Stick & Rudder Coordination

Keeping the Yaw String Straight

One of the first skills you start working on is coordinated use of the stick and rudder to make balanced turns.

While we were being towed the instructor showed me an exercise to check the coordination: a regular rhythm of coordinated stick and rudder to the left and right only results in opposite bank angles being generated - i.e. there is no turning effect. In other words: you move the stick and rudder right, and the glider starts to roll right. Then just before the turn starts, you move the stick and rudder left, and the glider levels out again. You keep holding the controls left, and the glider starts to roll left. Then just before the turn starts, you move the stick and rudder right again, and the glider levels out.

This is a good indication of how well coordinated the stick and rudder are. If you bank a glider with the ailerons alone, one wing generates more lift then the other, banking the aircraft. But since you can't get something for nothing, the higher wing also generates more drag. Because the wings on a glider are much longer than on a powered aircraft (compared to the body size), this aileron drag is sufficient to initially rotate the glider in the opposite direction to that of the turn.

So if you use right aileron to bank the glider right without using any rudder, initially the glider rotates slightly to the left. As the effect of the banked wings takes hold, a weak turn to the right starts - but the glider tends to remain weather-cocked into the slipstream so it starts to skid in the direction of the turn, rather than pointing cleanly into it. This is very inefficient and causes extra drag and loss of height. To overcome this effect, you need to use the rudder together with the ailerons to point the nose into the turn. But if you use too much rudder, the glider points towards the centre of the turn - again increasing drag and losing lift. In my case the natural tendency was to apply insufficient rudder and try to turn with mostly aileron alone, which is apparently commonplace, especially among powered pilots or people who've spent too long playing with flight simulators.

So how do you know if you are skidding or slipping ? On the canopy above your face a piece of coloured string is taped at one end. When flying it lies up the canopy due to the slipstream. If the glider is moving cleanly into the slipstream in a coordinated manoeuver, the string lies straight up the canopy. If however you do not have the stick and rudder correctly coordinated, the string swings one way or the other. For example, if the glider is skidding to the left, the string swings around so the top is off to the right. The trick is to imagine the string is an arrow with the head at the bottom (i.e. taped) end. If the string is pointing to the left, you need to apply more left rudder. If the string is pointing to the right, you need more right rudder. Keeping the string in the middle takes a fair amount of practice.

So far we had examined the effect of too much aileron. What about too much rudder ? If you are flying straight and use the rudder alone, the glider keeps on flying in the same direction, and simply rotates to point in the direction of the rudder, flying a little broadside to the slipstream. Again, this causes extra drag and loss of lift. So a glider is rather different to a powered aircraft with respect to turning. In a powered aircraft, you need very little rudder at all during turns. In fact, in a powered aircraft you spend most of the time flying in a straight line, whereas in a glider you spend most of the time turning, especially to stay in thermals. That's one reason why gliders are considered by many to be more interesting and challenging to fly than powered aircraft.

It is initially difficult to keep the rudder centred when you don't want any to be applied as there is no indication of the middle position. The easiest way to check if the rudder is centred is to lift your feet slightly off the pedals - if it is not centred the slipstream will centre it for you under normal conditions. The instructor pointed out that I was tending to keep a little left rudder applied when I thought it was centred. This is apparently another common fault for new students, and may be due to the greater left leg action when driving a car (the clutch takes a lot more effort than the accelerator).

So applying the correct amount of stick and rudder together is rather tricky and is really only learned through practice and experience - a bit like smooth gear changes in a car.

A little later in my flying I discovered how the outside view can muck up previously coordinated turns: After a short tow to only 1000 feet we skimmed along the underside of the blanket of cloud and very soon I found turn-coordination harder. In the past I had performed manoeuvers assisted by a visible horizon. When you turn, the angle of the horizon is a very helpful reference to pick and hold the desired bank angle - typically 30 degrees. However, today there was no horizon, just a fading gray distance. Furthermore, I had also been turning in previous flights at a rather higher altitude which reduced the contrast between low and high ground. Although the scenery around the airfield is lovely, it is definitely not flat. On this flight we were mainly above the ridge to the south of the airfield. As the tow had only been to 1000 feet, on one side the airfield was about 1000 feet below but on the other side the ridge-top was only a few hundred feet down. So when turning towards the ridge, the apparent horizon would suddenly rise up, encouraging me incorrectly to raise the nose, and at the same time distracting me from coordinating the turn

Earlier I had been using too little rudder, now I was using too much. After a while I began to settle down again - but I noted how much the outside view affected the whole process of entering and maintaining a turn. It was also clear that an inappropriate amount of rudder causes a lot of drag and slows the glider and also loses altitude. This is something to be avoided at lower altitudes, especially on the final approach when it can result in a stall and spin.