NavList:

I don't want to sound disparaging, but I tried this a while ago when first looking into the possibility of making a Bygrave SR.

It works, if the two scales are a different colour, but there is a serious problem - confusion between the closely packed scales one on top of the other it is very easy to make a blunder. I first made one with the same black scales - and that was almost impossible to use.

I feel sure that Bygrave must have tried this himself when making prototypes, but found the same difficulty. The separation physically of the scales into an upper and lower cylinder gives easier reading without the confusion of overlapping scales; and the clever use of the outer moveable cursor is so simple.

Also, the separation of the two scales physically has the immense advantage that the algorithm operation of changing from one scale to the other in sequence makes the procedure almost foolproof - something which he would have been acutely aware was important for a navigator in an aircraft under stress.

I am still thinking of pursuing making a copy Bygrave SR, and have the workshop machine tools to do reasonably sophisticated engineering operations, but the severe problem is that commercially available thin walled tubing in aluminium (or stainless steel) has fixed, standardised diameters; and those required for around that required for a Bygrave device are not possible with only a small difference in diameter such that one will slide into the other. The standard sizes will not allow it.
That is why I have not been able to go ahead until this problem is solved.
It is an irritating nuisance.

Henry Hughes and Sons probably had cylindrical tubing made to order for the job, something that could be done but no doubt today, but probably at ridiculous cost.

Douglas Denny.
Chichester. England.
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