# Knot (unit)

Last updated

Knot
An airspeed indicator, which shows speed in knots
General information
Unit system
• meteorology
• aviation
• maritime
Unit of speed
Symbolknorkt
Conversions
1 kn in ...... is equal to ...
km/h    1.852
mph    1.15078
m/s    0.514444
ft/s    1.68781

The knot () is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h (approximately 1.15078 mph or 0.514 m/s). [1] The ISO standard symbol for the knot is kn. [2] The same symbol is preferred by the Institute of Electrical and Electronics Engineers (IEEE); kt is also common, especially in aviation, where it is the form recommended by the International Civil Aviation Organization (ICAO). [3] The knot is a non-SI unit. [4] The knot is used in meteorology, and in maritime and air navigation. A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.

## Contents

Etymologically, the term derives from counting the number of knots in the line that unspooled from the reel of a chip log in a specific time.

## Definitions

1 international knot =
1  nautical mile per hour (by definition),
1852.000 metres per hour (exactly), [4]
0.51444 metres per second (approximately),
1.15078 miles per hour (approximately),
20.25372 inches per second (approximately)
1.68781 feet per second (approximately).

The length of the internationally agreed nautical mile is 1852 m. The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248 m). [5] The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080 ft or 1853.184 m).

Conversions between common units of speed
m/s km/h mph knot ft/s
1 m/s =13.62.236936*1.943844*3.280840*
1 km/h =0.277778*10.621371*0.539957*0.911344*
1 mph =0.447041.60934410.868976*1.466667*
1 knot =0.514444*1.8521.150779*11.687810*
1 ft/s =0.30481.097280.681818*0.592484*1

(* = approximate values)

## Usage

The speeds of vessels relative to the fluids in which they travel (boat speeds and air speeds) are measured in knots. For consistency, the speeds of navigational fluids (tidal streams, river currents and wind speeds) are also measured in knots. Thus, speed over the ground (SOG; ground speed (GS) in aircraft) and rate of progress towards a distant point ("velocity made good", VMG) are also given in knots.

## Origin

Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the moving vessel and the line allowed to pay out. [6] Knots placed at a distance of 47  feet 3  inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation. [7] The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%.

Derivation of knots spacing:

${\displaystyle 1~{\textrm {kn}}=1852~{\textrm {m/h}}=0.5144~{\textrm {m/s}}}$, so in ${\displaystyle 28}$ seconds that is ${\displaystyle 14.40}$ metres per knot.

## Modern use

Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because the length of a nautical mile, upon which the knot is based, is closely related to the longitude/latitude geographic coordinate system. As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship.

Standard nautical charts are on the Mercator projection and the horizontal (East-West) scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart. Recent British Admiralty charts have a latitude scale down the middle to make this even easier. [8]

Speed is sometimes incorrectly expressed as "knots per hour", [9] which would mean "nautical miles per hour per hour" and thus would refer to acceleration.

### Aeronautical terms

Prior to 1969, airworthiness standards for civil aircraft in the United States Federal Aviation Regulations specified that distances were to be in statute miles, and speeds in miles per hour. In 1969, these standards were progressively amended to specify that distances were to be in nautical miles, and speeds in knots. [10]

The following abbreviations are used to distinguish between various measurements of airspeed: [11]

The indicated airspeed is close to the true airspeed only at sea level in standard conditions and at low speeds. At 11000 m (36000 ft), an indicated airspeed of 300 kn may correspond to a true airspeed of 500 kn in standard conditions.

## Related Research Articles

The International Civil Aviation Organization is a specialized agency of the United Nations. It changes the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth. Its headquarters is located in the Quartier International of Montreal, Quebec, Canada.

The mile is an English unit of length of linear measure equal to 5,280 feet, or 1,760 yards, and standardised as exactly 1,609.344 metres by international agreement in 1959.

A nautical mile is a unit of measurement used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as one minute of latitude along any line of longitude. Today the international nautical mile is defined as exactly 1852 metres. The derived unit of speed is the knot, one nautical mile per hour.

Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with information about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading and much more other crucial information. They improve safety by allowing the pilot to fly the aircraft in level flight, and make turns, without a reference outside the aircraft such as the horizon. Visual flight rules (VFR) require an airspeed indicator, an altimeter, and a compass or other suitable magnetic direction indicator. Instrument flight rules (IFR) additionally require a gyroscopic pitch-bank, direction and rate of turn indicator, plus a slip-skid indicator, adjustable altimeter, and a clock. Flight into Instrument meteorological conditions (IMC) require radio navigation instruments for precise takeoffs and landings.

Wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Note that wind direction is usually almost parallel to isobars, due to Earth's rotation.

The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometers per hour (km/h), knots (kn), miles per hour (MPH) and/or meters per second (m/s). The recommendation by ICAO is to use km/h, however knots is currently the most used unit. The ASI measures the pressure differential between static pressure from the static port, and total pressure from the pitot tube. This difference in pressure is registered with the ASI pointer on the face of the instrument.

Speed to fly is a principle used by soaring pilots when flying between sources of lift, usually thermals, ridge lift and wave. The aim is to maximize the average cross-country speed by optimizing the airspeed in both rising and sinking air. The optimal airspeed is independent of the wind speed, because the fastest average speed achievable through the airmass corresponds to the fastest achievable average groundspeed.

Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are indicated airspeed ("IAS"), calibrated airspeed ("CAS"), equivalent airspeed ("EAS"), true airspeed ("TAS"), and density airspeed.

The true airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Traditionally it is measured using an analogue TAS indicator, but as the Global Positioning System has become available for civilian use, the importance of such analogue instruments has decreased. Since indicated airspeed is a better indicator of power used and lift available, True airspeed is not used for controlling the aircraft during taxiing, takeoff, climb, descent, approach or landing; for these purposes the Indicated airspeed – IAS or KIAS – is used. However, since indicated airspeed only shows true speed through the air at standard sea level pressure and temperature, a TAS meter is necessary for navigation purposes at cruising altitude in less dense air. The IAS meter reads very nearly the TAS at lower altitude and at lower speed. On jet airliners the TAS meter is usually hidden at speeds below 200 knots (370 km/h). Neither provides for accurate speed over the ground, since surface winds or winds aloft are not taken into account.

Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator (ASI) on an aircraft, driven by the pitot-static system. It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure dynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed. Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from true airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass. Calibrated airspeed (CAS) is the IAS corrected for instrument and position error.

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error.

Aircraft maneuvering is referenced to a standard rate turn, also known as a rate one turn (ROT).

Flight plans are documents filed by a pilot or flight dispatcher with the local Air Navigation Service Provider prior to departure which indicate the plane's planned route or flight path. Flight plan format is specified in ICAO Doc 4444. They generally include basic information such as departure and arrival points, estimated time en route, alternate airports in case of bad weather, type of flight, the pilot's information, number of people on board and information about the aircraft itself. In most countries, flight plans are required for flights under IFR, but may be optional for flying VFR unless crossing international borders. Flight plans are highly recommended, especially when flying over inhospitable areas, such as water, as they provide a way of alerting rescuers if the flight is overdue. In the United States and Canada, when an aircraft is crossing the Air Defense Identification Zone (ADIZ), either an IFR or a special type of VFR flight plan called a DVFR flight plan must be filed. For IFR flights, flight plans are used by air traffic control to initiate tracking and routing services. For VFR flights, their only purpose is to provide needed information should search and rescue operations be required, or for use by air traffic control when flying in a "Special Flight Rules Area".

Flight planning is the process of producing a flight plan to describe a proposed aircraft flight. It involves two safety-critical aspects: fuel calculation, to ensure that the aircraft can safely reach the destination, and compliance with air traffic control requirements, to minimise the risk of midair collision. In addition, flight planners normally wish to minimise flight cost through the appropriate choice of route, height, and speed, and by loading the minimum necessary fuel on board. Air Traffic Services (ATS) use the completed flight plan for separation of aircraft in air traffic management services, including tracking and finding lost aircraft, during search and rescue (SAR) missions.

In aviation, holding is a maneuver designed to delay an aircraft already in flight while keeping it within a specified airspace.

A World Aeronautical Chart (WAC) is a type of aeronautical chart used for navigation by pilots of moderate speed aircraft and aircraft at high altitudes. They are at a scale of 1:1,000,000.

In aviation, the rule of three or "3:1 rule of descent" is that 3 nautical miles (5.6 km) of travel should be allowed for every 1,000 feet (300 m) of descent.

A Machmeter is an aircraft pitot-static system flight instrument that shows the ratio of the true airspeed to the speed of sound, a dimensionless quantity called Mach number. This is shown on a Machmeter as a decimal fraction. An aircraft flying at the speed of sound is flying at a Mach number of one, expressed as Mach 1.

The kilometre per hour is a unit of speed, expressing the number of kilometres travelled in one hour.

## References

1. Bartlett, Tim (July 2008) [2003]. RYA Navigation Handbook. Southampton: Royal Yachting Association.
2. "ISO 80000-3:2006". International Organization for Standardization . Retrieved 20 July 2013.
3. International Standards and Recommended Practices, Annex 5 to the Convention on International Civil Aviation, “Units of measurement to be Used in Air and Ground Operations”, ICAO, 4th Edition, July 1979.
4. "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI brochure (8th ed.). International Bureau of Weights and Measures. The knot is defined as one nautical mile per hour. There is no internationally agreed symbol, but the symbol kn is commonly used.
5. Louis E. Barbow and Lewie V. Judson (1976). "Appendix 4 The international nautical mile" (PDF). Weights and Measures Standards of the United States, A brief history. NIST Physics Laboratory. Archived from the original (PDF) on 26 September 2007. Retrieved 2 August 2007.
6. Jacob Abbott (1858). Rollo on the Atlantic. DeWolfe, Fiske, & Co., Publishers. Retrieved 30 November 2011.
7. Kemp, Peter, ed. (1976). . Oxford University Press. p.  454. ISBN   0-19-282084-2.
8. e.g. BA Chart 73, Puerto de Huelva and Approaches, 2002
9. Wilson, Alastair (22 July 2009). "'Knots an hour'". The Kipling Society . Retrieved 1 December 2012. Since the 1890s or thereabouts, it has been drummed into the young seaman that a knot is a unit of speed, namely, one nautical mile per hour; and that consequently only the uneducated speak of "knots per hour" or "knots an hour". It was therefore inevitable that Kipling’s frequent use of this expression should grieve a number of seafaring readers, as the pages of the Kipling Journal testify.
10. For example, Part 23 of the Federal Aviation Regulations, amendment 23–7, 14 September 1969
11. "Abbreviations and symbols". edocket.access.gpo.gov.