The Driftmeter

Important among modern air navigation devices is the Drift Meter used to measure the angular distance that an airplane is forced from its compass course by the wind. Its indications are also utilized in calculating ground speed.

Air navigation is complicated by the action of winds. From any direction except dead ahead or dead astern, wind forces an airplane to drift sidewise — that is, action of the wind causes an angle to exist between the compass course and the actual path which an airplane describes over the ground.

In the left panel, an airplane is shown headed in the direction of its desired objective, O. The wind, W, however, causes the plane to drift sidewise. If the pilot follows his compass course without correction for this wind effect, the airplane will pass east of its objective toward E. Angle D is the drift of the aircraft, and the Drift Meter is used to measure this angle. Knowledge of the drift angle enables the pilot to steer the airplane in the direction necessary to "make good" the intended course and arrive at the scheduled destination. The adjacent panel shows that this is done by heading the plane into the wind, at such an angle that the effect of the wind is counterbalanced. Thus, the airplane exactly tracks the line AO, which is the intended course, and arrives at its planned destination. The compass course with allowance for wind is the compass heading, ie, the direction by compass in which the airplane is pointed to overcome the drift.

Operation of the Drift Meter

The Drift Meter is mounted in a vertical position with its objective end projecting below the fuselage. The instrument turns freely within the flange which holds it to the body of the aircraft. The housing of the Drift Meter incorporates an optical system which consists of an eyepiece lens, a reticle, two erector lenses, an objective lens, the index prism, and a reflecting prism. Rays of light from the object sighted pass through these various optical elements to the eyepiece. The image is viewed against the reticle which is etched with fore-and-aft lines, two speed lines, and a center athwartships line.

Drift Measurement

For measurement of drift, the index prism is adjusted by means of the sight handle so that the surface object is sighted along the fore-and-aft axis of the airplane. Since the grid lines etched on the reticle are parallel to the fore-and-aft axis of the aircraft, the surface object is seen — when the aircraft is drifting — at an angle to these lines. The instrument housing is rotated in the flange until the object sighted on the earth appears to parallel the grid lines. The angular displacement of the drift pointer to the right or left is read against the drift scale as the angle of drift.

Etched on the ground glass of the instrument are two lines called speed lines. A fixed object on the surface of the earth is sighted through the optical system of the Drift Meter. As the object crosses the leading ground-speed line, a stop watch is started. When the object crosses the trailing speed line, the watch is stopped, and elapsed time is noted. From this measurement — taking altitude into consideration — the ground speed of the aircraft is calculated.

This article was originally published in the April, 1944, issue of Air Tech magazine, vol 4, no 3, p 40.
Photo credited to Eclipse-Pioneer Div of Bendix Aviation Corp.
The PDF of this article includes a photo of a driftmeter and a notated phantom of a driftmeter, showing the internal arrangement of parts.