A drift meter, also drift indicator and drift sight, is an

optical device Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

used to improve

dead reckoning In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...

for aircraft

navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

by measuring

wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...

effect on

flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...

Design

A drift meter consists of a small telescope extended vertically through the bottom of the aircraft with the eyepiece inside the fuselage at the

navigator A navigator is the person on board a ship or aircraft responsible for its navigation.Grierson, MikeAviation History—Demise of the Flight Navigator FrancoFlyers.org website, October 14, 2008. Retrieved August 31, 2014. The navigator's primar ...

's station. A

reticle A reticle, or reticule also known as a graticule, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the screen of an oscilloscope, ...

, typically consisting of spaced parallel lines, is rotated until objects on the ground are seen to be moving parallel to the vertical lines. The angle of the reticle then indicates the aircraft's drift angle due to winds aloft. It is also used to calculate the

ground speed Ground speed is the horizontal speed of an aircraft relative to the Earth’s surface. It is vital for accurate navigation that the pilot has an estimate of the ground speed that will be achieved during each leg of a flight. An aircraft diving ve ...

by measuring the time it takes for an object on the ground to pass from the upper to the lower horizontal line of the reticle.

History

Drift meters were extensively used in

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

bomber aircraft A bomber is a military combat aircraft designed to attack ground and naval targets by dropping air-to-ground weaponry (such as bombs), launching torpedoes, or deploying air-launched cruise missiles. The first use of bombs dropped from an aircraf ...

. Typically,

fighter aircraft Fighter aircraft are fixed-wing military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air superiority of the battlespace. Domination of the airspace above a battlefield ...

were not equipped with this kind of relatively sophisticated

navigational equipment Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navi ...

. For instance, in the

Pacific Theater The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, where navigation was difficult due to scarcity of land or other prominent features, fighters were often guided by bombers that had a dedicated navigator and navigational equipment. Some drift meters were purely optical (e.g.,

USAAF The United States Army Air Forces (USAAF or AAF) was the major land-based aerial warfare service component of the United States Army and ''de facto'' aerial warfare service branch of the United States during and immediately after World War II ...

Type B-5 drift meter), while others included electrically powered gyroscope (e.g., USAAF Type B-3 drift meter). USAAF Type B-5 drift meter was also equipped with a simple

flight computer A flight computer is a form of circular slide rule used in aviation and one of a very few analog computers in widespread use in the 21st century. Sometimes it is called by the make or model name like E6B, CR, CRP-5 or in German, as the ''Dreieckrec ...

(a rotating wheel) to calculate the ground speed from the measured time that takes for a ground object to pass from the top to the bottom of the reticle.

Applications

Wind correction from drift angle

For practical purposes under ordinary conditions, the drift angle measured by the drift meter can be used for wind correction angle in the

heading Heading can refer to: * Heading (metalworking), a process which incorporates the extruding and upsetting processes * Headline, text at the top of a newspaper article * Heading (navigation), the direction a person or vehicle is facing, usually si ...

in order to put the aircraft back on the correct

course Course may refer to: Directions or navigation * Course (navigation), the path of travel * Course (orienteering), a series of control points visited by orienteers during a competition, marked with red/white flags in the terrain, and corresponding ...

, where the aircraft heading is turned into the opposite direction of the drift. However, theoretically there is a difference between the two angle values depending on conditions.

Wind vector from drift angles in two headings

Wind-from-aircraft-drift-angles-in-two-headings

It is possible to derive the

wind speed In meteorology, 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. Wind speed is now commonly measured with an anemometer. Wind speed ...

and the

wind direction Wind direction is generally reported by the direction from which it originates. For example, a ''north'' or ''northerly'' wind blows from the north to the south. The exceptions are onshore winds (blowing onto the shore from the water) and offsho ...

from the measurements of drift angle by the drift meter in two (or more) different headings. One method is to do it computationally by using several

trigonometric functions In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions) are real functions which relate an angle of a right-angled triangle to ratios of two side lengths. They are widely used in all ...

, which is not so trivial. An alternative method is to do it graphically by using a "double-drift diagram", which is a combination of two

wind triangle In air navigation, the wind triangle is a graphical representation of the relationship between aircraft motion and wind. It is used extensively in dead reckoning navigation. The wind triangle is a vector diagram, with three vectors. *The air ve ...

s (see the figure). In this method, two vectors are first drawn from the center of a plane, where each vector represents one of the headings at a given

airspeed In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: * Indicated airspeed ("IAS"), what is read on an airspeed gauge connected to a Pitot-static system; * Calibrated a ...

(scaling factor is used, e.g., 10 cm is 100 knots). The next step is to draw a drift line from the tip of the vector into the opposite direction at an angle equal to the drift (i.e., angle with respect to the line in the opposite direction of the vector), for each heading vector. The intersection of the two (or more) drift lines then indicates the beginning of the wind vector, while the center of the plane indicates the tip of the wind vector. The length of the wind vector provides the wind speed (using the same scaling factor as for the airspeed), while its direction gives the wind direction.Stanton (1946), pp. 30.

Wind vector from zero-drift heading

It is also possible to derive the wind vector by flying the heading, where the drift meter measures zero drift. This means that the aircraft is either flying directly into the wind or directly with the wind. Since the ground speed in that heading can be measured by the drift meter, one can calculate the wind speed by subtracting the airspeed from the ground speed. The sign of the difference indicates whether the wind direction is in the zero-drift heading or reciprocal to it. If the difference is zero, there is no wind present.

References

; Citations ; Bibliography * * * * {{refend ; External links *Type B-5: https://airandspace.si.edu/collection-objects/meter-drift-usaaf-type-b-5/nasm_A19601843000 *Type B-3: https://airandspace.si.edu/collection-objects/drift-meter-b-3-boeing-b-17d-swoose/nasm_A19500075025
"B-17 Pilot Training Manual: Duties and Responsibilities of THE NAVIGATOR"
Headquarters AAF
"Ellis Beymer, B-17 Navigator"
Combat Aircrews' Preservation Society, 2003 Navigational equipment Air navigation