Whiz Wheel – The Windy Side
By Christopher Hope

In these days of button pushing, GPS Systems, hand-held computers, and even cellular phone with features to spare, the lowly E6B wind computer often is lost in the bottom of the flight bag.  And this is a shame.  Because not only can the E6B perform all of the functions that the other computers do, it can do it with no batteries and, even more important, it can show the picture of what the wind is doing.  And while there are numerous features on the "numbers" side, I want to talk about the much-maligned wind side.

I think one reason that pilots have trouble working wind problems on the E6B is that they are trying to remember a step-by-step procedure, and not really thinking about what they are trying to accomplish.  So, let's step back and look at the big picture.

When you first started talking about the wind's effect on your path across the ground, your instructor probably likened it to paddling a canoe across the river, where the current moved you downstream as you tried to paddle straight across. (figure A.)  (Actually, I liked the analogy of paddling downstream rather than across, and then having the current move me even further in my desired direction.)  But in your instructor's picture, the current was always pushing perpendicular to your desired motion. The question that is generally presented in this problem is this:  If we know the speed of our paddling and direction of "my movement", ("straight across") and we know the speed and direction of the current ("downstream"), all we need to is draw the two to scale and draw the line from one end to the other.   Then, knowing a little geometry we can calculate the angle and length of the result to figure out where we end up. (I am already getting that  "too much geometry" hazy feeling.)

A more realistic situation might be where you are paddling to a point that is across the river, but somewhat downstream.  And you are moved even further downstream than you desired (figure B.)  No more nice 90-degree angle.  Now, unless your are the guy (or girl) who always aced the geonmetry test calculating the angle between north and "my movement", and then the angle between "my movement" and "current movement" and doing all sorts of math things to find the last angle, you are probably now finding your eyesight moving toward hazy..  (My brain is getting reeeeeaal hazy now.)  If we are going to convert this concept to moving through the air, we have to find a better way to compute.

So let's keep the idea that we can have one line to show our track through the air for one hour, (let's call that "air-line"), one line to show the effect of the wind over that same hour (wind-line), and then a third line to show, (for the same hour) our resulting track across the ground (ground-line.) 

We know everything about the wind-line.  We know how fast it is moving, and we know what direction it is coming from and going to, so we will be able to draw a one-hour picture of the wind.

As far as our movement through the air is concerned, we know our speed, and we know where we want to go.  For starters, that the direction we will assume we will head out.   So let's use that information for our one-hour picture of our movement through the air.

ALERT, ALERT-Just as adding plums and bananas give us meaningless information, so does adding knots and mph.  And for that matter, so does adding true headings to magnetic headings.  Keep all of the headings in the same family, and all of the speeds in the same family.  It doesn't matter which, but I recommend that you convert everything to magnetic headings and knots. 

So, grab that E6B, and let's find that easier system. 

First of all, we really don't need to see the entire triangle.  All we really care about is the ends of the three lines.  So our wind computer doesn't need to be big enough for the whole thing – just the ends.

To allow this explanation to make sense, let's use some real numbers, and let's move our picture to the E6B.

So, first we will draw the line that represents us going through the air, and we will let this line end at the middle of the circle.  So, turn the wheel until the heading that we want to fly (that is, the heading that we would fly if there was no wind) is at the top of the scale and place the center of the circle at the end of the "air arrow" (that is, put the true airspeed, on the slider under the center of the circle).  If you wish you can draw the air-line all the way to the bottom of the screen.  But it is not necessary.

Now, we can add the wind.  Just as the "movement through the air" line pointed in the direction that we were moving, so will the wind.  On the outside of the ring, find the direction that the wind is coming from, and draw a line across the end of the "movement through the air" line,  (which is the center of the ring) and extend it past the  end of the air-line the length of the wind line.

ALERT, ALERT.  OK, now you are saying, "I followed you through the first step.  But trying to draw the wind line just makes no sense."  You are absolutely correct.  So let's back up and see what happens if we draw the wind line first.

Rotate the circular screen to the point at which the "wind from" heading is at the top.  Move the slider up or down until a major number (100, 110, etc.) is lined up below the center.  Then draw a line straight DOWN for a distance corresponding to the wind velocity.  For example, with the winds forecast to be 230 ° at 20 knots, we would turn the scale until 230 ° shows on the upper ring at the top.  Then, we would move the slider up or down until the 100 line is under the center mark.  Then, we would draw a line straight down, 20 units long.

This makes more sense.  Now, if we turn the dial until the desired course is at the top of the case, and the true airspeed is under the center mark, we will have both the airspeed line and the wind line shown.  And with the air line and the wind lines drawn, we can see that the ground line would extend up from some point off of the scale to the end of the wind line.

Now, if we look at the end of the wind arrow, we can see at a glance how our ground speed compares with our true airspeed, (higher? lower?) and we can see which way the wind has blown us from our course. (left? right?) And, we can get exact. 

On the lines that are perpendicular to the air line, we can read our groundspeed underneath the end of the wind arrow.  And if we look at the long lines radiating from the bottom, we can see how many degrees off course we will move.  In our example, we will be blown 10° to the right, and we will get a 5 knot boost from the wind, resulting in a groundspeed of 115 knots

But there is one more step.  So far we have figured out how far we will blow off course if we assume that we are going to fly that no-wind heading.  So let's make that correction and see where we are.  We really don't want to blown off course.  We really want to be blown from some heading to "on-course."

If our calculations indicate that we will blow 10° to the right of our heading, let's make a correction at the beginning.  Let's fly 10° to the left of the course we first assumed.  That is, if the desired course that we first assumed was 330°, and we blew 10° to the right, or to a course of 340°, let's recompute that.  Turn the dial from 330° at the top, to 320° at the top.   Note

that this correction has rotated the wind line just a little.  But now, when the end of the wind line indicates that we will be blown 10° off course to the right, we will know that it is blowing us back onto our originally desired course of 330°.  And note also that, by making that 10° heading change to correct for the wind, we have lost 4 knots of groundspeed, leaving us with a groundspeed of 111 knots.

Usually, this is all there is to the procedure.  The air-line still indicates the true airspeed that we originally planned.  And now, the aircraft heading is shown at the top of the case.  The wind is still shown as we originally drew it.  And the end of wind arrow indicates the ground speed and the number of degrees blown to the right or left.

So, yes.  Usually this is all there is to the procedure.  But sometimes there is one more step.  If the wind is relatively high, compared to the true airspeed, and if it is close to a straight crosswind, there might be one more step.  Looking closely at the pictures, we can see that when we turned the dial to the new heading of 320° the wind line has shifted as well.  Most of the time, when we see the wind line rotate around, the number of degrees of drift does not appreciably change.  But sometimes it does.  In our example above, we assumed that we were blowing 10° to the right.  And so we made of a heading correction of 10° to the left.  After we make this correction on the computer, we need to look one more time to see how many degrees we are drifting.  If we are still indicating that we are drifting 10° to the right, all is well.  But if the wind shifts enough, relative to our ground track, we might see that our drift angle has changed to 8° or 9° to the right or maybe to 11° or 12° to the right.  If so, rotate the entire dial again a degree or two so that the numerical difference between the heading shown at the top of the circle and our desired ground track truly matches the number of degrees we are moving off course.

Now, if you are like me, you like to have a quick way of checking that you did not make some careless error in your calculations.  Take just a few seconds to make the "Am in the ballpark?" check.

First, is the ground speed answer reasonable?  If you have a direct tailwind, the ground speed will be exactly equal to the true airspeed value plus the wind value.  And if you have a wind right on the nose, the groundspeed will be equal to the true airspeed value minus the wind value. (In our example, the ground speed has to be between 90 and 130 knots.)  If your groundspeed value is not between these two, you have an error somewhere.  (Hint.  Did you ensure that the true airspeed value, shown on the slider, was under the center grommet?  Is the length of your wind line correct?)

Now make a quick check for the left \ right drift.  On a piece of scratch paper, imagine north at the top of the page and draw a short arrow more or less in the direction you will be traveling. (northwest in our case.)  Don't bother to get too fancy.  Just a northeast or southwest line sort of thing.  Then right next to that, draw an arrow in the direction that the wind is moving.  Make sure you don't do this backwards.  A wind from 230° for example, is coming FROM the southwest, and going TO the northeast.  With these two arrows, you will be able to see at a glance if you expect to be blown to the right or to the left as travel along.

Sound complicated?  Nah.  Nothing to it.  Try it next time you are planning your trip and you will wonder why you spent all of that money on an electronic wind computer.

Have fun and keep flying.

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