As someone else suggested above ... it doesn't matter which way you do the sums ... the answers are the same and the apparently reversed techniques are correct trigonometrically, or, at least, can be shown to be able to produce the correct answer .. provided you keep track of what you are doing.
The suggestion that one way is "professional" but that the other is "OK for private pilots" is just so much nonsense, unless there be some perceived elite clanishness involved.
You use whichever way you find more comfortable, although it is probably going to give you fewer troubles in the long run following the "conventional" way for the particular computer.
I would be more concerned that the individual pilot adopt a consistent set of techniques to minimise the likelihood of confusion and consequent error.
The Dalton (E6B) style of wheel provides a means to plot the W/V end of the navigation vector solution (navigation triangle).
The picture we get is the same as if we plotted the triangle directly onto a sheet of graph paper. The easiest way to think of it is to consider the instrument "floating" above the graph paper .. we look through the rose to obtain a view of the grid and the triangle, much in the same way as MS Windows provides a screen through which to look upon the "desktop" beneath.
The conventional solution requires that the centre of the compass rose viewer be located over the intersection of the TAS/HDG and W/V vectors. For this approach, when plotting the W/V vector, it is drawn "wind down" from the centre point.
However, because of the way the grid is drawn (r-theta, azimuth-range, angle-distance .. whatever descriptive term you might prefer) it doesn't matter if one positions the centremark over the other end of the W/V vector .. ie the intersection of the TR/GS and W/V vectors. This is what happens if the W/V vector is drawn "wind up" from the centre point .. actually it makes more sense to think of the W/V in this case as being plotted down from the end point to the centrepoint of the rose as the direction of the wind as plotted is exactly the same.
The instrument plots exactly the same triangle regardless of which approach is adopted. To see this more easily, try drawing an example on paper and then orient the paper with the instrument. To change from one technique to the other is just a matter of moving the instrument "over" the triangle to reposition the centrepoint while, at the same time, rotating it to remain aligned with the radial spokes of the grid.
The solution is read exactly the same for each case .. it doesn't matter which technique is used .. provided no careless mistakes are made, the correct answer results.
The suggestion that the unconventional technique can save a little effort is true, but rather illusory.
It may be helpful to look at the typical navigation problems which we routinely solve on the instrument.
(a) Given W/V, TR, and TAS .. find HDG and GS
The conventional "wind down" plot requires some iteration on the part of the pilot to obtain the solution. The alternative "wind up" technique gives the answer directly. Just be careful of confusing the decal markings.
(b) Given TR/GS and HDG/TAS .. find W/V
Either way works fine.
(c) Given HDG/TAS and W/V .. find TR/GS
The standard way works fine. If you use the "wind up" method, you end up having to do the same sort of iterative processing which the technique sought to avoid in (a).
(d) Given W/V and TR/GS .. find HDG/TAS
Similar to (a).
I really can't see that there is any significant advantage to be had in using the alternative method. If you take the small workload reduction in (a) and (d), you end up with the same sort of problem in (c). A matter of preference, I guess.
The CR works a little differently .. to save size .. which offers the advantage of its being able to be carried in one's shirt pocket.
To remove the need for a bulky slide, rather than plotting the standard (as in the Dalton solution) triangle, a line is drawn through the intersection of the HDG/TAS and W/V vectors and perpendicular to the TR/GS vector (or its extension). This results in a triangle to be plotted which is solved by a combination of figuring simple vector components (left/right crosswind and head/tailwind) and doing some basic trigonometry using the sine and cosine scales around the outside of the instrument (drift and effective TAS) .. the result is no need for a slide with the radial/range grid. It is, however, quite important to remember that we are solving a different triangle from the Dalton, although the end result is the same. A lot of people tend to get confused by this difference.
Unlike the Dalton, there is no need to plot the W/V vector as such and the idea of "wind up" and "wind down" is rather unnecessary. However, the distinction can be made by plotting the reciprocal of the conventional vector .. ie as if the wind is coming from the reciprocal direction but with the same magnitude (speed). Alternatively, this vector can be thought of in the same way as the Dalton situation in that it is just plotted, for curious inconvenience, downstream at a scale distance equal to the wind speed.
The result is that the unconventional plot ("wind down") causes the conventional CR triangle to be rotated 180 degrees about the intersection of the TR/GS and W/V vectors (the rose centre point regardless of which approach is adopted). The pilot must keep this firmly in mind .. while the resolved components of the W/V have the same magnitude the sense or direction, with respect to the instrument markings, is reversed. Provided that the pilot does not make any careless mistakes, the answers are going to end up the same regardless of which technique is adopted.
The standard problems listed above for the Dalton work fine for the CR, regardless of which technique is used. However, with the unconventional method, the mental housekeeping workload on the pilot increases significantly as does the likelihood of careless errors. For the life of me, I can see absolutely no advantage to be had in using the CR instrument in other than the conventional manner. If you want to do so, fine ... just be a little bit careful of the housekeeping workload .. but, surely, it is a bit like marching around the parade ground backwards ? .. a novelty but of little practical value.
The upshot is that the wind up and wind down approaches, for both computers, can be made to work fine for the typical problems we have to solve. I just can't see much point in using the unconventional technique for either instrument, especially in the case of the CR.
Last edited by john_tullamarine; 9th Apr 2002 at 07:29.