View Full Version : GS determination in flight- double drift method

25th Feb 2023, 16:31
Would anyone be able to elaborate on the following method of in-flight ground speed determination explained in Air Navigation by Weems (1943, p.128)?

The author explains the benefits of employing this method, but how it is used in practice to determine the GS/wind velocity has not been made clear.

Does anyone have any idea what the "double- (or triple-) drift method" entails?


The Determination of Ground Speed in Flight.

3. Indirect determinations by double drift, utilizing the air speed as the scale factor. The accuracy of this method, apart from the question of the exact value of the air speed, is good, and the error when using a well-made instrument should not exceed about 1 per cent. Further, as three drifts can be worked just as easily as two, the use of a third drift observation gives a measure of the accuracy obtained.

The value of the air speed to be used is the corrected or true air speed, and this can be readily obtained from the air-speed meter leading and a correction table, or air-speed computer; the true air speed so obtained is not absolutely accurate, but the error is not likely to exceed about 3 or 4 per cent, and is usually less at normal flying altitudes of, say, 4,000 ft. Thus by this method the ground speed can certainly be determined within 5 per cent.

In order to use the double- (or triple-) drift method, the course has to be altered twice, involving an increase in the distance flown, but this disadvantage is more apparent than real, as will be seen later. The increase of distance flown is not large if the courses are arranged as follows:

Course 1. Normal course.
Course 2. 45 to right for 3 min.
Course 3. 90 to left for 3 min.
Course 4. Revert to normal course

In the case of aircraft operating over wild uninhabited areas, the reduction of the uncertainty in the dead-reckoning position when using the above method of determining ground speed and wind is of the greatest value in those rare but inevitable cases where forced landings have to be made.

25th Feb 2023, 21:12
By deduction, this is written for airplanes equipped with drift meters. Think about the wind side of a flight computer. If you have a heading (from compass/DG) and crab angle (from drift meter), you can draw a line over the heading, for the corresponding course (which is in the middle). Do this for a second heading (and its drift), now you have 2 lines that make a cross, which is the wind.

28th Feb 2023, 10:45
Thank you Vessbot for the explanation. That makes sense.