Flying in Knots

When question time followed that very interesting KEITH ELLIS talk about the LINK TRAINER someone asked why is it that Air Speed Indicators were calibrated in KNOTS rather than Miles per Hour. Why, indeed

At the time I put it briefly, that navigation plotting charts are scaled in Nautical MILES whereas TOPOGRAPHICAL MAPS are scaled mainly in STATUTE MILES, (one must consult the Legend at the bottom of the map). However, that was received I felt, with some reservation (i.e., silence!) and that the question needed a filler answer

We know about the early sailors method of measuring ship speed through the water by the use of a rope with knots tied at measured intervals attached to a drag and thrown into the sea: the number of knots passing through the seaman’s fingers in a given time (27 seconds I think) gave a measure of distance traveled over a period of time. But, one of the oddities of measurement by knots is that, unlike MPH the speed is expressed merely in KNOTS. e.g. ‘a boat is making good at 12 knots’.

For the AIR NAVIGATOR the knotted rope method is a bit impractical; but he needs an instrument, an AIR SPEED INDICATOR, that will measure air speed in the same units that he must use when measuring distances over the surface of the earth. That is done on a map or chart and involves the use of linear units. Maps, (Topographical Maps) used mainly for land masses to show important features are commonly scaled in STATUTE MILES (which Queen Elizabeth 1st decreed must be 5280 feet!).

Charts; marine charts and MERCATOR charts, are scaled in NAUTICAL MILES (sometimes called SEA MILES) directly related to the curvature of the Earth.  It is the AVERAGE length of ONE MINUTE OF LATITUDE and taken to be 6080 feet. An average is used because the curvature of the Earth is less pronounced at the Poles than at the Equator In practice, a nautical mile is taken as the distance along ONE MINUTE OF ARC on ANY GREAT CIRCLE ON THE EARTH.

This formula is particularly useful for ASTRO NAVIGATION when the resolution of a spherical triangle to produce a POSITION LINE (passing through the aircraft position) has to be plotted on a chart (A GREAT CIRCLE is one that would cut a globe into two exact halves) It will be appreciated that using the same linear unit of measurement for a multiplicity of calculations, makes them easier to resolve and with less likelihood of error. If’ some information is presented in MPH. and others in knots (wind speed, air speed, ground speed), yet again others in fed or metric units, the necessity to convert them to a common unit will introduce the possibility of errors. This latter is an important factor for the AIR NAVIGATOR whose ‘working environment is hostile to concentration and rational thought

(The ratio of STATUTE MILES to NAUTICAL MILES is 76:66)

Of the many aircraft I flew in the course of 3O odd years, few had Air Speed Indicators calibrated in MPH. Of course, from a psychological view point there are times when there is something to be said for MPH. If, for example, having just done something nasty to the enemy one is making a hurried departure hotly pursued by irate and vengeful defenders, then it is better for the morale to see ones speed as 304 MPH than a mere 266 knots. At this point the NAVIGATOR is not interested in conversion!

What goes on, navigation wise, on Flight Decks these days I have little idea . A lot of buttons, I suspect! Talking to a chap on the air recently, who uses modern methods in light aircraft, he was aghast to hear that we often had to resort to DR (Deduced Reckoning or commonly DEAD RECKONING) to navigate over mountains and come down through cloud on the other side on ETA thus calculated! I know the feeling well!

So there you have it, why KNOTS and not M.P.H. Mind you, I still think my original, and SHORT, explanation was adequate and had it been received with a few grunts and general head-nodding I would not have gone to all this effort. The strain, on my brain has been terrible!

Rex G3MRS – July 2000.

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