hence his actual DR position would be 1.4 miles further from his objective than his chart suggests. As his wind finding was purely drift based and didn’t involve joining his plotted air position to a real fix, I don’t think inaccuracy in air position would have affected the accuracy of the wind he found.
Let me correct an earlier statement I made. I said his plotting was so neat that he possibly pre-drew some of his expected air positions before take-off. He couldn’t have. Further checking of his chart shows he restarted his manual air-plot from each DR position. This technique was shunned from the 1940s when automatic air position indicators (APIs) were introduced, because most APIs had no memory function to store mileage while the API was being reset. The API was kept running for long periods and manually plotted wind vectors were carried instead, but we needn’t go into that here. Nevertheless, the published charts remain relatively neat. One can only speculate that unless Chichester was a particularly neat pilot-navigator, someone might have tweaked up his charts a bit to prevent a print room disaster. He might also have preset a pair of bow-spring compass' to his anticipated TAS. I wish I still had my little aerplane; I could have tried some of this out in the air. DaveP