PRINCIPLES

by Pierre Verster (Paddy)

As you are flying at your assigned altitude from A to B, your aircraft is subject to a forward movement caused by the thrust delivered by the engines and a sideward movement, caused by the movement of the airmass you are flying in, called wind on flightlevel.

If there were no winds, your compass course would show the course of your groundtrack between A and B plus or minus the average magnetic variation enroute between A and B, allowing you to depart from A and arriving in B while letting the autopilot do the work. Magnetic deviation and inclination are not discussed here, because Microsoft Flightsimulator has not implemented these features.

Unfortunately there is always wind when flying in the real world, worse: wind is increasing in speed and changing direction as you climb. (Law of Buys Ballot).

So if you don’t take any action to compensate your compass course for any wind trying to blow you off your groundtrack, you will never arrive in B and you will drift off your track diagonally.

It is here that the Winddrift Meter comes into play.

Whenever we encounter wind, it can be:

- A full nosewind slowing down the groundspeed but not drifting us away from our groundtrack,

- A full tailwind increasing the groundspeed but not drifting us away from our groundtrack,

- A full crosswind coming in either from port or from starboard. This type of wind will blow us away from the groundtrack, but does not influence the groundspeed.

- Any other wind coming in between nose and wing or between wing and tail on either side.

This last mentioned type of wind can always be separated into a tail- and crosswind component or a nose- and crosswind component.

It follows that there is always a crosswind, be it either a full crosswind or a crosswind component, except in the cases of a full nosewind or a full tailwind.

In the old days it was common, that the aviation meteo services stated the wind as a "windvelocity", being the winddirection from where the wind is blowing, followed by the windspeed in knots.

For surface winds the winddirection is given in degrees magnetic, for winds aloft in degrees true.

An advantage of the drift meter is, that you have nothing to do with magnetic or true winddirections, because the measured winddrift is simply to add to or to subtract from the current magnetic course you are flying.

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