THE ILLUSION OF THE DOWN WIND TURN

This downwind turn thing comes up from time to time. Others try to explain it in terms of airplanes. If you GET it, don't read this. if not, maybe this will help. Think boat on a river. Water is a fluid, just like air, but you can see it. A lot more people have been in and driven boats than have piloted aircraft. So let's give this a try.

Your only steering control is the rudder. It only works if it is moving through the water. If you are stationary in the water there is no flow over the rudder so it doesn't do anything. If you are sitting in a still pond and move the rudder, nothing happens.

The river is flowing downstream at 10 mph relative to the shore. You are floating on the river with no engine or wind power. Relative to the water your speed is zero. This is identical to floating on that still pond since you are moving with the water. Relative to the land you are moving at 10 mph, but who cares. Your rudder doesn't know that. All it knows is that there is little or now water flow around it. You could be sitting in a still pond or you could be floating on a river. Same thing to the rudder.

You use a paddle to turn the nose of the boat so it faces up stream. You are now drifting backwards. But your rudder still has no control as it is still moving with the water. Doesn't matter which way you face. It is the movement of the rudder THROUGH the water that allows you to steer the boat with the rudder. Meanwhile the land continues to go buy.

You toss out an anchor. Your speed relative to the water is now 10 mph as that is how fast the water is flowing. It is still doing 10 but, you are stationary compared to the river bank. Relative to the water you are moving at 10 mph. NOW your rudder can wag the back of the boat and steer as water is flowing over it. Your land speed is zero, but who cares. Your water speed is all that matters.

You pull in the anchor and start to drift with the water again. No rudder authority at all! Do you know why?

You face the boat down river and turn on the engine and manage to get moving at 10 mph RELATIVE TO THE WATER. Who cares what your land speed, the rudder only cares about the water.

Since you are moving relative to the water your rudder works again. In fact it is just about as effective as it was when you were anchored.

What have we learned?

Land speed means nothing. Water speed means everything.

Airplanes?

Land speed means nothing. Air speed means everything. There is no down wind change in the handling of the plane. There is only air speed, your speed relative to the air. How it looks from the ground means nothing.

Any questions?