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, ultravio ...

used to improve

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

for aircraft

navigation 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 navig ...

by measuring

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

effect on

flight Flight or flying is the motion (physics), motion of an Physical object, object through an atmosphere, or through the vacuum of Outer space, space, without contacting any planetary surface. This can be achieved by generating aerodynamic lift ass ...

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 prim ...

's station. A

reticle A reticle or reticule, also known as a graticule or crosshair, 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 electronic v ...

, 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 component of the velocity of an aircraft relative to the Earth’s surface, also referred to as "speed over the ground". It is vital for accurate navigation that the pilot has an estimate of the ground speed that wil ...

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 (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

bomber aircraft A bomber is a military combat aircraft that utilizes air-to-ground weaponry to drop bombs, launch torpedoes, or deploy air-launched cruise missiles. There are two major classifications of bomber: strategic and tactical. Strategic bombing is d ...

. Typically,

fighter aircraft Fighter aircraft (early on also ''pursuit aircraft'') are military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air supremacy, air superiority of the battlespace. Domina ...

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 movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

. 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 the 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 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 ''Dreieckrechner' ...

(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 * Heading (navigation), the direction a person or vehicle is facing, usually similar to its course ** Aircraft heading, the directi ...

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 spe ...

and the

wind direction Wind direction is generally reported by the direction from which the wind 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 ...

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 v ...

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 it is flying through (which itself is usually moving relative to the ground due to wind). In contrast, the ground speed is the speed of an aircraft with respect to the sur ...

(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