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Old 30th Jun 2012, 15:54
  #104 (permalink)  
keith williams
Join Date: Jan 2011
Location: England
Posts: 643
Looking again at my description of the solution using a CRP5, I can see that I have made an error in that I assumed that the 240 knot TAS plus the 45 knot wind component would give a ground speed of 285 knots. This would be true if there was no drift, but it is not true with 15 degrees drift, because drift adds a lateral component to the ground speed.

If we split the TAS into one component along the 15 degree drift track, and another component at right angles to it we get 231.8 knots along the track and 62.2 knots across the track. Adding the 45 knot wind component to the along-track speed gives a ground speed of 231.8 + 45 = 276.8 knots.

The modified solution using the CRP5 become:

a. No heading is specified so select one at random, 000 degrees for example.

b. Set the high-speed slide with the centre dot at 240 kts.

c. The ground speed is 276.8 knots.

d. The question also specifies 150 left drift. So draw a cross where the 15 degrees left drift line crosses the 276.8 kt arc.

e. With 15 degrees left drift the track is 345 degrees. So the reverse track is 165 degrees.

f. Rotate window to align 165 degrees with true heading index.

g. The cross now indicates 19 degrees right drift. Align the 165 degrees track with 19 degrees right drift. The cross now indicates 15 degrees right drift. Align 165 degrees with 15 degrees right drift. The cross now indicates 16 degrees right drift. Align 165 degrees with 16 degrees right drift. Both the cross now indicates Move and the outer scale now indicate 16 degrees right drift, so the drift is balanced.

h. The cross is now on an arc at approximately 55 kts below the centre dot. The wind component was a headwind during the outward leg so it must be a tailwind during the return leg This means that the wind component is -55 kts.

This matches the solution that you got with your E6B.

It is curious that my error led me to answer that was marked as being correct. This makes me suspect that the author of this question made the same error.
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