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.

Return to the index of Vital Spark articles.

G3MGQ’s Month on the Air

Prepared by the clubs RSGB trainer, G3MGQ, you will find the latest DX contests including the ones to shoot for as well as ones to give a wide berth. Why not download the latest edition of Month on the Air and enjoy your DX just that little bit more.

Become a member of HERC

Join the Hastings Electronics and Radio Club.

Why not join one of the largest and most established Radio clubs in the South East of England? Very low joining cost, and free for a year to new licencees.

Vital Spark Archive

Vital Spark newsletter articles

Take a look through a large selection of articles written by club members over the years which have been published in the monthly Vital Spark newsletter .

Used Ham Radio Equipment

View HERC's Used Ham Radio Equipment for sale list..

Every four weeks, HERC's Used Ham Radio Equipment for sale list is updated on the site. Bookmark the gear for sale page to re-visit easily and take advantage of the used equipment on sale through the club.

Club Photographs

HERC Image Galleries.

Here is the official HERC photograph archive which contains multiple image galleries spanning several decades since the club was formed many years ago. Enjoy the images!

UK Amateur Radio Repeaters

UK repeaters

Click button above for full list, or a local repeater callsign below for info.

GB3EB 2m in Uckfield- Active
MB6EB 2m DStar Node in Eastbourne - Active
MB6RY Wires-X DigiGate in Broad Oak - Active
GB3HE 70cm in Hastings - Active
GB7HE 70cm DStar in Hastings - Coming soon
GB3ZX 70cm in Eastbourne - Low Power
GB3JT 23cm ATV in Hastings - NoV cleared
GB7RY 70cm X-Wires Repeater Rye - Active
GB7ES Eastbourne - DSTAR Rpt. - Active
GB3ES 2m in Hastings - Active

For a complete list of repeaters, head over to
the UK Amateur Radio repeaters list.

Popular pages

Get your amateur radio licence - Find out more about amateur radio licence training.
Month on the Air - G3MGQ's popular monthly DX contest/expedition list.
Wilf Gaye Memorial Cup - The clubs annual operating event in the memory of Wilf Gaye M0GYE.
St. Richard's College Buildathon/STEM/ARISS - HERC attends St. Richard's Catholic College for their various events surrounding the Tim Peake ARISS contact.
G3BDQ - John Hey's Rare QSL Cards.
Sussex Electronics Radio Fair - SERF Sussex Electronics Radio Fair 2016.
Vital Spark - A selection of articles re-published from the Vital Spark.
RSGB News - Find out how to get RSGB news on your mobile or PC.
Experimenters Corner - A selection of Proteus projects by Bob Gornal (G7DME)
BBADL - Bath Based Distance Learning Course.
Conquest Hospital Radio - Presented by HERC member Antony (G4CUS).
Radio Rallies 2016 - An up to date list of radio rallies scheduled for 2016.
Club QSL Cards - A selection of QSL cards the club has received over the years.
Other Newsletters - Excellent newsletters and magazines from other clubs.
TX Factor episodes - Take a look at the TX Factors YouTube videos.
John Taplin - A bio of the late John Taplin.

Amateur Radio Resources

Other Radio Clubs & RAYNET

BSARS - Brede Steam Amateur Radio Society

RAYNET - The Hastings and Rother RAYNET Group.

HERC members sites

Sigord - Gordon Sweet
Hastings Radio Comms - Andrew Haas-Campbell
Hoofbags - Liz Costa

Categories