Contest Corner

Contest Corner

Upwind and Downwind

“This can’t be right,” I thought. Flying in a regional contest at Ionia, our first leg took us into a 15 knot wind. “Take upwind turnpoints low,” all the books said, so I did. 10 minutes later, I found myself at 800 feet, drifting quickly away from the airport turnpoint,looking hard at the rather unpromising fields, crewless as usual, and praying for a knot of lift. Surely, there must be a better way!

Of course, there is. If the soaring gods offer up an 8 knot thermal on the way to an upwind turnpoint at Ionia, you take it, thankfully, as high as it will go!The question is choosey or desperate, not high or low. The question is, how strong a thermal upwind is the same thing as (say) a two knot thermal downwind? I spent some time on the math of this question (over the winter, not while drifting downwind at 800 feet!) The first graph gives the answer, using the polar of a dry ASW27.

Here’s how to read the graph. Suppose you’re headed into an upwind turnpoint. The GPS says that if you don’t stop to thermal, you’ll get there at 2000 feet. Thinking about what it will be like after the turn, 2000 feet on this day, you decide that a MacCready setting of about 2 will be right. You’ll take anything going up at a solid 2 knots or better, and you’ll be cruising at about 75 knots – hungry, but not desperate.

Now, locate the triangle at 0 wind speed and two knots on the graph – on the red line. Moving along the red line, you see how choosy to be on your way to the turnpoint. If you’re going in to a 10 knot headwind, the red line rises to 3.2 knots. You should head in to the turnpoint with a 3.2 MacCready setting:you should only take thermals stronger than 3.2 knots, and you should cruiseat around 80. If it’s a 20 knot headwind, the red line rises to MacCready 5. It’s only worth stopping for 5 knot thermals, and you should be gliding faster than you dare. Obviously, you will likely end up low if you follow this advice, but if a 6 knot thermal comes along, take it without regrets.

The lines work the other way too. If you’re going down a 20 knot wind, and you’ll be flying MacCready 2 after the turnpoint, then you should fly at about MacCready 0.5 on the way in. It’s better to take a 0.7 knot thermal while drifting down a 20 knot wind than it is to thermal at 2 knots going upwind. Obviously, you’re likely to end up high if you follow this advice.

Spending a little time with the graph helps to develop a seat of the pants sense of how important wind is. I carry this little graph around in the cockpit and look once before the flight, which seems enough. Of course, it is an easy enough number for our instrument makers to calculate for us.

Where does this come from? I’ll spare you equations, but the second pictureshows how to make the calculation.

When you’re going to a turnpoint, the glide over the groundmatters. Going into a 15 knot wind, it’s just as if your glider’s polar is shifted to the left (slower) by 15 knots, as shown by the red polar in the picture. When you round the turnpoint and go downwind, it will be just as if your glider’s polar is shifted to the right (faster) by 15 knots. This is shown by the green polar in the picture.

The MacCready setting now should always equal what you expect the MacCready setting to be later. There is no point in taking 2 knot thermals when you know you will be gliding at 4 knots up ahead. So you should set the MacCready value of the upwind glider, relative to the ground, equal to the MacCready value of the downwind glider, also relative to the ground. In the picture, you see the two MacCready lines cross at the black, ground-based vertical axis.

Our instruments and thermals give us MacCready values relative to the air. The upwind glider is thinking relative to the dashed red axis, and the downwind glider is thinking relative to the dashed green axis. We see that the upwind glider follows a much higher MacCready setting than the downwind glider. We knew that, but now we can calculate how much higher. To make the first graph, I simply calculated for each downwind (green) MacCready value what the corresponding upwind (red) MacCready value is.

At least the math can be comforting. Since making the graph, I’ve felt much better about taking the occasional really strong thermal, even though I was near an upwind turnpoint, and I’ve felt better about drifting downwind in 2 knots to a downwind turnpoint.Now I know that can be the right thing to do, not the wimpy thing. I’ve also been back to 800 feet, and it’s comforting to realize that the 2 knotter I’m struggling with isn’t as slow as it feels, because I would have needed5 knots upwind to do better.

So there is a better way. It’s not “take upwind turnpoints low and downwind turnpoints high.” It’s “take upwind turnpoints fast and choosey, and take downwind turnpoints slow and patient.”

Graph 1

Graph 2