THE EFFECT OF CG ON HIGH-START LAUNCHING

Let me share an experinece I had yesterday to illustrate a point. And I am going to take you through my thought processes as I discovered and corrected the problem.

My main planes are a Supra and an AVA. I use these for contest flying in the Eastern Soaring League where I launch off winches. My Thermal Dancer has been relegated to a back-up plane position. I had not flown it in months, but I had it out yesterday. What I relate here was experienced on a winch, but it applies to HS launching as well. What we are giont to discuss is the impact of moving the CG.

Before I launched the plane, I did a hand throw. Seemed it needed a couple clicks of up to give me the glide I wanted. Having done that and completing a good range check, it was time to launch. Well I popped off 3 times in a row. Once I did get the plane in the air, it was flying very poorly. A dive test indicated the plane was nose heavy.

I have done several repairs to the nose and fuselage area of the Thermal Dancer over the time I have owned it. This has resulted in the plane becoming somewhat nose heavy. Since I have no removeable weight in the nose, I added 1/4 oz of lead to the tail over a year ago.

After some trial and error, I realized the tail weight was missing and added it back. I added 7 grams, about 1/4 oz, right in front of the verticle fin. This would be like taking about 3/4 to 1 oz of weight out of the nose. This sifted the CG back, giving my plane a much better balance. Several hand throws confirmed this and allowed me to adjust the elevator trim, removing several clicks of up trim. Now I was getting a nice smooth glide.

Now I was getting great launches and the plane was flying much better. WHY????

Because the plane was nose heavy, this CG was shifted forward, as compred to my normal CG. This effectively moves the hook position rearward in relation to the CG. I may have actually had the hook behind the CG. The further back the hook, relative to the CG, the more unstable the plane becomes on launch. Most people have the hook 3/8 to 1/8 inch in front of the CG. On my competition planes I have it right on the CG. But having it behind the CG makes for extremely difficult to control launches and a great tendency to pop-off.

By putting that weight on the tail, I effectively shifted the CG rearward. Now the hook was back in its proper position relative to the CG and the launches were as I expected them to be, straight, steep and high.

The second issue that comes up is that a nose heavy plane requires a lot of up elevator trim. That up trim on the elevator can become very effective at the speeds that a hi-start produces during the launch. This can lead to pop-offs. By shifting the CG back, I was able to remove some of that up trim, thus removing my tendancy to over rotate and pop-off.

I share these experiences because they apply equally to the hi-start and to the winch. The position of the hook, relative to the CG is critical to a smooth, high launch. If you change the CG of your plane, be aware of how this will change the relative position of the hook.

If you have a high tendency to pop-off on launch, consider that your plane might be nose heavy. Or, if you like the balance of your plane, consider setting up a launch mix that adds a few clicks of down elevator during the launch. This will prevent that over rotation during the critical first 50 feet of the launch. You can flip that mix off somewhere along the arch of the launch.