Center of Gravity

There are those who are happy with a glider that is flown anywhere within the glider's center of gravity (CG) range, then there are the others, who think the glider performs best with the CofG as close to the rear limit permitted by the manufacturer. The question begs, which is correct, is there an optimum (& where is it?), finally, how does it affect the glider's performance & handling?

Tailplane

Let’s start with the tailplane, what does it do? It effectively provides stability & looks after the wings ever-changing aerodynamic moments & demands. What are the designer's goals when designing a tailplane? They’re effectively trying to ensure stability with the least amount of tailplane drag, whilst providing a fair & reasonable center of gravity range. Finally, aerodynamic loads generated by the tailplane should be kept to a minimum, this will provide the minimum drag position, which in turn gets you more performance. You will find there is an optimum for an empty sailplane & one for full of water.

Fly Your CG Envelope 

So how do we find this point? Sadly it’s far & beyond what I’m capable of explaining, I’m more of a practical pilot in this regard. I learnt these skills from Makato Ichikawa though, as he was preparing for the Benalla World Gliding Championships. He was flying my Ventus 2a, strangely, I was flying (also) my Ventus 1a. We would take off with a known CG position, fly the day, then rate how the glider would handle & perform in a straight line, also in general. My Ventus 1a stayed at a stable CG so we had a reference to work from. Task by task (& we flew many!), we worked out the optimum. The question was sometimes as simple as, was I harder or easier to keep up with today Adam?

If you’re unable to do this where you fly from, a simple trick is to take off in the most forward (with a margin) CG that’s possible & allowed, go for a fly, see what it feels like. Then repeat for the most aft too. Now you know what both extremities feel like & how to correct them when you start the fine-tuning. Typically a flapped glider likes 75% of the allowable range, so this is a good place to start.

CG for Different Days

What conditions do you expect for the days flying ahead, this plays a part too. If it’s going to be a day of streeting, then you’d typically choose a more forward CG of your optimum range. If it’s a day that is predicted to have tight cores or lots of thermalling time, then a more rearward CG (of your determined optimum range) would be best. Why is this? Typically the glider feels like it’s more alive or more agile.

CG and Safety

Don’t forget about the safety implications of all this though. A rear center of gravity (near the limits – have you allowed for a fudge factor in your calculations or a weather event at the wrong time?) will cause the glider to spin easier. This shouldn’t happen though with the modern glider design, but still should be in the back of your mind. So, even the most experienced pilot should use caution with the most extreme rearward CofG position on their glider.
 

 Adam Woolley was born into the gliding world, being the 3rd generation in his family. Going solo at 15, his thirst for efficiency in soaring flight & quest for a world championship title to his name has never wavered. One big passion is sharing his experiences & joy with other glider pilots all around the world. Adam is an airline pilot in Japan on the B767 & spends his off time chasing summer around the globe. He has now won 7 national Championships & represented Australia at 5 WGC's & 1 EGC.